THE WEATHER AND CLIMATE OF CHICAGOTHE UNIVERSITY OF CHICAGO PRESÖ CHICAGO, ILLINOIS Agents THE CAMBRIDGE UNIVERSITY PRESS LONDON AND EDINBURGH THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO KARL W. HIERSEMANN LEIPZIG THE BAKER & TAYLOR COMPANY NEW YORKExhibition Room, Weather Bureau Office, Chicago, III.THE GEOGRAPHIC SOCIETY OF CHICAGO Bulletin No. 4 THE WEATHER AND CLIMATE OF CHICAGO BY HENRY J. COX Professor of Meteorology United States Weather Bureau AND JOHN H. ARMINGTON Local Forecaster United States Weather Bureau PUBLISHED FOR THE GEOGRAPHIC SOCIETY OF CHICAGO BY THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS Copyright 1914 By The Geographic Society of Chicago All Rights Reserved Published July 1914 Composed and Printed By The University of Chicago Press Chicago, Illinois, U.S.A.S&hS G- 8>3> y>/ ACKNOWLEDGMENT In presenting this monograph to the public, the Geographic Society of Chicago desires to acknowledge the kind assistance of the persons and companies named below, who by generous contributions defrayed a large portion of the publication expenses of thè volume : X Mr. Charles R. Crane Miss Helen Culver Dr. Thomas L. Gilmer Mr. Charles F. Gunther Mr. Frank Hamlin Mr. James O. Heyworth Mr. Charles L. Hutchinson Mr. Ralph Isham Mr. S. J. Larned Mr. Victor F. Lawson Mr. LaVerne Noyes Mr. Henry J. Patten Mr. Julius Rosenwald Carson, Pirie, Scott & Company Q Chicago Association of Commerce Commonwealth-Edison Company Consumers Company r Albert Dickinson Company Marshall Field & Company People's Gas Light & Coke Company Swift & CompanyTABLE OF CONTENTS PAGE Acknowledgment.................................................. v Introduction.................................................xxiii Preparation of the Bulletin.......................... . xxiii Determination of Climate.................................xxiii Location and Environment of Chicago.......................xxiv Treatment of the Subject....................... . . xxiv PART I Temperature..................................................3-148 Definitions................................................ 3 Period of Observations...................................... 4 Mean Temperature, Annual, Seasonal, Monthly ... 5 Departure from Normal, Monthly and Annual .... 15 Warm and Cold Months and Seasons........................... 20 Succession of Seasons, Years, and Months................... 23 Mean Daily Temperatures . . . . . . . . 25 Mean Daily Change in Temperature ...... 30 Frequency of Changes of Stated Amounts in Mean Daily Temperature............................................ 31 Daily Normal Temperatures.................................. 33 Examples of Departures from Daily Normal Temperatures, Selected Years......................................... 34 Examples of Departures from Daily Normal Temperatures, Warmest and Coldest Months............................. 37 Influence of Lake Michigan on Temperature at Chicago . . 37 Comparisons of Temperature at Chicago with Temperatures of Other Places........................................ 46 (1) Near-by Locations: LaGrange.......................... 46 (2) Northern Illinois .................................. 53 (3) Other Portions of the United States.................. 55 Absolute Monthly Maximum and Minimum Temperatures . 65 Frequency of Days with Temperature of 90° or Over . . 69 Longest Periods of Consecutive Days with Maximum Temperature of 90° or Over........................................ 71 Frequency of Days with Zero Temperature, Maximum and Minimum................................................ 73 Longest Periods of Consecutive Days with Temperatures of Zero or Below.......................................... 76 viiviii TABLE OF CONTENTS PAGE Occurrence of Freezing Temperature, Seasonal .... 78 (1) Number of Days with Maximum Temperature of 32° or Below ................................................ 78 (2) Longest Periods of Consecutive Days with Maximum Temperature of 32° or Below........................... 80 (3) Number of Days with Minimum Temperature of 32° or Below............................................. 81 (4) Longest Periods of Consecutive Days with Minimum Temperature of 32° or Below........................... 84 Occurrence of Temperatures Favorable to Plant Growth . 85 List of Warm Days............................................. 87 Lists of Cold Days ........................................... 93 Daily Extremes of Temperature, Absolute ... . . 93 Interval between the Occurrence of Lowest and Highest Temperatures, Winter to Summer........................... 105 Occurrence of Frost.......................................... 107 (1) Minimum Temperature of 40° (Light Frost Tempera- ture) ............................................... 108 (2) Killing Frosts........................................ Ill (3) Minimum Temperature of 32°............................ 112 Influence of Lake Michigan on Frosts......................... 113 Range in Temperature......................................... 114 Frequency of Marked Rises and Falls of 20° or More in Temperature....................................... . 122 (1) Within 24 Hours....................................... 122 (2) Within 1 Hour ........................................ 130 Hourly Temperature Conditions................................ 133 (1) Mean Hourly Temperatures by Months .... 133 (2) Temperature phases.................................... 136 (3) Hourly Departures from Mean Hourly Temperatures . 138 (4) Mean Hourly Change in Temperature .... 141 Illustrations of Secondary Controls of Temperature . . . 141 (1) Effect of Lake and Land Winds......................... 142 (2) Temperatures of Cold Days ............................ 145 (3) Effect of Wind Direction on Temperature . . . 145 (4) Effect of Cloudiness and Rainfall on Temperature . 146 PART II Precipitation............................................. • 151-236 Definitions .......................................... . 151 Period of Observations....................................... 151 Annual, Seasonal, and Monthly Precipitation .... 152 Influence of Exposure in Measurement of Precipitation . . 160TABLE OF CONTENTS ix PAGE Comparisons of Precipitation at Chicago with Precipitation at Other Places........................................... 163 (1) Northern Illinois....................................... 163 (2) Other Portions of the United States . 165 Frequency of Precipitation, Annual and Monthly . . . 167 Wet Spells..................................................... 171 Annual Number of Days with Precipitation of Stated Amounts................................................. 177 Excessive Precipitation........................ . . . 178 (1) 1 Inch or More an Hour.................................. 178 (2) 2.50 Inches or More in 24 Consecutive Hours . . 179 (3) Greatest Precipitation in 24 Consecutive Hours, Monthly and Annual ................................... 180 (4) Maximum Precipitation in Short Periods . . . 183 (5) Accumulated Amounts of Excessive Pains . . . 183 Duration of Precipitation...................................... 185 Rain and Snow Storms of Long Duration . . . . . 191 Periods of Drouth.............................................. 192 Frequency of Precipitation, Daily.............................. 198 Normal Precipitation, Annual, Monthly, and Daily . . . 199 Examples of Departures from Daily Normal Precipitation, Selected Years............................................ 201 Hourly Precipitation........................................... 203 (1) Mean Hourly Rainfall.................................... 203 (2) Hourly Frequency of Precipitation....................... 205 Summary of Precipitation Data.................................. 209 Thunderstorms.................................................. 209 (1) Annual and Monthly Frequency............................ 209 (2) Hourly Frequency........................................ 209 Hail........................................................... 214 Snow...................................................... 215-236 How Measured................................................ 215 Seasonal and Monthly Snowfall............................... 216 Heavy Snowfalls............................................. 222 (1) Greatest Snowfall in 24 Consecutive Hours, Monthly 222 (2) Heavy and Damaging Storms of Snow, Sleet, and Ice.................................................. 224 Frequency of Snowfall....................................... 227 Depth of Snow on Ground..................................... 228 Dates of First Snowfall in Autumn and Last in Spring . 232 Proportion of Rainfall to Melted Snowfall .... 233 Distribution of Snowfall in City . . . . . 235 Summary of Snowfall Data.................................... 236x TABLE OF CONTENTS PAGE PART III Atmospheric Moisture ...................................... 239-249 Definitions................................................ 239 Relative Humidity, Annual and Monthly...................... 240 Relative Humidity, Hourly.................................. 243 Comparison of Relative Humidity at Chicago with That of Other Portions of the United States.................... 247 Dew Point, Annual and Monthly.............................. 249 PART IV Cloudiness and Sunshine.................................... 253-274 Average Cloudiness, Monthly and Annual..................... 253 Number of Clear, Partly Cloudy, and Cloudy Days. . . 254 Time of Sunrise and Sunset, Length of Twilight, and Equation of Time for Chicago.................................... 257 Average Sunshine, Monthly and Annual . . . . . 257 (1) Average Duration of Sunshine in Hours .... 258 (2) Percentage of Possible Sunshine.......................259 Hourly Sunshine Conditions . . 260 (1) Average Hourly Percentage of Sunshine , . . 260 (2) Sunshine Phases...................................... 262 Number of Days with 1 Hour or More of Sunshine, Monthly and Annual............................................ 262 Longest Periods of Continuous Sunshine . . . . . 264 Longest Periods without Sunshine........................... 265 Comparison of Percentage of Sunshine at Chicago with That of Other Portions of the United States .... 265 Occurrence of Dense Fog.................................... 268 Dark Days.................................................. 269 Effect of Cloudiness and Sunshine upon Temperature and Relative Humidity...................................... 272 PART V Wind Direction and Velocity................................ 277-317 Definitions............................................... 277 Prevailing Wind Direction, Monthly and Annual . . . 279 Total Wind Movement, Monthly and Annual .... 282 Greatest Daily Wind Movement............................... 289 Heavy Storm Winds.......................................... 289 Maximum Wind Velocities . . . . . . . . 292 Frequency of Gales......................................... 292 Mean Hourly Wind Velocity ..................................297 Prevalence and Velocity of Winds from Different Directions 297TABLE OF CONTENTS xi PAGE Resultant Direction and Movement.......................... 302 Prevailing Hourly Wind Direction.......................... 304 Comparison of Wind Velocity at Chicago with That at Other Cities of the United States...................... 309 Comparison of Wind Velocity with Other Conditions . . 310 Summary of Wind Data . . . . . . . . 314 Tornadoes . . 314 PART VI Barometric Pressure....................................... 321-333 Importance............................................... 321 Measurement............................................... 321 Mean Station Pressure, Monthly and Annual .... 323 Mean Departure from Normal Pressure, Monthly and Annual 323 Highest and Lowest Pressures, Sea Level, by Years . . . 323 Mean Hourly Pressure by Months............................ 326 Mean Hourly Pressure Departures............................327 Pressure Phases . . 327 Relation between Pressure and Other Conditions Illustrated 327 PART VII Storm Tracks............................................. 337-353 Average Storm Tracks...................................... 337 Tracks of Selected Storms, Cold Waves, and Hot Waves . 339 PART VIII Conclusion.................................................... 357 APPENDIXES Appendix I: Weather of Holidays................................361 Appendix II: Comparative Data on Temperature and Precipitation, World ................................................. 365 Appendix III: Journal Entries Relative to the Chicago Fire of 1871 ........................................ 367 Appendix IV: Notes Relative to Data, Observations, Instruments, Officials in Charge, and Forecast Service 370LIST OF TABLES Tempebatuke PAGE I. Mean Temperature, Annual, Seasonal, Monthly . . 6 II. Monthly and Annual Mean Temperatures, Four Exposures ................................. . . .10 III. Monthly and Annual Departures from Normal Tempera- ture ...........................................16 IV. Yearly Accumulated Departures from Normal Tempera- tures ..........................................18 Y. Warm Months and Seasons........................20 YI. Cold Months and Seasons........................20 VII. Mean Temperatures, Selected Warm and Cold Months 21 VIII. Warmest and Coldest Seasons and Years ... 22 IX. Succession of Seasons..........................23 X. Mean Daily Maximum Temperatures .... 25 XI. Mean Daily Minimum Temperatures . . . .26 XII. Mean Daily Temperatures........................27 XIII. Mean Daily Change in Temperature .... 30 XIY. Average Number of Changes in Daily Mean Temperature of Stated Amounts, Etc......................32 XV. Normal Temperature, Annual, Monthly, Daily . . 33 XVI. Daily Departures from Normal Temperature, 1911, Warmest Year..................................34 XVII. Daily Departures from Normal Temperature, 1875, Coldest Year..................................35 XVIII. Daily Departures from Normal Temperature, 1891, Nearly Normal Year . . . . . . .36 XIX. Comparison between Air Temperature and Water Temperature of Lake Michigan, 1884 .... 39 XIXa. Monthly and Annual Mean Temperatures, Lake and Air . 42 XX. Temperature and Prevailing Wind Direction, Chicago and LaGrange, 1904 47 XXI. Mean Temperatures, Stations in Northern Illinois . 54 xiixiii PAGE 57 65 65 67 68 70 72 74 75 76 77 77 79 81 82 84 86 88 94 97 99 101 # 106 LIST OF TABLES Monthly and Annual Normal Temperatures, Selected Cities in the United States...................... Average Number of Days Each Month with Maximum Temperature of 90° or Above, Selected Cities . Average Number of Days Each Month with Minimum Temperature of Zero or Below, Selected Cities . Absolute Maximum Temperatures, Monthly . Absolute Minimum Temperatures, Monthly . Number of Days with Maximum Temperature of 90° or Over.......................................... Longest Periods of Consecutive Days with Maximum Temperature of 90° or Over....................... Number of Days with Minimum Temperature of Zero or Below......................................... Number of Days with Maximum Temperature of Zero or Below......................................... Longest Periods of .Consecutive Days with Maximum Temperature of Zero or Below . . Longest Periods of Consecutive Days with Minimum Temperature of Zero or Below..................... Longest Periods of Consecutive Hourly Temperatures of Zero or Below, by Winters........................ Number of Days with Maximum Temperature of 32° or Below, Seasonal.................................. Longest Periods of Consecutive Days with Maximum Temperature of 32° or Below, Seasonal . Number of Days with Minimum Temperature of 32° or Below, Seasonal :................... Longest Periods of Consecutive Days with Minimum Temperature of 32° or Below, Seasonal Annual Number of Days with Mean Temperature above 42° ....................................... List of Warm Days, May to September .... List of Cold Days, December to February List of Cold Days, November and March List of Cold Days, April and October .... Daily Extremes of Temperature.................... Time of Occurrence of Annual Minimum and Maximum Temperatures, with Interval......................PAGE 109 111 113 115 119 123 123 126 126 127 127 128 131 131 132 132 134 137 139 140 153 155 156 156 161 163 165 168 LIST OF TABLES Last and First Occurrence of Minimum Temperature of 40° (Light Frost Temperature)...................... Last and First Occurrence of Killing Frosts . Last and First Occurrence of Minimum Temperature of 32°............................................. Absolute Monthly and, Annual Ranges in Temperature Greatest Daily Ranges in Temperature, Monthly and Annual........................................... Number of 24-Hour Temperature Rises of 20° or More Number of 24-Hour Temperature Falls of 20° or More Number of 24-Hour Temperature Rises of 30° to 39° . Number of 24-Hour Temperature Rises of 40° or More Number of 24-Hour Temperature Falls of 30° to 39° . Number of 24-Hour Temperature Falls of 40° to 49° . Number of 24-Hour Temperature Falls of 50° or More Greatest 24-Hour Falls in Temperature, Monthly and Annual . .............................. Greatest 24-Hour Rises in Temperature, Monthly and Annual........................................... Temperature Rises of 15° or Over in 1 Hour . Temperature Falls of 20° or Over in 1 Hour . Mean Hourly Temperatures........................... Temperature Phases................................. Hourly Departures from Mean of Hourly Temperatures, Monthly . .................................. Mean Hourly Changes in Temperature .... Precipitation Monthly and Annual Precipitation................... Seasonal Precipitation......................... Seasons of Excessive Precipitation................. Seasons of Deficient Precipitation . Monthly and Annual Precipitation, 4 Exposures . Monthly and Annual Precipitation, Northern Illinois and Eastern Iowa............................... Monthly and Annual Precipitation, Selected Cities of the United States . . . Monthly and Annual Frequency of Precipitation .LIST OF TABLES xv PAGE LXXIII. Average Monthly and Annual Number of Days with Precipitation of 0.01 Inch or More, Selected Cities of the United States...............................................170 LXXIY. Longest Periods of Consecutive Days with 0.01 Inch or More Precipitation, Monthly and Annual . . .173 LXXY. Longest Periods of Consecutive Days with “Trace” or More Precipitation, Monthly and Annual . . . 174 LXXYI. Wet Spells.................................................175 LXXYII. Annual Number of Days with Precipitation of Stated Amounts..................................... 177 LXXVIII. Excessive Precipitation: 1 Inch or More in 1 Hour . 179 LXXIX. Excessive Precipitation: 2.50 Inches or More in 24 Hours 180 LXXX. Greatest Precipitation in 24 Consecutive Hours . .181 LXXXI. Maximum Precipitation in Stated Periods . . .184 LXXXII. Excessive Precipitation: Accumulated Amounts in 5- Minute Periods.....................................186 LXXXIII. Summary of Excessive Precipitation, 5-Minute Periods 189 LXXXIY. Length of Precipitation.....................................190 LXXXY. Rain and Snow Storms of Long Duration . . . 192 LXXXYI. Periods of Drouth with No Appreciable Precipitation . 193 LXXXYII. Periods of Drouth with No Precipitation . . . 193 LXXXYIII. Dry Spells of 2 Weeks or Longer.............................194 LXXXIX. Drouth Preceding Chicago Fire of 1871 .... 197 XC. Summary of Dry Spells, by Months .... 198 XCI. Frequency of Precipitation on Each Day of the Year . 199 XCII. Normal Precipitation, Daily, Monthly, and Annual . 200 XCIII. Daily Departures of Precipitation, 1909, Wet Year . 201 XCIV. Daily Departures of Precipitation, 1910, Dry Year . 202 XCY. Daily Departures of Precipitation, 1911, Nearly Normal Year..................................................203 XCYI. Mean and Total Hourly Rainfall, by Months . . 204 XCYII. Hourly Frequency of Precipitation (Total Occurrences) 206 XCYIII. Hourly Frequency of Precipitation (Percentages) . . 208 XCIX. Summary of Precipitation Data..............................210 C. Number of Thunderstorms, Monthly and Annual . . 211 CL Frequency and Time Distribution of Thunderstorms . 213 CII. Frequency of Occurrence of Hail..........................215 CIII. Monthly and Seasonal Snowfall............................217XVI LIST OF TABLES CIV. CV. CVI. CVII. CVIII. CIX. CX. CXI. CXII. CXIII. CXIV. CXV. CXVI. CXVII. CXVIII. CXIX. cxx. CXXI. CXXII. CXXIIa. CXXIII. CXXIV. exxv. Annual Depth of Snowfall ....... Greatest Snowfalls in 24 Consecutive Hours . Monthly and Seasonal Number of Days with Snowfall of 0.1 Inch or More................................ Amount of Snow on Ground at End of Month Greatest Depth of Snow on Ground, and Date, Monthly Dates of First Snowfall in Autumn and Last in Spring Summary of Snowfall Data.......................... Atmospheric Moisture Mean Monthly and Annual Relative Humidity . Relative Humidity and Temperature, May, 1911, to April, 1912........................................ Mean Monthly and Annual Relative Humidity, Selected Cities in the United States .... Mean Monthly and Annual Dew Point .... Cloudiness and Sunshine Mean Monthly and Annual Cloudiness .... Annual Number of Clear, Partly Cloudy, and Cloudy Days............................................... Average Monthly and Annual Number of Clear, Partly Cloudy, and Cloudy Days..................... Times of Sunrise and Sunset, Length of Twilight, Number of Hours of Sunshine Possible, and Equation of Time............................................ Average Monthly and Annual Number of Hours of Sunshine.............................. Monthly and Annual Percentage of Possible Sunshine Average Hourly Duration of Sunshine .... Sunshine Phases........................... Number of Days with 1 Hour or More of Sunshine, Monthly and Annual................................. Periods of 4 or More Consecutive Days with 100 Per Cent Sunshine ..................................... Periods of 4 or More Consecutive Days without Sunshine ............................................. Monthly and Annual Percentage of Possible Sunshine, Selected Cities in the United States .... PAGE 220 223 228 230 231 233 235 241 243 247 249 253 255 256 256 258 260 261 263 264 265 266 266LIST OF TABLES xvii PAGE CXXVI. Number of Days with Dense Fog.............269 CXXVII. Dark Days.................................270 Wind Direction and Velocity CXXVIII. Monthly and Annual Prevailing Direction of Wind . 280 CXXVIIIa. Frequency of Monthly Prevailing Wind Directions . 281 CXXIX. Total Wind Movement, Annual and Monthly . . 284 CXXX. Greatest Daily Wind Movement, Each Month and Year 290 CXXXI. Maximum Wind Velocities . . . . . . . 293 CXXXII. Monthly and Annual Frequency of Gales . / . 294 CXXXIII. Mean Hourly Wind Velocity.............................298 CXXXIV. Average Number of Miles of Wind, and Time of Blowing, from Each Direction, Monthly and Annual . 300 CXXXV. Prevailing Hourly Wind Direction, Chicago, 111. . . 305 CXXXVI. Prevailing Hourly Wind Direction, Davenport, Iowa . 307 CXXXVII. Average Hourly Wind Velocity, by Months, 1909, Selected Cities in the United States . . . .310 CXXXVIII. Monthly Maximum Wind Velocities, 1909, Selected Cities in the United States.......................312 CXXXIX. Summary *of Wind Velocity and Direction Data . .315 Barometric Pressure Mean Monthly and Annual Station Pressure . . . 322 Monthly and Annual Departures from Normal Station Pressure...........................................324 Highest and Lowest Observed Pressures, Sea Level . 324 Mean Hourly Pressure, by Months.....................325 Mean Hourly Departures from Mean Daily Pressures, by Months..........................................318 Pressure Phases.....................................330 Appendixes CXLVI. Weather of New Year’s Day, Fourth of July, and Christmas Day.....................................362 CXLVII. Rainfall and Temperature Data for Various Selected Cities of the World...............................366 CXL. CXLI. CXLII. CXLIII. CXLIV. CXLV.LIST OF PLATES PAGE I. Departures of Mean Monthly, Seasonal, and Annual Temperature from the Normal..............................facing 5 II. Contrast between Daily Mean Temperatures of Warmest and Coldest Months.......................................38 III. Normal Temperature of Air at Surface, United States, Annual 56 IV. Highest Temperatures Ever Observed, United States . . 59 Y. Lowest Temperatures Ever Observed, United States . . 60 VI. Maximum Temperatures in Hot Wave of July, 1901 . . 62 VII. Temperatures at 7 a.m. in Cold Wave of February, 1906 . . 64 VIII. Departures of Monthly, Seasonal, and Annual Precipitation from the Normal..............................facing 156 IX. Normal Annual Precipitation in the United States . . . 164 XVlllLIST OF FIGURES Temperature 1. Annual Mean Temperatures................................ 2. Average Monthly Temperatures, 4 Exposures............... 3. Maximum and Minimum Temperatures, Auditorium and Federal Building, August and December, 1905 ............... 4. Daily March of Temperature . .................... 4a. Relation of Lake and Air Temperatures, Mean .... 46. Relation of Lake and Air Temperatures, 1911, Warm Year 4c. Relation of Lake and Air Temperatures, 1904, Cold Year . 5. Monthly and Annual Normal Temperatures, Selected Cities of the United States.............................. 6. Average Number of Days with Temperature of 90° or Above, and of Zero or Below, Selected Cities of the United States 7. Absolute Maximum, Mean, and Minimum Temperatures, Annual 8. Annual Number of Days with Maximum Temperature of 90° or Over .................................................. 9. Annual Number of Days with Minimum Temperature of Zero or Below................................... 10. Seasonal Number of Days with Maximum Temperature of 32° or Below.................................................. 11. Seasonal Number of Days with Minimum Temperature of 32° or Below.................................................. 12. Longest Period in Each Winter with Minimum Temperature of 32° or Below........................ 13. Annual Number of Days with Mean Temperature above 42° 14. Interval between Occurrence of Lowest and Highest Tempera- tures .....................................' 15. Interval between Last and First Occurrence of Temperature of 40° (Light Frost Temperature).......................... 16. Interval between Last and First Occurrence of Temperature of 32° 17. Annual Range in Temperature.............................. 18. Greatest Monthly Range in Temperature.................... 19. Days of Greatest and Least Range in Temperature . 20. Average Number of 24-Hour Temperature Rises and Falls of 20° or Over................................................ 21. Annual Frequency of 24-Hour Temperature Changes of 20° or Over .................................................. PAGE 6 10 11 28 43 45 45 58 66 69 71 74 80 83 85 87 107 110 114 117 118 121 124 124 xixXX LIST OF FIGURES PAGE 22. Greatest Falls in Temperature in Any 24 Consecutive Hours or Less................................................. . . 129 23. Greatest Rises in Temperature in Any 24 Consecutive Hours or Less...........................................................130 24. Mean Hourly Temperature . .................................135 25. Mean Hourly Change of Temperature................................141 26. Lake Influence, July 21, 1901....................................143 27. Effect of Lake and Land Winds........................... . . 144 28. Examples of Temperatures on Cold Days ........................146 29. Effect of Wind Direction on Temperature . . . . 147 30. Effect of Character of Day (Cloudiness, Sunshine, and Rainfall) on Temperature . . . . 147 Precipitation 31. Variation in Annual Amount of Precipitation......................152 32. Mean Monthly Precipitation . ..........................158 33. Monthly Precipitation, Highest, Lowest, and Mean . . .159 34. Total Precipitation during Dry Year, 1867, Normal Year, 1896, apd Wet Year, 1858 ... y ..... . 159 35. Monthly and Annual Precipitation, 4 Exposures . . . .162 36. Monthly and Annual Precipitation, Selected Cities in the United States....................................................166 37. Variation in Annual Frequency of Days with Appreciable Pre- cipitation ....................................................169 38. Monthly Frequency of Appreciable Precipitation . . . .170 39. Average Number of Days with Precipitation, Selected Cities of the United States . . ..............................172 40. Heaviest Precipitation in 24 Consecutive Hours .... 183. 41. Average Hourly Precipitation................................205 42. Hourly Frequency of Precipitation...........................207 43. Frequency of Thunderstorms, Annual . . . . . . 212 44. Frequency of Thunderstorms, Monthly .............................212 45. Frequency and Time Distribution of Thunderstorms . . .214 46. Total Snowfall by Winters, and Average Monthly Snowfall . . 218 47. Annual Depth of Snowfall ........................................221 48. Average Annual Snowfall in the United States................222 49. Monthly Frequency and Amount of Snowfall . . . . . 229 50. Distribution of Snowfall in Chicago, November 26, 1903 . . 234 Atmospheric Moisture 51. Mean Relative Humidity and Temperature, by Months, May, 1911, to April, 1912 ..........................................244LIST OF FIGURES xxi PAGE 52. Relative Humidity and Temperature, Bi-Hourly, May, 1911, to April, 1912............................................ 245 53. Average Monthly and Annual Relative Humidity, Selected Cities in the United States.....................................248 Cloudiness and Sunshine 54. Relative Frequency of Clear, Partly Cloudy, and Cloudy Days . 255 55. Relative Duration of Daylight, Twilight, and Darkness . . 257 56. Mean Hourly Sunshine . ...............................'. 263 57. Average Monthly and Annual Percentage of Possible Sunshine, Selected Cities in the United States........................267 58. Effect of Cloudiness and Sunshine on Temperature and Relative Humidity.............................................273 Wind Direction and Velocity 59. Average Hourly Wind Velocity, 4 Exposures..................286 60. Wind Roses, Auditorium Tower and Federal Building . . . 287 61. Frequency of Storm Winds ......................................294 62. Mean Hourly Wind Velocity..................................299 63. Average Duration and Velocity of Wind, Different Directions . 302 64. Resultant Wind Direction . 303 65. Prevailing Hourly Wind Direction, Chicago..................306 66. Prevailing Hourly Wind Direction, Davenport................308 67. Mean Hourly Wind Velocity..................................311 68. Average Monthly Values, Temperature, Precipitation, Sunshine, and Wind.................................................313 69. Average Hourly Values, Temperature and Wind .... 314 70. Tornado Track, Cook County, 111., May 25, 1896 . . . .316 Barometric Pressure 71. Mean Hourly Pressure..................................326 72. Mean Hourly Pressure Curves, January, April, July, October, and Year....................... 329 73. Pressure, Temperature, Weather, and Wind, in Cold Wave . .331 74. Examples of Warm Days........................... . . 332 75. Daily Local Record, December 27, 1904 333 Storm Tracks and Storm Movement 76. Mean Tracks and Daily Movement of Storms in the United States 337 77. 78. Weather Maps of November 21 and 23, 1903, Storm from Northwest...............................................340 79, 80. Weather Maps of January 20 and 21, 1895, Storm from Colorado .................................................341LIST OF FIGURES xxii PAGE 81, 82. Weather Maps of December 26 and 27, 1904, Storm from West Texas.................................................344 83, 84. Weather Maps of February 18 and 19, 1908, Storm from Arkansas...................................... ........345 85. Weather Map of September 8, 1900, Galveston Hurricane . . 346 86-88. Weather Maps of April 7, 8, and 9, 1900, Development of High- Pressure Area over Great Lakes in Spring .... 347-48 89-92. Weather Maps of February 11, 12, 13, and 14,1899, Movement of Cold Wave from the Northwest . ..... 348-50 93, 94. Weather Maps of February 7 and 8, 1895, Cold Wave at Chicago...............................................351 95, 96. Weather Maps of July 21 and 22, 1901, Hot Wave at Chicago 352 Appendixes 97. Weather of New Year’s Day...............................363 98. Weather of Fourth of July...............................363 99» Weather of Christmas Day................................364INTRODUCTION The task of preparing this publication has progressed rather slowly, because it was necessary to do the work at such times as regular office duties permitted. The preparation of the illustrations and tabular matter has covered a period of nearly five years, and it is because of the early completion of some of the graphs that they have not in every case been brought down to date. An effort has been made, however, to include in the various tables the most recent data available, and the records of 1913 have been entered as far as possible. Were it not for the varying storm movement of the earth’s atmosphere, the climate of any place would be determined largely by its distance from the equator, its elevation above sea level, and its surroundings, such as large bodies of water, level plains, and mountain ranges. In the middle latitudes, especially, the influence of storm movement is strong, and as a result endless changes in winds, clouds, rain and snow, heat and cold, etc., sweep the belt in which Chicago is situated, and cause variety in its weather conditions. The city is located in latitude 41°35' North, somewhat less than half-way from the equator to the pole, on a crescent-shaped plain gradually rising from the level of Lake Michigan, whose altitude is about 581 feet above mean sea level. This plain at its highest point is considerably less than 100 feet above the surface of the lake, and its greatest width is approximately 15 miles in a northeast-southwest direction. The whole plain is bordered inland by a glacial moraine which rises in places to about 150 feet above the higher portions of the plain itself. This rim is far too low to exert any appreciable effect upon the climatic or weather conditions of the city, and forms no barrier to either cold-wave areas from the west or hot winds from the southwest and south. Such barrier, however, is but infrequently needed, as many times the waters of the lake serve to soften the rigors of the wintry storms or to moderate the intensity of the summer’s heat. Located as it is at the southwest corner of the lake, Chicago is justly proud of its climate. As a consequence of the cool expanse of water in summer, the city often enjoys delightful and refreshing XXlllXXIV INTRODUCTION breezes while the interior of the country away from the lake is sweltering in an air hot and still almost to the point of suffocation. The city is close to certain well-defined storm tracks, and the passage of these disturbances insures ample precipitation and interrupts the otherwise monotonous cycles of temperature and weather change. Chicago has been called the “ Windy City,” and the ensuing pages will show that there is some reason for the sobriquet, although the wind movement here is not much greater than it is at other places in the Great Lakes region. Its changes in weather are often sudden and pronounced, but usually are of such a character as to have a stimulating effect upon the average person of health; and it is not at all improbable that the great energy of its people, which has resulted in the rapid upbuilding of Chicago, is due largely to the peculiar nature of its climate. In the various topics in this bulletin, the features of climate and weather are treated together. By weather we mean the passing conditions from hour to hour, and from day to day, while by climate we mean the sum total, as it were, of weather for many years. Hann says in his Handbook of Climatology (translated by Ward, p. 1): By climate we mean the sum total of the meteorological phenomena that characterize the average condition of the atmosphere at any one place on the earth’s surface. That which we call weather is only one phase in the succession of phenomena whose complete cycle, occurring with greater or less uniformity each year, constitutes the climate of any locality. Climate is the sum total of the weather as usually experienced during a longer or shorter period of time at any given season. By the method of treating climate and weather together, the data can easily be grouped or correlated under principal topics; that is, all temperature data, whether displaying climatic features or weather conditions, will be gathered under one general head of temperature, and similarly those for rainfall and snowfall will be gathered under precipitation, etc. For purposes of study this method will be found more satisfactory than that in which an effort is made to separate the different phases of the subject into two divisions, and it will be. equally useful for reference. It is often very difficult to decide just where the dividing line between climate and weather runs; and, as a matter of fact, a compilation may in many cases be used to demonstrate the qualities of both. In the chief divisions of this work, the order adopted is that of annual, seasonal, and monthly values, followed by the discussion ofINTRODUCTION XXV daily conditions. It is recognized that the common custom in this respect has been reversed, but the arrangement makes possible the grouping of the daily values in great detail, so that the panorama of weather changes presented to the reader is unbroken, while at the same time the delineation of the various climatic features is as comprehensive as would be the case in any other order. Temperature, precipitation, atmospheric moisture, cloudiness and sunshine, wind direction and velocity, and barometric pressure will be discussed in turn. The data on which these discussions are based are chiefly those of the official records, beginning with the establishment of the local Weather Office in October, 1870. A series of temperature and precipitation records prior to that time, obtained from observations made at certain points in Chicago or its vicinity, were available, and such have been used in the discussion of those subjects, in addition to the official data. The Weather Office has been moved from one location to another within the city on several occasions, and for various reasons, but chiefly in order to secure the best possible exposure for the wind instruments. Detailed references to these changes will be made at appropriate places in this bulletin. The authors desire to express their indebtedness to Professor Charles F. Marvin, Chief of the United States Weather Bureau, for his kindness in reading the manuscript of this bulletin, and for many valuable suggestions in connection with the several subjects. Much use was made of Professor Oliver L. Fassig’s work on the climate and weather of Baltimore (Maryland Weather Service, Vol. II), in determining the form and character of a great deal of the statistical data used in this volume. The authors desire also to thank the following members of the force of the Chicago Weather Bureau office for their generous assistance in the preparation of the statistical matter and in reading the proofs: Mr. E. H. Haines, Mr. A. R. Thorson, Mr. E. Q. Band, Jr., Mr. A. G. J. Kautz, Mr. F. J. Thomas, Mr. Wilfred P. Day, Mr. L. A. Cambridge, and Mr. C. I. Dague. Especial acknowledgment is due Mr. Band, who drew most of the graphs and figures.PART I TEMPERATURETEMPERATURE When we speak of the temperature of the air we mean the measure of its condition with respect to heat at auy particular instant.' If, however, we wish to extend the term so as to convey knowledge of general conditions covering many successive instants— a day, a month, a season, or a year—we must combine the measures of the individual cases into average or mean values. The average of the temperatures observed at each of the twenty-four hours of the day gives the most satisfactory mean temperature for that period, but, in the absence of an automatically recording instrument, it is not at all times practicable to make the computation in such manner. Instead, the average of certain temperature values, which by long comparison have been found to differ but slightly from the mean of all the hourly readings, is substituted. The most common method in the United States is to divide the sum of the highest (maximum) and the lowest (minimum) temperatures of the day by 2, and this has been the practice of the Weather Bureau from 1872 to the present time. Previous to 1872 the average of the temperatures observed at 7 a.m., 2 p.m., and 9 p.m. was taken as the daily mean. Results obtained by either method ordinarily differ from the mean of the twenty-four hourly readings by less than |° throughout the year. In presenting temperature data for the various months, the terms mean maximum and mean minimum are in common use, and express the average of the daily maxima and minima, respectively. The mean temperature of any month is the average of its daily mean temperatures. We may obtain a close approximation of this by dividing the sum of the mean maximum and the mean minimum by 2, while the average of the mean temperatures of the twelve months of the year gives the mean temperature of that year. When a series of mean temperatures for any month is averaged, the result is the mean monthly temperature for that month for that period, and its deviation from the normal—that is, the mean of a very large number of months—depends upon the length of the record. Similarly, the mean temperature of a series of years may be averaged, producing the mean annual temperature for that period, and if the period be sufficiently long, the mean value is the normal temperature of the year. 34 THE WEATHER AND CLIMATE OF CHICAGO It is obvious that these various means, daily, monthly, and annual, must change slightly from year to year as long as there is not a sufficient record to smooth out the effects of abnormal conditions, and consequently they cannot be used to advantage in making comparisons. For this reason, mean values for certain periods are arbitrarily assumed or adopted as normals, and comparisons are referred to them rather than to the changing means. This distinction should be clearly borne in mind, as the terms mean and normal are sometimes confused. In this work, the word normal in the subsequent pages will designate the adopted normal, as the period of record is for most elements too short to produce mean values not liable to some change by succeeding observations. Previous to the commencement of the observations of the Weather Bureau in Chicago in 1870, temperature readings as far back as 1830 were made at Fort Dearborn, the University, and at other places in the city, but prior to 1859 these observations were far from continuous. Since 1859 there has been no break in the series. During the periods of broken temperature records at Chicago, readings were made at various places in northern Illinois, mostly in co-operation with the Smithsonian Institution at Washington, D.C. In 1893, the late Professor Henry A. Hazen, in preparing his bulletin on the climate of Chicago, gathered up these old records and supplied for the city temperature readings for the years in which local observations were unavailable, by interpolating the data from the records made outside the city, but in near-by locations. The temperature data appearing in this book for the period prior to 1871 are taken from Hazen’s Climate of Chicago. They are not official, and the manner of exposing the thermometers, and their environment, doubtless differed considerably from that of the present day. .No definite information is at hand on the subject, so that we can take the readings merely for what they are worth. The thermometers of the Weather Office from its establishment until the removal to the Chicago Opera House Building, January 1, 1887, were exposed in a latticed shelter outside the office windows; but since the latter date the shelter has been located on the roof, the object being to place the thermometers in such a position as to protect them from sunshine or other direct radiated heat, and yet furnish the advantage of free air circulation. Apparent discrepancies in the unofficial records will later be pointed out, and examples given to show how much depends upon the location of the thermometer, and how the readings of instru-18 50 1840 tBSo I860 I&TO f880 f 890 f9 00 1910 ita hi ¿*3.7 Sept. Oct Nov. 52.3 Year. 48.5 1850 184-0 J850 I860 ¡8 70 1880 189 O 1900 1910 Plate I.—Departures of mean monthly, seasonal, and annual temperature from normal from 1830 to 1910. Normals are shown in figures under names of months, seasons, and years.TEMPERATURE 5 ments, even in shelters not far apart, may differ appreciably. Because of these facts, the temperature data in this bulletin, and the discussion relative thereto, are confined chiefly to the period of official record, and a similar treatment is given the precipitation data which follow the subject of temperature. All entries relative to temperature are in the Fahrenheit scale. MEAN TEMPERATURE, ANNUAL, SEASONAL, AND MONTHLY Fig. 1 shows graphically the mean temperature of each year from 1830 to 1910, inclusive. The mean of all these years is 47?7, while the adopted normal, based upon the Weather Bureau records from 1873 to 1905, inclusive, is 48?5, and the mean of the official period from 1871 to 1910 is 48?8. Table I presents the mean monthly and annual temperatures from 1830 to 1913, inclusive, and the means of these values by decades, as well as similar data for the different seasons. The monthly and annual adopted normals, 1873 to 1905, are shown at the top of the table. A month, year, or season having the normal temperature is, however, the exception rather than the rule, and it is interesting to note the departure (excess above or deficiency below) of temperature from the normal values for the various periods. These departures are presented graphically in Plate I. Reliable temperature records kept in Europe at various places for more than a hundred years show that there has been no permanent change during that time, and meteorologists are generally agreed that no appreciable permanent change has occurred there during even the entire period of written and legendary history. Yet, were the data in Table I strictly comparable, we should be forced to the conclusion that the years are gradually becoming warmer, with but slight and temporary reversions to lower temperature. The mass of evidence from many other localities of long record is, however, against such a conclusion, and we must seek the explanation of the increasing means in changing conditions which might affect the readings of the thermometers used in securing the data. Granting that the observations of the unofficial period were made with the utmost accuracy, and that the interpolations are absolutely correct, and that the instruments used were all of the same standard, the question resolves itself into one of the nature of the locations of the thermometers and the influence of their environment upon the readings. Repeated experiments have demonstrated that the mean temperature ,in a large city is higher than that of the surroundingJFig. 1.—Annual mean temperatures. Fig. 1 shows the mean annual temperature values based upon the data contained in Table I. The mean temperature line, 47?7, represents the mean for the entire period from 1830 to 1910, while the normal temperature line, 48?5, is the adopted normal, the mean of official observations from 1873 to 1905. TABLE 1 Mean Temperature, Annual, Seasonal, and Monthly, 1830-1912 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Winter Spring Summer Autumn Seasons Nnrmn.1 tftmpftrature 23.7 25.4 34.4 45.9 56.5 66.3 72.4 71.2 64.6 53.2 39.2 29.3 48.5 26.1 45.6 70.0 52.3 1830 23 30 37 53 59 64 75 72 58 57 45 26 50 49.7 70.3 35.3 1830 1831 18 20 37 43 57 69 72 70 61 49 32 15 45 21.3 45.7 70.3 47:3 1830-31 1832 24 15 37 49 55 68 70.6 71.4 62.9 54.0 39.9 33.2 48.0 18.0 47.0 70.0 52.3 1831-32 1833 29.7 27.6 33.3 50.5 60.1 63.6 72.4 70.3 64.0 45.5 39.8 34.1 49.2 30.2 48.0 68.8 49.8 1832-33 1834 13.3 34.9 36.6 47.4 54.6 62.9 74.3 71.2 60.1 46.3 40.3 29.6 47.6 27.4 46.2 69.5 48.9 1833-34 1835 28.1 14.0 32.0 42.5 54.6 63.1 67.1 65.2 54.3 47.9 34.2 24.8 44.0 23.9 43.0 65.1 45.5 1834-35 1836 22.4 21.7 26.1 42.4 53.5 58.6 66.5 61.9 56.7 46.8 34.3 24.2 42.9 23.0 40.7 62.3 45.9 1835-36 1837 23 25 28 38 48 61 66 65 59 50 40 26 44 24.1 38.0 64.0 49.7 1836-37 1838 25 11 42 40 50 66 74 69 58 48 25 19 44 20.7 44.0 69.7 43.7 1837-38 1839 29 28 35 53 54 61 73 66 54 59 31 27 48 25.3 47.3 66.7 48.0 1838-39 THE WEATHER AND CLIMATE OF CHICAGO1840 21 28 37 46 58 66 68 66 56 1841 22 23 35 41 55 67 69 67 57 1842 25 28 45 52 52 59 67 65 61 1843 26 13 16 45 53 65 71 67 65 1844 22 30.8 38.4 55.3 58.7 64 73.6 68 68.6 1845 35 33 39 51 58 68 75 70 63 1846 39 31 42 49 62 64 78 75 62 1847 20 30 32 47 53 65 76 67 64 1848 32 31 36 46 58 68 69 69 58 1849.'. 22 21 ,38 42 50 66 70 66 62 1850 30 32 34 41 51 66 74 71 61 1851 28 35 40 45 53 64 71 69 67 1852 22 32 34 39 55 66 72 69 59 1853 29.7 29.1 37 45 52 67 68 68 62 1854 19 29 38 44 54 66 74 72 67 1855 26 18 31 48 56 62 70 67 62 1856 13 17 27 44 51 68 71 65 59 1857 10.7 30.6 27.9 34.6 50.4 63.1 71.5 67.7 62.8 1858 33 19 36 43 52 68 73 70 63 1859 27 29 38 41 • 55 62 74 71 59 1860 18.1 26.8 35.1 42.6 57.7 63.0 68.2 68.8 57.6 1861 21.9 29.4 31.7 43.0 49.1 63.2 66.2 68.5 61.0 1862 18.2 20.9 32.1 43.0 51.8 57.0 68.0 70.9 63.8 1863 33.5 20.6 31.9 41.9 52.7 59.4 65.6 65.6 56.9 1864 16.2 23.6 27.4 38.6 55.7 60.6 67.9 68.0 58.4 1865 17.2 26.0 32.2 42.5 51.3 66.1 62,9 65.2 66.3 1866. 17.7 17.9 26.4 43.8 51.4 69.4 77.2 68.9 60.8 1867 19.9 30.8 29.5 46.5 50.7 72.4 73.1 74.5 67.4 1868 17.9 24.7 42.4 44.4 54.2 66.0 80.6 71.8 61.3 1869 31.2 29.3 29.5 45.2 53.1 64.3 71.2 72.1 64.9 1870 25.9 27.5 32.5 48.4 62.4 69.3 76.8 72.7 68.7 1871 30.5 31.7 41.1 50.7 57.7 67.5 72.0 71.7 60.5 1872 22.8 25.3 28.0 47.3 56.2 69.3 72.0 71.4 63.6 1873 19.6 23.6 32.3 42.7 52.1 68.3 68.8 71.0 62.6 1874 28.1 30.5 36.4 38.8 58.2 70.4 74.8 72.6 67.1 1875 16.1 14.6 31.0 43.2 54.5 63.0 68.8 67.9 60.9 1876 32.8 31.9 34.0 47.4 59.4 68.0 74.2 74.0 61.8 1877 22.4 37.3 28.9 46.6 57.2 66.9 73.6 71.8 66.8 1878 31.8 36.4 45.2 52.4 55.2 65.3 75.3 74.1 66.2 1879 21.2 27.0 40.6 47.4 57.8 64.3 75.6 72.3 61.0 1880 39.8 34.8 38.4 49.2 65,0 70.6 73.2 72.7 62.9 1881 19.4 25.0 32.4 41.8 61.0 63.2 73.3 75.4 70.2 1882 28.6 39.0 39.6 46.0 51.4 64.0 68.8 71.4 65.0 1883 16.4 22.6 32.0 46.9 53.0 64.5 70.6 67.8 60.6 1884 18.8 27.8 34.2 44.3 56.1 64.2 68.6 68.3 68.8 1885 18.6 17.8 31.3 46.3 53.4 65.4 73.2 68.4 64.4 1886 ..... 22.4 29.4 37.2 49.7 57.2 66.2 71.8 72.6 66.0 1887 16.4 26.0 32.2 48.7 60.0 67.4- 75.3 68.6 62.3 1888 14.8 22.6 29.9 46.4 53.1 67.2 72.6 69.4 59.8 1889 29.0 19.9 38.4 46.8 56.8 62.3 70.5 70.6 62.8 49 35 26 46 25.3 47.0 66.7 46.7 1839-40 48 37 27 46 23.7 43.7 67.7 47.3 , 1840-41 52 30 22 46 26.7 49.7 63.7 47.7 1841-42 44 33 32 44 20.4 38.0 67.7 47.3 1842-43 48 34 31 49 28.3 50.8 68.5 50.2 1843-44 51 37 23 50 33.0 49.3 71.0 50.3 1844-45 53 43 34 53 31.0 51.0 72.3 54.0 1845-46 51 41 30 48 28.0 44.0 69.3 52.0 1846-47 55 36 27 49 31.0 46.7 68.7 49.7 1847-48 50 45 22 46 ' 23.3 43.3 67.3 52.3 1848-49 51 43 26 48 28.0 42.0 70.4 51.7 1849-50 51 36 23 49 29.7 46.0 68.0 51.3 1850-51 54 35 28 47 25.7 42.7 69.0 51.0 1851-52 50 39 30 48 28.9 44.7 67.7 50.4 1852-53 55 38 28 49 26.0 45.3 70.7 53.3 1853-54 46 37 22 45 24.0 45.0 66.6 48.3 1854-55 49 35 18.4 43 17.3 40.7 68.0 47.7 1855-56 48.9 29.9 31.4 44 19.9 37.6 67.8 47.2 1856-57 50 35 28 48. 27.8 43.7 70.4 49.3 1857-58 49 32.9 15.7 46 28.0 44.7 69.0 47.0 1858-59 48.5 31.3 20.5 44.9 20.2 45.1 66.7 45.8 1859-60 48.4 34.1 28.4 45.4 23.9 41.3 66.0 47.8 1860-61 51.2 34.4 30.3 45.1 22.5 42.3 65.3 49.8 1861-62 39.9 33.4 26.3 44.3 28.1 42.2 63.5 43.4 1862-63 43.0 32.5 17.8 42.5 22.0 40.6 65.5 44.6 1863-64 46.6 35.4 20.2 44.3 20.3 42.0 64.7 49.4 1864-65 53.8 40.9 25.8 46.2 18.6 40.5 71.8 51.8 1865-66 56.1 43.5 28.8 49.4 25.5 42.2 73.3 55.7 1866-67 50.8 38.0 22.6 47.9 23.8 47.0 72.8 50.0 1867-68 43.2- 33.9 28.6 47.2 27.7 42.6 69.2 47.3 1868-69 55.1 42.3 27.1 50.7 27.3 47.8 72.9 55.4 1869-70 49,5 33.6 20.7 48.9 29.8 49.8 70.4 47.9 1870-71 50.2 31.6 18.4 46.3 22.9 43.8 70.9 48.5 1871-72 49.6 34.2 32.0 46.5 20.5 42.4 69.4 48.8 1872-73 53.7 40.0 32.8 50.3 30.2 44.5 72.6 53.6 1873-74 48.1 36.6 36.0 45.1 21.2 42.9 66.6 48.5 1874-75 49.4 39.5 19.4 49.3 33.6 46.9 72.1 50.2 1875-76 55.6 40.0 43.4 50.9 26.4 44.2 70.8 54.1 1876-77 52.8 44.0 23.8 51.9 37.2 50.9 71.6 54.3 1877-78 60.8 42.4 31.1 50.1 24.0 48.6 70.7 54.7 1878-79 51.4 31.6 24.0 51.1 35.2 50.9 72.2 48.6 1879-80 56.2 40.1 37.7 49.6 22.8 45.1 70.6 55.5 1880-81 58.0 42.6 26.6 50.1 35.1 45.7 68.1 55.2 1881-82 52.3 42.2 31.0 46.7 21.9 44.0 67.6 51.7 1882-83 56.4 41.0 30.0 48.2 25.9 44.9 67.0 55.4 1883-84 51.6 42.7 31.8 47.0 22.1 43.7 69.0 52.9 1884-85 57.0 38.4 25.3 49.4 27.9 48.0 70.2 53.8 1885-86 47.2 .36.2 26.7 47.2 22.6 47.0 70.4 48.6 1886-87 49.1 41.6 32.2 46.5 21.4 43.1 . 69.7 50.2 1887-88 49.4 38.6 40.6 48.8 27.0 47.3 67.8 50.3 1888-89 TEMPERATURETABLE I—Continued 00 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Winter Spring Summer Autumn Seasons Normal temperature 23.7 25.4 34.4 45.9 56.5 66.3 72.4 71.2 64.6 53.2 39.2 29.3 48.5 26.1 45.6 70.0 52.3 > 1890 30.8 32.4 29.5 45.6 53.4 70.2 72.1 67.6 60.4 51.4 41.9 30.6 48.8 34.6 42.8 70.0 51.2 1889-90 1891 30.2 28.6 30.6 47.0 53.4 65.7 67.0 69.0 69.0 52.6 33.8 35.4 48.5 29.8 43.7 67.2 51.8 1890-91 1892 19.5 30.2 31.0 44.0 52.4 64.4 71.6 70.9 63.9 53.6 34.7 23.4 46.6 28.4 42.5 69.0 50.7 1891-92 1893 12.0 21.5 33.2 44.3 52.4 67.8 73.8 69.8 64.1 52.6 36.0 25.4 46.1 19.0 43.3 70.5 50.9 1892-93 1894 27.5 23.0 41.2 46.8 56.1 71.4 73.4 70.8 66.2 52.1 34.4 32.4 49.6 25.3 48.0 71.9 50.9 1893-94 1895 17.6 17.0 31.7 46.3 59.2 70.2 70.2 72.4 68.6 46.2 36.5 29.6 47.1 22.3 45.7 70.9 50.4 1894-95 1896 27.0 26.6 31.2 53.4 65.5 67.0 72.2 72.8 60.6 49.6 38.4 32.9 49.8 27.7 50.0 70.7 49.5 1895-96 1897 21.8 28.6 34.7 46.0 55.0 65.2 74.2 69.0 69.5 58.4 38.7 25.0 48.8 27.8 45.2 69.5 55.5 1896-97 1898 28.6 27.6 40.4 44.4 56.2 68.8 73.4 71.4 67.6 50.6 37.0 24.3 49.2 27.1 47.0 71.2 51.7 1897-98 1899 23.0 17.9 29.8 50.0 59.0 69.9 72.2 73.5 62.7 58.0 44.8 27.2 49.0 21.7 46.3 71.9 55.2 1898-99 1900 28.7 20.1 29.0 46.8 58.1 64.0 71.6 76.3 65.6 61.4 38.4 30.0 49.2 25.3 44.6 70.6 55.1 1899-1900 1901 26.0 17.0 34.0 45.0 54.1 69.1 77.4 71.6 64.3 55.4 37.6 24.0 48.0 24.3 44.4 72.7 52.4 1900-1901 1902 25.2 20.8 38.6 46.4 59.0 64.2 72.4 68.4 60.8 55.2 47.0 26.5 48.7 23.3 48.0 68.3 54.3 1901-2 1903 24.0 25.0 40.4 47.2 59.8 61.2 72.2 68.4 64.4 53.6 36.4 20.0 47.7 25.2 49.1 67.3 51.5 1902-3 1904 17.7 17.2 35.2 40.7 57.3 64.2 71.0 68.3 64.5 53.4 42.8 26.8 46.6 18.3 44.4 67.8 53.6 1903-4 1905 18.1 17.0 39.2 45.5 56.4 65.4 71.2 74.0 67.5 53.4 40.6 31.8 48.3 20.6 47.0 70.2 53.8 1904-5 1906 32.6 27.6 30.2 50.7 59.8 68.0 71.6 75.6 70.1 52.8 41.8 32.8 51.1 30.7 46.9 71.7 54.9 1905-6 1907 27.8 26.4 42.6 39.8 51.6 66.2 73.3 71.2 64.6 52.6 41.0 32.8 49.2 29.0 44.7 70.2 52.7 1906-7 1908 28.6 26.7 40.6 49.0 58.7 68.4 74.3 73.4 70.6 55.2 43.5 31.3 51.7 29.4 49.4 72.0 56.4 1907-8 1909 28.8 32.4 36.0 45.0 55.9 66.6 72.3 74.8 64.0 50.6 48.5 21.6 49.7 30.8 45.6 71.2 54.4 1908-9 1910 25.6 24.8 48.6 51.2 53.4 68.2 76,0 73.3 65.2 58.6 36.0 26.4 50.6 24.0 51.1 72.5 53.3 1909-10 1911* 29.2 32.4 39.2 46.1 65.9 72.4 76.0 71.8 67.0 53.2 35.4 35.0 52.0 29.3 50.4 73.4 51.9 1910-11 1912* 11.9 21.8 28.8 48.8 59.9 66.0 72.9 71.0 67.7 55.8 42.8 33.4 48.4 22.9 45.8 70.0 55.4 1911-12 1913* 29.3 24.8 35.2 48.8 57.6 70.5 74.9 74.3 65.4 53.3 47.2 37.4 51.6 29.2 47.2 73.2 55.3 1912-13 Mean 1830-39 23.6 22.7 34.4 45.7 54.6 63.7 71.1 68.2 58.8 50.4 36.2 25.9 46.3 23.8 45.0 67.7 48.4 1830-39 Mean 1840-49 26.4 26.9 35.8 47.4 55.8 65.2 71.7 68.0 61.7 50.1 37.1 27.4 47.7 27.1 46.4 68.3 49.8 1840-49 Mean 1850-59 23.8 27.1 34.3 42.5 52.9 65.2 71.8 69.0 62.2 50.4 36.1 25.0 46.7 25.5 43.2 68.8 49.7 1850-59 Mean 1860-69 21.2 25.0 31.8 43.2 52.8 64.1 70.1 69.4 61.8 48.2 35.7 24.9 45.7 23.3 42.6 67.9 48.6 1860-69 Mean 1870-79 25.1 28.6 35.0 46.5 57.1 67.2 73.2 72.0 63.9 52.5 38.4 28.5 49.0 27.3 46.2 70.8 51.6 1870-79 Mean 1880-89 22.4 26.5 34.6 46.6 56.7 65.5 71.8 70.5 64.3 52.9 39.5 30.6 48.5 26.2 46.0 69.3 52.2 1880-89 Mean 1890-99 -23.8 25.3 33.3 46.8 56.3 68.1 72.0 70.7 65.3 52.5 37.6 28.6 48.4 26.4 45.4 70.3 51.8 1890-99 Mean 1900-1909 25.8 23.0 36.6 45.6 57.1 65.7 72.7 72.2 65.6 54.4 41.8 27.8 49.0 25.7 46.4 70.2 53.9 1900-1909 Mean of official record, 1871-1910 24.1 25.6 35.3 46.3 56.5 66.5 72.4 71.4 64.8 53.2 39.3 29.1 48.8 26.3 46.0 70.1 52.4 1871-1910 Means, 1830-1910 24.0 25.6 34.7 45.6 55.4 65.6 71.9 -70.0 63.0 51.5 37.8 27.3 47.7 25.3 45.2 69.2 50.7 1830-1910 * Not included in means. Table I shows the mean monthly, seasonal, and annual temperature for Chicago for the years from 1830 to 1913, inclusive, and the monthly and annual normals, 1873-1905, inclusive. The data previous to November, 1870, when the Signal Service (now Weather Bureau) record begins have been taken from Hazen’s Climate of Chicago, which contains observations as far back as 1830. Some of these observations were made at Fort Dearborn, others by volunteer observers of the Smithsonian Institution, while still others have been interpolated from temperature records at other points in Northern Illinois which overlap those of the Weather Bureau. Since 1859 the temperature record at Chicago has been continuous, secured, of course, from instruments at various exposures. THE WEATHER AND CLIMATE OF CHICAGOTEMPERATURE 9 country, provided the topography is fairly uniform. This is largely because of the lower night temperatures in the country, due to freer radiation of heat as compared with the city, where radiation is retarded by smoke and otherwise influenced by the character of pavements and buildings. These city conditions change gradually, of course, with the growth of the place and as it passes through the village stage to that of a large city, and ultimately becomes a great metropolis, the temperatures in the central portion are held up higher and higher each year of growth, so that the readings of a thermometer kept in the same position throughout the whole time would indicate a climatic change to warmer, while one similarly placed in the open country would maintain a fairly even record throughout the years. Moreover, the locations of the Chicago thermometers were frequently changed, and, as will be pointed out later in more detail, the position of the instrument with respect to surface inequality, height above the ground, protection from direct radiated heat, and air movement, affects its action to a greater or lesser extent, and produces different results in thermometers separated by comparatively short distances. The mean annual temperatures, even during the period of official readings, have doubtless been influenced in a large measure by changes in the exposure of the instruments, and a comparison of the mean temperatures at the four different locations in the city where the Weather Bureau has successively maintained its office should prove interesting, and bear out the statement of the previous paragraph. These data (Table II, Fig. 2) are not synchronous, nor do they even partially overlap, and as a result no exact allowance can be made for change of location, but the evidence regarding the general effect of such changes is plain. According to the actual means the temperature during the three years at the Chicago Opera House Building was lower by 2?6 than it was during the six years at the Federal Building, while the means at the Major Block and at the Auditorium Tower lie between these extremes. Owing to several remarkably warm, and but few cold Februaries, the mean of that month at the Major Block is higher than that of any of the other locations; but, as a rule, the means are highest at the Federal Building. The length of record at the latter, however, is entirely too brief for the determination of normals for that location, and this is also the case at the Chicago Opera House Building. The very short period of record at this place will account for some of the apparent discrepancies in the10 THE WEATHER AND CLIMATE OF CHICAGO means, such as the high average for December, as when the number of factors entering into the mean is small, accidental abnormalities control the resulting value to a large extent. TABLE II Monthly and Annual Mean Temperatures, Four Different Exposures Location Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Major Block 24.3 28.8 35.5 46.2 56.9 65.8 72.2 71.4 64.6 53.8 39.6 30.4 49.1 Chicago Opera House 22.8 22.8 33.5 47.3 56.6 65.6 72.8 69.5 61.6 48.6 38.8 33.2 47.8 Auditorium Tower 23.1 23.2 34.4 46.2 56.7 66.8 72.3 70.7 64.8 53.6 38.6 27.6 48.2 Federal Building 28.7 27.6 39.6 47.1 55.9 67.5 73.1 73.2 67.0 53.9 41.9 29.4 50.4 Table II contains the average monthly and annual temperatures at Chicago for four different exposures, not synchronous, located as follows: Major Block, June 8, 1873, to December 31, 1886; Chicago Opera House Building, January 1,1887, to January 31,1890; Auditorium, February 1,1890, to June 30,1905; Federal Building, July 1, 1905, to 1912. Elevation of thermometers above street as follows: Major Block, 70 feet; Chicago Opera House Building, 146 feet; Auditorium, 241 feet; Federal Building, 140 feet. Thermometers in each case exposed in lattice-work shelter on the roof of the building, except at the Major Block where the shelter was outside an office window on the north side. For graphic comparison see Fig. 2. Fig. 2.—Average monthly temperature; four exposures. First column of each set, Major Block; second, Chicago Opera House Building; third, Auditorium Tower; fourth, Federal Building. Previous to 1905, upon the removal of Weather Bureau offices from one location to another, it was not customary to make synchronous comparative observations between the instruments at the old and the new quarters. Following the removal from the Auditorium Tower to the Federal Building on July 1,1905, however, comparative readings were made for a period of six months at both old and new locations, and they afford an interesting study of relative conditions. During this time the mean temperatures averaged 1?7 higher at the Federal Building than at the Auditorium. The daily maximum and minimum temperatures for August and December, 1905, respectively, are shown graphically in Fig. 3, the Federal Building values averagingTEMPERATURE 11 1?7 higher in August, and 1?8 higher in December. There seems no reason to doubt that the difference in location alone is responsible for the temperature at the Federal Building averaging 1?7 higher than is the case at the Auditorium, and applying this difference to the annual mean at the former, as shown in Table II, the value would be reduced to 48?7, thus indicating the true mean for the 5^ years at the Federal Building on the basis of the Auditorium record of the previous 15 years, 48?2. In other words, if the record had been continued without change of location after July 1, 1905, the mean temperature for the period of 5| years ending with December, 1910, 95 90 85 80 75 70 65 35 30 25 20 15 10 S Maxima August, 1905 F. 80.2° A. 78.5° Minima December, 1905 F. 26.0° A. 24.2° Fig. 3.—Maximum and minimum temperatures. Auditorium -........... ‘ Federal Building ------------- would have been about 48?7 instead of 50 ?4 as obtained at the Federal Building; and the difference is caused by the change of location of the thermometers, the present one being three blocks farther from the lake, and in the midst of the loop or business district, where the instruments are more affected by the heat of great buildings and other city conditions. Then, too, the corrected mean of 48?7 for the Federal Building is 0?4 lower than the 14-year mean at the Major Block, 49?1, instead of 1?3 higher, as shown by the available records. The periods of observation at both Major Block and Auditorium are sufficiently long to establish fairly accurate mean temperatures for their respective localities, and while they are separated by an interval of four years, we should expect the mean at the12 THE WEATHER AND CLIMATE OF CHICAGO former to be the higher, and this is found to be the case, the values being 49 ?1 and 48?2, respectively. The Major Block is well within the loop district, and even farther from the lake than is the Federal Building. As a consequence, its location favored a higher temperature than that at the Auditorium, and one slightly higher than that at the Federal Building; while the fact that the thermometers were exposed in a window shelter at the Major Block would in itself be responsible for slightly higher temperatures than at the other locations, where roof shelters were used. The activities were not as intense in the business section from 1873 to 1887 as in later years, and therefore heat from the buildings could not have affected the situation quite as much as is the case at present at the Federal Building. The period of observations at the Chicago Opera House Building was only three years—entirely too short to give a reliable mean, and the value of 47?8, the lowest of any of the locations, may be misleading unless this fact is considered. The location, in fact, would favor a higher temperature than that at the Auditorium, but the average was 0?4 lower, and is due rather to actually colder weather during the period in which these readings were made. A number of the months during i 1887 and 1888 were exceptionally cold, the low temperatures of which served to lower considerably the mean temperature, not only of the years themselves, but also of the mean for the three-year period. The foregoing paragraphs illustrate the influence of location and exposure upon mean temperature at points several blocks apart, but differences are often caused by the surroundings of the observation point within quite small limits, and are due to irregularity of surface, character of street pavings, height and facings of buildings, and many other features peculiar to the particular area under consideration; and, in addition, to the varying conditions of cloudiness, sunshine, and air movement. In 1909 the Weather Bureau established for the benefit of the public in Chicago a set of instruments in a specially constructed shelter, or kiosk, at the southwest corner of Adams and Dearborn streets, under the walls of the Federal Building, on the roof of which, 140 feet above, are exposed the official thermometers. A series of readings, taken during the months following in that year, to determine the difference between the kiosk and the official instruments, furnishes interesting confirmation of the statements above. The observations indicate daytime temperatures (maxima), whoseTEMPERATURE 13 variation from the readings of the station instruments depends closely upon cloudiness and sunshine, and night time temperatures (minima) of nearly constant difference. Throughout the period the daytime temperatures averaged at the kiosk on clear days 3?7 higher than the station instruments on the roof near by; while on cloudy days the excess was but 1?3. Some portion of these differences, probably somewhat less than 1°, may be directly attributed to the difference in elevation between the kiosk and the roof shelter, and would reduce the excess due to other causes on cloudy days to practically nothing, and point to the conclusion that the environment of the kiosk—the pavements, the building walls as artificial barriers to air movement—together with the heating by insolation of the iron structure of the kiosk itself, is directly responsible for the pronounced differences in temperature on days of sunshine. It was noted, however, that the temperatures did not begin to rise in the morning at the kiosk as early as in the shelter on the roof, as on account of the high buildings the direct rays of the sun did not reach the street until some time later; while on account of shading soon after the middle of the day, the fall began sooner than on the roof, and was rather more gradual. Occasionally, also, when the official maximum was delayed until very late in the afternoon, the kiosk maximum, even on a day of bright sunshine, did not, on account of this shading at street level, rise as high as that on the roof. The night temperatures showed an average excess at the kiosk of 0?9 and 1?1 for clear and cloudy weather, respectively, indicating a greater loss of heat at the street location when conditions for radiation were good. It will be noticed that the excess is practically constant in cloudy weather for both day and night, the cloud blanket above preventing excessive heating at the kiosk during the day, and equalizing the conditions of radiation at night. Differences such as those described may be noted in many portions of the city, and in some locations are even more pronounced, especially in so far as night radiation is concerned. Thermometers exposed in the more sparsely built sections of the city, or in many of the suburban districts, show often much lower temperatures on quiet, clear nights than do the official instruments in the loop, influenced as are the latter by overhanging city smoke and the immense structures near by. This difference has been noted in special cases to be as much as 14°, and explains why at certain times the Weather Bureau thermometers in the city are well above the freezing point,14 THE WEATHER AND CLIMATE OF CHICAGO while ice and frozen surfaces are general in the outlying sections. It will be apparent from this that the official record must be regarded simply as a statement of the temperature conditions at the point of exposure of the instrument; and the person interested in any particular locality of the city must establish, if only roughly, his own “table of differences” if he wishes to use the local forecasts and temperature data intelligently. While the best exposure for a thermometer is in a latticed shelter on the roof of a building or over sod, a satisfactory location may often be found under a veranda with a north exposure, the position being such that the direct rays of the sun are kept from the instrument. Should the sun shine upon the thermometer, the reading will be in excess of the air temperature at that point, the difference depending on the character and proximity of radiating surfaces similarly exposed. It would, however, indicate approximately the degree of heat experienced by a person exposed to the sun’s rays at that location. Radiating surfaces of dark color influence the readings to a much greater extent than those of lighter shades. During cloudy weather the character of the exposure is not of the importance shown above, but the clearer the atmosphere the greater is the difference between the readings of a thermometer under shelter and one in the open. During the two warm days of September 2 and 7, 1913, at Chicago, when the official maximum readings were 97? 1 and 92?7 a thermometer exposed nearby in the sun registered 112?8 and 110?2, respectively, the difference being as much as 17?5 on the 7th, a Sunday when the atmosphere was clear with the exception of a few fleecy upper clouds. On week days, when there is more smoke in the atmosphere, the differences between sun and shade temperatures are not often great, while on clear, dry days, especially in the suburbs away from the smoke of the city, it is probable that the differences will range sometimes from 20° to 30°. Greely in his American Weather cites an instance in the Arctic of snow melting on a dark surface when the temperature of the air was 18° below zero. In view of the various cases of differences described above, it will readily be understood why we are loath to use unqualifiedly for purposes of comparison the old records of temperature made prior to 1871, and it is for the reasons outlined that very little attention will be given them in this bulletin; while in the detailed statements concerning the official records, the reader should bear in mind theTEMPERATURE 15 various locations of the Weather Bureau office, and the length of time it remained at each place. Fig. 2 will serve to show the monthly march of temperature, from the minimum in winter to the maximum in summer. The variations, of course, are not all of the same amount. Based upon the normals, as derived from the record of 1873-1905 (see Table I), the average changes from month to month are given in Auxiliary Table A. AUXILIARY TABLE A January to February. + 1?7 July to August - 1?2 February to March... + 9?0 August to September :.. - 6?6 March to April +11?5 September to October —11?4 April to May +10?6 October to November —14?0 May to June + 9?8 November to December - 9?9 June to July + 6?1 December to January - 5?6 January to July +48? 7 July to January —48?7 DEPARTURE FROM NORMAL TEMPERATURE, MONTHLY AND ANNUAL There are two ways in common use of showing the abnormal features of a month or year: one is to use the difference between the mean temperature of the period and the normal temperature, the other is to carry the algebraic sum of the daily departures throughout the period, the final value representing the departure for the month or year. These are called accumulated departures, and possess the quality of being more vivid, or easier to understand. To say th^t a year has exceeded the normal by 3 does not at first strike the average person as much out of the ordinary; but to say that during the year 1,262° of heat more than usual were experienced attracts the attention at once as exceptional. In Table III are given the accumulated monthly departures for each month, and the accumulated annual departures for each year of the official record. The highest and lowest departures are printed in heavy type, and indicate the warmest and coldest months and years, respectively. A study of this table and of the ohe following (Table IV), which give the departures accumulated through the months of each year reaching finally the annual value in December, should serve to illustrate further the character of the monthly and annual temperatures as shown in Table I, Fig. 1, and Plate I. The mean values at the bottom of the table prove, as might naturally be expected, that the most variable of the months are those of the winter season, and that the summer departures are scarcely more than one-third in amount as compared with those of the cold period. The16 THE WEATHER AND CLIMATE OF CHICAGO decrease and increase in the amount of departure are fairly uniform throughout the year with the exception of May, which shows TABLE III Monthly and Annual Departures from Normal Temperature, Expressed in Degrees, 1871-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Normal monthly heat | 734 711 1066 1376 1753 1990 2243 2208 1939 1650 1175 909 17754 1871 +211 - 28 +176 - 3 +207 +144 + 37 + 36 - 12 + 16 -123 -115 -168 -267 + 142 1872 -198 + 42 - 9 + 90 - 12 + 6 - 30 - 93 -229 -338 - 802 1873 -127 - 50 - 64 - 91 -137 + 59 -111 - 2 - 60 -110 -151 + 84 - 760 1874 +138 -233 +144 -297 + 66 -200 + 49 +121 + 75 + 52 + 75 + 14 + 36 +109 + 679 1875 -108 - 77 - 59 -102 -115 - 97 -111 -161 - 78 +207 -1231 1876 +276 - 48 +184 - 14 + 47 + 92 + 48 + 56 + 85 - 82 -120 - 14 -308 + 278 1877 +332 +312 -169 + 19 + 19 + 20 + 38 + 19 + 63 + 72 + 28 +436 + 829 1878 +252 - 75 +333 +193 - 39 - 30 + 92 + 91 + 46 - 11 +142 -170 +1211 1879 + 44 +191 + 41 + 40 - 58 +101 + 34 -104 +235 +101 + 55 + 605 1880 +501 -132 +265 +122 +102 +259 +129 + 27 + 47 - 53 - 56 -229 -163 + 951 1881 - 9 - 61 -123 +142 - 94 + 34 +127 +152 + 88 + 25 +262 + 411 1882 +154 -227 +382 +159 + 8 -156 - 69 -114 + 1 + 9 +149 +100 - 83 + 540 1883 - 79 - 71 + 30 -110 - 57 - 52 -103 -123 - 33 + 88 + 52 - 685 1884 -151 + 72 - 6 - 45 - 9 - 64 -120 - 87 +122 + 98 + 50 + 19 - 121 1885 -151 -224 - 95 *-b 7 -100 - 31 + 22 - 89 - 7 - 50 +102 + 77 - 539 1886 - 40 +136 + 83 +116 + 17 ± 0 -15 + 43 + 40 +111 - 23 -124 + 344 1887 -226 + 14 - 69 + 84 +102 + 28 + 96 - 83 - 69 -185 - 81 - 81 - 470 1888 -274 - 86 -140 + 17 -111 + 27 + 7 - 62 -145 -118 + 72 + 90 - 723 1889 +164 +223 +202 -129 -165 +128 + 32 + 9 -122 - 58 - 19 - 58 -120 - 19 +347 + 119 1890 +198 -154 - 11 - 99 +116 - 9 -114 -126 - 58 + 83 + 36 + 85 1891 + 94 -113 + 35 - 95 - 19 -169 - 68 +129 - 19 -167 +186 - 4 1892 +136 -108 -104 - 55 -126 - 58 - 24 - 13 - 20 + H -134 -182 - 698 1893 -364 - 37 - 47 -128 + 47 + 44 - 30 - 19 - 19 - 98 -125 - 884 1894 +119 -191 - 68 +215 + 33 - 11 +153 + 30 - 12 + 44 - 42 -146 + 93 + 408 1895 -236 - 82 + 7 + 80 +110 - 68 + 38 +121 -217 - 80 + 6 - 512 1896 +103 - 54 + 30 - 97 +226 +275 + 20 - 2 + 48 -123 -117 - 22 +116 + 457 1897 + 88 + 9 + 4 - 48 - 39 + 57 - 70 +143 +158 - 15 -132 + 101 1898 +153 - 27 + 61 +184 - 45 - 12 + 72 + 31 + 10 + 91 - 77 - 62 -155 + 251 1899 -211 -145 +124 + 78 +107 - 7 + 70 - 59 +150 +173 - 62 + 191 1900 ... +150 + 72 + 47 + 10 -182 -134 -173 + 26 + 48 - 65 - 20 +159 + 28 +258 - 25 + 25 + 277 1901 -236 - 9 - 23 - 76 + 83 +152 + 13 - 11 + 62 - 46 -164 - 183 1902 -129 +130 + 18 + 74 - 62 + 5 - 90 -112 + 62 +234 - 88 + 89 1903 - 11 +177 + 41 + 98 -153 - 8 - 89 - 8 + 16 - 81 -287 - 295 1904 -242 + 26 -157 + 21 - 60 - 49 - 88 - 5 + 5 +111 - 88 - 698 1905 -174 -248 +151 - 13 - 1 - 38 - 37 + 87 + 89 + 2 + 40 + 78 - 64 1906 +276 +129 +152 +160 + 62 +168 -368 + 61 -126 +140 + 99 + 45 - 24 +134 +166 - 13 + 77 +105 + 940 1907 + 30 +255 -182 -155 - 7 + 27 - 2 - 2 - 21 + 56 +107 + 235 1908 + 33 +195 + 87 + 66 + 64 + 59 + 67 +179 + 62 +125 + 60 +1149 1909 +196 - 20 + 46 - 24 - 19 + 7 - 3 +114 - 18 - 83 +279 -237 + 418 1910 +440 +159 - 98 + 57 +113 + 65 + 19 +167 - 96 - 96 + 772 1911 +194 +152 + 7 +291 +183 +114 + 18 + 70 + 1 -109 +173 +1262 1912* -113 -174 + 91 +100 - 10 + 16 - 5 + 95 + 77 +107 +127 - 57 1913* +176 - 14 + 24 + 86 + 34 +125 + 79 + 97 + 24 - 1 +238 +252 +1120 Means ±160 ±140 ±129 ± 70 ± 85 ± 67 ± 54 ± 60 ± 74 ± 87 ± 97 ±143 ± 522 Table III contains the monthly departures from normal temperature, expressed in degrees. Plus sign (+) indicates excess of temperature, above normal; minus sign (—) indicates deficiency of temperature, below normal. The extremes are shown by bold-faced type. The normal heat of each period is the sum of its daily normal temperatures (p. 33). * Not included in means. abnormal qualities greater than either April or June. This condition is due to proximity to the lake, whose influence is at its greatest in May, as will be brought out later, and causes changes of a wider range than during either the preceding or the following month.TEMPERATURE 17 The warmest years are easily 1911 and 1878, with accumulated departures of +1,262° and +1,211°, respectively, the mean temperatures being 52?0 and 51?9. In 1911 each month except November was above normal on the average, while in 1878 only eight months averaged above normal, although of the remaining four, three had but a slight deficiency. By far the coldest year is 1875, with an accumulated deficiency in temperature of 1,231°, and every month except December was below normal. The mean temperature of that year, 45? 1, compared with the mean of 1911, 52?0, gives an absolute range in annual mean temperature of 6?9. The years 1875 and 1878, however, are more strictly comparable, as the observations in both cases were made at the same location, the Major Block, and the difference between these two years is 6?8. In fact, if the correction of 1?7 were applied to the Federal Building record of 1911, as might properly be done, the average of that year would be reduced to 50?3. The average temperatures of the months having the greatest departures, together with the ranges, are as appear in Auxiliary Table B. Here again the great variability of the winter months is AUXILIARY TABLE B Month Warmest Coldest Range Year Degrees Year Degrees Degrees January 1880 39.8 1912 11.9 27.9 February 1882 39.0 1875 14.6 24.4 March 1910 48.6 1872 28.0 20.6 April 1896 53.4 1874 38.8 14.6 May 1911 65.9 1882 51.4 14.5 June 1911 72.4 1903 61.2 11.2 July 1901 77.4 1891 67.0 10.4 August 1900 76.3 1890 67.6 8.7 September 1908 70.6 1888 59.8 10.8 October 1900 61.4 1895 46.2 15.2 November 1909 48.5 *1872 31.6 16.9 December 1877 43.4 1872 18.4 25.0 *Also November, 1880. brought out, January having the greatest absolute monthly range, 27?9, with December and February following. The gradual change to smaller variability in summer is plainly evident here, as it is in the averages at the bottom of Table III, the absolute monthly range in August being only 8?7. It is also interesting to note that the fluctuations experienced in May are almost as great as those of the preceding month of April. Table IV brings out more vividly than is possible in Table III prolonged periods of excess or deficiency in temperature, as slight18 THE WEATHER AND CLIMATE OF CHICAGO reversions in periods of heat or cold are often lost in the accumulations through the entire year. In a study of this table it should be remembered that the accumulations for December are the accumulations for the year also, and the data appearing in the last column are TABLE IV Yearly Accumulated Departures by Months from Normal Temperatures, Expressed in Degrees, 1871-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Normal accu- Ì mulated \ 734 1445 2511 3887 5640 7630 9873 12081 14020 15670 16845 17754 heat...... J 1871 +211 +387 +594 + 738 + 775 + 811 + 799 + 815 + 692 + 577 + 409 + 142 1872 - 28 - 31 -229 - 187 — 196 — 106 — 118 — 112 — 142 — 235 — 464 - 802 1873 -127 -177 -241 - 332 — 469 — 410 — 521 — 523 — 583 — 693 — 844 - 760 1874 +138 +282 +348 + 148 + 197 + 318 + 393 + 445 + 520 + 534 + 570 + 679 1875 -233 -530 -638 - 715 — 774 — 876 — 991 -1088 L199 -1360 1438 -1231 1876 +276 +460 +446 + 493 + 585 + 633 + 689 + 774 + 692 + 572 + 586 + 278 1877 - 48 +284 +115 + 134 + 153 + 173 + 211 + 230 + 293 + 365 + 393 + 829 1878 +252 +564 +897 +1090 +1051 +1021 +1113 +1204 +1250 +1239. +1381 +1211 1879.. - 75 - 31 +160 + 201 + 241 + 183 + 285 + 318 + 214 + 449 + 550 + 605 1880 +501 +766 +888 + 990 +1249 +1378 +1405 +1452 +1399 +1343 +1114 + 951 1881 -132 -141 -202 - 325 — 183 — 277 — 243 — 116 + 36 + 124 + 149 + 411 1882.: +154 +536 +695 + 703 + 547 + 478 + 364 + 365 + 374 + 523 + 623 + 540 1883 -227 -306 -377 - 347 — 457 — 514 — 566 — 669 — 792 — 825 — 737 - 685 1884 -151 - 79 - 85 — 130 — 139 — 203 — 323 — 410 — 288 — 190 — 140 - 121 1885 -151 -375 -470 - 463 — 563 + 594 — 572 — 661 — 668 — 718 — 616 - 539 1886 - 40 + 96 +179 + 295 + 312 + 312 + 297 + 340 + 380 + 491 + 468 + 344 1887 -226 -212 -281 - 197 — 95 — 67 + 29 — 54 — 123 — 308 — 389 - 470 1888 -274 -360 -500 - 483 — 594 — 567 560 — 622 — 767 — 885 — 813 - 723 1889 +164 - 1 +127 + 159 + 168 + 46 — 12 — 31 — 89 — 209 — 228 + 119 1890 +223 +421 +267 + 256 + 157 + 273 + 264 + 150 + 24 — 34 + 49 + 85 1891 +202 +296 +183 + 218 + 123 + 104 — 65 — 133 — 4 — 23 — 190 - 4 1892 -129 + 7 - 97 - 152 — 278 — 336 — 360 — 373 — 393 — 382 — 516 - 698 1893 -364 -472 -509 - 556 — 684 — 637 — 593 — 623 — 642 — 661 — 759 - 884 1894 +119 + 51 +266 + 299 + 288 + 441 + 471 + 459 + 503 + 461 + 315 + 408 1895 -191 -427 -509 - 502 — 422 — 312 — 380 — 342 — 221 — 438 — 518 - 512 1896 +103 +133 + 36 + 262 + 537 + 557 + 555 + 603 + 480 + 363 + 341 + 457 1897 - 54 + 34 + 43 + 47 — 1 — 40 + 17 — 53 + 90 + 248 + 233 + 101 1898 +153 +214 +398 + 353 + 341 + 413 + 444 + 454 + 545 + 468 + 406 + 251 1899 - 27 -238 -383 - 259 — 181 — 74 — 81 — 11 — 70 + 80 + 253 + 191 1900........ +150 + 16 -157 - 131 — 83 — 148 — 168 — 9 + 19 + 277 + 252 +.. 277 1901 + 72 -164 -173 - 196 — 272 — 189 — 37 — 24 — 35 + 27 — 19 - 183 1902........ + 47 - 82 + 48 + 66 + 140 + 78 "h 83 — 7 — 119 — 57 + 177 + 89 1903 + 10 - 1 +176 + 217 + 315 + 162 + 154 + 65 + 57 + 73 — 8 - 295 1904 -182 -424 -398 - 555 — 534 — 594 — 643 — 731 — 736 — 731 — 620 - 708 1905 -174 -422 -271 - 284 — 285 — 323 — 360 — 273 — 184 — 182 — 142 - 64 1906 +276 +337 +211 + 351 + 450 + 495 + 471 + 605 + 771 + 758 + 835 + 940 1907 +129 +159 +414 + 232 + 77 + 70 + 97 + 95 + 93 + 72 + 128 + 235 1908 +152 +185 +380 + 467 + 533 + 597 + 656 + 723 + 902 + 964 +1089 +1149 1909 +160 +356 +402 + -378 + 359 + 366 + 363 + 477 + 459 + 376 + 655 + 412 1910 + 62 + 42 +482 + 641 + 543 + 600 + 713 + 778, + 797 + 964 + 868 + 778 1911 +168 +362 +514 + 521 + 812 + 995 +1109 +1127 +1197 +1198 +1089 +1262 1912 -368 -481 -655 - 564 — 464 — 474 — 458 — 463 — 368 — 291 — 184 - 57 1913 +176 +162 +186 + 272 + 306 + 431 + 510 + 607 + 631 + 630 + 868 +1120 Table IV contains the accumulated departures from the normal temperature by months, expressed in degrees. Plus sign (+) indicates accumulated excess of temperature, above normal; minus sign (—) indicates accumulated deficiency of temperature, below normal. The extremes are shown by bold-faced type. therefore identical with the annual values in Table III. The years 1875,1878,1880,1906,1908,1911, and 1913 have the most pronounced accumulated departures, 1875 being a cold year and the remainderTEMPERATURE 19 warm years. At the end of the month of October, the year 1880 was warmer than either 1878 or 1911. The year 1878 was the warmest at the end of November, but because of a cold period in December, the annual mean was lower than that of 1911. The uniform excess in temperature in the six years from 1906 to 1911, inclusive, as shown in both Tables III and IV, is to a large extent due to the present exposure of the thermometers on the roof of the Federal Building, which, as has already been shown (p. 11), averages 1?7 higher than the previous exposure at the Auditorium Tower. This average excess would amount to about 52° for the month, and 620°, nearly, for the year. Such correction would make the years 1907 and 1909 cold years instead of warm years, by deficiencies of 385° and 202°, respectively; and reduce 1908 and the record year of 1911 far below the warm years of 1878 and 1880 as recorded at the Major Block. Both the warm period of 1906 to 1911, and 1876 to 1880, inclusive, were broken somewhat by the occurrence of cold months here and there. There has been no other period than that of 1906 to 1911 where the accumulated departures continued above the normal for three successive years. As regards cold periods, the most pronounced are those from January, 1883, to January, 1886, from July, 1891, to December, 1893, and from November, 1903, to December, 1905, the last two being less than three years in extent. Space will not permit the discussion in detail at this time of the conditions of the warmest and coldest months, but a brief reference should nevertheless be made to certain of the most striking ones, and later more information will be given. The month of January, 1912, with its mean temperature of 11?9, was not only the coldest of all months in the official records, but there were only two with lower temperature during the period of the unofficial records, January, 1857, 10?7, and February, 1838, 11?0. The period of cold in January, 1912, was continuous throughout the month, there being 13 days with minimum temperatures of zero or below, and the absolute minimum was —16°. The December of 1872 was also marked by a period of prolonged cold. On the 24th of that month a temperature of —23° occurred, the lowest minimum of official record in Chicago. The warmest July, that of 1901, 77?4, was exceeded twice in mean temperature during the period of the old records, in 1846, 78?0, and in 1868, 80?6. In July, 1901, the highest absolute maximum temperature of the official record was reached, 103° on the 21st.20 THE WEATHER AND CLIMATE OF CHICAGO These data, as well as additional figures bearing on the warmest and coldest days in the various years, will be found in later tables (XXXIX to XLIV). WARM AND COLD MONTHS AND SEASONS It is easy to pick out the warmest and coldest months and seasons from the tables already given, but to list the months and seasons TABLE V Warm Months and Seasons, 1871-1913 Departure above Normal January................................ 6°+ 1871, 1876, 1878, 1880, 1890, 1891, 1906 February............................... 6°+ 1871, 1876, 1877, 1878, 1880, 1882, 1890, 1909, 1911 March................................ 6°+ 1871, 1878, 1879, 1894, 1898, 1903, 1907, 1908, 1910 April.................................. 5°+ 1878, 1896, 1910 May.................................... 4°+ 1880,1881, 1896,1911 June................................... 4°+ 1874, 1880, 1894,1911, 1913 July................................... 3°+ 1879,1901,1910,1911 August............................... 3°+ 1881, 1900, 1906, 1909, 1913 September.............................. 4°+ 1881,1884,1891,1895,1897, 1906,1908 October................................ 4°+ 1879, 1882, 1897, 1899,1900, 1910 November............................... 5°+ 1899, 1902,1909, 1913 December............................... 6°+ 1875, 1877, 1881, 1889, 1891, 1913 Winter........................... 4°-j- 1873-74, 1875-76, 1877-78,1879-80, 1881-82,1889-90, 1905-6, 1908-9 Spring........................... 3°+ 1871, 1878, 1879, 1880, 1896, 1903, 1908, 1910, 1911 Summer........................... 2°+ 1874,1876,1880,1901,1908,1910,1911,1913 Autumn........................... 3°+ 1881, 1884, 1897, 1908, 1912, 1913 Year................................ 2°+ 1878, 1880, 1882, 1906, 1908, 1911, 1913 (By seasons, beginning with previous December) Year. .............................. 2°+ 1870, 1877,1878, 1880, 1906, 1908,1910, 1911,1913 (Calendar) TABLE VI Cold Months and Seasons, 1871-1913 Departure below Normal January................................ 6°+ 1875, 1883, 1887, 1888, 1893,1895,1904, 1912 February............................... 6°+ 1875, 1885, 1895, 1899, 1901, 1904, 1905 March.................................. 6°+ 1872 April................................ 5°-j- 1874,1904,1907 May.................................... 4°+ 1873, 1882, 1892, 1893, 1907 June................................... 4°+ 1889, 1903 July................................... 3°+ 1873, 1875, 1882, 1884, 1891 August................................. 3°+ 1875,1883,1890 September.............................. 4°+ 1871, 1883,1888, 1890, 1896 October................................ 4°+ 1875, 1887, 1888, 1895 November............................... 5°+ 1871, 1872, 1873, 1880, 1891 December............................... 6°+ 1871, 1872, 1876, 1903, 1909 Winter................................. 4°+ 1874-75, 1882-83, 1884-85, 1887-88, 1892-93, 1898-99 1903-4, 1904-5 Spring................................. 3°+ 1873,1892 Summer................................. 2°+ 1875, 1883, 1884, 1889, 1891, 1903, 1904 Autumn................................. 3°+ 1871, 1872, 1873, 1875, 1880, 1887 Year................................. 2°+ 1873, 1875, 1883, 1888, 1893, 1904 (By seasons, beginning with previous December) Year................................. 2°+ 1872, 1873, 1875, 1888, 1893 (Calendar)TEMPERATURE 21 TABLE VII Mean Temperature of Warm and Cold Months, during Period from 1871 to 1913 Covering Data in Tables V and VI Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Normal temperature.. 23.7 25.4 34.4 45.9 56.5 66.3 72.4 71.2 64.6 53.2 39.2 29.3 1871 . 30.5 31.7 41.1 60.5 33.6 20.7 1872 28.0 31.6 18.4 1873 52.1 68.8 34.2 1874 38.8 70.4 1875 16.1 14.6 68.8 67.9 48.1 36.0 1876 32.8 31.9 19.4 1877 37.3 43.4 1878 31.8 36.4 45.2 52.4 1879 40.6 75.6 60.8 1880 39.8 34.8 65.0 70.6 31.6 1881 61.0 75.4 <5 70.2 37.7 1882 39.0 51.4 68.8 58.0 1883 16.4 67.8 60.6 1884 ¿8.è 68.8 1885. 17.8 1886 1887 16.4 47.2 1888 14.8 59.8 49.1 1889 62.3 40.6 1890 30.8 32.4 67.6 60.4 1891 30.2 67.0 69.0 33.8 35.4 1892 52.4 1893 12.0 52.4 1894 41.2 71.4 1895 17.6 17.0 68.6. 46.2 1896 53.4 65.5 60.6 1897 69.5 58.4 1898 40.4 1899 17.9 58.0 44.8 1900 76.3 61.4 1901 17.0 77.4 1902 47.0 1903 40.4 61.2 20.0 1904 17.7 17.2 40.7 1905 17.0 1906 32.6 75.6 70.1 1907 42.6 39.8 51.6 1908 40.6 70.6 1909 32.4 74.8 48.5 21.6 1910 48.6 51.2 76.0 58.6 1911...... 32.4 65.9 72.4 76.0 1912 11.9 1913 70.5 74.3 47.2 37.4 Warm months in bold-faced type. which can properly be characterized as warm or cold requires first the determination of what shall define a warm month, a cold month, a warm season, and a cold season. The mean departures shown in Table III would indicate that greater ranges occur in the winter months, and therefore that a warm or cold winter month must vary from its normal by considerably more than should a warm or cold summer month. Allowing for the number of days in the month, these departures average about 5° from December to March, inclusive, and about 2° for July and August. By adding an extra degree beyond the limit of which the monthly values will designate warm and cold months, and sliding the scale uniformly throughout the22 THE WEATHER AND CLIMATE OF CHICAGO year, we have as the limits for December, January, February, and March, 6°; April and November, 5°; May, June, September, and October, 4°; and July and August, 3°. As cold and warmth are not so frequently protracted through seasons and years, a reduction must be made from these limits, and the following were adopted: winter, 4°; spring and autumn, 3°; summer, 2°; the year, 2°. On this basis the list of warm and cold months, seasons, and years was made up, and the data are presented in Tables V and VI. The same data for the months are entered in Table VII, for some purposes a more usable form, as the actual mean temperatures for the warm and cold months are entered in their proper places. In Table VIII are given the warmest and coldest seasons and years with the ranges between the extreme mean temperatures in each case. The principal feature brought out in this table is the great range between the warmest and coldest winters, 18?9, as compared with the small range for the other seasons, and the year as a whole. The mean annual temperature of 53?0 in 1846 exceeded that shown in the table by 1?0, while there are several years in the old record with a lower mean than 45? 1. TABLE VIII Warmest and Coldest Seasons and Years, 1871-1913 Winter Spring Summer Autumn Year Normal temperature 26.1 45.6 70.0 52.3 48.5 Warmest 37.2 51.1 73.4 56.4 52.0 Departure +11.1 + 5.5 + 3.4 + 4.1 + 3.5 Year 1877-78 1910 1911 1908 1911 Coldest 18.3 42.4 66.6 48.5 45.1 Departure - 7.8 - 3.2 - 3.4 - 3.8 - 3.4 Year 1903-4 1873 1875 1872 (1875) 1875 Range 18.9 8.7 6.8 7.9 6.9 The winter of 1877-78 and the spring of 1910 are the warmest on record for these seasons, not excluding the entire period of unofficial record, and there are only two winters during that time of lower mean temperature than that of 1903-4 shown in the table: 1831-32, 18?0, and 1855-56, 17?3. Several spring seasons in the early days were colder than the spring of 1873. The means of the summers of 1867, 1868, and 1870 nearly equal that of 1911, and it is unfortunate that the daily maximum temperatures for those seasons are not available. Also, there are several summers in the old records that are colder than that of 1875, and several autumns colder thanTEMPERATURE 23 those of 1872 and 1875, but the mean of the latter season in 1908 is higher than that of any autumn in the old records. These comparisons cover the entire period of record from 1830 to 1913, as given in Table I, although, as previously shown, exact comparisons are not possible owing to the conditions under which the data prior to 1871 were obtained. SUCCESSION OF SEASONS, MONTHS, AND YEARS There are many maxims to the effect that pronounced seasonal conditions govern or indicate the character of the following seasons or year. Table IX was prepared for the purpose of determining TABLE IX Succession op Seasons, 1872-1910 Spring Summer Autumn Winter Total ( ±1.5+) (= ±1.0+) ( ±1.5+) ( ±2.0+) Num- BER Cold Ave. Warm Cold Ave. Warm Cold Ave. Warm Cold Ave. Warm Nnrmn.l 26.1 45.6 70.0 52.3 26.1 Cold winters (—2.0+) 15 6 6 3 4 9 2 5 6 4 2 7 6 Warm winters (+2.0+) No. with departure less 12 3 6 3 2 3 7 3 4 5 6 1 5 than 2.0 : 12 Autumn Winter Spring Summer ( ±1.5+) ( ±2.0+) ( ±1.5+) ( ±1.0+) Cold Ave. Warm Cold Ave. Warm Cold Ave. Warm Cold Ave. Warm Normal 70.0 52.3 26.1 45.6 70.0 Cold summers (—1.0+) Warm summers 11 3 5 3 5 3 3 5 4 2 4 6 1 (+1.0+) 12 2 5 5 7 2 3 1 7 4 2 6 4 Year Normal 48.5 Cold years (—1.0+)... Warm years (+1.0+).. No. with departure less 11 3 5 3 13 3 2 8 than 1.0 15 whether there is any uniformity in the succession of warm and cold seasons and years, and a careful examination fails to afford any clue by which the nature of a season or year may be foretold from any of its predecessors. Out of 15 cold winters, only 2 were followed by cold winters in the ensuing year, while only 6 were followed by warm winters. Of the 12 warm winters, 6 were followed by cold, and 5 by warm winters. The same irregularity is evident in the character of24 THE WEATHER AND CLIMATE OF CHICAGO the other seasons of the year succeeding these cold and warm winters, and also in the character of any of the seasons following the cold and warm summers. This may also be said regarding the succession of the years, although, as pointed out in the discussion of Table IV, years warmer than the normal have followed each other in two marked instances of several years' duration. Reference to Tables I and III will show that there is no more uniformity in the succession of months of given character than there is in that of seasons or years. There have been several years in which the months have been successively above or below the normal temperature, while in other instances warm months are followed by cold months, and vice versa. It is often said that Nature tends to average up, but the average of temperature may be made in a mtfnth, less frequently in a season, and it is seldom spread evenly over the extent of a year or two years. For instance, the warm period beginning in the summer of 1906 continued through March, 1907, but was followed by such cold weather that the accumulated departures were reduced to nearly normal conditions by June 1. This March, in fact, was the second warmest on record up to that year, and caused an unseasonably early start in all kinds of vegetation. The succeeding months of April and May were marked by abnormal cold and severe frosts that did much damage in the fruit sections of the entire Middle West. The mean of April of this year, 39?8, was below that of March, 42?6, by 2?8, a phenomenal occurrence when it is considered that the normal for April is 11?5 higher than the normal for March. March, 1910, was even warmer than the same month in 1907, but the warmth continued through the first half of April, a cold wave appearing on the 22d, with snow and severe frosts on the ensuing days, which caused damage similar to that in the spring of 1907. The high mean temperature of March, 1907, may be contrasted with that of the same month in 1906, 42?6 and 30?2, respectively, the latter being lower than that of the preceding January, 32?6. In 1877, March, with mean temperature of 28?9, followed a warm February with an average of 37?3. This is the most startling instance of reversion of temperature to be found in the record, the mean of this March being actually 8?4 lower than that of the preceding February, when under normal conditions for both months it should be 9?0 higher. It is interesting to note that the greatest -of the three instances of reversion of temperature in the fall and winter months alsoTEMPERATURE 25 occurred in 1877, the warm December of that year, 43?4, being higher by 3?4 than November, which was of nearly normal character. Following a cool period in 1889 from June to November, the temperature of the three succeeding months was considerably above the average, the mean for December exceeding that for November by 2?0. In 1891 the average for December was higher than that for November by 1?6. In the study of these various records one is ever impressed with the irregularities in temperature, but only in the few instances given above do we find in the march of temperature throughout the year a spring month to average lower, or an autumn month to average higher, than its predecessor. MEAN DAILY TEMPERATURES Tables X, XI, and XII give the average daily maximum, average daily minimum, and average daily mean temperatures for each day of the year, as determined from the period 1872 to 1910, inclusive, TABLE X Mean Daily Maximum Temperatures, 1872-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 34.3 29.1 38.2 47.6 57.7 66.8 78.2 78.0 76.6 65.8 54.6 36.7 2 32.1 29.6 38.9 46.7 58.0 66.8 78.7 77.8 76.6 67.7 53.7 37.4 3 30.6 28.8 37.1 49.3 60.1 70.6 79.9 78.9 75.4 67.8 52.7 38.1 4 30.6 28.5 35.4 50.5 58.6 72.7 77.9 77.7 74.7 64.1 53.3 37.5 5 30.8 30.3 38.3 50.2 61.6 70.9 78.3 79.1 75.3 63.6 51.8 38.3 6 29.6 31.6 41.2 52.0 59.0 71.1 79.4 78.2 76.2 62.3 51.4 38.8 7 31.8 32.9 39.9 49.1 60.2 71.9 79.5 79.5 75.7 64.5 51.9 35.6 8 32.5 31.5 41.1 49.6 64.3 70,6 79.0 80.6 74.3 64.1 49.6 36.5 9 30.0 30.9 43.4 50.5 65.5 70.3 79.3 81.0 74.4 63.6 49.8 36.2 10 29.7 32.6 43.8 50.5 65.6 70.8 78.7 78.8 73.3 63.9 48.5 37.2 11 30.3 34.1 43.4 51.9 63.9 72.3 79.5 78.3 73.3 61.1 49.4 37.3 12 31.7 33.2 39.9 52.7 65.9 73.3 81.4 77.7 71.9 59.8 47.5 38.2 13... 30.1 34.2 40.4 53.5 62.4 75.0 79.8 77.6 69.3 61.3 46.5 37.0 14 29.1 34.1 38.4 53.3 62.0 74.6 80.7 77.6 72.5 61.4 44.0 35.7 15 32.1 32.7 39.3 53.1 62.5 75.3 81.3 75.1 74.1 63.4 45.3 35.3 16 32.1 32.8 40.1 52.3 63.7 74.1 82.2 76.9 72.2 62.7 45.6 35.0 17 32.9 35.6 41.4 53.8 66.4 75.7 81.0 77.1 71.6 61.2 45.8 32.5 18 31.1 34.1 44.4 56.2 66.2 76.2 77.2 78.8 71.6 59.4 43.4 32.7 19 32.8 33.3 42.4 53.0 65.8 75.9 78.4 77.6 69.8 59.2 42.6 33.7 20 34.5 33.3 39.8 54.6 64.4 76.1 77.7 79.6 68.8 56.9 44.4 35.5 21. 33.9 33.7 42.9 54.9 64.2 74.8 79.1 79.0 71.5 58.5 44.9 35.9 22 31.9 34.4 44.4 57.7 67.6 75.6 79.9 78.0 70.4 58.3 44.4 35.8 23 29.8 31.8 45.8 56.5 65.7 78.1 78.1 77.7 69.8 56.9 43.3 35.7 24 29.9 34.6 43.8 54.9 68.2 78.6 80.1 77.6 69.7 56.4 39.9 34.2 25 30.9 34.3 46.0 54.8 67.2 78.3 81.1 76.2 69.3 56.8 41.7 32.4 26 31.6 35.3 47.9 57.8 66.3 76.1 80.4 74.9 68.9 56.3 42.4 32.3 27 29.5 35.7 46.3 60.0 66.6 76.1 79.6 75.5 67.0 52.6 40.9 34.1 28 30.7 37.2 42.4 61.5 67.4 76.5 80.5 77.4 65.7 54.2 37.7 32.3 29 31.1 31.2 45.8 60.3 69.3 77.4 79.1 76.2 66.1 54.3 35.5 34.5 30 31.7 45.3 58.5 67.4 77.3 79.1 76.3 65.8 53.6 33.4 33.1 31 30.0 47.1 67.4 77.9 75.7 53.4 31.9 Means 31.3 32.7 42.1 53.6 64.3 74.0 79.4 77.7 71.7 60.2 45.9 35.5 Mean annual maximum, 55?7 Table X contains the mean daily maximum temperature based upon readings of the maximum thermometer from 1872 to 1910. The value for each day is found by adding all the maximums on that date together, and dividing by the number of years, 39.26 THE WEATHER AND CLIMATE OF CHICAGO and Fig. 4 presents the same data so as to illustrate the daily march of temperature throughout the year. On the average the lowest minimum, 13?8, occurs on February 1, and the highest maximum, 82?2, on July 16. Similarly the lowest daily mean temperature, 21 ?2, and the highest daily mean temperature, 74?6, occur respectively on these same dates. This is also true of the highest mean minimum temperature, 66?9, which falls on July 16; but the lowest TABLE XI Mean Daily Minimum Temperatures, 1872-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 20.2 13.8 25.5 33.7 42.6 53.2 63.7 66.2 63.0 52.3 39.8 24.6 2 17.8 14.2 26.0 33.0 41.9 53.2 64.3 65.6 61.9 53.3 39.6 25.8 3 16.1 16.2 24.7 34.6 43.7 54.4 64.4 65.1 61.8 53.6 37.7 26.6 4 15.9 14.1 23.1 35.1 45.1 55.6 64.6 65.5 61.7 51.5 39.4 25.1 5 16.7 14.3 25.8 35.7 45.9 55.3 63.7 65.7 61.5 50.9 38.3 24.9 6 17.3 16.2 26.8 35.6 45.8 55.2 64.3 65.1 61.8 49.6 36.5 24.0 7 17.3 19.4 27.5 35.9 45.9 55.5 64.2 65.7 61.6 49.5 38.3 23.1 8 17.7 18.0 28.1 35.9 47.3 56.5 65.9 66.6 61.2 49.2 38.4 23.7 9 14.9 16.6 29.6 37.0 48.8 55.4 65.0 65.9 61.2 49.5 37.5 22.9 10 15.2 18.1 29.1 37.1 49.5 55.1 64.5 65.3 60.1 48.9 37.2 24.2 11 16.6 18.9 28.6 37.3 48.7 56.9 64.3 65.1 59.4 48.6 35.9 25.2 12 16.3 18.4 27.7 38.7 49.3 57.9 65.3 65.3 60.0 47.5 33.5 24.8 13 16.0 18.3 27.3 38.6 48.1 59.3 65.6 65.2 57.5 46.8 31.9 24.6 14 16.1 19.1 25.2 39.6 46.8 59.7 66.0 65.2 57.7 46.5 31.8 22.9 15 18.9 18.7 25.7 38.5 46.9 60.3 66.5 64.9 58.0 48.5 32.3 22.1 16..: 17.9 17.4 25.7 37.9 47.6 59.8 66.9 64.5 57.1 47.4 32.6 21.3 17 16.8 19.6 26.5 39.0 50.3 60.4 66.5 64.8 56.7 46.6 32.5 21.2 18 19.6 19.9 29.2 41.7 50.5 60.3 64.8 65.6 56.9 45.3 31.4 19.9 19 17.5 20.2 30.4 40.2 50.7 61.2 64.5 65.9 56.8 44.2 30.4 21.1 20 19.8 19.2 26.9 39.6 50.1 60.9 64.9 64.7 55.1 43.8 31.2 22.7 21 19.4 19.6 28.1 40.9 50.1 60.8 64.6 65.4 55.1 44.3 31.6 24.0 22 18.5 19.3 29.5 42.2 50.3 60.5 65.6 64.9 56.5 44.4 30.6 22.9 23 16.3 19.9 30.1 41.6 51.3 62.6 65.3 63.9 55.9 42.9 28.6 22.8 24 14.6 20.0 30.8 40.9 51.2 63.2 66.2 64.3 56.0 42.7 27.7 21.0 25; 15.8 20.3 31.2 41.4 52.4 63.4 66.7 63.4 55.4 42.6 29.0 21.4 26 : 18.3 21.4 32.4 42.9 50.6 62.8 66.9 63.2 54.0 43.2 29.6 20.0 27 17.6 21.8 32.8 44.3 51.1 62.9 66.3 63.2 52.5 40.7 26.8 19.5 28 15.9 23.9 31.6 45.3 51.5 62.6 66.7 64.6 54.1 39.9 24.9 19.5 29 15.7 21.8 31.7 44.3 52.6 62.9 66.1 64.1 53.4 40.6 22.8 20.2 30 15.4 32.8 41.9 52.3 63.9 66.0 64.0 52.0 40.9 22.9 19.6 31 16.1 34.4 52.1 66.5 63.9 40.4 19.7 Means 17.0 18.5 28.5 39.0 48.7 59.0 65.4 65.0 57.9 46.3 32.7 22.6 Mean annual minimum, 41?7 Table XI contains the mean daily minimum temperature based upon the readings of the minimum thermometer from 1872 to 1910. The value for each day is found by adding all the minimums on that date together and dividing by the number of years, 39. mean maximum temperature, 28?5, occurs on February 4. The average highest minimum is seen in the table to occur on July 26, as well, but on the average, February 1 and July 16 are the dates , of highest and lowest temperatures. The temperature therefore rises throughout 165 days of the year, and falls throughout 200 days. It is interesting to note in this connection that at Baltimore, 2° 35' farther south in latitude, the period of rise is 4 days shorter, fromTEMPERATURE 27 February 5 to July 16, and the period of fall 4 days longer {Maryland Weather Service, II, 78). The change from the lowest temperature of winter to the highest of summer and back again is not a steady march, the advance and retreat of the seasons being accomplished by a succession of waves of rising and falling temperature of unequal periods, but averaging from three to four days each, accompanying the movement across the country of areas of high and low barometric pressure with their attendant shifting winds. TABLE XII Mean Daily Temperatures, 1872-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 27.6 21.2 31.8 40.6 50.2 60.0 70.8 72.3 69.9 58.8 47.2 30.7 2 24.9 21.9 32.5 39.8 50.5 60.8 71.5 71.6 69.2 60.5 46.7 31.5 3 23.3 22.6 31.1 42.0 51.7 60.5 72.1 71.8 68.6 60.7 45.2 32.5 4 23.2 21.3 29.0 42.8 52.0 64.1 71.3 71.7 68.0 57.7 46.3 31.2 5 24.0 22.3 32.0 43.0 53.7 63.1 71.3 72.5 68.4 57.2 45.2 31.5 6 23.4 25.2 33.9 43.8 53.2 63.1 71.9 71.9 68.9 55.8 43.9 31.3 7 24.5 26.2 33.7 42.9 52.9 63.7 71.9 72.5 68.7 57.0 44.0 29.3 8 25.0 24.8 34.6 42.8 55.8 63.5 72.4 73.7 67.7 56.6 44.0 30.1 9 25.0 23.8 36.4 43.7 57.0 62.9 72.1 73.3 67.8 56.5 43.7 29.6 10 22.4 25.3 36.5 43.9 57.7 63.0 71.6 71.6 66.7 56.2 42.8 30.8 11 23.4 26.4 36.0 44.6 56.2 64.7 71.8 71.9 66.2 54.9 42.6 29.7 12 23.9 25.8 33.7 45.7 57.6 65.5 73.3 71.6 65.9 53.4 40.6 31.6 13.... 23.3 26.2 34.1 46.0 55.3 67.2 72.7 71.4 63.4 54.1 39.1 30.8 14 22.6 26.5 31.8 46.4 54.5 67.4 73.3 71.2 64.6 54.0 38.0 29.2 15 1... 25.5 25.8 32.6 45.8 54.7 67.7 73.8 69.8 66.0 55.5 38.7 28.7 16 24.9 25.0 32.9 45.1 55.7 67.1 74.6 70.6 64.6 55.1 39.1 28.2 17 24.8 26.3 33.9 46.4 58.4 67.8 73.7 70.9 64.1 53.9 39.0 26.8 18 25.4 27.0 36.8 48.9 58.4 67.9 71.0 72.3 64.3 52.4 37.4 26.4 19 25.2 26.7 36.4 46.5 58.2 68.2 71.5 71.7 63.3 51.7 36.5 27.1 20... 27.4 26.2 33.5 47.1 57.3 68.1 71.3 72.3 61.9 50.4 37.6 29.1 21 26.6 26.7 35.5 48.3 57.2 67.8 71.8 72.2 63.3 51.4 37.5 30.0 22 25.2 26.9 37.0 49.8 58.9 68.1 72.7 71.3 63.5 51.3 37.5 29.4 23. 23.1 25.9 37.9 48.8 58.5 70.5 71.7 70.9 62.8 49.9 35.9 29.3 24 22.2 27.3 37.3 47.9 59.7 71.0 73.1 70.9 62.7 49.6 33.8 27.5 25 23.4 27.3 38.6 48.1 59.8 70.8 73.9 69.7 62.3 49.7 35.3 26.8 26 23.7 28.3 40.2 50.3 58.4 69.3 73.6 69.0 61.5 49.8 36.1 26.2 27 23.6 28.8 39.5 52.2 58.9 69.4 72.9 69.4 59.8 46.6 33.9 26.8 28 26.0 30.4 37.0 53.3 59.4 69.2 73.7 70.8 60.0 47.1 31.3 25.9 29 23.4 36.1 52.4 61.1 69.8 72.5 70.2 59.7 47.5 29.3 24.7 30 23.6 39.2 50.2 59.9 69.0 72.5 70.0 58.8 46.9 29.3 26.4 31 23.1 40.4 59.8 72.1 70.1 46.9 27.0 Means 24.1 25.6 35.3 46.3 56.5 66.5 72.4 71.4 64.8 50.7 39.3 29.1 Mean annual mean, 48‘ ?7 Table XII contains the daily mean temperature—the mean of the maximum and the minimum from 1872 to 1910. The value for each day is found by adding all the mean temperatures on that date together and dividing by the number of years, 39. The mean daily temperature thus obtained is about one-half degree higher than the mean of the 24 hourly readings. The annual mean temperature shown above, 48?7, is lower by 0?1 than the annual mean shown in Table I, the period in the latter table being one year longer. It will be noticed that the highest and lowest mean temperatures do not occur at the times of the solstices, June 21 and December 21, when the northern hemisphere receives the maximum and minimum amount of heat from the sun, respectively, but lag behind these times by more than three weeks in summer and five weeks in winter.fco 00 5 lò 15 202550 S 10 IS 20 ZS S 10 IS 20 2530 S iO 15 20 2S30 S 10 IS 20 2S5Ò 5 >0 IS 202530 5 101520 2530 5 ¡0 15 20 SS 30 S ¡0 iS2025ZO S )0 15 202S3Ö £ 1Ò (520 2530 5 ¡0 15202530 Fig. 4.—Daily march of temperature. A=average daily maximum temperature; B=daily mean temperature; C=average daily minimum temperature. THE WEATHER AND CLIMATE OF CHICAGOTEMPERATURE .29 The atmosphere and earth are storing heat so long as the daily quantity received exceeds the loss by radiation, and as a consequence the time of maximum temperature is delayed until these two factors are equal. Similarly, there will be loss of heat during the winter so long as the daily amount of radiation from a given region exceeds the daily solar insolation, and the time of minimum temperature is delayed until the returning sun furnishes daily a sufficient amount of heat to balance or exceed that lost from the region in question. It will be evident that the maximum temperature of summer, July 16, occurs relatively earlier after the solstice than does the minimum temperature of winter, February 1; or, in other words, that the mean temperature rises through a shorter period than it falls, as stated in a previous paragraph. Other things being equal, these periods should be of the same length, and the times of highest and lowest temperatures should follow the solstices at the same interval, so that there must be some disturbing influence which retards the occurrence of the minimum temperature, or accelerates the occurrence of the maximum, or both. Von Bezold divides the earth into three zones, an equatorial zone in which insolation is considerably greater than radiation, and two polar zones in which the amount of radiation is greater than the amount of insolation (.Mechanics of the Earth’s Atmosphere, 3d Collection—Abbe, p. 388), and fixes the boundaries at between 35° and 40° of latitude. Plainly the equatorial or insolation zone would be marked by higher and higher temperatures each year, and the polar or radiation zones by temperatures becoming lower and lower, were it not that the excess of heat in the one is transferred by convection (circulation) to the two radiation zones, thus maintaining the mean annual temperature. Chicago is situated just within the north radiation zone, and this fact explains why the minimum temperature of winter is retarded to a seemingly undue extent; and also why the period of excess heat in summer is shortened, thereby advancing the time of maximum temperature. Table X shows that the maximum temperature in June averages 74?0, in July 79?4, in August 77?7, and in September 71?7. The average maximum in January is 31?3, and this is the only month in which the maximum temperature averages below freezing. Table XI shows that the minimum temperature averages in January 17?0, in February 18?5, in March 28?5, in November 32?7, and in December 22?6. The mean daily maximum for the entire year is 55?7, and the mean daily minimum 41 ?7. These values, however, do not express30 THE WEATHER AND CLIMATE OF CHICAGO much in themselves, but they serve to establish the mean daily range for the year, 14?0. MEAN DAILY CHANGE IN TEMPERATURE The figures in Table XIII show the mean changes in temperature from day to day throughout the year. They are obtained from TABLE XIII Mean Daily Change in Temperature, Expressed in Degrees and Tenths of a Degree, 1873-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 +0.6 -1.9 +1.4 +0.2 0 +0.2 +1.8 +0.2 -0.2 0 +0.3 +1.4 2 -2.7 +0.7 +0.7 -0.8 +0.3 +0.8 +0.7 -0.7 -0.7 +1.7 -0.5 +0.8 3 -1.6 +0.7 -1.4 +2.2 +1.2 -0.3 +0.6 +0.2 -0.6 +0.2 -1.5 +1.0 4 -0.1 -1.3 -2.1 +0.8 +0.3 +3.6 -0.8 -0.1 -0.6 -3.0 +1.1 -1.3 5 +0.8 +1.0 +3.0 +0.2 +1.7 -1.0 0 +0.8 +0.4 -0.5 -1.1 +0.3 6 \ -0.6 +2.9 +1.9 +0.8 -0.5 0 +0.6 -0.6 +0.5 -1.4 -1.3 -0.2 7 +1.1 +1.0 -0.2 -1.2 -0.3 +0.6 0 +0.6 -0.2 +1.2 +0.1 -2.0 8 +0.5 -1.4 +0.9 +0.2 +2.9 -0.2 +0.5 +1.2 -1.0 -0.4 0 +0.8 9 0 -1.0 +1.8 +0.9 +1.2 -0.6 -0.3 -0.4 +0.1 -0.1 -0.3 -0.5 10 -2.6 +1.5 +0.1 +0.2 +0.7 +0.1 -0.5 -1.7 -1.1 -0.3 -0.9 +1.2 11 +1.0 +1.1 -0.5 +0.7 -0.8 +1.7 +0.2 +0.3 -0.5 -1.3 -0.2 -1.1 12......., +0.5 -0.6 -2.3 +1.1 +1.4 +0.8 +1.5 -0.3 -0.3 -1.5 -2.0 +1.9 13.... -0.6 +0.4 +0.4 +0.3 -2.3 +1.7 -0.6 -0.2 -2.5 +0.7 -1.5 -0.8 14 -0.7 +0.3 -2.3 +0.4 -0.8 +0.2 +0.6 -0.2 +1.2 -0.1 -1.0 -1.6 15 +2.9 -0.7 +0.8 -0.6 +0.2 +0.3 +0.2 -1.4 +1.4 +1.5 +0.7 -0.5 16 -0.6 -0.8 +0.3 -0.7 +1.0 -0.6 +0.8 +0.8 -1.4 -0.4 +0.4 -0.5 17 -0.1 +1.3 +1.0 +1.3 +2.7 +0.7 -0.9 +0.3 -0.5 -1.2 -0.1 -1.4 18 +0.6 +0.7 +2.9 +2.5 0 +o. i -2.7 +1.4 +0.2 -1.5 -1.6 -0.4 19 -0.2 -0.3 -0.4 -2.4 -0.2 +0.3 +0.5 -0.6 -1.0 -0.7 -0.9 +0.7 20 +2.2 -0.5 -2.9 +0.6 -0.9 -0.1 -0.2 +0.6 -1.4 -1.3 +1.1 +2.0 21 -0.8 +0.5 +2.0 +1.2 -0.1 -0.3 +0.5 -0.1 +1.4 +1.0 -0.1 +0.9 22.. -1.4 +0.2 +1.5 +1.5 +1.7 +0.3 +0.9 -0.9 +0.2 -0.1 0 -0.6 23 -2.1 -1.0 +0.9 -1.0 -0.4 +2.4 -1.0 -0.4 -0.7 -1.4 -1.6 -0.1 24 -0.9 +1.4 -0.6 -0.9 +1.2 +0.5 +1.4 0 -0.1 -0.3 -2.1 -1.8 25 +1.2 0 +1.3 +0.2 +0.1 -0.2 -0.8 -1.2 -0.4 +0.1 +1.5 -0.7 26 +0.3 +1.0 +1.6 +2.2 -1.4 -1.5 -0.3 -0.7 -0.8 +0.1 +0.8 -0.6 27 -0.1 +0.5 -0.7 +1.9 +0.5 +0.1 -0.7 +0.4 -1.7 -3.2 -2.2 +0.6 28 +2.4 +1.6 -2.5 +1.1 +0.5 -0.2 +0.8 +1.4 +0.2 +0.5 -2.6 -0.9 29 -2>.6 -0.9 -0.9 +1.7 +0.6 +1.2 —0.6 -0.3 +0.4 -2.0 -1.2 30 +0.2 +3.1 -2.2 -1.2 -0,8 ' 0 -0.2 -0.8 -0.6 0 +1.7 31 -0.5 +1.2 -0.1 -0.4 +0.1 0 +0.6 Means =*=1.0 ±0.9 ±1.4 ±1.0 ±0.9 ±0.7 ±0.7 ±0.6 ±0.7 ±0.9 ±1.0 ±1.0 Table XIII contains the mean change in daily temperature, that is, the variations, plus or minus, between the means of succeeding days. SUMMARY OF TABLE XIII Mean Daily Change in Temperature Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Average daily change. ±1.0 ±0.9 ±1.4 ±1.0 ±0.9 ±0.7 ±0.7 ±0.6 ±0.7 ±0.9 ±1.0 ±1.0 Average number of positive changes.... Average amount of positive changes 13 17 19 21 17 18 17 13 9 10 8 13 (degrees) Average number of 1.1 0.9 1.4 1.0 1.1 0.8 0.8 0.6 0.6 0.7 0.8 1.1 negative changes... Average amount of negative changes 17 • 10 12 9 12 11 11 17 21 19 19 18 (degrees) 1.1 1.0 1.4 1.2 0.8 0.5 0.8 0.6 0.8 1.0 1.2 0.9 Average number of times with no change 1 1 0 0 * 2 1 3 1 0 2 3 0TEMPERATURE 31 Table XII by taking the differences as they occur, from each day to the following. The much greater variability of temperatures in winter has already been brought out (p. 15), and we should therefore expect these daily changes to be greater in that season. While this is true on the average, the extreme values of mean change occur at some other time: the greatest of either positive or negative changes on June 4, +3?6, and the greatest negative change on October 27, — 3?2. There is, of course, the greatest number of positive changes, or days warmer than the one immediately preceding, from midwinter to midsummer; and the greatest number of negative changes, or days cooler than the one immediately preceding, from midsummer to midwinter. This follows naturally from the succession of the seasons, but is by no means absolutely uniform, as an inspection of the table or of the graph in Fig. 4 will show. The greatest number of changes to warmer in any month, 21, occurs in April, with an average of 1?0 for each change; while the least number of changes to cooler, 9, also occurs in April, with an average of 1?9 for each change. September holds the record for the greatest number of changes to cooler, 21, with an average of 0?8 for each, and the least number of changes to warmer, 9, with an average of 0?6 each. FREQUENCY OF CHANGES OF STATED AMOUNTS IN MEAN DAILY TEMPERATURE It is important to know the average frequency of changes of certain amounts in mean temperature from one day to the next, and for this purpose Table XIV is given. It is a summary of extensive data for which there is hardly room in this bulletin, and adds interesting confirmation to the statements made in connection with Tables III and XIII, regarding the greater variations in temperature occurring during the winter than is the case during the summer. It will readily be seen that there are but few instances during the warm months where a change of 15° or more between the mean temperatures of two successive days has occurred. The number of such changes is least in August, when on the average one occurs in ten years, and greatest in January and February, when they average more than two for each month. In the case of the smaller changes of from 0° to 5°, the conditions are reversed, because by far the larger proportion of the daily changes in the summer season are of this kind, while in January, for instance, more than one-half of all the daily changes are of 6° or more. All portions of the table a, 6, and c show this to32 THE WEATHER AND CLIMATE OF CHICAGO be true, and all emphasize the great irregularity in temperature change, especially the last two sections, in that some months show 20 days or more on which the temperature change exceeds 6°, while in other months, even in colder weather, marked changes are comparatively few. A striking example is that of November, 1878, in which there was only one change of 6° or more during the entire month, while in August, 1901, there was not a single instance of such a change. It should be understood that the diurnal changes TABLE XIV (a) Average Number of Changes in Daily Mean Temperature of Stated Amounts, 1873-1910 Month Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Change of 0° to 5°, inch 14.6 13.6 17.2 16.0 17.8 18.6 21.5 23.1 19.8 18.8 17.8 17.0 Change of 6° plus 16.4 14.7 13.8 14.0 13.2 11.4 9.5 7.9 10.2 12.2 12.2 14.0 Change of 10° plus— 8.7 7.1 6.4 6.4 6.0 4.3 1.9 1.3 3.4 4.4 4.7 6.6 Change of 15° plus— 2.9 2.3 1.6 1.7 1.2 0.6 0.2 0.1 0.5 0.8 1.2 2.0 (6) Examples of Greatest Diurnal Changes in Temperature Year and Month Jan. 1875 Feb. 1888 Mar. 1888 April 1910 May 1907 June 1891 July 1874 Aug. 1888 Sept. 1873 Oct. 1895 Nov. 1894 Dec. 1893 Change of 0° to 5°, incl. 9 9 13 14 16 18 13 19 16 15 10 8 Change of 6° plus— 22 20 18 16 15 12 18 12 14 16 20 23 Change of 10° plus 14 10 12 10 10 6 6 3 8 7 11 16 Change of 15° plus 8 5 4 8 4 3 1 2 3 3 5 7 (c) Examples of Least Diurnal Changes in Temperature Year and Month Jan. 1891 Feb. 1897 Mar. 1881 April 1881 May 1877 June 1907 July 1882 Aug. 1901 Sept. 1877 Oct. 1882 Nov. 1878 Dec. 1896 Change of 0° to 5°, incl. 23 22 25 25 25 24 28 31 26 25 29 23 Change of 6° plus.... 8 6 6 .5 6 6 3 0 4 6 1 8 Change of 10° plus 1' 2 1 0 3 2 1 0 0 1 0 2 Change of 15° plus 1 1 0 0 0 0 0 0 0 0 0 0 referred to in Tables XIII and XIV are merely the differences in mean daily temperature between two successive days, and that they have nothing to do with daily range in temperature, which will be discussed later: In order to comprehend properly Table XIV, the reader should bear in mind that the sum of the number of changes from 0° to 5°, and the number of changes of 6°+, in every case equals the total number of days in the month; and that the changes of 6°+ include those of 10°+ and 15°+, and that the changes of 10°+ include those of 15°+•TEMPERATURE 33 DAILY NORMAL TEMPERATURES, ADOPTED As stated on p. 4, mean temperatures cannot well be used for purposes of comparison, as they change somewhat from year to year unless the period of observations is long enough to smooth out accidental irregularities, and certain mean temperatures are adopted TABLE XV Normal Temperature Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 25 23 29 40 51 62 71 73 68 60 45 34 2 25 23 30 41 52 62 71 73 68 59 45 34 3 25 23 30 41 52 63 71 73 68 59 44 33 4 25 23 30 42 52 63 71 73 68 59 44 33 5 25 24 30 42 53 63 71 73 68 58 43 32 6 24 24 31 42 53 64 71 72 67 58 43 32 7 24 24 31 43 54 64 72- 72 67 58 42 32 8 24 24 32 43 54 64 72 72 67 57 42 32 9 24 24 32 43 54 65 72 72 66 57 42 31 10 24 24 32 44 54 65 72 72 66 56 41 31 11 24 24 32 44 55 65 72 72 66 56 41 31 12 24 25 33 45 55 66 72 72 66 55 40 30 13 24 25 33 45 55 66 72 72 66 55 40 30 14 24 25 34 46 56 66 72 72 65 54 39 30 15 24 25 34 46 56 66 73 72 . 65 54 39 30 16 24 25 34 46 57 66 73 72 65 54 39 29 17 23 26 35 46 57 67 73 71 64 53 38 29 18 23 26 35 47 57 67 73 71 64 53 38 29 19.... 23 26 35 47 58 68 73 71 64 52 38 28 20 23 26 36 48 58 68 73 71 64 52 37 28 21 23 27 36 48 58 68 73 71 63 51 37 28 22 23 27 36 48 59 68 73 70 63 50 37 28 23 23 27 37 49 59 68 73 70 62 50 36 27 24.... 23 28 37 49 59 69 73 70 62 49 36 27 25 23 28 38 49 60 . 69 73 70 62 49 36 27 26 23 28 38 50 60 69 73 70 62 48 35 26 27 23 28 38 50 60 69 73 70 61 48 35 26 28 23 29 39 50 61 70 73 69 61 47 35 26 29 23 39 51 61 70 73 69 61 47 34 26 30 23 40 51 61 70 73 69 60 46 34 25 31 23 40 62 73 69 46 25 Means 23.7 25.4 34.4 45.9 56.5 66.3 72.4 71.2 64.6 53.2 39.2 29.3 Annual normal, 48?5 • Table XV contains the daily normal temperature. The monthly mean temperatures for the period from 1873 to 1905 were taken as a means of comparison and established as “monthly normals.” The daily normal's were obtained by plotting down on'a large sheet the monthly normals, drawing a curve through the twelve points representing the months, and then scaling off the temperature for each day. The monthly means were then taken from these values and in case of any discrepancy between these monthly means and the original monthly normals the curve was slightly adjusted, so that these two monthly means should be in very close agreement. Ordinarily it requires a period of temperature observations of from 100 to 200 years in order to determine true means that may be accepted as normals without further change. as normals instead. Table XV gives the adopted daily normal temperatures for Chicago, and a comparison of the data with those in Table XII will show that the values have been so smoothed out as to present a steady rise from the time of lowest readings in winter to the highest readings in summer, and a steady fall thereafter. The normal daily minimum of the year, 23°, extends from January 1734 THE WEATHER AND CLIMATE OF CHICAGO to February 4, while the normal maximum, 73°, covers the period from July 15 to August 5. The normal maximum and minimum do not agree exactly with the actual mean maximum and minimum in Table XII, 74?6 and 21 ?2, respectively, and there are two reasons for the apparent inconsistency. The periods of observations on which the two tables are based are not of the same length, being 33 years for the normals and 39 years for the mean temperatures; and again, accidental irregularities have given an actual mean minimum slightly too low, and an actual mean maximum slightly too high. EXAMPLES OF DEPARTURES FROM DAILY NORMAL TEMPERATURES, SELECTED YEARS The use of the daily normal temperatures in characterizing the various days of a year or month is illustrated in Tables XVI, XVII, and XVIII. The warmest and coldest years, seasons, and months TABLE XVI Daily Departures from Normal Temperature for 1911, Warmest Year during Period of Official Records, 1871-1911 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 +13 +16 + 4 -10 -11 + 6 +15 + 1 + 5 - 1 -11 0 2 + 1 +13 + 6 - 9 -14 + 6 +17 - 5 + 9 - 3 -15 - 2 3 -17 + 9 + 6 - 6 - 4 +11 +18 - 3 0 + 7 -13 - 5 4 -15 + 7 + 2 - 6 - 3 +17 +19 + 6 + 2 + 3 - 6 - 5 5 -11 0 + 2 - 2 + 1 + 6 +21 + 2 + 7 - 4 + 2 + 4 6 0 + 1 + 5 - 5 + 7 + 6 + 9 + 4 — 2 + 6 + 7 + 8 7 + 6 + 2 + 2 -10 +12 + 1 + 4 +11 + 1 - 6 + 2 +12 8 + 4 + 2 + 8 - 7 +14 + 6 +10 + 2 — 3 - 5 0 +16 9 - 5 - 2 +16 - 5 +12 +17 +14 + 8 0 0 + 4 +21 10 +14 - 9 + 8 0 +16 +21 +10 + 7 + 4 + 5 +15 +22 11 +18 + 8 +20 + 6 +13 + 7 + 9 - 4 + 6 + 2 +12 +12 12 +12 +13 +12 + 7 + 7 - 1 + 3 - 2 0 + 4 -18 + 7 13 +12 +15 + 3 +15 - 3 0 + 4 - 1 — 4 + 5 -20 + 6 14 + 4 +20 +10 + 2 + 6 0 + 2 + 9 + 4 + 4 - 9 + 6 15 - 6 +13 - 4 0 +19 + 5 + 6 + 1 + 9 + 2 -11 + 6 16 - 6 +21 -12 0 +22 + 1 - 3 +10 + 7 +12 - 9 + 6 17 : + 2 +20 + 4 + 2 +23 - 3 - 7 + 5 + 9 + 6 + 4 + 1 18 - 1 + 9 - 1 +11 . +25 0 + 1 + 1 + 8 + 6 -10 + 1 19 + 5 + 4 + 9 + 3 +24 + 4 + 1 - 3 — 2 + 2 - 9 + 4 20 +13 - 1 +16 + 2 +14 +10 - 3 - 3 — 1 0 - 5 + 6 21 + 4 - 7 +24 0 +10 + 8 - 3 + 3 + 1 - 3 - 5 +12 22 - 5 + 1 +12 - 4 + 5 +16 - 3 - 2 0 - 5 + 1 + 8 23 + 7 + 7 - 3 - 6 + 8 +17 - 4 - 6 + 6 - 6 - 3 + 5 24 +13 +12 + 3 - 3 + 9 + 7 -11 - 8 + 2 - 4 - 8 + 9 25 +18 +14 +12 0 +20 + 7 -13 - 6 — 2 + 3 - 1 + 7 26 +24 + 9 +12 + 4 +22 + 9 -10 - 2 — 4 - 6 + 6 +11 27 +23 + 2 + 2 +14 +24 + 1 - 1 + 5 + 9 -11 + 9 - 6 28 +11 - 5 -12 + 6 + 9 - 8 - 1 ' 0 + 3 - 4 - 1 -11 29 +17 - 3 + 9 - 1 - 3 + 1 - 7 0 - 3 -11 + 4 30 + 2 - 6 - 1 + 3 + 9 + 4 - 6 4 - 2 - 6 + 9 31 +11 - 5 + 2 + 5 + 1 - 3 - 1 Means +5.5 +7.0 +4.8 +0.2 +9.4 +6.1 +3.6 +0.6 +2.4 ±0.0 -3.8 +5.7 Annual departure, +3?5 Days of greatest departure in each month entered in bold-faced type. The year 1878 may strictly be considered the warmest year, instead of 1911, if the proper correction is applied to the latter. (See p. 17.)TEMPERATURE 35 have already been discussed in a general way (p. 17), and it will now be interesting to note the temperature in these years from day to day. In order further to appreciate the great variation in daily temperature throughout the year, it was first necessary to have before us the daily normal temperatures discussed in the preceding topic. Tables XVI and XVII show the differences between these normal temperatures and the actual means for each day of the TABLE XYII Daily Departures from Normal Temperature for 1875, Coldest Year during Period of Official Records, 1871-1911 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 + 9 + 1 -13 + 8 -15 + 4 - 7 -11 +12 -11 - 3 - 1 2 - 3 +10 -14 + 1 -16 + 7 - 6 -12 + 8 -11 - 4 + 6 3 - 3 - 1 -16 +10 - 6 - 1 + 1 -10 + 8 - 3 - 3 + 9 4 -10 -25 -12 + 2 - 4 + 5 + 3 - 7 - 2 - 5 - 2 + 7 5 -22 - 7 - 4 + 4 - 5 - 5 - 4 - 3 + 4 - 6 - 5 +14 6.*. -10 -20 - 5 +18 -10 -10 -10 -10 + 2 - 8 + 1 +11 7 - 9 -22 -13 + 9 0 - 9 - 7 - 4 + 3 - 8 + 2 + 8 8 -14 -24 - 6 +17 +12 - 2 - 6 + 2 + 6 -10 + 4 + 4 9 -34 -11 + 2 + 6 + 2 - 2 + 1 0 + 2 -11 + 2 + 1 10 -16 - 8 - 2 + 5 - 2 + 1 0 - 5 - 7 -12 - 1 + 7 11 -12 -15 + 5 0 + 2 +13 - 7 - 4 - 9 -18 + 8 + 1 12 + 1 -27 + 5 - 5 - 7 - 8 - 2 - 4 - 6 -15 + 8 0 13 - 8 -15 +11 - 6 - 1 -17 - 2 + 1 - 8 -11 0 + 2 14 -25 -18 + 4 - 1 - 6 -13 - 2 0 + 2 - 4 + 3 - 4 15 -18 -23 0 - 4 -12 -10 + 5 + 1 + 4 -12 + 1 +10 16 -12 -17 -15 -24 - 6 - 8 - 3 - 2 - 5 -16 - 9 - 9 17 -17 -14 -19 -24 - 5 - 8 - 6 - 5 -15 - 4 -12 -23 18 -12 -14 -18 -13 - 5 -12 -11 - 8 -13 -15 0 -17 19 -17 0 -17 0 + 2 - 8 - 9 - 8 -14 - 6 + 2 - 6 20 - 5 - 6 -15 -14 + 6 - 3 - 5 - 3 -16 + 2 + 3 +15 21 +13 - 7 -13 -14 + 9 - 6 - 1 - 9 -17 + 7 -13 +20 22 - 1 + 5 - 8 - 6 - 1 - 8 + 1 - 8 -13 +12 -10 +20 23 + 6 + 5 - 1 - 3 - 2 + 8 - 5 -10 -10 +10 - 1 +15 24 + 3 0 - 3 - 8 0 + 8 + 1 - 4 - 5 + 5 -14 +21 25 -14 - 2 + 2 -13 0 - 2 + 3 - 1 - 6 + 7 - 3 + 9 26 + 2 - 9 + 8 -10 - 2 + 9 - 3 + 6 - 6 - 5 + 1 + 6 27 +11 -18 + 6 - 4 + 1 0 - 7 0 + 1 - 7 + 1 + 2 28 + 3 -15 - 1 + 4 + 9 -13 - 7 + 8 - 3 + 7 - 3 +10 29 + 2 + 5 - 2 - 3 -10 - 5 + 3 0 + 3 -23 +15 30.: — 1 +18 -10 - 1 - 2 - 6 + 1 - 8 - 6 - 8 +25 31 -20 +21 + 7 - 9 + 9 -10 +29 Means -7.6 -10.8 -3.4 -2.7 -2.0 -3.3 -3.6 -3.3 -3.7 -5.1 -2.6 +6.7 Annual departure, —3?4 Days of greatest departure in each month entered in bold-faced type. warmest year, 1911, and of the coldest year, 1875, in the official records. Table XVIII shows the departures for a nearly normal year, 1891, and accentuates the contrast. In 1911 there were several days on which the excess in temperature was more than 20°; and in May alone there were 7 such days, the greatest excess being 25° on the 18th, when a maximum of 92° was reached. In 1875 there were a few days on which the excess in temperature was greater than 20°, that on the last day of the year reaching 29°. There was36 THE WEATHER AND CLIMATE OF CHICAGO however, an unusually large number of days with deficient temperature, the greatest departure being —34° on January 9, when a minimum of —20° was reached. In the normal year, 1891 (Table XVIII), there were several well-marked excesses and deficiencies in temperature at various times during the course of the year, but these balanced so nearly that at the end of the period there was a deficiency of only 4°. This departure, if distributed equally among the 365 days of the year, would amount to practically nothing. TABLE XVIII Daily Departures from Normal Temperature for 1891, the Year Most Nearly Normal during Period of Official Records, 1871-1911 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 +20 + 7 -11 + 4. + 3 + 6 - 9 - 3 - 2 + 8 -11 - 2 2 - 5 + 8 - 8 - 2 - 2 -10 - 7 - 9 + 6 +16 - 6 + 6 3 - 3 -17 - 4 -13 - 6 + 5 - 7 - 7 -12 +16 - 6 +17 4 + 1 -20 -14 -16 -14 -17 - 5 - 7 -10 - 7 - 4 + 1 5 + 2 + 1 - 4 -11 -12 -13 - 7 - 3 - 8 - 8 0 + 4 6 + 6 +11 - 3 -10 -12 -11 - 6 + 4 - 5 - 4 + 1 - 3 7 + 3 +12 - 3 - 8 + 1 ^ 8 -12 + 8 - 7 - 6 +10 -18 8 + 6 + 8 0 - 9 + 8 - 6 -14 +10 - 7 - 4 +10 - 4 9 + 7 + 4 -11 - 7 +14 - 1 - 8 +12 - 4 - 5 + 4 + 6 10 +10 - 4 + 1 + 2 - 2 + 7 - 5 + 3 - 2 - 2 + 1 + 7 11 : + 6 + 8 + 6 - 2 - 5 + 1 + 2 + 3 0 - 2 - 1 + 8 12 + 4 +13 -12 - 1 + 3 0 + 4 - 8 + 2 - 3 - 2 + 7 13 - 4 +11 -13 +14 + 7 + 2 + 5 - 4 - 2 - 1 -12 +12 14 + 4 +14 -20 0 0 + 9 - 2 0 + 3 - 8 - 7 +20 15 + 6 +19 - 4 - 6 + 7 +10 - 9 - 2 + 5 -12 + 7 +14 16 + 5 +17 - 6 + 4 -17 +12 - 1 0 +11 - 6 - 5 + 3 17 + 3 + 9 + 7 +16 - 9 - 9 - 1 + 6 +15 - 3 -26 - 6 18 + 2 -10 - 1 +15 + 1 - 7 -11 + 7 +16 - 4 -20 0 19 +.11 0 - 4 - 2 + 6 - 2 - 9 + 5 +14 -10 -10 + 4 20 +14 +10 - 2 + 4 + 8 - 6 - 6 + 5 +14 - 4 - 1 + 8 21 +11 + 3 - 2 +12 + 3 0 - 3 - 3 + 9 - 3 + 3 +16 22 + 9 - 4 + 3 +14 -15 + 2 + 5 - 8 +13 -10 + 3 +12 23 + 5 +15 + 2 + 5 -11 + 5 + 2 -10 +16 + 1 0 +13 24 0 +23 - 5 - 5 - 7 + 7 - 9 -12 +19 + 3 -12 +13 25 + 7 - 5 - 4 + 5 -10 +10 - 9 - 6 +13 +11 - 9 +16 26... + 8 -11 - 5 +10 -16 - 3 -11 - 4 +Ì0 +11 - 1 -14 27 +12 -10 - 4 +10 -10 0 - 7 - 8 +17 - 8 - 9 - 2 28 +11 -20 - 3 - 4 - 7 + 4 - 8 -13 +11 - 1 -25 +12 29 +17 + 3 + 7 - 5 + 2 - 3 - 9 - 6 +11 -23 +10 30 +11 + 6 +13 - 3 - 8 -11 - 9 0 +14 -16 +10 31 "+13 + 2 + 7 - 7 - 9 + 1 +16 Means +6.5 +3.2 -3.8 +1.1 -3.1 -0.6 -5.4 -2.2 +4.4 -0.6 -5.4 +6.1 Annual departure, 0?0 Days of greatest departure given in bold-faced type. These tables show forcibly how changeable is the temperature from day to day, and how seldom the normal temperature is experienced. The seeming strangeness of this fact disappears when we consider that the normal is simply the mean value of many varying conditions. (For the relation of the various monthly and annual temperatures to their respective normals see Table III.)TEMPERATURE 37 EXAMPLES OF DEPARTURES FROM DAILY NORMAL TEMPERATURES, WARMEST AND COLDEST MONTHS In Plate II are given the curves of daily mean temperatures of the warmest and coldest months from July, 1872, to December, 1910, inclusive.. Daily mean temperatures are not available for the period prior to July, 1872. The line of normal temperature is drawn in each case, so that the abnormal features of the various months can be traced easily from day to day. The curves show graphically the wide variations in the mean temperature for the months, especially during the winter season; but while the daily readings in these particular instances were persistently high or low, there was not a single cold month in which the temperature did not rise above normal on at least one day,, nor was there a warm month, except January, 1880, in which the temperature did not at some time fall below the normal. INFLUENCE OF LAKE MICHIGAN ON TEMPERATURE AT CHICAGO The lake wind is so powerful a factor in determining the temperatures experienced in the city during certain seasons that it requires description in somewhat greater detail than would otherwise be necessary. For a number of years the temperature of the water in the lake was taken by the Weather Bureau. The readings were made at 1 p.m. each day when the surface was not frozen, at the old stone pier which was located at the foot of Monroe Street. The water here was about 8 feet deep, with rocky bottom. During the period of observations, which was ended in 1887, no continuous record of wind direction was made, but for all purposes of comparison, the direction as observed at 2 p.m., one hour after the reading of the water temperature, will serve admirably. Table XIX gives the daily maximum temperature and the daily mean temperature of the air, the temperature of the surface water in the lake, and the wind direction at 2 p.m., as observed in 1884, and a study of the table will bring out the manner in which the lake wind influences the air temperature over the city. The mean temperatures in the table do not show the effect of the lake wind as markedly as do the maximum readings, but the former more closely correspond to the temperature of the water. The temperature of the water near the shore of the lake, when above the point of maximum density of water, 39°, is always higher with an easterly wind of moderate or greater force, as the warm surface water is brought in by it. On the other hand it is lower withPlate II.—Contrast between the warmest and coldest months. Plate II shows the contrast between the warmest and the coldest months during the period of available daily mean temperatures, from July, 1872, to 1910. The heavy lines indicate the average temperature each day of the month, the upper line representing the warmest month and the lower, the coldest. The dotted line in between these two shows the normal daily temperature.TEMPERATURE 39 an offshore wind, because the warmer surface water is blown out and the colder strata of lower depths take its place. August 16 and 19 are good examples of these conditions. The maximum air temperature on the 16th was 80°, and on that day the maximum temperature of the water for the season was reached, 73?7, with an easterly wind. On the 19th, just three days later, the maximum air temperature of the month, 91°, occurred with a southwest wind, but the temperature of the water was considerably lower, 68?8. These effects are noticeable only when the wind is of sufficient force to drive the surface water either toward or from the shore, a light wind having no appreciable influence, no matter what its direction, in controlling the temperature of the water. Air moving over a great expanse of water surface tends to acquire the temperature of that surface. If warmer, the air will lose a portion of its heat to the water by conduction; if colder, it will receive a portion of the water’s heat by the same method. The specific heat of air being less than one-quarter that of water, the interchange of heat just mentioned will result in a larger change of air temperature than of water temperature. The temperatures of the overlying air and of the water in Lake Michigan are not often equal, and as a consequence there is a practically constant transmission of heat from I TABLE XIX Comparison between the Temperature op the Air and the Temperature op the Water, Lake Michigan, 1884 ' March April March April Temperature J Wind Direction Temperature Wind Direction j Temperature Wind Direction Temperature Wind Direction Maximum Average Surface of Water Maximum Average Surface of Water Maximum Average Surface of Water Maximum Average Surface of Water 1 54 45 40.9 N 18... 49 44 45.1 E 2 39 35 40.5 N 19... 49 44 46.0 E 3 55 44 38.8 W 20... 42 38 43.0 N 4 49 44 40.7 N 21... 41 38 42.4 N 5 42 38 41.6 NE 22... 43 39 41.1 N 6 39 36 42.1 NE 23... 46 42 41.7 N 7.... 45 39 41.4 W 24... 49 44 39.2 E 52 46 43.0 N 8 40 37 40.9 N 25... 48 43 38.5 E 55 48 48.2 E 9.... 40 37 41.1 N 26... 59 49 40.5 NW 67 58 45.0 SE 10.... 40 38 40.4 N 27... 59 52 39.9 SE 64 58 45.0 SW 11.... 52 *44 41.6 SW 28... 54 46 41.8 E 55 48 48.8 E 12.... 50 45 42.3 E 29... 43 36 N 64 52 47.9 E 13.... 48 44 42.8 E 30... 35 32 42.7 NE 77 60 48.8 SW 14.... 70 57 43.9 S 31... 55 44 41.5 S 1 ^ 00 51 44 3 SW 16.... 48 42 ll'.l NW Means 50.7 44.4 43.3 N 17.... 46 40 44.8 E 40 THE WEATHER AND CLIMATE OF CHICAGO TABLE XIX—Continued May June May June Temperature Wind Direction Temperature Wind Direction Temperature Wind Direction Temperature Wind Direction Maximum Average Surface of Water Maximum I Average 1 Surface of Water Maximum Average Surface of Water 1 Maximum j Average Surface of Water 1.... 71 58 50.9 NE 76 68 54.0 SW 18... 70 64 55.6 SE 83 74 60.2 SW 2.... 59 50 46.8 E 69 60 59.9 SW 19... 64 60 53.0 NW 74 66 60.8 N 3.... 58 52 52.3 E 66 60 58.8 N 20... 76 62 51.8 SW 72 64 62.4 NE 4.... 65 56 50.0 S 67 60 59.4 E 21... 74 68 56.0 SE 79 73 65.7 E 5.... 70 62 50.8 S 72 64 56.2 E 22... 78 70 53.1 s 80 74 70.8 NW 6.... 64 56 51.9 NW 69 65 62.4 NE 23... 73 68 55.0 sw 86 78 65.6 W 7.... 52 46 50.5 N 72 66 61.8 E 24... 66 58 55.6 sw 84 76 62.1 SW 8.... 59 51 52.4 NE 78 72 60.1 SW 25... 56 52 56.2 N 70 60 59.6 N 9.... 66 59 52.5 N 72 60 55.2 NW 26... 56 51 56.0 N 60 56 59.0 N 10.... 65 58 51.7 S 56 52 53.5 NW 27... 55 50 54.9 N 64 60 60.6 N 11.... 56 50 54.3 E 66 56 55.7 N 28... 47 45 53.7 N 69 62 62.9 N 12.... 60 55 56.9 E 70 61 62.3 NE 29... 51 46 53.1 N 77 69 70.7 NE 13.... 58 54 53.6 NW 59 54 57.8 N 30... 73 58 53.5 W 70 68 71.4 E 14 . 67 55 53.8 sw 60 54 57.8 N 31... 75 64 53.0 SW 15.... 68 56 54.6 NE 63 58 59.6 NE 16.... 55 48 54.5 SE 71 64 66.8 E Means 63.9 56.3 53.3 N 71.1 64.2 61.3 N 17.... 74 62 54.7 SW 78 72 64.6 SE July August July August Temperature Wind Direction Temperature Wind Direction Temperature Wind Direction Temperature Wind Direction Maximum Average Surface of Water Maximum Average Surface of Water 1 Maximum Average Surface of Water j Maximum Average Surface of Water 1.... 77 70 71.9 E 75 70 71.7 E 18... 81 72 67.6 SW 87 79 68.3 S 2.... 73 69 70.9 E 83 76 71.8 S 19... 74 68 67.9 E 91 82 68.8 SW 3.... 75 70 71.6 NE 77 70 66.7 W 20... 75 68 69.6 E 88 80 69.0 s 4.... 68 64 69.1 NE 67 62 60.6 NW 21... 81 70 68.0 SW 77 72 65.1 NE 5.... 75 69 69.1 W 69 61 62.3 E 22... 89 79 67.7 W 73 68 67.5 E 6.... 70 62 64.7 NW 70 64 61.7 SW 23... 83 73 63.7 sw 70 66 68.4 NE 7.... 67 62 66.5 NE 67 62 62.3 NE 24... 83 76 65.0 w 70 62 67.9 SE 8.... 72 66 65.6 S 62 60 61.9 N 25... 76 70 70.0 E 80 71 65.6 W 9.... 70 66 64.8 N 64 58 64.9 NE 26... 81 70 68.6 N 70 66 67.0 NE 10.... 71 63 66.4 E 68 60 65.7 SE 27... 80 72 70.1 E 70 63 67.7 E 11.... 83 72 65.6 NW 78 68 65.6 W 28... 73 70 71.7 N 70 66 66.6 N 12.... 78 73 66.9 W 77 69 68.3 E 29... 75 69 70.3 S 76 72 66.6 W 13.... 72 66 65.2 NW 77 72 69.4 NE 30... 81 72 71.3 SW 71 64 66.5 NE 14.... 68 63 65.3 NE 76 72 71.0 NE 31... 72 67 69.8 NE 70 62. 66.3 E 15.... 68 64 66.9 NE 78 73 72.4 NE — 16.... 66 62 67.6 N 80 74 73.7 E Means 75 68.5 67.9 E 74.7 68.5 67.3 NE 17.... 72 66 66.6 S 86 77 73.5 S TEMPERATURE 41 TABLE XIX—Continued September October September October Temperature Wind Direction Temperature Wind Direction Temperature Wind Direction i Temperature Wind Direction Maximum Average Surface of Water Maximum Average Surface of Water - Maximum j Average 1 Surface of Water Maximum Average Surface of Water 1.... 77 68 66.8 S 71 65 59.7 N 18... 62 56 64.7 E 59 52 56.6 E 2.... 82 72 66.6 sw 83 74 61.0 SW 19... 71 64 62.9 S 71 62 56.7 SW 3.... 84 74 67.6 sw 83 76 61.7 SW 20... 66 62 63.0 N 73 63 56.8 SW 4.... 80 75 63.5 NE 77 72 61.2 SW 21... 74 62 62.7 SW 66 57 57.4 sw 5.... 86 77 63.9 S 83 74 62.7 SW 22... 70 66 64.0 N 52 45 56.4 w 6.... 86 79 64.0 s 73 64 61.7 E 23... 69 64 62.8 S 39 34 53.8 w 7.... 79 74 64.8 NE 71 66 60.2 NE 24... 70 66 63.0 w 57 45 52.0 sw 8.... 84 75 68.2 E 69 58 56.7 NW 25... 66 60 63.7 E 52 42 52.1 E 9.... 89 80 65.4 SW 57 50 54.6 W 26... 74 66 63.9 S 55 50 49.8 SE 10... 88 80 62.7 SW 64 54 63.9 s 27... 75 70 64.4 w 57 48 50.3 W 11.... 76 67 58.9 N 74 62 54.6 sw 28... 74 70 62.2 sw 51 42 49.3 s 12.... 68 64 59.9 NE 67 62 57.7 NE 29... 70 66 63.2 E 55 48 49.2 SE 13.... 68 65 62.8 NE 62 56 57.1 N 30... 74 70 62.2 W 58 53 49.9 N 14 ... 68 63 63.2 E 58 54 56.6 E 31... 55 48 49.3 W 15.... 85 74 64.5 SW 63 54 55.7 SW 16.... 77 68 66.6 W 69 60 55.9 w Means 75.5 68.7 63.9 SW 64.2 56.4 56.0 SW 17.... 72 64 65.0 w 67 58 56.7 sw November December Temperature d Temperature a Maximum Average Surface of Water Wind Directi< Maximum Average 1 Surface of Water 1 Wind Direct« 1.... 58 49 48.6 NW 38 30 33.1 SW 18... 2.... 51 44 48.4 SE 46 40 34.1 SW 19... 3.... 57 50: 48.8 E 52 42 35.0 sw 20... 4.... 50 44 47.4 W 54 46 34.9 s 21... 5.... 45 40 46.6 NW 59 52 36.5 s 22... 6.... 47 40 46.5 SW 55 50 36.7 SE 23... 7.... 56 47 45.6 SW 44 39 34.7 w 24... 8 56 50 44.7 E 42 38 34.7 w 25... 9.... 64 54 45.3 SW 44 34 35.1 E 26... 10.... 49 46 45.8 N 45 40 33.0 SW 27... 11.... 53 47 46.1 W 39 33 33.9 NE 28... 12.... 47 42 45.5 SW 42 36 33.3 NE 29... 13.... 50 45 45.5 N 39 32 34.3 W 30... 14.. .. 15.. .. 58 60 48 51 46.3 45.7 w sw 41 36 36 26 34.8 34.1 SW w 31... 16.... 17.... 58 53 52 44 45.4 44.5 sw N 16 12 32.2 NW Means November December Temperature O bù c5 M C3 -5 02 42 S-e m ns a g 40 42 48 57 55 55 28 40 39 43 30 33 38 36 36 42 48 50 34 16 30 27 34 24 26 29 43.0 42.0 41.7 41.8 41.3 42.3 38.8 38.3 35.6 35.7 35.8 35.0 34.7 NE S SW SW S w SW w SW NW NW W w 48.7 40.8 43.1 SW Table XIX contains for a single year 1884, from March 24 to December 16, the period during which the surface was free from ice, the maximum and average daily air temperature in shelter, the temperature of the water at the surface of Lake Michigan at 1 p.m., and the wind direction at 2 p.m. Figures in bold-faced type indicate highest and lowest temperatures.42 THE WEATHER AND CLIMATE OF CHICAGO the one to the other, which results in the air temperature there differing from that over the shores. A lake wind, therefore, tends usually to raise or lower the air temperature in the city. The temperature of the air at Chicago is not influenced by the lake wind in the same degree throughout the year. A comparison of the means of air and water temperatures in Table XIX shows that the TABLE XlXa Monthly and Annual Mean Temperatures of Lake Michigan at Two-Mile Crib and Monthly and Annual Air Temperatures at Weather Bureau Office, Federal Building, 1902-11 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual lgo2 {Water 32.0 32.2 32.7 41.5 50.4 61.2 63.6 69.1 62.7 56.0 51.4 35.0 49.0 25.2 20.8 38.6 46.4 59.0 64.2 72.4 68.4 60.8 55.2 47.0 26.5 48.7 1903{^ter 32.0 32.0 36.5 43.8 50.8 55.7 64.2 66.2 65.0 57.4 43.7 32.3 48.3 24.0 25.0 40.4 47.2 59.8 61.2 72.2 68.4 64.4 53.6 36.4 20.0 47.7 1904{^ater 32.0 32.0 32.2 38.2 46.8 56.6 62.7 62.6 61.5 55.7 44.5 33.4 46.5 17.7 17.2 35.2 40.7 57.3 64.2 71.0 68.3 64.5 53.4 42.8 26.8 46.6 1905{^ter 32.0 32.0 34.3 42.1 50.8 59.3 65.6 69.4 62.0 54.6 41.3 33.5 48.1 18.1 17.0 39.2 45.5 56.4 65.4 71.2 74.0 67.5 53.4 40.6 31.8 48.3 1906{^ter 32.1 32.3 32.4 39.9 50.9 55.8 64.3 70.6 64.8 55.1 43.0 34.8 48.0 32.6 27.6 30.2 50.7 59.8 68.0 71.6 75.6 70.1 52.8 41.8 32.8 51.1 1907 (Ster 32.3 32.0 34.9 39.5 45.6 53.8 61.7 63.7 61.9 52.4 42.2 34.2 46.2 27.8 26.4 42.6 39.8 51.6 66.2 73.3 71.2 64.6 52.6 41.0 32.8 49.2 1908{S?t!r::::: 32.0 32.0 34.6 41.3 48.5 57.6 66.4 71.8 65.8 55.0 42.1 33.2 48.4 28.6 26.7 40.6 49.0 58.7 68.4 74.3 73.4 70.6 55.2 43.5 31.3 51.7 1909 (Water 32.4 32.2 33.3 39.8 48.2 56.3 65.4 70.6 64.1 52.6 46.4 34.8 48.0 28.8 32.4 36.0 45.0 55.9 66.6 72.3 74.8 64.0 50.6 48.5 21.6 49.7 1910{Water.;:;. 32.0 32.0 35.9 45.0 48.0 57.6 66.7 70.2 64.2 59.2 42.3 32.9 48.8 25.6 24.8 48.6 51.2 53.4 68.2 76.0 73.3 65.2 58.6 36.0 26.4 50.6 191l{£te 32.2 32.5 34.5 39.7 50.6 62.2 65.4 65.0 66.6 57.2 41.0 34.2 48.4 29.2 32.4 39.2 46.1 65.9 72.4 76.0 71.8 67.0 53.2 35.4 35.0 52.0 32.1 32.1 34.1 41.1 49.1 57.6 64.6 67.9 63.8 55.5 43.8 33.8 48.0 25.8 25.0 39.1 46.2 57.8 66.5 73.0 71.9 65.9 53.9 41.3 28.5 49.6 Departure from mean of water temperature.. -6.3 -7.1 +5.0 +5.1 +8.7 +8.9 +8.4 +4.0 +2.1 -1.6 -2.5 -5.3 +1.6 Monthly averages of lake-water temperatures obtained from tri-daily observations (9 a.m., 2 p.m., and 7 p.m.), taken 24 feet below the lake surface. For this period the temperature of the air averaged 1?6 higher than that of the water in the lake. differences grow smaller as the heated season advances, becoming of the opposite kind before the beginning of winter. Table XlXa and Fig. 4a show the mean temperatures of the water in Lake Michigan and of the air at the Weather Bureau station in Chicago for the period 1902 to 1911, inclusive, by months, and the average differences between the two. The data for the water of the lake was furnished by the City Engineer, from tri-daily observations taken at 9 a.m., 2 p.m., and 7 p.m. at the Two-Mile Crib, 24 feet below theTEMPERATURE 43 surface, and indicate more nearly than do the old shore readings in Table XIX the condition of the body of the water toward which the lake wind adjusts its temperature. It will be noticed that the differences are greatest in the spring and early summer, the water being colder than the air by 8?7 in May, 8?9 in June, and 8?4 in July. It Fig. 4a—Relation of lake and air temperatures. A=monthly mean temperature of air; W=monthly mean temperature of water in Lake Michigan. Record, 1902-11, inclusive. Water temperatures taken at Two-Mile Crib, 24 ft. below the surface, three times daily: 9:00 a.m.; 2:00 p.m.; 7:00 p.m. About February 20 and September 25, as indicated by the lines crossing each other, the air and water temperatures are the same. is during the first part of this period, in May and June, that the lake wind influences the air temperatures of the city to the greatest extent. By the end of July the differences have begun to lessen rapidly, and by the first of October the water has become warmer than the air, but is not much in excess until December. Throughout44 THE WEATHER AND CLIMATE OF CHICAGO the autumn the on shore breeze exercises but little effect upon the air temperature, although it moderates severe conditions very much, and tends to hold off frosts until a later time than would otherwise be the case. In December, January, and February, the air temperature over the city averages much below that of the lake water, but the decided moderating influence exerted by lake winds during the early portion of this period is occasionally lost in January and February, when the surface of the lake becomes frozen far enough out to assume the characteristics of a large expanse of land, and has therefore but little effect in raising the temperature of the air above. It will thus be seen that the lake often serves to break up hot waves in summer, and, when not extensively frozen, cold waves in winter; and because of it hot and cold waves seldom last longer than three or four days at a time. Whenever the wind is from the lake, the tendency is for the air over the city to acquire the temperature of the water in the lake, and the relation is sure to be very close when the wind velocity is high. There was only one pronounced warm wave in the summer of 1884, August 17 to 20, inclusive, and this was broken up in the ensuing days by the lake wind. Table XIX shows that when the wind shifted to the lake on the 21st, the temperature fell rapidly and continued to fall for several days, the maximum temperatures on the 23d and 24th not exceeding 70°, although the temperature of the water changed but little. While the lake wind is usually felt over the entire city, it is occasionally so light as not to be noticeable over the west and southwest portions, and, as a consequence, local temperature conditions at such times vary considerably. With moderate wind, however, the effect is carried far into the interior, and in the spring its influence is sometimes felt as far inland as Joliet. The chill of the wind from the lake in early spring is rather disagreeable, but in the late spring and summer its coolness is most refreshing. At all times it brings pure air in abundance, and this, together with its moderating effect, is one of the city’s valuable assets. As water acquires and loses heat more slowly than does the air, we should expect that on the average the rise and fall of temperature of the water in Lake Michigan would follow throughout the year the various changes in mean air temperature by a shorter or longer interval, and this is found to be the case except in the coldest weather of winter when the water temperature has reached the freezing point,TEMPERATURE 45 below which it cannot fall and remain in the liquid state. Figs. 46 and 4c show the mean daily temperatures of the air at the Weather Fig. 46.—Relation of lake temperature and air temperature for 1911, warm year. Lake temperature at Two-Mile Crib; air temperature at Weather Bureau office. A=mean daily air temperature; B=mean daily water temperature. Fig. 4c.—Relation of lake temperature and air temperature for 1904, cold year. Lake temperature at Two-Mile Crib; air temperature at Weather Bureau office. A=mean daily air temperature; B=mean daily water temperature.46 THE WEATHER AND CLIMATE OF CHICAGO Bureau office, and the mean daily water temperatures at the Two-Mile Crib, for 1911, an exceptionally warm year, and for 1904, a cold year, respectively. An inspection of these graphs will bring out the lagging of the change in temperature in the lake behind that of the mean air temperature, the interval ranging from two or three days to nearly a week. COMPARISONS OF TEMPERATURE AT CHICAGO WITH TEMPERATURES OF OTHER PLACES 1. Near-by locations: LaGrange.—In the previous section devoted to the influence of Lake Michigan on the temperature at Chicago, it was noted that the cooling effects of the lake wind in summer are sometimes felt far inland. As a rule, however, the influence of this wind weakens rapidly as the interior is approached, and nearly or quite disappears at 15 or 20 miles. Then again, as the city is left behind, the spread of the open country and the abundant vegetation offer much better conditions for the free radiation of heat, and at night considerably lower minimum temperatures might be expected than would occur in the closely built-up sections of the business district. Table XX, giving the maximum and minimum temperatures for Chicago and LaGrange, and the prevailing wind direction for the former, for the year 1904, will prove interesting after the study of Tables XIX and XlXa, as it brings out clearly the remarkable difference a distance of a few miles makes when the city is under the dominance of the lake breeze. The year 1904 was selected for no particular reason except that the record at LaGrange is complete for that entire year, while in most other years it is more or less broken. LaGrange is located on the inner margin of the Valparaiso moraine about 15 miles directly west of the city, and we should therefore expect to find a lower maximum temperature at Chicago throughout the spring and summer whenever lake winds prevail, and in the table this is almost invariably found to be the case. The maximum temperature is usually higher at LaGrange, even when the wind is from some other direction, but there is very little difference during the winter season. March 23, April 24 to 28, May 1 to 3, and June 8 to 19 furnish excellent examples of the pronounced lake-wind effect in spring and summer, for both short and long periods. In the period last named the maximum temperature at LaGrange ranged from 72° to 88°, while during the same time the highest readings at the Chicago Weather office oscillated between 61° and 77°. For thisTEMPERATURE 47 12-day interval during which northeasterly winds prevailed, the daytime maximum temperatures at Chicago averaged 67°, while at LaGrange the average was 80°, or 13° higher for the inland station. On the other hand minimum temperatures at LaGrange averaged lower throughout the entire year than at Chicago, although there were days here and there when these readings were higher. As previously indicated (p. 9), the minimum temperature should ordinarily be lower in the country than in the city, especially during quiet, clear nights, and numerous instances of this kind can be found in the table. The average difference for the year between Chicago and LaGrange is 3?9, and ranges somewhat greater during the summer than during the winter, although in exceptional cases differences of 20° or more are to be noted in both seasons, as on January 4, May 25, TABLE XX Temperatures and Prevailing Wind Direction at Chicago and LaGrange, III., for a Selected Year, 1904 January, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 1.... ■ 27 26 NE 20 14 2.... 21 24 NE 4 4 3.... 10 12 NW - 1 - 9 4.... 16 15 SW 2 -18 5.... 24 26 SW 7 - 3 6.... 30 32 SW 22 13 7.... 36 36 SW 16 18 8.... 36 37 w 28 24 9.... 30 34 E 20 11 10.... 26 28 E 22 21 11.... 27 30 E 23 23 12.... 34 30 S 25 24 13.... 27 25 NW 10 10 14.... 20 22 NW 6 - 1 15.... 31 29 S 12 - 1 16.... 29 29 NW 11 13 17.... 23 26 NE, NW 8 7 18.... 24 22 SE 14 15 19.... 39 39 SW 15 13 20.... 39 39 NE 32 32 21.... 37 35 NE 31 32 22.... 34 33 N 25 25 23.... 25 29 W - 2 2 24.... - 2 2 NW, W -13 -11 25.... 11 4 NE -15 -14 26.... 12 9 NW 2 1 27.... 12 9 s, w - 6 -12 28.... 14 10 N 4 - 3 29.... 13 15 s 5 - 2 30.... 26 28 s 9 0 31.... 25 25 NW 6 13 Means 24.4 24.5 Chicago NW LaGrange NW 11.0 7.8 February, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 10 9 S, NW - 6 - 8 15 15 W 6 5 15 18 NW 1 - 1 16 16 SW - 4 - 5 43 42 SW 15 10 53 52 SW 42 41 49 49 NW 7 6 14 12 NE 2 2 14 19 NE 10 10 ,14 17 NE 8 10 17 19 N 7 6 18 21 SE 7 0 37 34 S 18 18 35 33 NW 11 12 11 14 NW - 2 - 1 5 10 NE - 4 - 7 13 13 E 2 - 1 15 16 NE 12 13 15 22 NE 8 - 4 24 32 E 5 - 6 31 32 W 20 23 31 24 W 0 - 2 38 36 NE 19 19 21 21 NW 10 10 16 16 E 5 4 23 25 NE 14 11 41 38 S 14 5 39 43 NE 33 33 35 35 E, NE 27 28 24.4 25.2 Chicago NE LaGrange N 9.9 8.048 THE WEATHER AND CLIMATE OF CHICAGO TABLE XX—Continued March, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 1.... 35 35 SW 25 23 2.... 51 50 s 32 28 3.... 50 50 NW 13 14 4.... 34 36 w 19 14 5.... 44 40 s 31 30 6.... 40 38 N 35 33 7.... 46 46 NW 32 32 8.... 40 42 NW 31 28 9.... 36 41 E, SE 28 24 10.... 52 52 SE, S 34 35 11.... 34 35 N 27 26 12.... 27 27 NE 23 23 13.... 29 32 NE 23 21 14.... 27 30 N 24 22 15.... 30 34 NW 21 21 16.... 33 37 SE 26 24 17.... 32 33 E 30 24 18.... 40 42 S 28 . 25 19.... 44 45 . W 33 33 20.... 33 39 E 26 26 21.... 59 56 SE 31 31 22.... 60 58 SW 36 32 23.... 44 55 E 34 28 24.... 65 66 SE 36 33 25.... 57 43 W 31 29 26.... 31 29 NW 18 18 27.... 26 33 NW 16 16 28.... 41 46 SW 22 21 29.... 57 49 s 35 29 30.... 45 45 SE 38 39 31.... 57 . 58 W, SW 41 43 Chicago NW LaGrange Means 41.9 42.6 NW 28.4 26.6 April, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 49 53 W 40 40 41 43 NW 32 32 34 39 . NE 27 25 48 42 S 30 28 53 59 S 38 32 57 61 W 41 37 53 64 E 31 31 49 52 S 38 37 40 42 NW 33 33 39 42 NW 33 33 50 54 SW, W 33 31 37 45 N 32 31 36 41 NE 30 30 45 44 SW 29 26 51 51. N, S 29 28 32 43 NE 23 21 46 51 SW 28 24 50 63 NE, SW 33 33 34 - 44 NE 28 25 32 40 NE 25 25 39 53 NE 31 25 52 55 S, SE 37 33 79 81 S 51 42 63 69 NE 39 42 40 48 NE 36 37 40 47 NE 37 37 46 58 NE 38 34 52 64 NE 41 35 62 71 N 44 36 60 65 N 46 36 47.0 52.8 Chicago NE LaGrange N 34.4 32.0TEMPERATURE 49 TABLE XX—Continued May, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 1.... 51 64 NE 43 42 2.... 55 67 NE 42 40 3.... 54 71 NE 42 43 4.... 66 80 SE 50 42 5.... 69 81 SE 57 49 6.... 79 82 S 59 54 7.... 77 78 SW 56 59 8.... 71 73 SE 43 47 9.... 62 66 NW 45 46 10.... 52 58 SE 40 40 11.... 69 72 S 45 40 12.... 80 71 SW 60 -58 13.... 67 66 w 47 46 14.... 52 55 NW 44 42 15.... 44 52 NE 38 36 16.... 51 56 NE 39 32 17.... 50 51 NE 42 36 18.... 50 63 N 42 38 19.... 66 66 NW 47 45 20.... 68 77 NE 55 50 21.... 74 86 SE, W 57 52 22.... 83 85 SW 61 58 23.... 70 75 NE 55 56 24...'. 72 75 SE 49 46 25.... 87 89 SW 70 41 26.... 74 75 SW 45 49 27.... 71 74 NW 51 46 28.... 75 77 S 55 48 29.... 74 75 s 58 56 30.... 58 60 NE 42 39 31.... 56 68 NE 45 40 Chicago NE LaGrange Means 65.4 70.6 W 49.2 45.7 June, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 55 68 NE 48 42 65 76 E 50 46 87 90 SE 58 45 82 83 S 64 63 77 SW 64 65 79 w 57 54 67 69 NW 55 50 62 72 NE 54 52 63 76 NE 52 50 61 75 NE 53 51 62 77 NE 53 50 67 82 NE 53 48 76 88 SE 59 52 68 82 N,NE,SE 54 56 73 82 NE , 53 53 60 73 NE 53 50 65 82 NE 53 41 77 86 SE 62 41 70 82 NE 58 58 83 88 SW 60 57' 76 79 NW 58 61 60 75 NE 52 50 86 75 S 59 50 88 91 SW 67 71 82 83 SW 70 67 71 77 NE 62 60 67 76 NE 60 54 67 75 SE 61 54 81 84 W 62 54 71 77 w 58 55 Chicago NE LaGrange 71.1 79.4 SE 57.4 52.950 THE WEATHER AND CLIMATE OF CHICAGO TABLE XX—Continued July, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 1.... 61 66 NE 53 49 2.... 68 76 SE 56 44 3.... 83 89 S 60 49 4.... 80 89 SW 70 69 5.... 76 89 SE 67 56 6.... 70 88 NE 57 57 7.... 62 71 NE 58 57 8.... 78 80 SW 60 56 9.... 79 85 W 66 68 10.... 71 85 E 64 58 11.... 80 88 SE 65 59 12.... 68 80 N, NE 60 61 13.... 80 84 W 62 55 14.... 85 89 S 65 60 15.... 86 91 w 66 63 16.... 90 98 SW 71 62 17.... 94 97 SW 75 73 18.... 92 99 SW 75 72 19.... 80 93 NE 71 68 20.... 84 93 NW 70 63 21.... 74 85 NE 67 62 22.... 72 85 NE 62 57 23.... 66 77 NE 59 53 24.... 69 81 NE 64 53 25.... 82 88 SW 61 51 26.... 79 84 SW 65 60 27.... 80 88 SW 62 63 28.... 69 82 NE 62 58 29.... 80 86 SE 65 56 30.... 88 90 SW 67 66 31.... 87 90 W 68 68 Chicago SW LaGrange Means 77.8 86.0 W 64.3 59.5 August, 1904 Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange 78 84 NW 62 58 66 76 NE 62 55 74 82 SE 61 51 82 85 SW 63 60 81 84 W 62 57 78 84 NW 65 58 74 80 W 59 54 64 76 NE 55 46 82 88 S 59 46 70 77 NE 61 60 66 78 NE 58 48 85 89 SE, SW 62 50 85 81 SW 66 66 72 86 N, NE 64 58 86 89 SW 65 60 75 87 N 62 61 76 89 NE 64 64 67 81 E 64 61 72 75 SE 62 61 77 80 N 60 60 90 91 SW 66 62 67 77 NE 59 59 76 80 SW 58 48 83 87 SW 60 50 78 80 NW 61 66 66 77 SE 56 48 73 81 SW 58 48 81 84 SW 60 50 73 75 ' NE 61 61 66 77 NE 60 57 71 81 SE 64 61 75.3 82.2 Chicago NE LaGrange NE 61.3 56.1TEMPERATURE 51 TABLE XX—Continued September, 1904 October, 1904 Maximum P.W.D. Minimum Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange Chicago LaGrange Chicago Chicago LaGrange 1.... 82 89 SW 67 66 72 74 W 52 48 2.... 77 81 W 64 65 60 59 NE 52 44 3.... 71 74 w 59 56 59 64 S 45 38 4 76 78 w 58 51 73 73 S, SW 50 44 5.... 68 78 NE 59 55 64 63 NE 49 47 6.... 74 82 SE 63 54 49 52 NE,E 45 37 7 72 75 NW 62 54 60 60 S 42 31 8.... 65 73 NE 60 58 72 74 SW 54 51 9.... 80 80 SW 58 51 77 80 SW 65 60 10.... 87 88 SW 64 51 78 81 SW 59 60 11.... 73 , 73 NE 60 56 59 61 NE 53 52 12.... 60 64 E 52 44 53 57 NE 50 47 13.... 65 65 SE 54 42 53 58 E 50 43 14.... 58 60 NW 47 44 53 56 E 47 37 15.... 67 69 SW 47 39 58 * 66 SE 50 37 16.... 76 78 SW 52 51 69 72 S 48 37 17.... 85 87 SW 59 48 78 77 SW 56 46 18.... 73 73 NE 60 61 75 76 SW 57 48 19.... 72 78 SE 60 60 74 73 S 54 44 20.... 70 66 NE 52 44 54 63 SW 44 43 21.... 55 57 NE 47 44 50 51 w 43 40 22.... 60 64 SE 47 42 45 49 NW 39 35 23.... 76 78 S 51 43 50 54 w 35 32 24.... 72 73 SW 67 67 55 57 w 44 42 25.... 67 77 NE 58 59 48 52 w 37 33 26.... 66 69 NE' 58 51 50 55 NE 37 33 27.... 63 72 NE 55 52 46 49 SW 34 30 28.... 83 83 S 62 59 57 59 SW 34 31 29.... 79 81 W 69 64 59 67 NE 42 34 30.... 69 69 NW 58 55 50 57 SE 44 35 31.... 62 65 SW 39 28 Chicago Chicago SW SW LaGrange LaGrange Means 71.4 74.5 SW 57.6 52.9 60.1 63.0 SW 46.8 40.952 THE WEATHER AND CLIMATE OF CHICAGO TABLE XX—Continued November, 1904 December, 1904 Maximum P.W.D. Minimum Maximum P.W.D. Minimum Chicago LaGrange Chicago Chicago LaGrange Chicago LaGrange Chicago Chicago LaGrange 1.... 67 70 SW 48 38 31 30 NE 23 17 2.... 67 69 sw 49 38 31 31 NE 25 21 3.... 60 69 NE 50 39 29 30 NE 26 25 4.... 58 69 NE 45 39 26 26 W 21 14 5.... 47 49 NE 42 37 32 32 W 20 16 6.... 45 48 N 38 30 36 36 sw 20 13 7.... 55 57 NE 37 32 36 36 w 27 22 8.... 47 45 NE 40 36 35 36 NE 30 26 9.... 44 47 NE 36 36 31 33 E 24 25 10.... 42 43 NE 38 36 30 32 NW 24 22 11.... 45 46 NW 32 27 28 28 B 24 20 12.... 45 46 W 27 24 29 26 NW 21 20 13.... 40 41 N 31 27 29 21 SW 19 5 14.... 45 47 NW 34 36 24 25 NE 16 7 15.... 54 55 SW 33 28 23 25 SW 16 2 16.... 53 62 SE 40 24 29 29 SE 15 - 2 17.... 58 59 SW 42 37 32 31 NE 17 15 18.... 65 66 sw 40 30 34 34 SW 16 11 19.... 66 68 sw 48 40 26 29 NW 15 15 20.... 60 60 N 43 38 32 30 NW 19 17 21.... 49 51 SW 35 28 28 28 SW 14 5 22.... 55 57 W 39 34 51 49 SW 26 24 23.... 58 59 NW 43 35 54 53 SW 31 32 24.... 43 47 NE 39 35 31 32 E 25 25 25.... 42 43 NW 30 27 29 30 E 25 29 26.... 32 32 NW 23 20 37 ' 35 SE 29 28 27.... 26 30 NW 19 17 43 42 SW 11 11 28.... 44 41 S 25 24 11 11 W 4 6 29.... 44 44 W 22 19 30 29 sw 2 2 30.... 27 30 W 19 16 42 42 sw 28 28 31.... 49 49 sw 38 29 * Chicago Chicago SW SW LaGrange LaGrange Means 49.4 51.7 SW 36.2 30.9 32.5 32.3 SW 21.0 17.1 Table XX contains the daily maximum and minimum temperatures at Chicago and LaGrange, 111., and the prevailing wind direction at Chicago for a selected year, 1904. Thermometers at Chicago were exposed in a shelter on the Auditorium Tower,, and those at LaGrange were in a shelter a few feet above the sod. Summary of Table XX Stations Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Prev. Wd. Dir., Chicago.. NW NE NW NE NE NE SW NE SW SW SW w SW Mean Max. Chicago.. 24.4 24.4 41.9 47.0 65.4 71.1 77.8 75.3 71.4 60.1 49.4 32.5 53.4 Temp _ LaGrange 24.5 25.2 42.6 52.8 70.6 79.4 86.0 82.2 74.5 63.0 51.7 32.3 57.1 Mean Min. f Chicago.. 11.0 9.9 28.4 34.4 49.2 57.4 64.3 61.3 57.6 46.8 36.2 21.0 39.8 Temp k LaGrange 7.8 8.0 26.6 32.0 45.7 52.9 59.5 56.1 52.9 40.9 30.9 17.1 35.9 Mean Monthly Chicago.. .17.7 17.2 35.2 40.7 57.3 64.2 71.0 68.3 64.5 53.4 42.8 26.8 46.6 Temp , LaGrange 16.2 16.6 34.6 42.4 58.2 66.2 72.8 69.2 63.7 52.0 41.3 24.7 46.5 Mean Daily f Chicago.. 13.4 14.5 13.5 12.6 16.2 13.7 13.5 14.0 13.8 13.3 13.2 11.5 13.6 Range ^LaGrange 16.7 17.2 16.0 20.8 24.9 26.5 26.5 26.1 21.6 22.1 20.8 15.2 21.2TEMPERATURE 53 and June 18, 1904. During the summer of 1904 there were but 4 days on which the temperature reached a maximum of 90° or over at Chicago, while there were 11 such days at LaGrange. On some of these warm days, when with a prevailing offshore wind the temperature was higher at LaGrange than in the city, the wind temporarily shifted to the lake for a few hours during the afternoon, thus lowering the maximum temperature sufficiently to affect the mean for the day as well. The temperature data for LaGrange for the warm summer of 1913 are not available, but a comparison between the readings at Joliet, about 18 miles farther inland, and Chicago shows that during the months of July and August there were at the former place a total of 24 days in which the maximum reached 90°, while in Chicago the total equaled only 11. This example is given here, as the heat of the summer of 1913 was pronounced over a large area, and the data emphasize the moderating influence of the lake as brought out in the table. The greater frequency of severe temperatures in the winter at LaGrange is also apparent from Table XX, where it will be seen that there were 22 days at this inland station with a minimum of zero or below in 1904, while there were but 10 days of similar character at Chicago. As a consequence of the ordinarily higher maximum and lower minimum at LaGrange, its daily range of temperature is correspondingly greater, averaging for the year, 21?2, as compared with 13?6 in the city. The average maximum temperature for 1904, 57? 1, was 3?7 higher than that at Chicago, while the average minimum, 35?9, was 3?9 lower. In the summer months the mean daily range at LaGrange was 26° or more, and averaged about 10° greater than in the colder season, while at Chicago the range in comparison was fairly even throughout the year. Yet, in spite of the great difference between the maximum and minimum temperatures of the two places, it is interesting to note that the mean temperatures for the year as a whole differed only by 0?1, Chicago averaging 46?6 and LaGrange 46?5. From this it will readily be seen that mean annual temperatures alone are far from conclusive in the comparison of climates, but that the range of temperature, the upper and lower limits, and the frequency of extreme conditions must be taken into consideration. 2. Northern Illinois.—Table XXI gives the mean annual temperatures for 23 stations over northern Illinois, including Chicago, for the entire period of their records up to and including 1906. The length of these records necessarily varies considerably for the different54 THE WEATHER AND CLIMATE OF CHICAGO stations, most of the observers rendering voluntary services, which have been more or less interrupted. Unfortunately, the mean maximum and mean minimum temperatures for these places are not available, but the greater daily range holds good for these inland stations just as was shown to be the case at LaGrange in the previous paragraph. This statement is borne out by the highest and lowest readings observed during the period of record, the minimums ranging from 1° to 9° lower than the absolute record of —23° at Chicago, TABLE XXI Mean Temperatures at Stations in Northern Illinois Station Length of Record, Years January February March April May June July August September October November December Annual Aledo 8 24.1 21.6 39.5 49.7 61.2 68.6 74.1 71.8 65.6 53.5 37.4 27.3 49.5 Antioch 7 20.7 19.1 36.5 44.4 57.0 65.4 72.1 69.2 63.8 49.6 37.6 25.7 46.8 Ashton 14 22.1 19.9 34.7 47.8 59.8 67.9 72.8 70.9 64.1 51.5 37.1 24.4 47.8 Aurora 29 20.8 22.4 34.6 48.0 59.2 68.9 73.2 71.0 63.8 51.5 37.2 26.4 48.1 Cambridge.. . 16 22.3 22.1 36.3 50.4 62.4 70.0 75.2 73.1 65.7 53.5 38.4 26.4 49.6 Chicago . 36 24.0 25.5 34.9 46.2 56.6 66.5 72.3 71.2 64.8 53.2 39.2 29.3 48.7 Davenport, la. 35 21.4 24.0 35.5 50.2 61.1 70.2 75.1 72.8 65.4 53.4 38.5 27.6 49.6 Dixon 18 20.5 20.6 34.8 48.7 59.8 69.5 74.0 72.0 64.7 51.7 36.6 25.4 48.2 Dubuque, la.. 35 18.6 21.5 33.6 49.1 60.4 69.2 74.2 71.8 63.9 51.9 36.1 25.1 48.0 Galva 16 22.3 21.0 36.6 49.8 61.2 69.8 74.4 72.5 6ff.6 53.1 37.6 25.9 49.2 Henry 20 23.1 23.7 36.9 50.0 61.4 70.4 74.4 72.3 65.5 52.6 38.4 28.6 49.8 Joliet 17 24.4 22.6 37.0 48.7 61.0 69.5 73.9 71.7 64.8 52.1 38.8 27.7 49.4 Kishwaukee. . 20 20.3 20.8 33.8 47.7 58.3 68.5 73.1 70.8 63.6 51.3 36.9 26.2 47.6 LaGrange 16 23.0 21.8 35.5 47.5 58.5 68.2 73.1 71.1 64.2 52.1 38.0 26.8 48.3 Lanark 19 20.8 20.6 33.3 47.7 58.9 67.9 72.6 70.2 62.7 50.3 35.9 25.3 47.2 LaSalle 33 22.3 25.0 36.8 49.8 60.8 70.1 75.3 72.0 63.9 51.9 37.8 27.5 49.4 Morrison..... 14 21.2 20.0 35.0 49.3 60.4 68.6 73.3 71.7 64.7 53.1 37.9 25.0 48.4 Ottawa 22 24.0 24.4 37.0 50.6 61.4 70.9 75.2 72.2 65.4 53.2 39.0 29.2 50,2 Streator 15 25.1 22.7 38.5 49.8 61.5 70.5 75.0 72.9 66.6 53.9 39.7 27.9 50.3 Sycamore 28 19.6 20.9 33.1 47.1 58.2 67.8 71.9 69.6 62.6 50.3 35.9 25.6 46.9 Tiskilwa 14 23.6 21.1 36.0 49.1 61.5 69.6 74.0 72.1 65.1 52.6 38.4 26.3 49.1 Walnut 17 23.0 22.2 37.5 51.2 62.3 71.0 75.5 73.5 66.9 54.7 38.7 27.2 50.3 Winnebago... 21 20.7 20.0 33.7 47.8 58.3 68.3 72.8 70.7 63.4 51.2 36.0 25.5 47.4 Means 22.1 21.9 35.7 48.7 60.0 69.0 73.8 71.6 64.6 52.3 37.7 26.6 48.7 Table XXI gives the mean monthly temperatures of the various stations in northern Illinois, together with the annual values. The data are based upon different periods of observation, as the stations were not all established at the same time, but the record in each case extends to and includes the year 1906. and the maximums exceeding the city’s record of 103° by the same amounts (.Bulletin W, Summary of Climatological Data for the United States, sec. 64, p. 6, U.S. Weather Bureau). It is a remarkable fact that the average of the annual means for all these various stations is 48?7, exactly the same as the 36-year mean for Chicago, and this is even more significant as to the indefiniteness of annual values for purposes of comparison than was the small difference between the means for the year 1904 at LaGrange and Chicago. In individual cases, too, the maximum and minimum temperatures at manyTEMPERATURE 55 of these places show greater departures from the readings observed at Chicago than does LaGrange, yet the annual means do not differ appreciably. 3. Other 'portions of the United States.—To extend in a very general way the comparison of the mean temperature of Chicago with that in other portions of the United States, Plate III has been introduced at this point, giving the annual isothermal lines for the entire country. It will be noted that the isotherm representing the mean annual temperature of 50° passes east-westerly through northern Illinois a short distance south of Chicago, and this is in harmony with the conditions prevailing in that section of the state as brought out in the discussion of Table XXI (p. 53). This plate shows the southward dip of the lines in the mountain regions, indicating the influence of the greater elevations in lowering the temperatures of those districts; and the northward dip in the great valley of California, where bright and almost uninterrupted sunshine during the daytime results in a region of unusual warmth. As, however, the map gives no information whatever as to daily ranges, or seasonal ranges, of temperature, comparisons between the climatic conditions of different places must be made with caution if it is used alone. Space prevents the publication of isothermal charts of the various seasons, but in those of spring the influence of the cold water of the Great Lakes is distinctly to be seen in the southward bending of the lines in that region, while in the summer the opposite trend is given to the lines passing over the superheated areas of the*plains states stretching eastward from the foothills of the Rockies. Table XXII and Fig. 5 give more detailed data regarding the temperature conditions of various sections of the country, as represented by 15 cities selected in the different regions. The monthly and annual normal temperatures are shown for these places, including Chicago, and will serve to particularize more clearly the information to be gained from a study of Plate III, but even monthly normals or means, present the same disadvantages in comparisons as do annual values, only in lesser degree. The mean temperatures of two places may be exactly the same and yet, as previously pointed out, their variation in temperature may differ radically. For instance, the mean temperature of Chicago is only 4° lower than that of Portland, Ore., and to the casual observer this might be taken to indicate that higher temperatures prevail at the latter place. This is, indeed, pronouncedly the case in winter, but in the summer season theNORMAL TEMPERATURE, ANNUAL, 1871-1908 Plate IIITEMPERATURE 57 relation is reversed. At Portland the range in temperature from month to month and from day to day is very small, especially during the colder weather of the rainy season, from November to May. A city whose average annual maximum temperature is 70° and whose average minimum temperature is 30° will have an annual mean of 50°; another city may have the same annual value, but its average maximum and minimum may be 60° and 40°, respectively. We must therefore study the ranges in temperature as well as the average monthly and annual means in order to make proper comparisons. TABLE XXII Monthly and Annual Normal Temperatures, Selected Cities in the United States Station Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Portland, Ore 39 41 46 51 57 61 66 66 61 53 46 41 52 San Francisco, Cal 50 51 53 54 56 57 57 58 59 58 56 51 55 Yuma, Ariz 55 59 64 70 77 85 91 90 84 72 62 56 72 Havre, Mont 14 15 27 43 54 62 68 67 58 44 31 21 42 Denver, Colo 29 31 39 48 57 66 72 70 63 51 39 32 50 El Paso, Tex 44 49 56 64 72 80 80 79 73 62 51 45 63 Moorhead, Minn 3 7 21 41 55 64 69 66 57 43 24 11 38 Omaha, Neb 20 24 36 50 62 72 76 74 66 54 38 27 50 Galveston, Tex 53 56 62 69 75 81 83 83 79 72 63 56 69 Marquette, Mich 16 16 24 38 49 58 65 64 57 46 32 23 40 Chicago, 111 24 25 34 46 56 66 72 71 65 53 39 29 48 New Orleans, La 53 56 62 68 74 80 81 81 78 69 61 54 68 Northfield, Vt 15 17 26 40 54 63 67 63 55 44 32 20 41 New York, N.Y 30 31 38 48 59 68 74 72 66 56 44 34 52 Jacksonville, Fla 54 57 62 68 74 79 81 80 77 70 61 55 68 Table XXII contains the monthly and annual normal temperatures for 15 selected cities in the United States. The values are shown graphically in Fig. 5, the cities being arranged from west to east over the northern, central, and southern portions of the country. Plates IV and V show the extremes in temperature which have been experienced in the various portions of the country, and emphasize features which cannot be brought out in the plate, figure, and table immediately preceding. The highest temperatures, as shown on Plate IV, occur in the desert and semiarid regions of Arizona and California, while the lowest temperatures, as shown on Plate V, are to be found in northeastern Montana. Moreover, in the plains states as far north as the Dakotas summer maximum temperatures are higher than is the case in the extreme South. There is practically no portion of the country in which the temperature has not at some time or other fallen as low as 30°, and zero temperatures have occurred as far south as the east-central Gulf coast. In endeavoring to comprehend the temperature conditions of Chicago in relation to those of other portions of the United States, it will now be helpful to secure a view, bird’s-eye, as it were, of actualÖl 00 Fig. 5.—Monthly and annual normal temperatures for 15 selected cities (see Table XXII). Mean annual temperature given in figures under the name of the city. THE WEATHER AND CLIMATE OF CHICAGOHIGHEST TEMPERATURES OBSERVED AT WEATHER BUREAU STATIONS, 1871-1908 Plate IV.LOWEST TEMPERATURES OBSERVED AT WEATHER BUREAU STATIONS, 1871-1908 Plate V.TEMPERATURE 61 periods of extreme heat and cold. During July, 1901 (Plate YI), one of the most severe hot waves on record prevailed over the greater portion of the country, a maximum temperature of 103° being reached at Chicago on the 21st, and readings of from 104° to 106° being general in the middle Mississippi and lower Missouri valleys, as indicated on the second map of the plate. The maps of July 22 and 23 show the hot wave continuing over the interior portions of the country with even greater intensity than on the two preceding days, but broken over much of the Great Lakes region. The break in temperature was especially pronounced at Chicago, where the temperature on the last two days did not rise above 76° and 82°, respectively. The low temperature at Chicago was ushered in and maintained by the lake breeze, which blew steadily throughout the daytime hours, and in this case proved itself indeed the “ life-saver of the community.” The pressure conditions which produced this great heat period, and the adjustment which permitted the lake wind, are discussed later (p. 353). A severe cold wave swept over the northern and central states during the early part of February, 1906, the morning temperatures of which on the 5th and 6th are shown in Plate VII. The cold was very intense throughout the northern tier of states, as indicated by the lines and figures in the plate. On the morning of the 5th the readings were as low as —48° in the northern Lake Superior region, and had reached —2° in southwestern Missouri, and low temperatures continued generally during the 6th with but slight moderation, yet on the two days in question 12° and 10° above zero were the morning readings in Chicago, the wind shifting to the lake and tempering decidedly the cold in the city. Such conditions, as indicated on p. 44, happen frequently during the winter season/ In addition to mean temperature and the extent of temperature range, the frequency of hot and cold periods is important in establishing the temperature features of any locality. Fig. 6 and Tables XXIII and XXIV give the average monthly and annual number of days with maximum temperature of 90° or over, and minimum temperature of zero or below, for certain stations selected so as to cover practically the entire country. Chicago averages 8 days in each summer with maximum temperature of 90° or over, as does St. Paul, Minn., while Bismarck, N.D., farther to the northwest, averages 14.2 such days. On the other hand, while Chicago averages in winter 9.2 days with temperatures of zero or below, St. Paul experiences nearly 4 times as many, 34.5, and Bismarck nearly 6Plate VI.—Continuous lines, or isotherms, connect places of equal temperatures.Plate VI {continued).—Continuous lines, or isotherms, connect places of equal temperatures.Plate VII.—Continuous lines, or isotherms, connect places of equal temperature.TEMPERATURE 65 times as many, 54.3. With the interior cities of the West and South, Denver and St. Louis average more than three times as many hot days as does Chicago, and Denver has the greater number of days with zero temperature also. New York City averages 6.8 days with maximums of 90° or over, somewhat less than Chicago, while Buffalo, N.Y., and San Francisco, Cal., have low averages TABLE XXIII Average Number of Days Each Month with Maximum Temperature 90° or Above (These values are shown graphically in Fig. 6) Station Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Bismarck, N.D 0 0 0 0.1 0.4 1.7 5.4 5.1 1.6 0 0 0 14.2 St. Paul, Minn , 0 0 0 0 0.1 1.5 3.4 2.2 1.1 0 0 0 8.0 8.0 Chicago, 111 0 0 0 0 0.1 1.2 3.8 2.1 0.8 0 0 0 Buffalo, N.Y 0 0 0 0 0 0 0.3 0.1 0 0 0 0 0.4 New York, N.Y 0 0 0 0 0.2 1.5 3.1 1.3 0.6 0 0 0 6.8 San Francisco, Cal 0 0 0 0 0.1 0.3 0.1 0.1 0.4 0.1 0 0 1.1 Denver, Colo 0 0 0 0 0.2 4.7 10.9 8.0 1.4 0 0 0 25.2 St. Louis, Mo 0 0 0 0 0.5 5.4 10.4 8.5 3.5 0.2 0 0 28.5 New Orleans, La 0 0 0 0 0.8 9.0 14.8 13.6 5.0 0.2 0 0 43.3 TABLE XXIV Average Number of Days Each Month with Minimum Temperature Zero or Below (These values are shown, graphically in Fig. 6) Station Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Bismarck, N.D 18.1 15.2 6.6 0 0 0 0 0 0 0 3.4 10.9 54.3 St. Paul, Minn 13.1 10.3 2.4 0 0 0 0 0 0 0 1.6 6.4 34.5 Chicago, 111 4.1 3.1 0.2 0 0 0 0 0 0 0 0.1 1.6 9.2 Buffalo, N.Y 1.2 2.0 0.1 0 0 0 0 0 0 0 0 0.3 3.6 New York, N.Y 0.3 0.3 0 0 0 0 0 0 0 0 0 0.1 0.7 Denver, Colo 3.7 2.7 0.5 0 0 0 0 0 0 0 0.6 2.3 9.6 St. Louis, Mo 1.4 1.0 0 0 0 0 0 0 0 0 0 0.5 2.9 of 0.4 and 1.1 days, respectively, because their prevailing winds are from water surfaces. The great expanse of the Pacific Ocean protects San Francisco, in common with all other western coast cities, from the extreme rigors of the winter season, and it does not experience zero weather; and while New Orleans is too near the tropics to expect such conditions, it has experienced temperatures differing from zero by less than 10°. ABSOLUTE MONTHLY MAXIMUM AND MINIMUM TEMPERATURES We have now reached a point where greater details can be given regarding the temperature records of the Chicago Weather Bureau office. Tables XXV and XXVI give the highest maximum and lowest minimum temperature, respectively, occurring in each month66 THE WEATHER AND CLIMATE OF CHICAGO Average number of days with maximum temperature of 90° or more. Fig. 6. Average number of days with minimum temperature of zero or below.TEMPERATURE 67 of the year during the period of official record, and the annual values together with the mean annual temperatures are graphically portrayed TABLE XXV Absolute Monthly Maximum Temperatures, 1871-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1871 62 56 67 80 84 87 92 90 87 75 61 45 92 1872 45 52 45 81 83 98 97 93 93 81 59 46 98 1873 51 53 60 83 87 92 93 92 88 75 59 60 93 1874 60 56 64 67 89 95 99 98 89 78 72 52 99 1875 44 45 73 72 79 89 88, 86 87 73 57 68 89 1876 65 63 69 70 87 88 93 92 78 73 64 45 93 1877 56 58 65 78 86 87 91 89 86 80 58 67 91 1878... 49 55 68 75 78 85 97 91 87 79 57 46 97 1879 49 51 71 80 87 87 93 91 83 84 69 62 93 1880 61 63 60 80 85 91 95 93 85 78 65 50 95 1881 41 51 48 77 87 89 93 98 94 77 64 59 98 1882 58 62 63 76 76 88 90 87 87 77 72 45 90 1883 40 57 62 78 80 84 91 89 84 .78 62 57 91 1884 49 53 59 77 78 86 89 91 89 83 64 61 91 1885 50 47 58 76 80 88 94 85 81 69 66 50 94 1886 48 56 70 81 82 87 94 92 86 79 69 60 94 1887 52 58 68 82 86 96 100 98 92 82 67 53 100 1888 44 47 64 83 81 90 94 91 88 76 75 53 94 1889 55 48 68 73' 88 86 90 88 84 79 57 64 90 1890 62 59 56 75 86 92 93 96 88 73 67 53 96 1891 54 58 57 75 81 88 87 96 91 86 60 57 96 1892 55 49 53 78 76 91 94 93 88 80 58 57 94 1893 46 46 69 84 83 85 94 95 95 81 67 58 95 1894 60 48 74 84 88 93 96 95 90 76 60 55 96 1895 51 61 80 81 94 95 92 93 92 71 67 56 95 1896 50 58 58 84 89 89 93 98 88 . 75 70 57 98 1897 57 44 58 78 81 93 95 90 94 87 66 57 95 1898 55 60 68 76 79 90 94 93 92 75 67 46 94 1899 48 49 64 88 83 90 90 91 98 84 63 56 98 1900 56 62 55 79 86 88 92 94 90 86 63 53 94 1901 54 35 68 83 87 97 103 90 87 85 66 57 103 1902 50 48 65 83 86 91 90 86 81 76 70 50 91 1903 51 49 74 78 85 90 92 92 86 83 63 40 92 1904 39 53 65 79 87 88 94 90 87 78 67 54 94 1905 48 47 75 77 84 91 95 92 86 83 69 52 95 i 1906. 63 54 56 80 90 93 92 92 89 75 64 56 93 1907 59 53 80 70 83 91 90 92 92 81 56 58 92 1908 49 50 71 76 87 93 96 96 92 82 66 53 96 1909 65 57 55 74 86 87 92 93 89 76 71 55 93 1910 46 51 81 86 78 91 97 89 83 84 67 43 97 1911* 54 58 73 70 94 98 102 92 86 79 74 60 102 1912* 38 50 56 75 88 87 92 95 94 84 70 57 95 1913* 55 62 65 80 85 99 99 97 97 83 72 57 99 Means 52.4 53.0 64.6 78.4 84.0 90.0 94.4 91.8 87.9 78.8 64.6 54.2 94.5 Highest monthly maximum... Year Lowest monthly maximum... Year 65 /1876\ 11909/ 38 1912 63 /1876Ì 11880/ 35 1901 81 1910 45 1872 88 1899 67 1874 94 fl895\ 11911/ 76 /1882Ì 11892/ 99 1913 84 1883 103 1901 87 1891 98 Ì1874) 11881 11887 11896J 85 1885 98 1899 78 1876 87 1897 69 1885 75 1888 56 1907 68 1875 40 1903 103 1901 35 1901 * Not included in means. Highest and lowest maxima in bold-faced type. in Fig. 7. Table XXV shows that on the average a maximum temperature of 90?0 is reached some time during the month of June, 94?4 during the month of July, and 91?8 during the month of August, and68 THE WEATHER AND CLIMATE OF CHICAGO in no other months does the temperature average to reach or exceed 90°. Table XXVI shows that oh the average a minimum tempera- TABLE XXVI Absolute Monthly Minimum Temperatures, 1871-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1871 10 6 32 36 38 56 59 54 40 26 i - 8 - 8 1872 -12 - 2 8 27 38 51 52 56 37 28 - 2 -23 -23 1873 -16 -18 -12 25 35 44 50 53 41 23 8 13 -18 1874 - 6 9 17 22 38 46 60 58 44 30 0 1 - 6 1875 -20 -13 9 17 27 40 56 52 40 30 0 - 1 -20 1876 4 - 3 10 32 35 47 57 54 37 28 14 -14 -14 1877 - 4 21 5 27 33 45 57 55 44 35 14 22 - 4 1878 - 1 17 25 36 38 50 59 57 43 27 31 - 9 - 9 1879 -18 - 6 16 17 39 43 60 52 39 28 16 - 2 -18 1880 19 12 19 27 . 37 52 57 53 40 28 1 -15 -15 1881 -13 8 11 17 37 46 57 58 49 41 14 13 -13 1882 1 10 22 25 34 42 55 51 42 40 21 - 7 - 7 1883 -17 - 9 10 28 36 48 51 54 42 38 10 0 -17 1884 -18 - 3 - 1 31 40 47 54 51 51 28 5 -11 -18 1885 -13 -14 4 27 34 42 53 55 47 36 28 - 2 -14 1886 -14 - 6 15 23 40 49 55 53 42 32 16 -10 -14 1887 -15 - 7 9 19 42 48 61 47 38 14 - 1 - 5 -15 1888 -17 -18 - 1 30 32 43 56 51 36 32 20 15 -18 1889 0 -11 20 29 36 42 54 54 35 35 12 15 -11 1890 - 5 3 0 28 34 52 56 51 39 28 27 8 - 5 1891 10 - 8 7 23 35 44 55 49 48 33 3 9 - 8 1892 - 5 2 5 23 37 43 53 50 46 32 12 -10 -10 1893 -16 - 9 9 27 37 48 60 54 39 26 4 - 6 -16 1894 - 9 - 4 9 31 35 40 54 48 44 32 10 - 3 - 9 • 1895 - 9 -15 9 29 32 50 50 58 36 24 8 0 -15 1896 - 9 - 9 6 18 46 50 56 54 40 29 6 8 - 9 1897 -20 - 6 17 24 33 44 56 54 42 38 7 - 2 -20 1898 6 - 8 21 19 37 52 59 59 49 30 2 0 - 8 1899 -13 -21 3 18 42 49 59 63 32 36 30 - 3 -21 . 1900 - 8 - 9 - 1 28 36 48 55 64 41 41 14 2 - 9 1901 - 5 - 1 1 30 41 43 56 58 40 33 16 -12 -12 1902 - 8 - 7 3 23 36 48 54 55 42 35 26 - 1 - 8' 1903 - 6 -11 13 28 33 44 59 55 40 33 12 -13 -13 1904 -15 - 6 13 23 38 48 53 55 47 34 19 2 -15 1905 - 5 -18 15 30 38 49 56 62 52 29 14 7 -18 1906.... 11 6 10 34 36 51 59 56 55 27 25 10 6 1907 - 3 - 2 23 23 34 47 55 54 39 35 25 19 - 3 1908 0 - 2 21 24 34 47 60 58 36 34 25 10 - 2 1909 -10 12 14 25 32 48 55 59 47 29 28 - 7 -10 1910 - 5 - 6 26 26 38 43 62 53 49 27 20 7 - 6 1911* 0 8 13 27 33 55 53 59 51 32 12 5 0 1912* -16 - 7 6 31 39 49 57 55 39 38 25 3 -16 1913* 0 - 2 - 4 32 38 44 59 60 38 27 20 18 - 4 Means - 6.8 - 3.9 11.0 25.7 36.3 46.7 56.1 54.7 42.2 31.1 13.7 - 0.1 -11.8 Lowest monthly minimum Year Highest monthly minimum.... Year -20 /1875Ì 11897/ 19 1880 -21 1899 21 1877 -12 1873 32 1871 17 [18751 [ 1879 [ (l88lj 36 (18711 11878/ 27 1875 46 1896 40 /18751 11894/ 56 1871 50 /18731 11895/ 62 1910 47 1887 64 1900 32 1899 55 1906 14 1887 41 (18811 11900/ - 2 1872 31 1878 -23 1872 22 1877 -23 1872 ‘ 64 1900 * Not included in means. Highest and lowest minima in bold-faced type. ture of — 6?8 is reached some time during the month of January, —3?9 during the month of February, and — 0?1 during the month of December, and that the temperature does not average to fall with-TEMPERATURE 69 in 10° of zero in any other month. These statements must not be construed as contradictory in any sense to the discussion in connection with Tables X and XI (p. 25), and must be understood as (871 IB7S 1880 (805 ¡680 189S 1900 I90S (9/0 IÓ 0° •90* 80* 70* €»0* SO* 40* SO* 20 id* O* -to* -SO* A=absolute annual maximum temperature; B = average annual temperature; C = absolute annual minimum temperature. applying only to the absolute extremes of temperature for each month, whereas in the previous case the mean maximum and mean minimum temperatures were under consideration. FREQUENCY OF DAYS WITH TEMPERATURE OF 90° OR OVER Fig. 8 shows graphically the great variation in the frequency of high temperatures from one year to another, and the detailed data in Table XXVII supplement, as far as Chicago is concerned, the averages found in Table XXIII. While the temperature reaches 90° on an average of 8 times each year, it did not once reach 90° in 1875, the coldest year on record, and it reached it only once in 1882 and only once in 1884. The greatest number of times on which70 THE WEATHER AND CLIMATE OF CHICAGO 90° or over has been reached, exclusive of 1911 and 1913, is 18 days in each of the years 1895 and 1900, although the monthly frequency in the two years was quite unequal. In 1895, of the 18 days, 4 occurred in May, 3 in June, 2 in July, 3 in August, and 6 in September; while in 1900, none occurred in either May or June, and there were 7 in July, 10 in August, and 1 in September. The year 1900 was on the whole warmer than the normal, with its August the warmest month of that name on record, but the year 1895 showed quite a deficiency in temperature. The averages of the monthly records in Table XXVII, however, give the greatest frequency for days with r TABLE XXVII Number op Days with Maximum Temperature op 90° or Over, 1873-1913 (These values are shown graphically in Fig. 8) Year May June July Aug. Sept. Annual Year May June July Aug. Sept. Annual 1873 2 2 3 7 1895 4 3 2 3 6 18 1874 5 6 6 17 1896 0 5 6 11 1875 0 0 0 0 1897 2 4 1 5 12 1876 0 6 1 7 1898.... 1 6 4 3 14 1877 0 2 0 2 1899 2 1 2 3 8 1878 0 3 1 4 1900 0 7 10 1 18 1879.... 0 6 1 7 1901 4 11 1 16 1880 1 4 1 6 1902 1 2 0 3 1881 0 5 5 2 12 1903 1 '7 2 10 1882.... 0 1 0 1 1904 0 3 1 4 1883 0 4 0 4 1905 3 3 3 9 1884 0 0 1 1 1906 1 3 1 4 9 1885 0 4 0 4 1907 1 1 1 1 4 1886 0 3 1 4 1908 2 3 4 4 13 1887 2 5 3 1 11 1909 0 3 2 5 1888/... 1 4 1 6 1910 3 6 0 9 1889 0 2 0 2 1911*... 6 5 9 2 22 1890 3 4 2 9 1912*... 0 3 3 5 11 1891 0 0 3 2 5 1913*... 8 7 4 3 22 1 809 o K 9 Q 1893 0 6 1 2 9 Average.. 0.1 1.2 3.8 2.1 0.8 8.0 1894.... 5 6 3 1 15 * Not included in averages. See note under Table XXXIX. high temperatures to July, 3.8 days. August follows with 2.1 days, and June is next with 1.2 days; September averages 0.8 day, while 90Q or over is comparatively rare during May, the average being but 0.1 day. By months, the greatest number of occurrences of 90° or over is as follows: May, 6 in 1911; June, 8 in 1913; July, 11 in 1901; August, 10 in 1900; and September, 6 in 1895. In 1878, one of the warmest years from the standpoint of the annual mean, there were but 4 days with a maximum temperature of 90° or over; while in 1901, a year slightly below normal, there were 16 such days, 4 occurring in June, 11 in July, and 1 in August. July of that year was the warmest month on record, and had the highest absolute maximum temperature ever officially observed at Chicago, 103°.TEMPERATURE 71 These variations with those noted above lead to the conclusion that some years are warmer than the average because of sustained moderate heat, while others are warmer because of the relative frequency of hot days. The year 1911 in Chicago was the warmest since the establishment of the Weather Bureau office, if we accept the Federal Building record without correction, and it was marked by an extraordinarily large number of hot days, 90° or over being reached 22 times, 4 days more than in either 1900 or 1905, although the number was equaled in 1913. Of these 22 days, 6 occurred in May, 5 in 1875 1680 1885 1890 183S 1300 1905 19)0 PAYS 20 15 10 „11 II n if nni hbb irim 111 ! m I inn in days 20 IS IO s Fig. 8.^-Annual number of days with maximum temperature of 90° or over (see Table XXVII). June, 9 in July, and 2 in August, and May and June were the warmest of those months on record. In 1913, the occurrence was June, 8; July, 7; August, 4, and September, 3. LONGEST PERIODS OF CONSECUTIVE DAYS WITH MAXIMUM TEMPERATURE OF 90° OR ABOVE In addition to the relative annual and monthly frequency of hot days, it is important to secure an idea of the length of periods of such extreme conditions, and the longest of each year are shown in Table XXVIII. The average length is 2.8 days, and the longest period experienced is 8 days, from August 4 to 11, 1900, the absolute maximum during that time being 94°. There are several summers apparent in the table in which the temperature did not reach 90° on any two successive days. In fact, warm waves in Chicago are of such brief duration, being interrupted by changes to moderate and pleasant weather, that residences and other buildings seldom become heated to an uncomfortable degree when proper precautions72 THE WEATHER AND CLIMATE OF CHICAGO are exercised. It will be noted that by far the greater proportion of the periods recorded in Table XXVIII occurred either in July or August, 24 of the 31 occurring in these two months falling in July, and indicating a greater frequency of periods of extreme and sustained heat in that month. On such days the temperature remains at 90° or over for a variable period, depending largely upon the absolute maximum. During the hot waves of July, 1901, and June-July, 1911, when the thermometer in Chicago passed the 100° mark, reaching 103° and 102°, respectively, the temperature remained at 90° or above for from 10 to 14 hours on TABLE XXVIII Longest Periods of Consecutive Days with a Maximum Temperature of 90° or Above, 1873-1913 Year Began Ended Length in Days Tempei ture 1873 July 16 July 17 2 93 1874 Aug. 19 Aug. 21 3 94 1875 None occurred 1876 July 6 July 9 4 93 1877 July 8 July 8 1 91 1878...... July 16 July 17 2 97 1879 July 14 July 16 3 93 1880 July 10 July 13 4 95 1881 July 7 July 9 3 93 1882 July 27 July 27 1 90 1883 July 2 July 4 3 91 1884 Aug. 19 Aug. 19 1 91 1885 July 19 July 20 2 94 1886 July 6 July 6 1* 94 1887 July 15 July 17 3 100 1888 July 30 July 31 2 94 1889 July 8 July 9 2 90 1890 June 28 June 30 3 92 1891 Aug. 7 Aug. 9 3 96 1892 July 23 July 27 5 94 1893 July 12 July 14 3 92 1894 June 21 June 23 3 93 Year Began Ended Length in Days Tempera- ture 1895 Sept. 19 Sept. 22 4 91 1896 Aug. 8 Aug. 11 4 98 1897 July 2 July 4 3* 95 1898 Sept. 1 Sept. 3 3 92 1899 Sept. 5 Sept. 5 1* 98 1900 Aug. 4 Aug. 11 8 94 1901 July 20 July 21 2* 103 1902 June 12 June 12 1* 91 1903 July 1 July 4 4 92 1904 July 16 July 18 3 94 1905 July 16 July 18 3 95 1906 Aug. 20 Aug. 22 3 92 1907 Aug. 11 Aug. 11 1* 92 1908 Sept. 9 Sept. 11 3 92 1909 July 28 July 29 2 92 1910 July 24 July 26 3 97 1911 June 30 July 5 6 102 1912f Aug. 31 Sept. 1 2* 95 1913f July 3 July 5 3 94 Average.... 2.8 94.0 * Two periods of equal length; the period with the highest maximum temperature selected, f Not included in averages. each day, as shown in Auxiliary Table C, which gives the actual times and extent of the high temperatures. On July 4 and 5, 1911, the temperature remained above 90° for 14 hours each day, reaching a maximum of 102° at 4 p.m. on the 4th, and at 3 p.m. on the following day. On the 4th the last reading of 90° was at 11 p.m., the latest at night that this degree has occurred within the period of record, although on the 5th it continued as late as 10 p.m. In the morning 90° was reached exceptionally early during both hot waves: at 9 a.m. on July 21, 1901, and at the same hour on July 2 to 5, inclusive, 1911. Fortunately, such days are very rare in Chicago, and can happen only when conditions are unfavorable for the setting in of the lake breeze.TEMPERATURE 73 Extreme heat would occur in the city much more frequently were it not for this beneficent wind, and would often extend over a much longer period, as is the case in the interior sections away from the lake. In this connection it is interesting to note that on July 5, 1911, the last day of the second period shown in the table, the minimum temperature for the entire twenty-four hours was 82° at 5 a.m., the highest minimum ever recorded at the Chicago Weather Bureau office. AUXILIARY TABLE C Length of Time with Temperature of 90° or Over, During Hot Waves of 1901 and 1911, Chicago Date From To Number of Hours Max. Temp. Time of Max. July 20, 1901 12 NOON / 9 A.M. \ 3 P.M. 7 P.M. 1 P.M. 9 P.M. 1 P.M. 8 P.M. 7 P.M. 9 A.M. 7 P.M. 8 P.M. 11 P.M. 10 P.M. 8 97 4, 5 p.m. ¿1, iyui June 30, 1911 12 1 10 11 103 90 96 98 5 P.M. 1 P.M. 2, 3, 4 p.m. 4 P.M. July 1, 1911 11 A.M. 9 A.M. / 2, 1911 o, lyily A 1A11 + 1 11 A.M. i 9 A.M. \ 10 P.M. 9 A.M. 10 100 3 P.M. 5, 1911 14 14 102 102 4 P.M. 3 P.M. * On July 21, 1901, temperature fell to 84° between 1 and 3 p.m. t On July 3, 1911, temperature fell to 88° between 9 and 11 a.m. X On July 4, 1911, temperature fell to 89° between 8 and 10 p.m. Indeed, on only two other days have minimum temperatures as high as 80° occurréd', this mark being just reached on July 16 and 17, 1878, when the maximum readings were 97° amd *95°, respectively. Ordinarily, in hot-wave periods with daily maximum temperatures of 90° or over the temperature falls below 80° for at least several hours during the night; and it has been found that the daily minimums in such conditions average about 73?2. FREQUENCY OF DAYS WITH ZERO TEMPERATURES, MINIMUM AND MAXIMUM Turning now from the consideration of conditions of extreme heat to those of extreme cold, Fig. 9 and Table XXIX give in detail data regarding the number of days,, monthly and annually, of minimum temperatures of zero or below, and supplement for Chicago Fig. 6 and Table XXIV. The average number of such days occurring throughout the year is 9.2, distributed as follows: January, 4.1, February, 3.1, March, 0.2, November, 0.1, and December, 1.6 days. The time of greatest frequency is therefore during the month of January, with February following closely. One in about74 THE WEATHER AND CLIMATE OF CHICAGO every 8 days in January, and one in about every 10 days in February, are days on which the temperature reaches zero or goes below for some portion of the time, although the zero weather of February is TABLE XXIX Number op Days with Minimum Temperature op Zero or Below, 1873-1913 (These values shown graphically in Fig. 9) Year Jan. Feb. Mar. Nov. Dec. Total 1873 6 7 3 0 16 1874 2 0 1 0 3 1875 11 14 1 1 27 1876 0 2 4 6 1877 4 0 0 4 1878 1 0 2 3 1879 7 2 1 10 1880 0 0 6 6 1881..... 3 0 0 3 1882 0 0 2 2 1883 8 3 1 12 1884 6 2 . 1 4 13 1885 12 9 2 23 1886:... 5 3 3 11 1887 13 4 2 3 22 1888 13 5 1 0 19 1889 1 4 0 5 1890 1 0 2 0 3 1891 0 2 0 2 1892 3 0 3 6 1893 10 3 4 17 1894. ,.. 2 1 1 4 Year Jan. Feb. Mar. Nov. Dec. Total 1895 7 8 1 16 1896 3 3 0 6 1897 6 1 3 10 1898 0 3 1 4 1899 6 8 2 16 1900.... 4 5 2 0 11 1901 1 2 7 10 1902 3 5 1 9 1903 4 3 7 14 1904 5 5 0 10 1905 6 9 0 15 1906 0 0 0 0 1907 2 3 0 5 1908 1 1 0 2 1909 2 ' 0 3 5 1910 2 2 0 4 1911*... 1 0 0 1 1912*... 13 3 0 16 1913*... 1 2 1 0 4 Average. 4.1 3.1 0.2 0.1 1.6 9.2 * Not included in average. See note under Table XL. Fig. 9.—Annual number of days with minimum temperature of zero or below (see Table XXIX). confined chiefly to the first half of that month. In 1875, the record-breaking year for cold weather, temperatures of zero or below occurred on 27 days, 11 of these being in January, 14 in February, andTEMPERATURE 75 1 each in November and December. On the other hand, in 1906 zero did not occur on any day of the year, and there are a number of years within the period of record in which zero was reached on less than 5 days. By months, February holds the individual record, zero or lower having occurred on 14 days in that month in 1875, as noted above, but January is really the month of greatest frequency, having 13 days in each of the years 1887, 1888, and 1912. In March, 1873, there were 3 such days; in November, 1887, 2, and in December, 1901 and 1903, 7 days each. A day on which the minimum temperature reaches zero, or goes below it, marks, of course, severe winter weather in the city of Chicago, and it is but seldom that the mercury remains so low throughout the entire period of twenty-four hours. There have been a few instances of this kind, however, on which the maximum temperature for the day did not rise above the zero point, and these dates are set forth in Table XXX. Although the total number of such days TABLE XXX Number op Days with Maximum Temperature op Zero or Below, 1873-1913 Year Jan. Feb. Dec. Total Year Jan. Feb. Dec. Total Year Jan. Feb. Dec. Total 1873.. 0 0 0 0 1888 1 1 0 2 1903 0 0 0 0 1874.. 0 0 0 0 1889 0 1 0 1 1904 1 0 0 1 1875.. 0 0 0 0 1890 0 0 0 0 1905 0 2 0 2 1876.. 0 0 0 0 1891 0 0 0 0 1906 0 0 0 0 1877.. 0 0 0 0 1892 0 0 0 0 1907 0 0 0 0 1878.. 0 0 0 0 1893 3 1 0 4 1908 0 0 0 0 1879.. 1 0 0 1 1894 0 0 0 0 1909 0 0 0 0 1880.. 0 0 1 1 1895 1 1 0 2 1910 0 0 0 0 1881.. 0 0 0 0 1896 0 0 0 0 1911 0 0 0 0 1882.. 0 0 0 0 1897 2 0 0 2 1912 2 0 0 2 1883.. 3 0 0 3 1898 0 0 0 0 1913 0 0 0 0 1884.. 2 0 0 2 1899 0 2 0 2 1885.. 0 0 0 0 1900 0 0 0 0 Total 16 8 2 26 1886.. 0 0 0 0 1901 0 0 1 1 1887.. 0 0 0 0 1902 0 0 0 0 within the entire period of official record is only 26, the decidedly larger proportion, 16, have occurred in January, as was the case in the occurrence of zero minimum temperatures. Of the remaining cases, 8 occurred in February and 2 in December, which follow the order of frequency shown in the preceding table. No other months have been marked by such low maximum temperatures, and these 26 cases in the winter months were confined to 14 years. This would indicate a frequency of about one year in every three, but it will be noticed that in only 5 years has a single instance occurred, the other 9 years having 2, 3, or 4 each. January, 1883, and January, 1893,76 THE WEATHER AND CLIMATE OF CHICAGO had each 3 such days, but in no other individual month have there been more than 2. LONGEST PERIODS OF CONSECUTIVE DAYS WITH TEMPERATURES OF ZERO OR BELOW As is the case in regard to hot weather of the summer season (p. 71), a study of the frequency of severe cold weather during the opposite season must be incomplete unless an idea is gained relative to its length. Table XXXI gives the longest period in consecutive days for each of the fourteen years shown in Table XXX, when daily maximum temperatures of zero or lower were recorded. In only 5 of these years were there 2 or more consecutive days, and in only one year were there more than 2. This was in 1883, January 21 to 23, inclusive, during which time the temperature fluctuated between TABLE XXXI Longest Periods of Consecutive Days with Maximum Temperature of Zero or Below, 1873-1913 Year Began Ended Length Lowest Maxi- mum Lowest Mini- mum Year Began Ended Length Lowest Maxi- mum Lowest Mini- mum 1879... Jan. 3 Jan. 3 1 0 -18 1895.. Jan. 27 Jan. 27 If - 1 - 9 1880... Dec. 28 Dec. 28 1 -1 -12 1897.. Jan. 25 Jan. 26 2 -10 -20 1883... Jan. 21 Jan. 23 3 -4 -17 1899.. Feb. 9 Feb. 10 2 - 8 -21 1884... Jan. 4 Jan. 5 2 -6 -18 - 1901.. Dec. 15 Dec. 15 1 - 2 -12 1888... Feb. 9 Feb. 9 1 -4 -18 1904.. Jan. 24 Jan. 24 1 — ; -13 1889... Feb. 23 Feb. 23 1 -2 -11 1905.. Feb. 13 Feb. 13 It - 4 -18 1893... Jan. 15 Jan. 15 1* -4 -16 1912.. Jan. 5 Jan. 6 2 - 5 -11 * Also three other dates, t Also one other date. zero and —17°, not rising above —4° on either the 21st or 22d. While during the cold wave of January 25-26, 1897, the maximum temperature was below zero on but 2 days, the temperature did not rise above —10° on the 25th. This is the lowest maximum temperature on record at Chicago, and as the minimum on this day was —20°, the mean temperature for the day was —15°. January 25, 1897, therefore, as determined by its mean temperature, was the coldest day in Chicago during the history of the Weather Bureau office, although the absolute minimum was —23°, on December 24, 1872, with a mean temperature of —9°. Table XXXII gives for each year the longest period of consecutive days with minimum temperature of zero or below. The feature of this table is the long period of 10 days extending from January 4 to January 13, 1912, which is 2 days longer than the next longestTEMPERATURE 77 period, in February, 1875. Both these months were the coldest of their respective names on record, the former having 13 days of zero temperature and the latter 14 (p. 75). TABLE XXXII Longest Peeiods of Consecutive Days with Minimum Temperature of Zero or Below, 1873-1913 Year Began Ended Length Lowest Tempera- ture 1873 Feb. 21 Feb. 25 5 -18 1874 Jan. 14 Jan. 15 2 - 6 1875 Feb. 11 Feb. 18 8 -13 1876 Dec. 8 Dec. 10 3 -14 1877 Jan. 8 Jan. 9 2 - 3 1878 Dec. 23 Dec. 24 2 - 9 1879 Jan. 2 Jan. 6 5 -18 1880 Dec. 27 Dec. 31 5 -15 1881 Jan. 14 Jan. 14 1* -13 1882 Dec. 7 Dec. 8 2 - 7 1883 Jan. 19 Jan. 24 6 -17 1884 Jan. 3 Jan. 7 5 -18 1885 Jan. 17 Jan. 22 6 -13 1886 Jan. 10 Jan. 12 3 - 9 1887 Jan. 1 Jan. 3 3* -15 1888 Jan. 11 Jan. 16 6 -17 1889 Feb. 22 Feb. 24 3 -11 1890 Mar. 1 Mar. 2 2 0 1891 Feb. 3 Feb. 4 2 - 8 1892 Dec. 25 Dec. 27 3 -10 1893 Jan. 13 Jan. 17 5 -16 Year Began Ended Length Lowest Tempera- ture 1894 Jan. 24 Jan. 25 2 - 9 1895 Feb. 7 Feb. 9 3 -15 1896 Feb. 19 Feb. 21 3* - 9 1897.'..... Jan. 24 Jan. 29 6 -20 1898 Feb. 1 Feb. 3 3 - 8 1899 Feb. 7 Feb. 13 7 -21 1900 Jan. 28 Feb. 1 5 - 8 1901 Dec. 18 Dec. 21 4 - 8 1902 Feb. 2 Feb. 5 4 - 7 1903 Jan. 10 Jan. 13 4 - 6 1904 Jan. 23 Jan. 25 3. -15 1905 Feb. 12 Feb. 16 5 -18 1906f 1907.' Feb. 2 Feb. 4 3 - 2 1908 Feb. 2 Feb. 2 1* - 2 1909 Jan. 6 Jan. 7 2* -10 1910 Jan. 6 Jan. 7 2* - 5 1911 Jan. 5 Jan. 5 1 0 1912 Jan. 4 Jan. 13 10 -16 1913 Mar. 2 Mar. 2 1* - 4 * Other periods of same length occurred during the year, t No record of zero in 1906. TABLE XXXIII Longest Periods of Consecutive Hourly Temperatures, in Each Winter, of Zero or Below, 1890-91 to 1912-13 Season From To Hours Temperature 1890-91 8 :00 p.m. Feb. 3 11:00 a.m. Feb. 4 16 - 6 1891-92 3:00 a.m. Jan. 9 10:00 a.m. Jan. 9 8 - 4 1892-93 6:00 a.m. Jan. 13 11:00 a.m. Jan. 16 78 -16 1893-94 9:00 a.m. Jan. 24 10:00 a.m. Jan. 25 26 - 9 1894-95 5:00 a.m. Feb. 7 10:00 a.m. Feb. 9 54 -15 1895-96 5:00 a.m. Feb. 19 10:00 a.m. Feb. 20 30 - 9 1896-97* 1:00 a.m. Jan. 24 10:00 a.m. Jan. 28 106 -20 1897-98 7:00 p.m. Feb. 2 8:00 a.m. Feb. 3 14 - 8 1898-99 1:00 a.m. Feb. 8 2:00 p.m. Feb. 11 86 -21 1899-1900 6:00 a.m. Feb. 24 8:00 a.m. Feb. 25 27 - 9 1900-1901 12:00 mdt. Dec. 31 10:00 a.m. Jan. 1 11 - 5 1901-2 2:00 a.m. Dec. 14 8:00 a.m. Dec. 16 55 -12 1902-3 12:00 mdt. Feb. 16 1:00 a.m. Feb. 18 26 -11 1903-4 10:00 p.m. Jan. 23 12:00 noon Jan. 25 39 -15 1904-5 7:00 p.m. Feb. 1 9:00 a.m. Feb. 3 39 -14 1905-6f 1906- 7 | 1907- 8 11:00 p.m. 11:00 p.m. 2:00 a.m. Jan. Feb. Feb. 25 2 2 9:00 a.m. Jan. 9:00 a.m. Feb. 8:00 a.m. Feb. 26 3 2 11 11 7 - 3 - 2 - 2 1908-9 2:00 a.m. Jan. 6 4:00 a.m. Jan. 7 27 -10 1909-10 11:00 p.m. Jan. 6 10:00 a.m. Jan. 7 12 - 5 ' 1910—Ilf 6:00 a.m. Jan. 5 1 0 1911-12] 4:00 p.m. Jan. 4 10:00 p.m. Jan. 7 79 -16 1912-13 4:00 a.m. Mar. 2 11:00 a.m. Mar. 2 8 - 4 * Temperature reached 1° above zero between hourly readings on the 27th. f Zero temperature did not occur during periods for which no entries are made.78 THE WEATHER AND CLIMATE OF CHICAGO The preceding discussion has been confined to the length of cold periods by days. Since 1890, however, hourly temperature data are available, and from that date it is possible to record the actual number of hours in the longest periods of zero temperatures. This has been done by winters, and the results are presented in Table XXXIII. The longest period of this kind was in the winter of 1896-97, from 1 a.m. of January 24 to 10 a.m. of January 28, inclusive, extending over 106 hours, although once between the hourly readings on the 27th the thermometer indicated 1° above zero for a few minutes. The minimum during this period was—20°. The next longest period was 86 hours, in the cold February of 1899, with a minimum of —21°; and the third, 79 hours, during January, 1912, with a minimum of —16°. OCCURRENCE OF FREEZING TEMPERATURE, SEASONAL Between the conditions of extreme heat and extreme cold, marked usually by the limits of 90° and zero, respectively, there are certain other established degrees by which it is customary to characterize the temperature phases of a given period of time, and one of the more common of these is the freezing point, or 32°. Accordingly, around this point we have grouped several tables of data for the winter season, a study of which with their accompanying graphs will bring out still other features of the colder months than have yet been presented. 1. Number of days with maximum temperature of 32° or below.— The number of days in each winter season from October to April, inclusive, on which the temperature did not rise above the freezing point is shown by months in Table XXXIV, and Fig. 10 gives a graphic view of their seasonal frequency. Such days, on the average, appear to have been more frequent during the period from 1891 to 1905, during which time the Weather Bureau office was located at the Auditorium Tower, the average for the first 10 years of that period being much higher than that of any other decade, although there was but little difference in the mean temperatures, as shown by Table I. The mean daily maximum temperatures for the months of January and February, however, 31?3 and 32?7, respectively (Table X), are so near the freezing point that the average excess in temperature of the Federal Building' 1?7, and of the Major Block, 0?9 (p. 10), over that of the Auditorium, might be expected to show in aTEMPERATURE 79 relatively smaller number of days with maximum temperatures at 32° or below at those locations during the months in question. An TABLE XXXIV Number of Days with a Maximum Temperature of 32° or Below, October to April, 1871-1913 (These values, not including 1910-13, are shown graphically in Fig. 10) Season Oct. Nov. Dec. Jan. Feb. Mar. April Season 1871-72 7 17 16 16 14 70 1872-73 9 18 18 7 5 57 1873-74. 1 6 8 8 11 6 4 44 1874-75 3 22 20 20 12 2 79 1875-76 1 3 6 7 8 25 1876-77....... 2 22 15 0 13 52 1877-78 2 1 4 1 0 8 1878-79 0 ■ 15 17 11 2 1 46 1879-80 1 7 0 6 1 15 1880-81 10 11 21 14 3 1 60 1881-82... 1 2 9 2 0 14 1882-83 0 12 20 13 6 51 1883-84 2 8 18 8 9 45 1884-85 2 10 18 18 7 55 1885-86 0 7 17 8 3 35 1886-87 3 13 19 11 6 52 1887-88 1 2 11 23 10 11 58 1888-89 1 6 11 16 2 36 1889-90 3 1 9 6 9 28 1890-91 0 10 9 9 10 2 40 1891-92 7 4 23 6 10 50 1892-93 5 16 28 17 8 1 75 1893-94 6 16 11 17 5 55 1894-95 6 9 22 20 8 65 1895-96 3 12 12 14 11 1 53 1896-97 :. 5 8 16 10 5 44 1897-98 4 16 10 12 3 45 1898-99 6 13 15 17 10 3 64 1899-1900 0 13 * 8 17 9 47 1900-1901 2 8 17 26 5 58 1901-2 2 10 14 21 5 52 1902-3 0 13 18 13 1 45 1903-4 10 18 24 20 6 2 80 1904-5 2 19 24 20 4 69 1905-6 1 7 4 11 14 37 1906-7 0 6 13 10 0 29 1907-8 0 6 10 16 0 32 1908-9 1 6 11 6 2 26 1909-10 0 20 15 14 0 49 1910-11 4 16 10* 8* 2* 40 1911-12* 3 3 28 19 14 67 1912-13* 0 5 12 15 8 40 Means 0 O CO 11.0 14.7 12.3 6.0 0.4 47.3 * Not included in means. SUMMARY OF TABLE XXXIV Average Frequency op Days with a Maximum op 32° or Below Decade Nov. Dec. Jan. Feb. Mar. Season 1871-72 to 1880-81 4.1 12.4 12.5 6.3 6.4 41.7 1881-82 to 1890-91 1.4 8.0 15.3 10.1 6.3 41.1 1891-92 to 1900-1901 4.4 11.5 16.2 15.6 7.4 55.1 1901-2 to 1910-11 2.0 12.1 14.3 13.9 3.4 45.7 1871-72 to 1909-10 3.0 11.0 14.7 12.3 6.0 47.3 Greatest number (1903-4) 10 18 24 20 6 80 Least number (1877-78) 2 1 4 1 0 880 THE WEATHER AND CLIMATE OF CHICAGO examination of the table proves this to be the case, and the earlier and later months of the winter season do not present the same disproportion of occurrences at the different locations, because the mean maximum temperatures are then considerably higher than the freezing point. The greatest number of such days occurring in any one season is 80, during 1903-4, and the next largest number, 79 in the season of 1874-75, while the least number is 8 in 1877-78. The greatest number in any one month is 28 in January, 1893, and also in January, 1912, while in January, 1880, and in February, 1877, there was not a single day on which the temperature did not exceed 32°. On the I875-7& l880-$f 1885-86' 189091 1895-96 |900-0f 1905-06 11 mi mini mmmiimmimmiii Fig. 10.—Number of days with a maximum temperature of 32° or below, October, 1871, to April, 1910 (see Table XXXIV). average, the greatest frequency occurs in January, with a mean of 14.7 days. The averages of the entire winter season increase and decrease as follows: November, 3.0; December, 11.0; January, 14.7; February, 12.3; March, 6.0; April, 0.4 days. In October, throughout the whole period of record, the temperature remained at 32° or below in only two instances, once in 1873 and once in 1887, but the mean occurrence is too small to record. 2. Longest periods of consecutive days with maximum temperature of 32° or below.—-Table XXXV will serve to supplement the discussion immediately preceding. The longest periods of the winter season during which the temperature remained at or below the freezing pointTEMPERATURE 81 were those from December 18, 1878, to January 15, 1879, and from January 22 to February 19, 1895, and were of 29 days each. Several other periods of 20 days’ length or over will be noted in the table, and with the exception of that in the extremely cold weather of January and February, 1912, all occurred during the occupancy by the Weather Bureau office of the Auditorium Tower. There were three winter seasons, 1877-78, 1879-80 and 1908-9, in which the maximum temperature did not remain below 32° for any period longer than 3 consecutive days. On the whole the temperature averages so as to remain at freezing or below for at least 12.6 consecutive days during the winter. TABLE XXXV Longest Periods of Consécutive Days with Maximum Temperature of 32° or Below, 1872-73 to 1912-13 Winter No. of Days Time of Occurrence 1872-73 15 Dec. 15-29 1873-74 4 Jan. 30-Feb.2 1874-75 18 Jan. 3-20 1875-76 5 Mar. 17-21 1876-77 16 Dec. 16-31 1877-78 3 Nov. 29-Dec. 1., Jan. 5-7 1878-79 29 Dec. 18-Jan. 15 1879-80 3 Feb. 3-5 1880-81 7 Jan. 14-20 1881-82 6 Dec. 30-Jan. 4 1882-83 13 Dec. 31-Jan. 12 1883-84 11 Feb. 28-Mar. 9 1884-85 18 Feb. 6-23 1885-86 8 Jan. 5-12, Jan. 29-Feb. 5 1886-87 17 Dec. 27-Jan. 12 1887-88 16 Jan. 14-29 1888-89 5 Jan. 17-21, Feb. 5-9 1889-90 5 Jan. 20-24, Mar. 4-8 1890-91 8 Feb. 25-Mar. 4 1891-92 20 Jan. 2-21 1892-93 23 Jan. 1-23 1893-94 7 Feb. 10-16, Feb. 19-25 Winter No. of Days Time of Occurrence 1894-95....... 29 Jan. 22-Feb. 19 1895-96 6 Jan. 3-8, Feb. 16-21 1896-97 8 Jan. 23-30 1897-98 12 Dec. 17-28 1898-99 19 Jan. 27-Feb. 14 1899-1900... ;. 10 Dec. 25-Jan. 3 1900-1901 23 Jan. 25-Feb. 16 1901-2 26 Jan.27-Feb. 21 1902-3 10 Feb. 13-22 1903-4 13 Jan.23-Feb.4 1904-5 28 Jan. 22-Feb.18 1905-6 7 Mar. 11-17 1906-7 6 Jan. 25-30, Feb. 3-8 1907-8 7 Jan. 29-Feb. 4 1908-9 3 Dec. 7-9, Jan. 11-13, Jan. 6-8, Jan. 30-Feb. 1, Feb. 14-16 1909-10 15 Dec. 17-31 1910-11 5 Nov. 29-Dec. 3, Dec. 5-9 1911-12 21 Jan. 24-Feb. 13 1912-13* 10 Jan. 31-Feb. 9 Average 12.6 * Not included in means. 3. Number of days with minimum temperature of 32° or below.— The discussion of the two preceding topics has been confined to the days on which the temperature was at freezing or below during the entire twenty-four hours. It is useful also to know the frequency of days on which the lowest temperature touches or is below this point. Such days would, of course, include those shown in Table XXXV, but there would be many in addition on which the temperature was at freezing or below only a portion of the time. Table XXXVI and Fig. 11 give the data in the same manner as was done with the occurrence of a maximum of 32° or lower. It will be noted here also that82 THE WEATHER AND CLIMATE OF CHICAGO the Auditorium period of the record displays the greatest frequency, but the excess occurs chiefly in November, during the latter half of TABLE XXXVI Number of Days with a Minimum Temperature of 32° or Below, October to May, 1871-1913 (These values, not including 1910-13, are shown graphically in Fig. 11) Season Oct. Nov. Dec. Jan. Feb. Mar. April May Season 1871-72 12 30 29 27 27 2 127 1872-73 3 17 31 30 28 24 11 144 1873-74 / 5 18 19 23 23 18 10 116 1874-75 1 14 25 30 28 21 8 1 128 1875-76 1 13 18 23 17 20 102 1876-77 1 11 30 28 12 28 2 112 1877-78 11 7 18 11 2 49 1878-79 3 1 25 27 26 11 3 96 1879-80 1 13 22 12 17 14 3 82 1880-81 •. 2 20 27 31 27 28 8 143 1881-82. 10 11 28 14 13 3 79 1882-83 10 26 31 26 26 3 122 1883-84 12 22 29 25 15 1 104 1884-85 1 9 20 28 26 25 4 113 1885-86 4 20 28 23 17 5 97 1886-87 16 27 30 28 27 6 134 1887-88. 9 19 25 30 26 27 6 142 1888-89 1 7 24 28 27 11 1 99 1889-90 11 7 21 20 24 3 86 1890-91 2 7 25 25 23 24 5, 111 1891-92 17 16 30 24 23 3 113 1892-93 21 29 31 27 22 4 i 134 1893-94 3 18 26 23 24 10 2 106 1894-95 1 22 22 31 24 25 5 135 1895-96 7 17 20 28 26 26 7 131 1896-97 4 18 18 28 26 19 4 117 1897-98 14 28 27 25 12 4 110 1898-99 1 13 28 27 24 29 5 126 1899-1900 1 22 23 27 30 5 108 1900-1901 15 24 30 28 22 1 120 1901-2 ' 15 26 29 23 9 4 106 1902-3 6 25 29 23 11 3 97 1903-4 19 30 31 27 20 12 139 1904-5 8 30 30 27 18 4 117 1905-6 1 11 22 26 20 21 101 1906-7 2 8 24 24 24 14 11 107 1907-8 8 26 28 24 15 3 104 1908-9 9 25 22 20 19 3 1 99 1909-10 4 6 27 . 28 27 6 2 100 1910-11* 2 21 30 23 19 18 4 116 1911-12* 20 20 31 26 27 3 127 1912-13* 11 22 27 26 21 1 108 Means 1.4 12.5 23.2 27.0 23.7 18.8 4.2 0.1 111.7 * Not included in means. SUMMARY OF TABLE XXXVI Average Frequency of Days with a Minimum Temperature of 32° or Below (by Decades) Season Oct. Nov. Dec. Jan. Feb. Mar. April Season 1871-72 to 1880-81 1.7 13.0 23.4 25.1 21.6 19.3 4.7 109.9 1881-82 to 1890-91 1.3 • 10.5 20.7 27.8 23.8 20.9 3.7 108.7 1891-92 to 1900-1901 1.6 15.6 23.3 27.8 25.5 21.8 4.0 120.0 1901-2 to 1910-11 0.9 11.1 26.5 27.0 23.4 15.1 4.6 108.6 1871-72 to 1910-11 1.4 12.5 23.2 27.0 23.7 18.8 4.2 111.7 Greatest number (1872-73) 3 17 31 30 28 24 11 144 Least number (1877-78) 11 7 18 11 2 49TEMPERATURE 83 which the mean daily minimum is very near the freezing point, and the influence of the various locations of the Weather Bureau office affects the occurrence at this time, just as was the case with the maximum temperatures of 32° or lower in January and February (p. 78). This excess is noticeable in March, as well, but not to so great an extent, as the mean daily minimum temperature, on account of the more rapid advance of the season in spring than in autumn (p. 26), is close to the freezing point for a much shorter interval than is the case in November. The excess shown in the month of February during the Auditorium Tower period, however, is due to really greater frequency in those years, as the average minimum temperature is well below 32° throughout the month. I87J-72 187.5- 76 1880-81 I8BS-8E 1890-91 1895-96 1900-01 1905-Ofi (909/0 Fig. 11.—Number of days with a minimum temperature of 32° or below, from October, 1871, to May, 1910 (see Table XXXVI). The average number of days from October to May on which the minimum temperature touches or goes below the freezing point, as shown by the table, is 112, but the actual frequency has varied greatly. In the season of 1872-73 there were as many as 144, while in that of 1877-78 there were only 49. The month of greatest frequency is, of course, January, with 27.0 days, followed in order by February, with 23.7, March with 18.8, April with 4.2, and May with 0.1 in the spring. In the first portion of the season, October averages 1.4 days, November 12.5, and December 23.2. In December, 1872, January, 1881, 1883, 1893, 1895, 1904, and 1912, and February, 1873, 1875, 1887, and 1901, the minimum temperature of every day reached 32° or lower; and in the three winter months of 1872-7384 THE WEATHER AND CLIMATE OF CHICAGO there was only one day on which the temperature remained above the freezing point. In the warm January of 1880 there were only 12 such days, and in February of 1878, only 11. There are several Aprils and Octobers apparent in the table in which freezing weather did not occur, but October, 1887, and April, 1904, were marked by an unusually large number, 9 and 12, respectively. 4. Longest periods of consecutive days with minimum temperature of 32° or below.—'These periods, as shown in Table XXXVII and Fig. 12, include usually the longest intervals of maximum temperatures of TABLE XXXVII Longest Periods of Consecutive Days with Minimum Temperature of 32° or Below, 1872-73 to 1912-13 (These values, up to and including 1909-10, are shown graphically in Fig. 12) Winter No. of Days Time of Occurrence 1872-73 52 Nov. 25-Jan. 14 1873-74 15 Jan. 28-Feb. 11 1874-75 74 Dec. 28-Mar. 11 1875-76....... 14 Jan. 23-Feb. 5 1876-77 46 Dec. 14-Jan. 28 1877-78 11 Jan. 26-Feb. 5 1878-79 48 Dec. 10-Jan. 26 1879-80 24 Dec. 10-Jan. 2 1880-81 56 Dec. 15-Feb. 8 1881-82 15 Jan. 11-Jan. 25 1882-83 49 Dec. 27-Feb. 13 1883-84 28 Dec. 31-Jan. 27 1884-85 52 Jan. 7-Feb. 27 1885-86 37 Jan. 4-Feb. 9 1886-87 39 Dec. 14-Jan. 21 1887-88 42 Jan. 7-Feb. 17 1888-89 30 Jan. 17-Feb. 15 1889-90 13 Feb. 26-Mar. 10 1890-91 18 Dec. 1-18, Jan. 2-19 Jan. 2-31 1891-92 30 1892-93 68 Dec. 8-Feb. 13 1893-94 32 Nov. 13-Dec. 14 Winter No. of Days Time of Occurrence 1894-95. 65 Dec. 22-Feb. 24 1895-96 29 Dec. 26-Jan. 23 1896-97 61 Jan. 3-Mar. 4 1897-98 27 Dec. 15-Jan. 10 1898-99....... 26 Dec. 3-28, Jan. 22-Feb. 16 1899-1900 52 Feb. 8-Mar. 31 1900-1901 51 Jan. 16-Mar. 7 1901-2 45 Jan. 10-Feb. 23 1902-3 36 Dec. 22-Jan. 26 1903-4 48 Dec. 20-Feb. 5 1904-5 57 Jan. 2-Feb. 27 1905-6 16 Mar. 10-25 1906-7 28 Jan. 19-Feb. 15 1907-8 20 Jan. 22-Feb.10 1908-9........ 16 Jafa. 5-20 1909-10 38 Dec. 5-Jan. 11 1910-11 30 Nov. 28-Dec. 27 1911-12 49 Dec. 31-Feb. 17 1912-13* 31 Jan. 18-Feb. 17 Average 37.2 * Not included in means. 32° or below which were presented in Table XXXV, as the minimum temperature at night ordinarily passes the freezing point for several days before and after a cold spell in which the maximum readings are below that point. The exceptions to this rule are confined to the shorter periods, as will be seen readily in the comparison of the two tables. On the average the temperature reaches the freezing point during the winter season for an interval of 37.2 consecutive days. That is, out of a total average seasonal number of 112 days with minimum temperature of 32° or below (Table XXXVI), about one-third occur consecutively at some time or other during the course of the winter; and these periods are about three times the length of the longestTEMPERATURE 85 intervals of temperature continuously below the freezing point (Table XXXV). From season to season there is considerable variation in the number of consecutive days with minimum temperature of 32° or lower, the largest number being 74, from December 28, 1874, to March 11, 1875; and the least, 11, from January 26 to February 5, 1878. The latter period occurred during the warmest winter season on record; but in the former case, while the winter of 1874-75 averaged considerably below normal, it was not by several instances the coldest winter within the period of official observations. In the very cold winter of 1892-93, whose mean temperature was 19?0, the longest Fig. 12.—Longest period of consecutive days in each winter with a minimum temperature of 32° or below (see Table XXXVII). period with minimums of 32° or below was 68 days, while in the still colder winter of 1903-4, whose mean temperature was 18?3, the longest period was only 48 days. These seasons had 16 and 17 days with minimums of zero or below, and 4 and 1 days with maximums of zero or below, respectively (Tables XXIX and XXX). From such instances as those given in connection with the discussion of the present and previous topics, we must conclude that, analogous to the occurrence of years with an excess of heat (p. 71), some winters are below normal because of the relative frequency of severe days, and others because of a long continuance of moderately cold weather. OCCURRENCE OF TEMPERATURES FAVORABLE TO PLANT GROWTH The temperature of 42?8 is one of the critical temperatures in the advancement of vegetation. There is very little plant growth below this temperature, but as soon as it is reached the various processes86 THE WEATHER AND CLIMATE OF CHICAGO of plant activity necessary to growth are begun and proceed more and more rapidly as the temperature rises, up to certain limits depending upon the character of the vegetation. Fig. 13 and the data given in Table XXXVIII show the number of days in each year since 1872 on which the mean temperature was above 42°. The average for the entire period is 221 days annually, but there is a rather wide range of fluctuation above and below this figure—from 249 days in 1878 to 198 days in 1873, which makes a difference of 51 days between the highest and lowest records. By referring to the tables of monthly mean temperatures (Tables I and III) it will be seen that the latter was an abnormally cold year, while the former was one of thé warmest years within the period of observations. The table TABLE XXXVIII Annual Number op Days with Mean Temperature above 42°, 1872-1913 (These values, not including 1911-13, are graphically shown in Fig. 13) Year No. of Days Year No. of Days Year No. of Days Year No. of Days 1873 198 1884... 230 1895 208 1906 221 1874 208 1885 213 1896 221 1907 215 1875 210 1886 231 1897 221 1908 236 1876..... 231 1887 206 1898 216 1909 232 1877 237 1888 210 1899 228 1910 223 1878 249 1889 222 1900 216 1911* 235 1879 235 1890 213 1901 210 1912* 221 1880 238 1891 210 1902 233 1913* 241 1881 218 1892 204 1902 222 1882 236 1893 207 1904 213 Average 220.7 1883 223 1894 223 1905 218 * Not included in averages. Table XXXVIII shows the annual number of days with a mean temperature above 42°. The mean temperature of 42?8 is the point above which temperatures are conducive to plant growth, and below which very little growth takes place. includes all days of mean temperature of more than 42°, no matter in what portion of the year such days occur, and, of course, some proportion of them happen in the colder season, even in the months of January and February. However, the average number, 221 days, may be taken to represent the extreme length of the interval each year, other things being favorable, during which effective plant growth may take place. In Table XII it will be noted that the first occurrence of a daily mean temperature of 42?8 is on April 4, the last on November 10, and that all temperatures between are higher than the critical temperature of 42?8, and all recorded prior to the first and after the last are below that point. The interval from April 4 to November 10, inclusive, is exactly 221 days. So far as the growthTEMPERATURE 87 of annuals is concerned, however, a period of such length is seldom experienced in Chicago, as the season for such plants is materially shortened in most years by the occurrence of minimum temperatures below freezing, or sufficiently low to permit the formation of damaging frosts (Tables XLV, XLYI, and XLVII). In latitudes farther north the average number of days with mean temperature above 42° is considerably less than it is in this city, but the hours during which the sun shines through the summer days are correspondingly more, so that as a result the amount of heat necessary for the proper maturing of the crops of such regions is received. However, crops accli- 1875 1680 teas ¡890 ¡895 l9Cp ¡90S ¡910 da vs rnTTi i r'T I""}1 c i-i1 '] . p«.. imu 250 1--1—1-1 !-l~l. 1 -I—1-1 1 1 1 I M 1 bl I 11 1 1 1 I 1 I I I 1 1 I M 1 iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii llllllllllllllllllllllllllllllllllllll DAYS 250 200 ISO IOO SO Fig. 13.—Annual number of days with mean temperature above 42°, Fig. 13 shows the total annual number of days having a mean temperature above 42°, the degree of heat which marks the beginning of plant growth (see Table XXXVIII). mated to northern latitudes require in maturing a less amount of heat than do those of similar character accustomed to regions farther south. LIST OF WARM DAYS The frequency of days with a maximum temperature of 90° or over has already been discussed (p. 69), so that no great enlargement on the list presented in Table XXXIX is necessary. The table serves the purpose of setting out sharply the various periods of hot weather which the city has experienced, and conveys in addition a general idea of the intensity of each. The figures show the absolute maximums for the days against which the entries are made, and may therefore be used also in connection with Table XLIII, in the study of daily extremes of temperature.00 00 TABLE XXXIX List of Warm Days—May, 1873-1913 (Temperatures of 90° or above) Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1895.... 90 93 91 94 4 1906.... 90 1 1911.... 9Ò 92 91 94 94 94 6 11 See note at foot of table, p. 92. THE WEATHER AND CLIMATE OF CHICAGOTABLE XXXIX—Continued List of Warm Days—June, 1873-1913 (Temperatures of 90° or above) Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1873 .. 91 92 2 1874 90 90 94 95 93 5 1875 0 1876 0 1877 0 1878 0 1879 0 1880 91 1 1881 .. 0 1882 .. 0 1883 ... 0 1884 0 1885 ... 0 1886 0 1887 91 96 2 1888 90 .. 1 1889.... 0 1890 ■ 92 90 90 3 1891.... 0 1892 90 91 2 1893.... 0 1894 90 91 93 92 92 5 1895 92 95 93 3 1896.... 0 1897.... 90 93 2 1898.... 90 1 1899 90 90 2 1900.... 0 1901 93 93 93 97 4 1902.... 91 1 1903.... 90 1 1904.... 0 1905.... 90 91 90 3 1906.... 91 93 92 3 1907.... 91 1 1908.... 93 90 2 1909.... 0 1910.... 91 90 91 3 1911 95 98 98 93 94 90 6 1912.... 0 1913.... 91 94 98 95 91 96 90 99 8 61 00 CO TEMPERATURETABLE XXXIX—Continued List op Warm Days—July, 1873-1913 (Temperatures of 90° or above) Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1873.... - 93 90 o 1874 94 99 97 93 92 98 A 1875.... o 1876.... 90 93 93 93 90 91 A 1877.... 91 90 2 1878.... 97 95 90 Q 1879.... 90 92 90 91 93 90 A 1880.... 92 91 93 95 4 1881.... 92 92 92 93 92 5 1882.... 90 1 1883.... 90 91 90 90 4 1884.... o 1885.... 90 92 94 92 4 1886.... 94 90 94 3 1887.... 96 96 100 100 94 R 1888.... 91 90 92 94 4 1889.... 90 90 2 1890.... 93 90 9i 92 4 1891.... o 1892.... 92 91 94 93 92 R 1893.... 94 90 92 92 94 91 R 1894.... 92 93 90 92 96 93 R 1895.... 91 92 2 1896.... 91 92 93 91 92 5 1897.... 91 95 94 91 4 1898.... 92 91 91 94 90 91 .... 6 1899.... 90 1 1900..,. 91 90 91 91 91 92 90 7 1901.... 92 95 91 102 92 92 97 103 94 95 90 11 1902.... 90 90 2 1903.... 92 90 91 91 92 90 90 7 1904.... 90 94 92 3 1905.... 93 94 95 3 1906... 92 1 1907.... 90 1 1908.... 92 96 91 3 1909.... 91 90 92 3 1910.... 92 94 94 97 92 95 6 1911.... 96 98 100 102 102 90 94 91 91 g 1912.... 92 91 92 3 1913.... 94 94 92 92 92 94 99 7 162 THE WEATHER AND CLIMATE OF CHICAGOTABLE XXXIX—Continued List op Warm Days—August, 1873-1913 (Temperatures of 90° or above) Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1873.... 90 92 92 3 1874.... 98 90 91 93 94 91 6 1875.... 0 1876.... 92 1 1877.... 0 1878.... 9Ì - 1 1879.... 91 1 1880.... 93 1 1881.... 94 98 91 91 90 5 1882.... 0 1883.... 0 1884.... 91 1 1885.... 0 1886.... 92 1 1887 95 90 98 3 1888.... 91 1 1889.... 0 1890 96 95 2 1891.... 91 94 96 3 1892 93 92 » 2 1893 95 1 1894 93 92 95 3 1895.... 91 92 93 3 1896 94 95 98 93 93 92 6 1897 90 1 1898 90 93 90 90 4 1899.... 9Ì 90 2 1900 94 94 93 92 92 94 ,94 91 93 92 10 1901 90 1 1902.... 0 1903 90 92 2 1904.... 90 1 1905.... 90 91 92 3 1906.... 90 91 91 92 4 1907 92 1 1908 96 94 94 92 4 1909 93 91 2 1910 0 1911.... 92 90 90 92 4 1912.... 90 90 90 95 4 1913 95 94 97 93 4 91 TEMPERATURECO fcO TABLE XXXIX—Continued List of Warm Days—September, 1873-1913 (Temperatures of 90° or above) Table XXXIX contains a complete list of days from 1873 to 1913 during which the temperature rose to 90° or above, together with the maximum temperature reached on such days and the monthly frequency of occurrence. The figures in this table are entered to the nearest whole degree, while in Table XXVII the entries are made to the nearest enth of a degree. Temperature of 90° or above did not occur in years omitted from May and September lists. THE WEATHER AND CLIMATE OF CHICAGOTEMPERATURE 93 LISTS OF COLD DAYS The list of cold days with minimum temperatures of zero or lower, as set forth in Table XL, bears the same relation to the discussion of the frequency of such days (p. 73) that Table XXXIX of the previous paragraph does to that of the occurrence of hot days. This list covers the cold days of December, January, and February. In order to extend the record over the periods of severe weather in the early spring and late autumn months, however, it was necessary to use different limiting temperatures. Zero represents roughly a point about 25° below the daily mean temperatures of the winter season, and this difference makes the approximate measure of a cold day in March and November, 10° or below, and of a cold day in April and October, 24° or below, which are respectively the bases used in Tables XLI and XLII. The entries are in all cases the absolute minimum temperatures for the days in question and may be used in the study of the following subject. DAILY EXTREMES OF TEMPERATURE, ABSOLUTE Table XLIII contains the highest and lowest temperatures as recorded for each of the days of the year, the year of such record being given in each case, and the absolute range for the day. The table will be found useful in locating the earliest and latest occurrence of any given temperature, and the interval through which any given extreme has been reached or exceeded. For instance, it will be seen readily that the occurrence of a temperature of 100° or over has in all cases been confined to the first three weeks in July. The highest temperature ever recorded in this city by the official instruments was 103° on July 21, 1901. Other records of 100° or over are as follows: 102° on July 10, 1901, and on July 4 and 5, 1911; 100° on July 3, 1911, and on July 16 and 17, 1887. A temperature of 90° has occurred as early as May 9, and as late as September 24, in 1895 and 1891, respectively. The earliest protracted periods of 90° or over occurred from May 17 to 19, and from May 25 to 27, 1911; the latest was from September 19 to 22, 1895, during one of the warmest Septembers on record. This September had in all 6 days with readings of 90° or over, while there were but 3 in the preceding August, and 2 in July (Table XXXIX). In fact, the most pronounced hot weather of the year 1895 occurred, not in the summer, so much as in the late spring and early autumn. The temperature<ù fABLE XL List of Cold Days—December, 1872-1913 (Temperatures of zero or below) See note at foot of table, p. 96. THE WEATHER AND CLIMATE OF CHICAGOTABLE XL—Continued List of Cold Days—January, 1873-1913 (Temperatures of zero or below) Year 1 2 3 4 5 6 7 . 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1873.... - 8 - 7 — 4 ' K -16 Q e 1874.... 0 - 6 o .... 0 o 1875.... 0 - 7 -11 -20 - 3 - 8 - 9 - 1 - 4 0 7 11 1877.... - 3 - 3 - 1 — 4 A 1878.... - 1 i 1879.... -16 -18 - 8 -11 - 1 - 2 7 1881.... -10 -13 o i Q 1883.... - 2 - 2 0 — 4 -15 -17 -14 _ i O Q 1884.... - 6 —Ì4 —Ì8 —Ì4 - 3 0 O a 1885.... - 2 - 3 — 2 — 6 -13 — 7 - 8 —12 5 O -11 A 1886.... - 8 - 9 - 7 -11 -14 Z — 4 IZ 1887.... - 6 —ié -15 * • • - 3 -ió -15 -10 -11 - 2 - 9 - 8 A - 8 0 io 1888.... - l 0 -11 - 4 - 9 -ii -14 -17 - 6 - 1 — 4 — 12 1 1 9 1889.... 0 lo 1 1890.... — 5 1 1 1892.... 0 - 5 0 1 o 1893.... - 3 0 - 9 -7 - 8 - 9 -16 - 8 - 8 - 1 0 m 1894.... _ 3 — Q 9 1895.... - 3 - 9 o 0 - 9 - 8 0 1896.... - 4 - 9 - 3 y .... / q 1897.... —Ì5 -20 -16 K 9 Ó 1 Q o 6 a 1899.... 0 0 O a mi 10 1900.... 9 lo 1 1U A o 0 A 1901.... - 5 o 1 1902.... - 0 - 8 A Q 1903.... 0 - 4 - 6 0 0 A 1904.... - 1 — 2 —13 ■j * - 6 c 1905.... - 1 - 5 - 5 - 5 _ 2 o O a 1907.... _ 1 Q 0 9 1908.... * 0 ù 1 1909.... -10 - 3 9 1910.... - 4 - 5 9 1911.... 0 1 1912.... - 6 -10 -11 -16 - 3 - 4 - 6 - 3 - 3 - 1 - 2 - 2 0 1913.... 0 ì 175 CO Cm See note at foot of table, p. 96. TEMPERATURECO TABLE XL—Continued ^ List op Cold Days—February, 1873-1913 (Temperatures of zero or below) Table XL contains a complete list of days from December, 1872 to 1913, for December, January, and February, during which a temperature of zero or below occurred, together with the minimum temperature reached on such days and the monthly frequency of occurrence. Temperatures of zero or below occurred in the months of November and March during but three and five years, respectively, and the dates of such occurrences will be found in Table XLI. The figures in this table are entered to the nearest whole degree, while in Table XXIX the entries are made to the nearest tenth of a degree. Temperature of zero or below did not occur in years omitted from the list. THE WEATHER AND CLIMATE OF CHICAGOTABLE XLI List of Cold Days—November, 1872-1913 (Temperatures of 10° or below) CO TEMPERATURECD 00 TABLE XLI—Continued List of Cold Days—March, 1873-1913 (Temperatures of 10° or below) Table XLI contains a complete list of days from November, 1872 to 1913, for the months of March and November, during which a temperature of 10° or below occurred, together with the minimum temperature reached on such days and the monthly frequency of occurrence. Temperature of 10° or below did not occur in years omitted from the list. THE WEATHER AND CLIMATE OF CHICAGOTABLE XLII List of Cold Days—April, 1873-1913 (Temperatures of 24° or below) H § g d § CO COo o TABLE XLII—Continued List op Cold Days—October, 1872-1913 (Temperatures of 24° or below) Year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total 1873.. 23 1 1887.. 23 14 22 23 4 1895 24 24 24 3 8 Table XLII contains a complete list of days from October, 1872 to 1913, for the months of April and October, during which a temperature of 24° or below occurred, together with the minimum temperature on such dates and the monthly frequency. Temperature of 24° or below did not occur in years omitted from this table. THE WEATHER, AND CLIMATE OF CHICAGOTEMPERATURE 101 of 80° has been experienced as early as March 23 (1907), and as late as October 23 (1899 and 1901), when the maximum for each day was 82°. The earliest occurrence of 70° or over is March 8 (1879), and the latest is November 21 (1913), when the maximum was 72°. The freezing point, 32°, has occurred as early as September 30 (1899), and as late as May 14 (1895); zero, as early as November 28 (1872 and 1887), and as late as March 22 (1888). The absolute minimum for the city is —23°, which occurred on December 24, 1872, TABLE XLIII Daily Extremes op Temperature, 1872-1913 January February March Ex- Ex- Ex- Max. Year Min. Year treme Max. Year Min. Year treme Max. Year Min. Year treme Range Range Range 1.. 65 1876 - 6 1887 71 55 1877 -11 1905 66 60 1882 0 1890 60 2.. 57 1897 -16 1879 73 48 1877 -14 1905 62 61 1878 - 4 1913 65 3., 60 1874 -18 1879 78 47 /1882 \1885 1890 -10 1905 57 64 1894 - 6 1873 70 4.. 59 1880 -14 1884 73 59 -13 1895 72 60 1894 -12 1873 72 5.. 56 1890 -18 1884 74 53 1909 -15 1895 68 66 1894 0 1873 66 6.. 54 1907 -14 1884 68 57 1882 - 1 [1875 \1901 1875 58 66 1876 3 1901 63 7.. 59 ' 1907 -16 1912 75 53 1882 -10 63 60 1876 3 1899 57 8.. 56 1876 -11 1875 67 62 1900 -17 1899 79 71 1879 14 1899 57 9.. 60 1880 -20 1875 80 56 1886 -21 1899 77 66 1879 9 1877 57 10.. 46 1880 -10 /1881 11887 56 63 1876 -18 1899 81 69 [1876 Ì1879 5 1892 64 11.. 61 1880 -11 /1887 \1888 72 60 1876 -14 1885 74 65 [1902 \1911 11 1892 54 12.. 62 1890 - 9 1895 71 62 1882 -17 1899 79 68 '1887 6 1896 62 [1893 13.. 47 1894 - 9 1888 56 60 1882 -18 1905 78 60 ] 1903 6 1896 54 [1913 14.. 48 1894 -13 1881 61 58 1887 -11 1905 69 63 1913 7 1891 56 15.. 53 1906 -16 1893 69 58 1882 - 9 [1875 \1905 67 62 [1878 \1889 6 [1877 \1890 56 16.. 55 1889 —17 1888 72 57 /1880 \l883 -10 1885 67 68 [1889 \1898 1894 - 1 1900 69 17.. 60 1894 - 8 1893 68 60 1880 -11 1903 71 74 - 1 1900 75 18.. 55 1880 - 9 1887 64 58 1877 - 9 1903 67 74 1903 10 1876 64 19.. 55 1907 -13 1885 68 62 1913 - 6 1896 68 73 1903 9 1885 64 20.. 63 1906 - 7 1885 70 54 1906 - 9 1896 63 64 1886 4 1885 60 21.. 62 1906 -15 1883 77 57 1877 -11 1873 68 76 1907 9 [1885 \1888 67 22.. 59 1909 -17 1883 76 58 1877 -14 1873 72 71 1907 - 1 '1888 72 23.. 65 1909 -14 (1883 \1887 79 53 1891 oo 1 1873 71 80 1907 10 1888 70 24.. 62 1909 -15 1897 77 58 1891 -11 1873 69 79 1910 17 [1874 \1899 62 25.. 51 1880 -20 1897 71 57 1880 - 9 1900 66 72 1907 13 '1894 59 26.. 58 1882 -16 1897 74 63 1880 - 1 1888 64 71 1908 7 1873 64 27.. 53 1911 - 9 1895 62 58 1896 - 6 [1879 \1897 1884 64 81 1910 15 1873 66 28.. 60 1876 -11 1885 71 61 1895 0 61 75 1905 9 1887 66 29.. 51 1903 -16 1873 67 38* 1888 - 3 1884 41 80 1895 9 1887 71 [1877 60 30.. 50 -j 1896 -10 1899 60 76 1910 16 1887 [1913 31.. 56 1877 -12 1899 68 69 1893 21 1899 48 * 8 years.102 THE WEATHER AND CLIMATE OF CHICAGO TABLE XLIII—Continued Daily Extremes of Temperature, 1872-1913 April May June Ex- Ex- Ex- Max. Year Min. Year treme Max. Year Min. Year treme Max. Year Min. Year treme Range Range Range 1.. 73 1882 17 1881 56 85 /1887 [1901 32 /1888 [1909 53 85 1895 42 /1882 [1889 43 2.. 77 1903 18 /1896 [1899 59 87 1901 27' 1875 60 92 1895 43 1879 1888 [1910 49 3.. 73 1887 17 1879 56 87 1895 34 1907 53 95 1895 43 1910 52 4.. 77 1873 19 1881 58 84 /1895 \1905 35 [1891 \1907 49 95 1911 44 1891 51 5.. 83' 1873 19 [1881 •j 1887 [1898 64 86 1909 36 (1890 [1891 50 90 1905 41 1894 49 6.. 76 1882 21 1881 55 85 1889 35 [1875 [1890 50 91 [1905 ■11906 [1913 40 1894 51 7.. 84 1893 23 1902 61 85 1880 34 1885 51 89 1896 44 /1894 [1913 45 8.. 75 1910 26 1902 49 88 1889 37 1885 51 90 1874 42 1885 48 9.. 80 1887 23 1892 57 90 1895 35 1885 55 98 1911 43 1901 55 10.. 78 1887 25 1909 53 89 1896 36 1907 53 98 1911 45 /1877 [1903 53 11.. 78 1910 25 1882 53 85 /1879 [1881 36 1907 49 91 1880 45 1903 46 12.. 80 /1887 [1896 28 /1882 [1900 52 87 /1879 [1881 36 1895 51 91 1902 44 1903 47 13.. 82 1887 27 1885 55 86 1900 34 1888 52 90 1897 46 1875 44 [1875 14.. 79 1887 23 1907 56 86 1900 32 1895 54 87 1888 48 i 1883 39 /1906 [1911 [1909 15.. 80 1896 27 1893 53 86 35 1895 51 94 1913 48 1909 46 16.. 84 1896 18 1875 66 89 1911 34 1888 55 98 1913 48 1879 50 17.. 83 1896 17 1875 66 90 /1911 [1906 38 1873 52 96 1887 47 1879 49 18.. 83 1896 23 1875 60 92 1911 35 1894 57 91 1872 47 1876 44 19.. 77 1906 26 1897 51 91 1911 36 1894 55 98 1872 46 1882 52 20.. 74 1899 24 1897 50 86 1887 37 1892 49 92 1872 50 1903 42 21.. 83 1902 30 1893 53 83 1876 36 1883 47 91 1894 48 1902 43 [1894 22.. 78 1913 29 1873 49 87 1912 39 1883 48 93 1908 40 1875 53 [1911 23.. 81 1886 25 1873 56 88 1912 38 1882 50 94 /1874 \1911 50 1885 44 24.. 81 1895 29 1910 52 84 1896 41 1901 43 93 1901 51 1887 42 25.. 80 1906 27 1887 53 94 1911 39 1893 55 93 /1895 [1901 50 1883 43 26.. 80 1888 31 /1875 \1887 49 94 1911 41 /1891 \1893 53 91 1913 51 1883 40 27.. 82 1888 35 1874 47 94 1911 38 '1907 56 96 1913 51 1905 45 [1874 28.. 83 1888 35 1874 48 88 1895 38 1894 50 93 •{1901 50 1875 43 [1906 29.. 88 1899 34 1874 54 93 1895 40 1873 53 92 /1894 [1906 50 1875 42 30.. 84 1894 30 1873 54 91 1895 35 1873 56 99 1913 53 /1885 [1902 46 31 94 1895 35 1873 59 TEMPERATURE 103 TABLE XLIII—Continued Daily Extremes op Temperature, 1872-1913 July August September Ex- Ex- Ex- Max. Year Min. Year treme Max. Year Min. Year treme Max. Year Min. Year treme Range Range Range (1885 (1872 1.. 96 1911 53 U892 43 93 1894 57 1875 36 95 1913 52 1885 43 [1904 [1895 2.. 98 1911 56 /1882 \1904 42 96 1890 54 1907 42 97 1913 50 1888 47 3.. 100 1911 53 1892 47 96 1908 48 1894 48 92 1898 52 i 1877 \1893 40 4.. 102 1911 56 /1872 \1882 46 98 1881 53 /1880 \1894 45 91 1893 48 1902 43 5.. 102 1911 52 1872 50 95 1896 53 '1884 42 98 1899 52 1896 46 6.. 99 1874 52 1873 47 93 1900 52 1875 41 94 1912 47 1885 47 7.. 94 1893 54 (1883 \1894 40 92 (1894 1900 [1911 57 /1875 \1902 35 95 1899 49 1898 46 8.. 94 1910 53 1883 41 98 1896 55 1904 43 92 1897 46 1883 46 9.. 94 /1910 \1911 50 1895 44 97 1913 52 /1884 11888 45 94 1897 48 1883 46 10.. 102 1901 52 1873 50 98 1887 51 /1882 11884 47 94 1897 48 1883 46 11.. 96 1908 53 1873 43 98 1874 55 1882 i 1902 [1903 43 92 /1895 \1908 47 1878 45 12.. 96 1887 56 1900 40 92 1905 57 1902 35 87 1874 48 1902 39 13.. 95 1880 56 (1883 U888 1897 39 86 (1876 \1912 54 1888 32 89 1909 39 1890 50 /18741 \1896j (1882 14.. 93 58 11883 11884 35 93 1895 56 1885 37 95 1893 42 1873 53 [l888 15.. 96 1887 54 1889 42 86 1904 55 1885 31 91 1897 42 1873 49 16.. 100 1887 55 1889 45 94 1908 56 1883 38 89 /1891 \1899 46 1900 43 17.. 100 1887 59 1886 41 91 1874 52 1879 39 90 '1891 41 1900 49 18.. 95 1905 54 /1873Ì \1883j 41 93 /1880 \1900 54 1897 39 90 1908 40 (1889 1901 [1903 50 19.. 95 1913 50 1873 45 93 1874 58 1875 35 91 1895 41 1873 50 20.. 97 1901 56 1873 41 % 94 1874 56 /1890 \1897 38 91 1895 42 1896 49 (1877 - 1880 21.. 103 1901 . 57 1882 ]1889 46 93 1872 53 1875 40 90 1895 39 1889 51 1900 1902 22.. 92 1906 56 1872 36 92 1906 51 1890 41 90 1895 38 1913 ! 52 23.. 92 1892 57 1896 35 93 1898 51 1888 42 88 1892 38 1887 50 24.. 97 1910 56 1905 41 92 /1873 \1903 49 1887 43 91 1891 40 /1879 \1887 51 1879 25.. 98 1874 56 1890 42 90 1912 50 1887 40 89 1900 39 1893 50 [1907 26.. 95 1910 58 1890 37 89 (1881 1888 [1913 47 1887 42 86 1908 39 /1893 \1912 47 27.. 95 1901 57 1872 38 89 1877 54 /1876 \1896 35 87 1891 35 1889 52 28.. 94 1886 56 1891 38 90 1897 49 1891 41 83 /1904 \1905 39 1888 44 29.. 92' /1896 \1909 56 1883 36 88 /1889 \1905 53 1891 35 87 1898. 36 J1888 \1908 51 30.. 92 /1888 \1890 58 (1883 1889 [1895 34 90 /1881 \1898 49 1872 41 86 1877 32 1899 54 31.. 94 1888 54 1895 40 95 1912 47 1872 48 * 104 THE WEATHER AND CLIMATE OF CHICAGO TABLE XLIII—Continued Daily Extremes of Temperature, 1872-1913 October November December Ex- Ex- Ex- Max. Year Min. Year treme Max. Year Min. Year treme Max. Year Min. Year treme Range Range Range 1.. 85 1901 32 1886 53 75 1888 21 1879 54 57 1901 - 6 1893 63 2.. 86 1891 37 1886 49 69 /1886 [1896 23 1911 46 57 1873 - 1 1893 58 3.. 84 1891 32 1888 52 67 1887 20 1901 47 60 1873 0 1895 60 4.. 86 1897 36 1901 50 67 /1895 [1898 16 1901 51 57 1913 0 1893 57 5.. 86 1900 34 1876 52 68 1874 24 1901 44 62 1879 6 1895 56 6.. 84 /1879 [1912 38 1885 46 67 1887 22 /1877 [1910 45 56 /1879 \1883 0 1885 56 7... 82 1887 34 1876 48 72 1874 25 1886 47 57 /1883 [1892 - ,6 1882 63 8.. 83 1905 33 1876 50 69 1879 17 1892 52 59 '1889 - 7 1882 ’66 9.. 82 1879 33 1895 49 67 1910 23 1896 44 62 1879 -14 1876 76 10.. 83 1913 30 1906 53 66 1909 22 1913 44 62 1879 0 1876 62 11.. 81 1893 27 1906 54 74 1911 16 1894 58 60 1886 8 1892 52 [1893 12.. 79 1886 30 1887 49 70 1902 13 1911 57 61 1877 3 •! 1903 58 [1912 72 13.. 83 1899 29 1909 54 70 1909 12 1911 58 59 1881 -13 1903 14.. 86 1897 27 1872 59 65 il 902 [1909 14 1900 51 57 1891 - 9 1901 66 15.. 87 1897 28 1876 59 62 1896 14 /1883 \l900 48 59 1877 -12 1901 71 16.. 83 1910 32 1876 51 70 1896 10 '1883 60 58 1877 - 8 1876 66 17.. 84 1910 32 1880 52 64 1881 8 (1880 [1891 56 55 1877 - 1 /1875 \1884 56 18.. 78 1910 28 1880 50 65 Î1896 [1904 8 1880 57 62 1877 - 9 1884 71 19.. 81 1910 30 1880 51 67 1913 10 1894 57 64 1877 -11 1884 75 20.. 75 1872 24 1895 51 68 1913 8 1873 60 67 1877 - 8 1901 75 21.. 82 1908 27 1913 55 72 1913 1 1880 71 55 (1875 [1894 -12 1872 67 22.. 79 1901 26 1887 53 69 1913 4 1880 65 57 1875 -21 1872 78 23.. 82 /1899 [1901 25 1895 57 61 1886 10 1898 51 57 1877 -18 1872 75 24.. 76 ¡1899 [1902 23 1887 53 59 1908 4 1893 55 64 1889 -23 1872' 87 25.. 74 1899 14 1887 60 65 1908 7 1893 58 56 1895 - 2 /1892 [1903 58 26.. 74 1900 , 22 .1887 52 63 1896 2 -1898 61 53 1888 -10 (1892 [1903 63 27.. 70 1896 28 1878 '42 63 1909 2 1887 61 58 1907 -10 1886 68 28.. 75 /1874 [1896 24 1895 51 69 1905 0 /1872 [1887 69 50 1889 -12 1880 62 29.. 78 1901 24 /1873 [1895 54 59 1913 - 2 1872 61 61 1889 -15 1880 76 30.. 75 1901 23 1887 52 58 /1908 [1911 0 1874 58 61 1884 - 7 1909 68 31.. 76 1888 23 1873 53 68 1875 - 1 /1880 [1899 69 Table XLIII shows the highest and the lowest temperatures recorded on each day of the year from 1872 to 1913, with year of occurrence and the extreme range for the day.TEMPERATURE 105 SUMMARY OF TABLE XLIII Absolute Extremes, of Temperature : • 1872-1913 January February . . . March....... April...... May........ June....... July....... August ..... September .. October.... November . . December... Year Absolute Maximum Year Day Absolute Minimum Year Day Absolute Range O CD /1876 11909 1 23 o O oq 1 /1875 [1897 9 25 OO o 63° /1876 \1880 10 26 -21° 1899 9 o OO 00 0 1910 27 -12° 1873 4 93° f 1875 17 O 00 00 1899 29 17° 1879 3 71° [1881 1 94° /1895 31 \1911 25* 27° 1875 2 67° /1875 22 99° 1913 30 o O [1894 6 58° (1873 19 103° 1901 21 50° 11895 9 53° f 1874 11 (1872 31 CO OO O 11881 4 47° [1887 26 51° 11887 10 [1896 8 98° 1899 5 . 32° 1899 30 66° 87° 1897 15 14° 1887 25 73° 75° 1888' 1 - 2° 1872 29 77° 68° 1875 31 -23° 1872 24 91° 103° 1901 July 21 -23° 1872 Dec. 24 126° * Also on the 26th and 27th. about a month before the average date for the absolute winter minimum (Table XLIV). Zero or lower has been recorded in some year or other on every day of January and February, and on every day of December except the 5th, 11th, and 12th. The greatest difference between the extremes of temperature for any one day throughout the year, that is, the greatest absolute range, is 87° on December 24, from 64° in 1889 to -23° in 1872. INTERVAL BETWEEN OCCURRENCE OF LOWEST AND HIGHEST TEMPERATURES, WINTER TO SUMMER The lowest mean temperature of the year occurs on February 1, and the highest mean on July 16, 165 days later (p. 26), but it is seldom that the extreme temperatures of winter and summer are actually experienced on those dates. In fact, as will be seen in Table XLIV and Fig. 14, the absolute minimum of the winter season has never occurred on February 1, and the absolute maximum of summer has fallen on July 16 only three times since the official records began. The winter minimum usually accompanies the passage of a more or less severe cold wave, and the summer maximum ordinarily attends a period of pronounced heat in the interior portions of the106 THE WEATHER AND CLIMATE OF CHICAGO country. Both occurrences are the result of conditions much more complicated than the simple swing of the sun from tropic to tropic, and are far from being coincident with the time of arrival of that TABLE XLIV Time of Occurrence of Annual Minimum and Maximum Temperatures, with Interval, 1871-1913 (These values, not including 1910-13, are shown graphically in Fig. 14) Minimum Date of Minimum Date of Maximum Maximum Summer Interval in Days Dec. Jan. Feb. Mar. June July Aug. Sept. 1871-72 -12 29 19 98 1872 142 1872-73 -23 24 16 93 1873 204 1873-74 - 6 15 6 99 1874 172 1874-75 -20 9 11 89 1875 153 1875-76 - 3 2 7 93 1876 155 1876-77 -14 9 8 91 1877 211 1877-78 - 1 7 16 97 1878 190 1878-79 -18 3 15 93 1879 193 1879-80 - 2 26 13 95 1880 200 1880-81 -15 29 4 98 1881 218 1881-82 1 17 27 90 1882 191 1882-83 -17 22 3 91 1883 162 1883-84 -18 5 19 91 1884 227 1884-85 -14 11 20 94 1885 159 1885-86 -14 23 6 94 1886 164 1886-87 -15 3 16 100 1887 194 1887-88 -18 9 31 94 1888 173 1888-89 -11 23 9 90 1889 136 1889-90 - 5 22 2 96 1890 192 1890-91.. - 8 4 9 96 1891 186 1891-92 - 5 9 25 94 1892 197 1892-93 -16 15 10* 95 1893 207 1893-94 - 9 25 24 96 1894 180 1894-95 -15 8 3 95 1895 115 1895-96 - 9 4* 8 98 1896 217 1896-97 -20 25 3 95 1897 159 1897-98 - 8 3 19 94 1898 166 1898-99 -21 9 5 98 1899 208 1899-1900 — 9 24* 5 94 1900 163 1900-1901 — 5 1 21 103 1901 201 1901-2 -12 15 12 91 1902 179 1902-3 -11 17 1* 92 1903 134 1903-4.... -15 25 17 94 1904 174 1904-5 -18 13 18 95 1905 155 1905-6 6 2 28 93 1906 146 1906-7 — 3 26 1 92 1907 218 1907-8 - 2 2 11* 96 1908 160 1908-9 -10 6 8 93 1909 214 1909-10 - 7 30 24 97 1910 206 1910-11f 0 5 5 102 1911 181 1911—12f -16 7 31 95 1912 236 1912-131 - 4 2 29* 99 1913 148 No. of occurrences ... 6 22 13 1 5 26 9 2 Mean 20 » 19 180 . * On other dates also; latest date of minimum and earliest date of maximum given in table, t Not included in means. luminary at either end of his journey. The lowest temperature of the winter season has occurred as early as December 9 and as late as March 2; the highest temperature of summer has occurred as early as June 3, and in one instance was delayed until September 5.TEMPERATURE 107 The average times of these extremes are January 20 and July 19, respectively, and the interval between extends over 180 days. There is, however, a considerable range in the interval between the occurrence of the lowest and highest temperatures, the longest on record Dec. jan. Feb Mar. Apr. May June July Aug Sept - 1871-72 1875-76 1880-81 1835-86 1890-91 1895-96 1300-01 1^05-06 Mean Fig. 14.—Interval between time of occurrence of lowest and highest temperatures. Fig. 14 shows the time of occurrence of the lowest temperature each winter and the highest temperature of each succeeding summer from 1871 to 1910; also the length of the intervening period. The lowest line marked “mean” shows the average time of occurrence and the average length of the intervening period (see Table XLIV). being 236 days in 1912, from January 7 to August 31, while the shortest, 115 days in 1895, from February 8 to June 3, was only a little less than half that time. OCCURRENCE OF FROST The length of time each year during which the temperature is sufficiently high to permit the growth of vegetation has been established in a previous topic (p. 86) as 221 days, but attention was there called to the fact that this period does not represent the actual growing season for annuals or plants susceptible to damage by low temperatures. It now remains to inquire into the occurrence of frosts and I87I-7Z I87S-76 /880-Si 1385-86 1890-91 1895-96 J9O0-OI 1305-06 Mean108 THE WEATHER AND CLIMATE OF CHICAGO frost temperatures, and to determine the limits within which tender plants may reasonably be regarded as safe. According to Weather Bureau usage, three kinds of frost are distinguished: light frost, which has no destructive effect, except on the tenderest plants and vines in exposed places; heavy frost, which is accompanied by a considerable deposit, but is yet not severe enough to affect the hardier annuals; and killing frost, which destroys all susceptible growth, even if there is no actual deposit of hoar frost. This last condition is usually attended by the formation of ice on shallow puddles and pools. A dry freeze unattended by the formation of hoar frost is, in fact, often the most damaging, because it is the result of low temperatures throughout a considerable portion of the night; whereas a lower reading may occur on some other night but the damage yet be much less because the temperature falls rapidly to its minimum and immediately rises to a point above the danger mark. From the above explanations it will be plain that the recording of the various classes of frost must depend largely upon the judgment of the observer as to the damage sustained by the different kinds of vegetation. Obviously, in a large city this is a very difficult task, as the extent of vegetation is limited, and much of that existing is to some extent at least under artificial protection. For this reason, while the actual occurrences of frosts are recorded as far as they can be observed, certain temperatures, called frost temperatures, are noted as the limits above which frost is not likely to occur. 1. Minimum temperature of Jfi°, or light frost.—Ordinarily, during the spring and fall light frost will occur in portions of the city when the air temperature at the Weather Bureau office has fallen to 40°, if the sky is clear enough to promote rapid radiation at night and the wind not of sufficient force to keep the lower strata of the atmosphere well mixed. In fact, there are certain localities in the outskirts where the relatively lower' surface of the ground gathers the colder, heavier air as in a cup, and in these sections light frost often occurs when the official thermometers record a minimum temperature considerably higher than 40° (p. 13). Even in the section where frost has formed the temperature of the air does not necessarily fall to the freezing point, but it is the surface of the object on which the frost forms that has cooled to 32°. The air itself is both a poor absorber and a poor radiator of heat, and most objects, such as leaves of plants, edges of boards, and loamy soils, become much cooler under a clear sky at night than does the air, because they areTEMPERATURE 109 more active in radiating the heat accumulated during the day. Thus the temperature of the surface of such objects may descend below the freezing point, and frost is therefore likely even though the temperature of the air is considerably above 32°. Table XLV and Fig. 15 show the dates of the last occurrence in spring and the first in autumn of the frost temperature of 40°. The interval between these dates is also shown, while the table indicates TABLE XLV Last and First Occurrence of a Minimum Temperature of 40° (Light Frost Temperature), 1871-1912 (See Fig. 15) Year a 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 << 27 Last in First in Spring Autumn O ■< H « « H , ft > « *3 O H « A 4 4 31 18 6 10 13 6 1 3 23 23 7 19 7 28 27 22 21 29 24 30 19 31 17 16 20 25 23 28 18 13 25 21 6 12 5 18 24 3 22 4 1 2 5 6 4 146 145 127 146 90 145 147 157 140 151 182 153 132 167 137 146 148 131 109 118 141 138 122 119 131 + 2 + 1 -17 + 1 -54 + 1 + 3 +13 - 4 + 7 +38 + 9 -12 +23 - 7 + 2 + 4 -13 -35 -26 + 7 - 6 -22 -25 -13 Year Last in Spring First in Autumn Interval in Days Departure 1 from Average April ! May June ft a> 02 Ü O Nov. 1896 10 23 165 +21 1897 25 29 156 +12 1898 6 13 159 +15 1899 22 29 159 +15 1900 20 2 165 +21 1901 23 18 147 + 3 1902 27 13 138 — 6 1903 3 18 137 - 7 1904 16 22 158 +14 1905 9 11 154 +10 1906 9 9 152 + 8 1907 27 25 120 -24 1908 6 28 144 0 1909 3 11 160 +16 1910 4 27 175 +31 1911* 3 22 171 +27 1912* 13 26 135 - 9 Average date: 1871-80.. 15 2 139 ±10.3 1881-90.. 15 5 142 ±16.9 1891-1900 14 8 146 ±15.7 1901-10.. 10 6 148 ±11.9 1871-1910 14 5 144 ±13.9 * Not included in means. for each season the excess length or the deficiency as based upon the average period of frost-free temperatures given at the bottom. The last frost temperature of spring has occurred in May 36 times during the 42 years from 1871 to 1912; in April, 4 times, once as early as the 10th; and in June only twice, the latest occurrence being June 22 in 1875. The average date of the last occurrence in spring is May 14. The average date of the first occurrence of the frost temperature in autumn is October 5. It will be seen from the table that there have been nearly as many occurrences in September as in October, but this does not indicate any greater irregularity in the autumn, because110 THE WEATHER AND CLIMATE OF CHICAGO the equated dates fall 9 days earlier in the month than is the case in the spring. Frost temperatures in the latter half of the year have been noted as early as September 13, and in two instances have been delayed until November 2. The average interval between the equated dates in spring and fall is 144 days, and this may be regarded as the length of time through which freedom from injury by low temperatures may reasonably be expected for the most susceptible plants common to the locality. In the 42 years of record, 25 seasons APR KAY JUNE JULY AUG SEPT OCT NOV »871 Í876 V88I 9880 1891 9886 *909 1906 MEAN Fig. 15.—Interval between last and first occurrence of a minimum temperature of 40° (light frost). Fig. 15 shows the time of last occurrence of a minimum temperature of 40° in spring and that of the first occurrence in fall, together with the length of the intervening period. The lowest line marked “mean” shows the date of the average occurrence and the average length of the intervening period (see Table XLV). have been longer than the average and only 16 have been shorter, but in the latter cases the departures have as a rule been the most pronounced. The season of 1908 was exactly of the average length, but with opening and closing earlier than the equated dates by about 8 days; the longest period was in 1881, from May 3 to October 2, 182 days; while in 1875, the year of lowest mean temperature, only 90 days intervened between the first and last occurrences of frost temperatures on June 22 and September 21, respectively. 1871 1876 1681 1886 1891 1896 1901 1906 MEANTEMPERATURE 111 2. Killing frosts.—No special data have been prepared on the occurrence of heavy frosts because of the personal error possible in judging the different degrees of intensity laid down for light and heavy deposits; and, indeed, both light and heavy frosts may occur TABLE XLVI Last and First Occurrence of Killing Frosts, 1871-1912 Year Last in Spring Min. Temp. First in Autumn Min. Temp. Interval in Days 1871 *.. Mar. 23 32° Oct. 28 26° 218 1872 Apr. 16 32° Oct. 10 31° 176 1873 Apr. 30 30° Oct. 23 28° 175 1874 Apr. 24 30° Nov. 1 29° 190 1875. May 2 27° Oct. 2 38° 152 1876 Apr. 2* 32° Oct. 4 38° ' 184 1877 Apr. 5* 32° Oct. 5 37° 182 1878 May 13 38° Oct. 19 34° 158 1879 Apr. 4* 25° Oct. 24 34° 202 1880 Apr. 12* 31° Oct. 18 28° 188 1881 Apr. 7* 26° Nov. 3 36° 209 1882 May 16 37° Nov. 13 25° 180 1883 Apr. 24 32° Nov. 1 31° 190 1884 Apr. 3* 32° Oct. 23 28° 202 1885 May 10 38° Oct. 6 38° 148 1886 Apr. 8* 32° Oct. 1 32° 175 1887 Apr. 25 27° Oct. 12 30° 169 1888 May 16 34° Oct.. 3 32° 139 1889 Apr. 6* 29° Sept. 27 35° 173 1890.: Apr. 14* 28° Oct. 27 34° 195 1891 Apr. 8* 31° Oct. 22 33° 196 1892 Apr. 24 32° Oct. 6 40° 164 1893 Apr. 22 31° Oct. 16 38° 176 1894 :.. Apr. 9* 31° Oct. 6 39° 179 1895 May 14 32° Sept. 30 36° 138 1896 Apr. 9* 32° Oct. 18 34° 191 1897 May 1 33° Nov. 3 36° 185 1898 Apr. 6* 27° Oct. 15 40° 191 1899 Apr. 5 29° Sept. 30 32° 177 1900 Apr. 13* 32° Nov. 6 38° 206 1901 Apr. 19 34° Oct. 17 33° 180 1902 Apr. 8* 26° Nov. 23 31° 228 1903 Apr. 5* 30° Oct. 27 33° 204 1904 Apr. 21 31° Oct. 28 34° 189 1905 Apr. 7* 31° Oct. 21 35° 196 1906 Mar. 31* 32° Oct. 10 30° 192 1907 May 4 35° Oct. 14 39° 162 1908..... Apr. 3* 26° Oct. 12 41° 191 1909 May 2 33° Oct. 14 29° 164 1910 Apr. 24 ,29° Oct. 29 27° 187 1911| Apr. 7 32° Oct. 24 36° 199 1912f Apr. 18 32° Oct. 24 39° 188 Average Apr. 18 Oct. 18 182 Earliest Mar. 23, 1871 Sept. 27, 1889 Longest, 228 Latest May 16i 1888 Nov. 23, 1900 Shortest, 138 * Not customary to record frosts until April 15, dates given being the last occurrence of freezing tempera- ture. f Not included in averages. at the same time in localities in close proximity to each other. In the case of a killing frost, however, the effects are practically in all cases easily discerned, especially in the autumn, when vegetation is in full development. Table XLVI will therefore be valuable in establishing the average and extreme limits of the growing season, so far as the112 THE WEATHER AND CLIMATE OF CHICAGO ordinary annual and deciduous plants and shrubs are concerned. The accompanying minimum temperatures are given in the table, which has reference to the actual occurrence of killing frost, without regard to the temperature at the time, and in this respect it differs from the preceding table and the one following, these being concerned only with the first and last occurrences of certain temperatures, 40° and the freezing point, respectively. The average interval between the last killing frost in the spring and the first in the autumn is 182 days, and extends from April 18 to October 18, but the season has ranged from 228 days in 1902 to only 138 days in 1895. It has not been customary to record frosts in this latitude prior to April 15 of each year, so that in the table all dates before April 15 are those of the last occurrence of a temperature of 32°. In 1871 this date fell as early as March 23, while in 1888 a killing frost occurred as late as May 16. In 1889 killing frost occurred on September 27, the earliest autumn record for this phenomenon, while in 1902 the season was extended until November 23. It will be noted that the minimum temperatures, counting only the actual occurrences of killing frost as recorded in the table, range from several degrees below the freezing point to nearly 10° above. In 1908 killing frost occurred with a minimum temperature at the Weather Bureau office of 41°, and a number of instances where the minimum was from 35° to 40° may be picked out. The explanation is simply that of the varying conditions of radiation due to differences in cloudiness and character of exposure, and the effect of wind movement, as has been brought out previously (pp. 13, 108). 3. Minimum temperature of 32°, or freezing point.—As frosts do not occur in cloudy weather or on nights in which the wind movement is brisk, damage to vegetation at such times is wrought by the actual occurrence of freezing temperatures, and under the conditions outlined temperatures in the various portions of the city do not differ appreciably from those recorded at the Weather Bureau office. In England the expression “degrees, of frost” has reference to the number of degrees the temperature falls below the freezing point. We do not make use of this term in the United States, and it would be difficult to prepare data relative to such a term, if unlimited. It will, however, be helpful in securing a knowledge of the temperature conditions in Chicago to note the interval between the first and last occurrence of the freezing point, 32°, each year, and these data are presented in Table XLVII and Fig. 16, in substantially the sameTEMPERATURE 113 manner as was used in the treatment of the occurrence of the frost temperature (Table XLV, Fig. 15). The average interval is 193 days in length, and extends from April 16 to October 27. The longest interval on record was 228 days, from April 8 to November 23, 1902, November of that year being exceptionally warm; while the shortest was 154 days, from May 1 to October 3, 1888. In 1871, 32° was not reached later than March 23, and in 1895 it occurred as late as May 14. In the fall the earliest recorded date is September 30, 1899; and the latest, November 23, 1902, mentioned above as the expiration of the longest interval of temperatures above freezing. TABLE XLYII Last and First Occurrence of a Temperature of 32° or Freezing Point, 1871-1912 (See Fig. 16) Year Last in Spring First in Autumn Interval in Days Departure from Average Year Last in Spring First in Autumn jInterval in | Days Departure from Average j Mar. April May j Sept. O O Nov. ] Mar. April May •4^ ft a> CO *+^> O o Nov. 1871 23 27 217 -1-24° 1896 9 19 192 - 1° 1872 16 10 176 -17° 1897 20 6 199 + 6° 1873 30 23 175 -18° 1898 6 23 199 + 6° 1874 24 31 189 - 4° 1899 8 30 174 —19° 1875 2 30 180 -13° 1900 13 8 208 +15° 1876 30 15 167 -26° 1901 18 3 198 + 5° 1877 30 3 186 - 7° 1902 8 23 228 +35° 1878 25 27 215 +22° 1903 5 6 214 +21° 1879 4 31 209 +16° 1904 21 11 203 +10° 1880 12 17 187 - 6° 1905 16 28 194 + 1° 1881 14 14 213 +20° 1906 31 10 192 - 1° 1882 12 12 213 +20° 1907 26 10 197 + 4° 1883 24 1 190 — 3° 1908 3 5 215 4-22° 1884 3 23 202 + 9° 1909 1 12 163' -30° 1885....:.. 13 13 213 +20° 1910 24 28 186 - 7° 1886 8 i 175 -18° 1911* 7 27 202 + 9° 1887 26 12 168 -25° 1912* 18 1 196 + 3° i Q 1 U 39° 1889 6 ' '4' 211 +18° Average 1890 14 30 198 + 5° date: 1891 8 1 206 +13° 1871-80.. 16 24 190 1892 24 30 188 - 5° 1881-90.. 15 27 194 1893 22 28 188 - 5° 1891-1900 16 24 190 1894 9 14 187 - 6° 1901-10.. 15 1 199 1895 14 20 158 -35° 1871-1910 16 27 193 * Not included in means. Both this record of the first and last occurrence of freezing temperature, and that in the previous paragraph on killing frosts, bear out all that has previously been stated regarding the influence of Lake Michigan in tending to maintain moderate temperatures near its shore. The average growing season at LaGrange, whose temperatures have been compared with those of Chicago (p. 46), as determined from its record of killing frosts, is an interval of 163 days,114 THE WEATHER AND CLIMATE OF CHICAGO and extends from May 1 to October 11; whereas, as shown in Tables XLVI and XLVII, in the city the period of freedom from killing frosts is 19 days longer, extending from April 18 to October 18. The average interval between the occurrence of last and first actual temperature of 32° in Chicago is 11 days longer than its season between killing 1871 187É» 1861 1886 i89l 1696 1901 1906 MgAN Fig. 16.—Interval between last and first occurrence of a temperature of 32°, 1871-1910. Fig. 16 shows the time of occurrence of the last freezing temperature in spring and the first in autumn; also the length of the intervening period. The lowest line marked “mean” shows the date of the average occurrence and the average length of the intervening period (see Table XLVII). frosts, but is fully 30 days longer than the record for LaGrange. Similarly, none of the stations in northern Illinois appearing in Table XXI (p. 54) has as long a season free from killing frosts as does Chicago (see Bulletin W, Summary of Climatological Data for the United States, sec. 64, p. 6). RANGE IN TEMPERATURE The difference between the maximum and minimum temperatures of any place is called the range in temperature for that point, and may be either absolute—that is, calculated for individual years, months, or days—or mean—that is, based upon the average maximum and minimum values. The term indicates the amount of temperatureTEMPERATURE 115 change experienced in a locality, and it is therefore an important factor in the discussion of climate and weather. In Table XLVIII the absolute annual range for Chicago is given in the last column for the entire period of official record, and the mean for the year shows an average annual fluctuation through 106?3. As the highest tempera- TABLE XLVIII Absolute Monthly and Annual Ranges in Temperature, 1871-1913 Year Jan. Feb. Mar. April May > June July Aug. Sept. Oct. Nov. Dec. Great- est Abso- lute Annual 1871 52 50 35 44 46 31 33 36 47 49 52 53 53 100 1872 ,... 57 54 37 54 45 47 45 37 56 53 61 69 69 121 1873 67 65 72 58 52 48 43 39 47 52 51 47 72 111 1874 66 47 47 45 51 49 39 40 45 48 72 51 72 105 1875. 64 58 64 55 52 49 32 34 47 43 57 69 69 109 1876 61 66 59 38 52 41 36 38 41 45 50 59 66 107 1877 60 37 60 51 53 42 34 34 42 45 44 45 60 95 1878 50 38 43 39 40 35 38 34 44 52 26 55 55 106 1879.. 67 57 55 63 48 44 33 39 44 56 53 64 67 111 1880 42 51 41 53 50 39 38 40 45 50 62 65 65 110 1881 54 43 37 60 50 43 36 40 45 36 50 46 60 111 1882 57 52 41 41 42 46 35 36 45 37 51 52 57 97 1883 57 66 52 50 44 36 40 35 42 40 52 57 66 108 1884 67 56 60 46 38 39 35 40 38 55 59 72 72 109 1885 63 61 54 49 46 46 41 30 34 33 38 52 63 108 1886 62 62 55 58 42 38 39 39 44 47 53 70 70 108 1887 67 65 59 63 44 48 39 51 54 68 68 58 68 115 1888 61 65 65 53 49 47 38 40 52 44 55 38 65 112 1889.. 55 59 48 44 52 44 36 34 49 44 45 49 59 101 1890 67 56 56 47 52 40 37 45 49 45 40 45 67 101 1891 44 66 50 52 46 44 32 47 43 53 57 48 66 104 1892 60 47 48 55 39 48 41 43 42 48 46 67 67 104 1893 62 55 60 57 46 37 34 41 56 55 63 64 64 111 1894 69 52 65 53 53 53 42 47 46 44 50 58 69 105 1895 60 76 71 52 62 45 42 35 56 47 59 56 76 110 1896 59 67 52 66 43 39 37 44 48 46 64 49 67 107 1897 77 50 41 54 48 49 39 36 52 49 59 59 77 115 1898 49 68 47 57 42 38 35 34 43 45 65 46 68 102 1899 61 70 61 70 41 41 31 28 66 48 33 59 70 119 1900 64 71 56 51 50 40 37 30 49 45 49 51 71 103 1901 59 36 67 53 46 54 47 32 47 52 50 69 69 115 1902 58 55 62 60 50 43 36 31 39 41 44 51 62 99 1903 57 60 61 50 52 46 • 33 37 46 50 51 53 61 105 1904. 54 59 52 56 49 40 41 35 40 44 48 52 59 109 1905 53 65 60 47 46 42 39 . 30 34 54 55 45 65 113 1906 52 48 46 . 46 54 42 33 36 34 48 39 46 54 87 1907 62 55 57 47 49 44 35 38 53 46 31 39 62 95 1908 49 52 50 52 53 46 36 38 56 48 41 43 56 98 1909 75 45 41 49 54 39 37 34 42 47 43 62 75 103 1910 51 57 55 60 40 48 35 36 34 57 47 36 60 103 1911* 54 50 60 43 61 43 49 33 35 47 62 55 62 102 1912* 54 57 50 44 49 38 35 40 55 46 45 54 57 111 1913* 55 64 69 48 47 55 40 37 59 56 52 39 69 103 Means 59.3 56.6 53.6 52.4 47.8 43.2 37.2 37.3 45.9 47.7 50.8 54.2 65.3 106.3 Greatest monthly range 77 76 72 70 62 55 49 51 66 68 72 72 77 121 Least monthly range 42 36 35 38 38 31 31 28 34 33 26 36 26 87 * Not included in means. Table XLVIII shows the absolute monthly range of temperature, that is, the difference between the absolute monthly maximum and the absolute monthly minimum; also the absolute monthly range for each year, and the absolute annual range, the latter being the difference between the lowest minimum and highest maximum during each year (see Tables XXV and XXVI).116 THE WEATHER AND CLIMATE OF CHICAGO SUMMARY OF TABLE XLVIII Extreme Monthly Ranges in Temperature, 1871-1913 (See Fig. 18) Month Year Range Maximum Minimum January 1897 77 57 —20 February 1895 76 61 -15 March 1873 72 60 — 12 April 1899 70 88 18 May 1895 62 94 32 June 1913 55 99 44 July 1911 49 102 53 August 1887 51 98 47 September 1899 66 98 32 October 1887 68 82 14 November 1874 72 72 0 December 1884 72 61 -11 ture on record for Chicago is 103° and the lowest —23° (Table XLIII), the recorded absolute extreme for the city is 126°, but this extreme range for the whole period of observations has never been experienced in any one year. In 1872, however, the range reached 121°, and there are a number of years in which it exceeded 110. In only 6 years of the record has the range been less than 100°, the least being a fluctuation through only 87° in 1906, from a maximum of 93° to a minimum of 6°. Fig. 17 shows graphically the annual range of temperature at Chicago, together with seven other cities of the United States, roughly in meridian and.parallel arrangement, for a period of 35 years ending with 1905, and serves to illustrate the variation in the annual oscillation of temperature in the different sections of the country represented. The average annual range at Chicago for this period was 108°, slightly higher than that shown in Table XLVIII, as the generally higher minimum temperatures of the last five years of record have reduced the mean range by nearly 2°, but this does not affect the comparison with the other cities shown, the records being all for the same period. From the upper portion of the figure it will be seen that the annual range in temperature steadily decreases from north to south, that Duluth experiences annually fluctuations nearly equal to the greatest yearly range in the record of Chicago, that at Memphis the average range of the year is just that of Chicago’s lowest record, while at New Orleans the amplitude is less than Chicago’s average by 38°. The extreme cold of winter in the northern sections and the high summer temperatures produced by long hours of sunshine falling upon the great land expanse of the level plains states results in a far greater annual range than is found in any region to the southward. In theTEMPERATURE 117 lower portion of the figure the effect of the westerly winds blowing from the Pacific Ocean may be seen in the very small average yearly range for San Francisco, which is less than half that for Chicago. At New York, while proximity to the Atlantic Ocean has reduced the annual range considerably, the effect is not nearly so great as is that of the Pacific along the western coast, because the prevailing winds at New York blow from the landward side; yet the ocean influence is sufficient to bring the average yearly range 14° under the 120° Duluth, Minn. Chicago, 111. 108° MomphiB, Tenn. 87r New Orleans, La. 70° _ 113® , Denver, Colo. Chicaf&oIn- New YOrk. N. Y. 94® San Francisco, Cal. 53° Fig. 17.—Annual range in temperature, 1875-1905. Fig. 17 shows the annual range in temperature of certain selected cities. The upper graph shows the range for cities passing from north to south from Duluth to New Orleans and including Chicago and Memphis, while the lower graph shows the range for cities from east to west and including New York, Chicago, Denver, and San Francisco. range for Chicago. Denver experiences annually fluctuations that average 5° greater than those in this city, and the greater range is due to the higher altitude and the rarer atmosphere in the case of the former, and the moderating influence of Lake Michigan in the case of the latter. While the annual range at Chicago is considerable, it is not as great, as a rule, as that observed on the Great Plains and in the eastern foothills of the Rockies; and it is, in fact, of such an extent as rather to stimulate the energies and activities of its people than otherwise.118 THE WEATHER AND CLIMATE OF CHICAGO Table XLVIII also contains the absolute range in temperature for each month since 1871, that is, the difference between the absolute maximum and the absolute minimum for the month. Just as the daily changes in mean temperature are greater in winter than in summer (p. 30), so are the mean monthly ranges in temperature, the average range for January being 59?3 and that for July 37?2. The greatest range in any one month was 77°, in January, 1897; and the least, 26°, curiously enough occurred in November, 1878, and is 2° 710° <00° 50° 40° 30* 20* 10° Ö* 1 1 1 1 1 1 1 1 1 1 1 1 1 1 70# Q><$ 50* 40* 30° 20° 10* O* Fig. 18.—Greatest monthly range of temperature, 1871-1913 (see summary of Table XLVIII). lowér than the least range for any of the summer months. The greatest absolute range for the various months of the year is given in Fig. 18, and in the summary of the table, the latter also showing the highest and lowest temperatures and the years of the months in question. The absolute daily range in temperature for the different days of the year has already been pointed out in connection with the discussion of daily extremes (p. 93), so that no further treatment of that phase of the subject is needed here. Table XLIX, however, givesTEMPERATURE 119 TABLE XLIX Greatest Daily Ranges in Temperature, Monthly and Annual, 1873-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1873 39 51 35 37 42 28 27 26 33 27 28 25 51 1874. 25 25 22 26 31 27 33 23 22 31 34 31 34 1875 42 47 29 27 27 40 27 21 26 27 24 31 47 1876 44 32 30 28 22 27 22 20 20 28 25 30 44 1877 31 28 33 30 24 25 23 26 26 25 31 22 33 1878 26 21 30 23 20 20 20 21 30 24 26 21 30 1879 30 27 29 25 38 26 21 21 30 22 22 34 38 1880 27 25 26 31 33 21 20 24 22 28 24 38 38 1881 39 26 17 27 31 30 26 22 32 24 25 19 39 1882 29 24 28 33 28 28 20 20 24 21 32 23 33 1883 36 35 31 26 28 22 28 21 22 24 36 28 36 1884 25 34 26 33 31 24 23 19 24 24 42 44 44 1885 30 33 29 28 29 29 33 22 22 20 24 30 33 1886 37 40 32 25 34 23 23 23 27 23 37 30 40 1887 43 40 33 45 33 27 30 30 28 26 42 34 45 1888 51 29 30 36 35 35 28 27 28 32 23 26 51 1889 27 30 30 27 40 26 20 26 25 28 23 33 40 1890 29 22 26 35 35 29 2A 27 23 21 33 25 35 1891 21 30 32 30 30 32 22 23 27 28 28 24 32 1892 19 32 30 31 28 29 21 23 28 28 22 20 32 1893 23 29 33 45 29 21 27 26 29 28 24 30 45 1894 23 23 29 34 35 36 31 26 23 30 30 21 36 1895 36 24 49 40 40 29 23 26 30 29 27 23 49 1896 25 24 24 36 33 26 25 25 25 25 44 22 44 1897 30 30 23 31 38 27 24 28 28 26 26 31 38 1898 25 28 30 28 30 28 30 24 20 23 37 32 37 1899 33 25 27 28 38 31 22 21 28 27 24 22 38 1900 29 52 27 27 36 22 25 24 23 29 24 30 52 1901 27 19 33 21 43 31 34 27 26 28 34 41 43 1902 35 23 25 36 31 34 25 24 30 23 22 26 36 1903 28 24 32 36 29 23 21 22 29 23 24 32 36 1904 27 42 37 28 29 29 23 24 26 23 25 32 42 1905 30 28 32 33 28 40 24 27 27 31 31 18 40 1906 31 27 23 31 27 25 22 21 19 30 26 39 39 1907 31 37 38 31 34 23 25 21 21 29 17 21 38 ^908 25 29 36 29 33 25 23 23 35 27 27 26 36 909 42 32 23 29 35 30 32 21 20 24 28 36 42 l910 ........ 27 26 36 37 29 24 22 23 24 25 29 20 37 1911* 34 23 32 18 35 31 24 20 24 25 42 27 42 1912* 29 19 25 31 39 33 24 27 26 23 20 26 39 1913* 32 26 33 29 29 41 29 25 24 21 30 29 41 Means 31.0 30.3 30.0 31.1 32.0 27.7 24.9 23.6 25.8 26.1 28.4 28.1 39.5 * Not included in means. Table XLIX shows the greatest daily range of temperature, that is, the difference between the maximum and minimum for any one day, for each month and year. SUMMARY OF TABLE XLIX Extreme Daily Range in Temperature, 1873-1913 Month January......... February........ March......, — April........... May............. June............ July............ August.......... September....... October........ November....... December........ Year........... Day 13 8 29 J12 \ 7 2 6 10 10 27 31 27 31 Feb. 8 Year 1888 1900 1895 1887 1893 1901 1913 1901 1887 1908 1888 1896 1884 Range 51° 52° 49° 45° 43° 41° 34° 30° 35° 32° 44° 44° Maximum 42° 62° 80° (80° \84° 87° 91° 102° 98° 80° 76° 58° 59° Minimum - 9° 10° 31° 35° 39° 44° 50° 68° 68° 45° 44° 14° 15° 1900 52° 62° 10°120 THE WEATHER AND CLIMATE OF CHICAGO the greatest range occurring on any one day for the various months of the period of record. Were it not for the location of Chicago upon the shore of the lake, whose waters in winter are relatively warmer than the air, the daily ranges of the colder season could confidently be expected to be the greatest of any throughout the year. As a matter of fact, they are considerably greater than the daily ranges of mid-summer, but yet are not as great as those occurring in the months of April, May, and June. This greater fluctuation, due to the cool lake breezes (p. 43), is readily seen in the mean values at the bottom of the table, and also from the mean daily ranges in temperature based upon the range of every day in the official record, as given in Auxiliary Table D. AUXILIARY TABLE D Mean Daily Range in Temperature January 14?3 May 15?6 September 13?9 February 14?2 June 14? 9 October 13?9 March 13?6 July 14? 1 November 13?3 April 14?6 August 12?8 December 13?0 For the year.....................13?9 This greater fluctuation in April, May, and June is not because there are greater individual ranges in the spring than occur in January and February, but rather because the well-marked changes are more frequent at that time of the year. Great ranges during the winter months are usually caused by the advance of general cold waves which have escaped the moderating influence of the lake, as was the case with the large ranges of February, 1900, and January, 1888. In the spring, however, the greatest ranges are caused by a shifting of the wind from land to lake at a time when the temperature has risen to a markedly high degree. Such conditions occur every year in the spring months, because the water in the lake is then much colder than the air, and the difference is greater than at any other season; while in winter, months occasionally pass without any decided temperature changes. The amount of range in temperature, of course, does not give any indication of the character of a month with respect to warmth or cold, but does describe the period with regard to changeableness. In fact, a very cold or very warm month will often have but little range in temperature if the cold or warmth has been persistent. For instance, in January, 1880, which was the warmest January on record at Chicago, the greatest daily range in temperature was onlyTEMPERATURE 121 27°, and the absolute range for the month was but 42°. There are, however, instances which present altogether different relations between the mean temperature and the range. In April, 1911, a very nearly normal month with respect to mean temperature, the greatest daily range was only 18°, and the absolute range was only 43°. There were but two other months in the history of the Weather Bureau office in Chicago in which at least one day did not exceed the record of April, 1911, in greatest daily range. Range is there- fore indicative rather of evenness of temperature than of the actual degree experienced, and in comparisons serves to point out equable climates. Fig. 19 illustrates the widely varying character of temperature changes which have been experienced in Chicago. The curved line shows the hourly course of temperature on February 8,1900, when the greatest daily range on record occurred, 52°, from a maximum of 62° to a minimum of 10°. This change actually occurred between the122 THE WEATHER AND CLIMATE OF CHICAGO hours of 8 a.m. and midnight, and did not require the entire space of the day to accomplish the fall. On the same graph, the straight line shows a temperature of 32° at every hour on March 24,1891, there being not a single degree of variation throughout that time. Because of a strong northeast wind during the whole period on the latter date, and a totally clouded sky which shielded the atmosphere from the rays of the sun, the air assumed the temperature of the water in the lake, then at the freezing point, and maintained it from one end of the day to the other. It is the only instance of the kind within the Weather Bureau record, and is therefore the day of least range in temperature. FREQUENCY OF MARKED RISES AND FALLS OF 20° OR MORE IN TEMPERATURE 1. Within twenty-four hours.—In the discussion of changes in temperature of stated amounts (p. 31) the mean temperature of the day was compared with that of the day following, while in the discussion of ranges just preceding this paragraph the change of temperature within the actual twenty-four hours of the day was used as a basis. A marked fall or rise, however, not infrequently extends from one day into the next, and the range for either day or the mean temperatures for both days, may give an entirely wrong idea of the real course of the temperature. For this reason the Weather Bureau has, since shortly after the beginning of its hourly records in 1890, taken note of marked changes in temperature for any period of twenty-four hours, whether such change occurs in one day or in portions of two days, and such portions of these data as are important to an understanding of Chicago's weather and climate are presented here. Tables L and LI and Figs. 20 and 21 show the frequency of 24-hour rises and falls of 20° or more in temperature by months and years from 1893 to 1910, inclusive. The number of such falls is seen to be much greater than the number of rises, in the ratio of 532 to 348, the average annual occurrences being 29.6 for falls and 19.2 for rises. Ordinarily a rise in temperature is more gradual than a fall. For instance, with the coming of a cold wave the temperature falls rapidly, but after the crest of the wave has passed the change back to normal is usually gradual and requires a considerably longer time than did the drop to the minimum. The greatest number of these marked falls in temperature occurs during the winter and springTEMPERATURE 123 months, just as the greatest daily and monthly ranges have been shown to occur (p. 118). In autumn, however, the number of marked TABLE L Number of 24-Hour Rises in Temperature of 20° or Over, Monthly and Annual, 1893-1910 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1893 3 2 4 4 5 2 3 2 1 2 6 34 1894 2 1 3 2 3 3 1 1 3 19 1895. 3 1 3 3 3 3 4 2 1 23 1896 1 2 1 3 1 3 2 1 14 1897 3 4 2 3 4 2 2 20 1898 2 2 1 3 1 1 i 11 1899 2 2 4 2 5 2 3 1 21 1900 3 3 3 3 2 1 2 1 1 3 22 1901 1 3 2 4 1 1 3 2 17 1902 1 1 2 6 4 1 1 2 18 1903 3 2 2 1 1 1 1 3 14 1904 4 5 3 1 4 2 2 1 2 24 1905 2 5 3 6 4 3 1 1 2 27 1906 3 4 4 5 1 1 1 19 1907 1 2 3 2 4 12 1908 1 2 4 3 3 1 2 16 1909 3 3 1 4 2 1 1 3 18 1910 3 2 5 4 2 1 2 19 Total 36 40 48 48 58 36 8 6 8 18 26 16 348 Average 2.0 2.2 2.7 2.7 3.2 2.0 0.4 0.3 0.4 1.0 1.4 0.9 19.2 TABLE LI Number of 24-Hour Falls in Temperature of 20° or Over, Monthly and Annual, 1893-1910 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1893 4 3 4 4 5 1 3 3 5 3 3 6 44 1894 5 2 4 2 5 3 3 3 2 3 2 3 37 1895 5 2 5 3 4 3 1 3 2 3 2 33 1896 1 2 2 3 3 2 3 2 2 4 1 25 1897 4 1 1 3 3 1 1 3 2 6 2 27 1898 1 2 5 2 4 1 1 i 2 2 2 23 1899 3 2 2 3 3 2 2 4 3 2 26 1900 3 3 2 2 5 3 2 1 4 2 3 3 33 1901 4 3 2 4 2 5 1 1 4 3 2 31 1902 2 1 3 3 3 3 2 1 2 3 2 1 26 1903 2 2 6 3 3 1 2 4 1 3 4 31 1904 1 5 4 2 4 3 2 3 2 2 28 1905 4 3 2 5 5 5 3 3 3 2 35 1906 3 3 5 3 3 1 1 2 1 2 24 1907 4 2 6 3 5 1 1 1 2 2 27 1908 3 1 4 3 2 2 1 3 2 21 1909 4 3 3 1 3 2 1 1 3 4 25 1910 4 2 7 6 3 1 3 1 2 2 2 3 36 Total 57 42 59 55 65 35 33 17 39 40 45 45 532 Average 3.2 2.3 3.3 3.1 3.6 1.9 1.8 0.9 2.2 2.2 2.5 2.5 29.6 The average and total number of both 24-hour temperature rises and falls are graphically shown in Figs. 20 and 21. falls is considerable, and this is to be expected, as the sun is then retreating to the winter solstice and the mean change is from warm to colder weather. So also, with the approach of the sun in spring124 THE WEATHER AND CLIMATE OF CHICAGO JAN. re 8 MAR. APR. MAY. Ol/N. JUL. AUG SEP* OCT NOV. DEC. 4 3 2 1 Rises. 0 Fa I Is. 1 a 3 4 -— — ! j_. ■ IT JL - r JL — " - r - - _T ir kr T_ MM l 1 1 1 l l 1 1 L.J 1 1 1 1 1 1 1 1 II I 1 1 1 1 L 1 L ■ m i j ■ ■ II 1 1 1 1 1 1 dhri ! ! i ■ 1 1 1 ■■ 1 1 ■ II q — i 4 3 2 1 R»ses. 0 Falls 1 2 . 3 4 Fig. 20.—Average number of 24-hour temperature rises and falls of 20° or more. Record of 1893-1910, inclusive. 1993 I89S 1900 190S 1910 Fig. 21.—Annual frequency of 24-hour temperature changes of 20° or more. Record of 1893-1910, inclusive.TEMPERATURE 125 and early summer, marked rises in temperature are far more numerous than those of autumn and the remainder of the summer season. In May both marked rises and marked falls are more frequent than in the other months of the year, with an average occurrence of 3.2 and 3.6, respectively, and this fact, together with its comparatively great variability in mean temperature (p. 16) and its large daily ranges (p. 120) and its response to lake influence (p. 43), marks it as the most changeable month of the year so far as the weather of Chicago is concerned. On the other hand, practically the opposite of all these characteristics show August to be the month of most even temperature conditions, it having only one such marked rise in temperature in about three years, and one such marked fall annually. To be exact, the average yearly occurrence of these rises and falls is 0.3 and 0.9, respectively. Fig. 21 illustrates at a glance the great variation in the number of these changes from year to year, and the large excess in the number of marked falls over the marked rises in temperature. It has been previously pointed out that the temperature rises throughout a considerably shorter period of the year than it falls (p. 26), and as the record of marked falls and rises shows a large excess of changes of the first class, it might hastily be concluded that the statements are inconsistent with each other, as such a condition would indicate a constantly decreasing mean annual temperature, which is not the case (p. 5). It must be remembered in this connection that these records of marked changes include only those in which the temperature rose or fell 20° or more in twenty-four hours, leaving out of consideration altogether the changes in temperature of lesser amounts. Now, as a matter of fact, in changes of the latter class the rises far outnumber the falls, and so counteract the effect of the greater number of instances in which the temperature declines markedly. It is, however, impracticable to present tables showing these lesser changes, as space would have to be given the entire mass of hourly data on temperature. Individually, the year 1893 holds the record for the greatest number of marked rises and falls in temperature, with 34 and 44, respectively. The year 1898 was that of the least number of marked rises, its total being only 11, while the least number of marked falls during the period of record was experienced in 1908, when only 21 occurred. It will be noted from the tables that the warmer months of the year are comparatively free from rises in temperature of 20° or more during any twenty-four hours.126 THE WEATHER AND CLIMATE OF CHICAGO Table LII gives the number of days in each month and year in which these marked rises amounted to from 30° to 39° in any twenty-four hours, and Table LIII shows the actual change in degrees when TABLE LII Monthly and Annual Number op 24-Hour Rises in Temperature op 30° to 39°, 1893-1910 TABLE LIII Number op 24-Hour Rises in Temperature op 40° or Over, 1893-1910, with the Amount in Each Case the rise exceeded or equaled 40°. In only one instance has the temperature risen as much as 48° in any twenty-four hours (March 29-30, 1895), and there are only 4 days in the record which show a rise of 40° or over. Table LIV gives the number of days in each month and year in which the marked falls amounted to from 30° to 39°. Table LV gives the same data for falls of from 40° to 49°, but shows the actualTEMPERATURE 127 amount of change in each case; and in a similar manner Table LVI shows the number of changes in which the fall was 50° or over in twenty-four hours, with the actual amount in degrees. A comparison of Tables LII and LIII with Tables LIY, LV, and LYI, shows TABLE LIV Monthly and Annual Number op 24-Hour Falls in Temperature op 30° to 39°, 1893-1910 TABLE LV Number op 24-Hour Falls in Temperature op 40° to 49°, 1893-1910, with the Amount in Each Case Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1893 45° O 1 O j O O rH 3 1894 44° 1 1895 42° 41742° 3 1896 44° 1 1897 . . o O 1 1898 0 1899 o O -*1 1 1900 45° £>- to o 2 1901 O CO 1 1902 ... 41° 1 1903 47°/42° 45° 3 1904 ... 44° . .40° 2 1905 44° 1 1906 o o 1 1907 o CO Tt< 42° 2 1908 41° 1 1909 co o 1 1910 42° 1 Tnt.a.l. 5 3 4 6 3 2 3 26 plainly how the marked falls in temperature of each class predominate over marked rises of like amounts. In the first class of from 30° to 39° change, the record shows 100 falls to 41 rises during the period of 18 years ; in the class of from 40° to 49° change, 26 falls to 4 rises ; and while there have been 3 instances in which the temperature fell128 THE WEATHER AND CLIMATE OF CHICAGO 50° or more in twenty-four hours, there has not been a single time in which the temperature rose 50° in that time. There has been one occurrence of a fall of more than 60° in twenty-four hours. This happened in November, 1911 (see Fig. 22), after the period shown in the tables, during the advent of the severe cold wave of the llth-12th, chronicled by one of the city newspapers as the one time in the history of Chicago when “ one man was overcome by heat and two others frozen to death within the short space of twenty-four hours.” At 4 p.m. of November 11, 1911, the official thermometers stood at 74°. At 10 a.m. on the following morning the mercury had dropped through 60°, and by 12:30 p.m., 20| hours after its highest point of the day before, the instruments showed 13°, making the TABLE LVI Number of 24-Hour Falls in Temperature of 50° or Over, 1893-1910, with the Amount in Each Case record fall of 61p during any 24-hour period covered by the observations of the Weather Bureau. Never before had a November 11 been so warm, and never before had a November 12 been so cold. No change of 30° or over has been observed in August in twenty-four hours, and the months of June to October, inclusive, appear to be exempt from any such changes of 40° or more. Figs. 22 and 23 illustrate the hourly course of temperature during the great changes shown in Table LVI and of three of the four shown in Table LIII. The great fall in November, 1911, has been added to the former, and a marked rise in March, 1903, to the latter figure. The general similarity of movement in the temperature change during these pronounced rises and falls can be seen at aTEMPERATURE 129 glance. Usually the movement begins between sunrise and sunset of the first day, the maximum of the rise being reached during the afternoon of the second day, and the minimum of the fall during the early morning hours. The influence of night radiation in the case of the fall, and of the sun’s rays during the following morning in the Mdt. £ 4 © 8 10 Noon 2 4 S 8 to Mdt- Z 4 6 8 /O Noon 2 4 6 8 IO Mdt Fig. 22.—Greatest falls in temperature in any twenty-four consecutive hours or less, 1893-1911. » November 11-12,1911, fall of 60°—'from 74° to 14° in 18 hrs., 4:00 p.m. to 10:00 a.m., and to 13° at 12:30 p.m., a total fall of 61°. .. February 8-9, 1900, fall of 58°—from 62° to 4° in 22 hrs., 8:00 a.m. to 6:00 a.m. — — — December 13-14, 1901, fall of 56°—from 48° to —8° in 23 hrs., 7:00 a.m. to 6:00 a.m. ™ « November 21-22, 1898, fall of 50°—from 66° to 16° in 15 hrs., 4:00 p.m. to 7:00 a.m. case of the rise, is quite plain from a study of the graphs, although, of course, the chief factor of control is the movement of the general storm areas past the city, with the attendant shifting winds that import air of greatly different temperatures. Tables. LVII and LYIII contain the greatest rises and falls in temperature for each month of the period of hourly record, when130 THE WEATHER AND CLIMATE OF CHICAGO such change has amounted to 20° or more, and has occurred in any 24-hour period or less. The data have no bearing on the greatest daily range for the month, as shown in Table XLIX, as already explained (p. 122). Mdt. 2 4 6 8 jo Noon 2 4 6 8 to Mdt.2 4 6 8 10 Noon 2 4 6 8 lo Mdt. Fig. 23.—Greatest rises in temperature in any twenty-four consecutive hours or less, 1893-1910. mmmmmm March 28-29, 1895, rise of 48°—from 31° to 79° in 22 hrs., 6:00 p.m. to 4:00 p.m. «* ■> /■> April 6-7, 1893, rise of 43°—from 41° to 84° in 11 hrs., 5:00 a.m. to 4:00 p.m. »■ - ■ - December 26-27, 1903, rise of 40°—from —6° to 34° in 23 hrs., 11:00 a.m. to 10:00 a.m. ----— March 17-18, 1903, rise of 38°—from 36° to 74° in 19 hrs., 7:00 p.m. to 2:00 p.m. 2. Within one hour.—It sometimes happens in the passage of storm areas, when conditions are such as to shift the wind suddenly, that much of the marked change in temperature ensuing occurs within a very short time, and the fall or rise thereafter to the extreme is much more gradual. Table LIX contains a list of the dates on which the temperature rose 15° or more within one hour, and Table LX shows the dates on which the temperature fell 20° or more withinTEMPERATURE 131 one hour, from 1902 to 1911, inclusive. As rises in temperature are not so rapid as pronounced falls, the limit of 15° has been taken for Table LIX instead of 20°, a rise of the latter amount having occurred TABLE LVII Greatest 24-Hour Falls in Temperature, Monthly and Annual (Degrees), 1893-1910 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Great- est 1893 31 45 40 40 34 32 21 23 25 27 29 36 45 1894 27 23 31 25 44 36 26 25 24 24 27 23 44 1895 42 22 42 30 33 22 20 32 29 24 24 42 1896 34 27 31 33 26 22 31 27 27 44 20 44 1897 39 20 23 40 33 24 20 24 32 38 29 40 1898 23 27 35 33 29 21 21 22 23 50 35 50 1899 38 35 38 40 34 27 25 28 28 27 40 1900 45 58 22 27 34 27 23 22 33 28 26 29 58 1901 33 23 33 43 24 34 24 20 35 33 56 56 1902 41 23 35 35 39 31 26 21 27 24 25 26 41 1903 33 25 37 47 28 20 24 26 24 29 45 47 1904 29 44 29 38 40 27 26 29 23 36 44 1905 30 32 33 35 25 36 20 27 44 21 44 1906 33 27 • 31 32 29 22 20 31 28 40 40 1907 43 38 42 33 38 20 22 24 27 27 43 1908 33 29 33 38 24 26 31 27 41 41 1909 43 30 27 21 31 28 21 20 27 36 43 1910 31 32 28 42 27 24 22 23 23 29 26 26 42 Greatest.... 45 58 42 47 44 36 34 27 33 35 50 56 58 No entry unless fall of 20° is recorded. TABLE LVIII Greatest 24-Hour Rises in Temperature, Monthly and Annual (Degrees), 1893-1910 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Great- est 1893 25 33 30 44 25 22 25 24 23 24 29 44 1894 23 28 25 26 28 30 22 20 31 31 1895 24 20 48 39 41 23 26 24 20 48 1896 21 24- 23 28 28 24 22 24 28 1897 29 25 30 25 24 21 23 30 1898 25 28 30 30 25 20 26 30 1899 26 29 24 28 31 32 25 28 32 1900 25 21 29 24 22 30 24 24 24 24 30 1901 32 25 30 29 21 21 24 25 32 1902 20 23 28 32 28 20 22 30 32 1903 23 38 28 24 22 23 25 40 40 1904 27 35 30 29 33 30 23 22 27 35 1905 25 24 28 31 33 24 20 24 27 33 1906 31 24 24 26 20 22 25 31 1907 20 24 38 23 37 38 1908 22 25 31 29 28 21 22 31 1909 22 26 25 30 39 26 22 24 39 1910. 30 22 30 33 29 23 24 33 Greatest.... 32 35 48 44 41 32 25 24 25 28 31 40 48 No entry unless rise of 20° is recorded. in one hour only once in the period of 10 years shown. This instance occurred on March 26, 1907, from 3:15 to 4:15 p.m., when the rise was from 45° to 65°, and is the most pronounced rise of the record.132 THE WEATHER AND CLIMATE OF CHICAGO The change was due to a shifting of the wind from the lake to a southerly direction. The rise on May 10, 1911, was occasioned by similar conditions, the temperature rising 18° during the first hour, and continuing thereafter at such a rate that at the close of the second hour the total rise amounted to 27°. When the change in temperature is due to lake effect, by far the greater portion, whether rise or fall, occurs during the first hour. Marked falls in temperature, TABLE LIX Temperature Rises of 15° or Over in 1 Hour, 1902-11 Date Time Temperature Rise (Degrees) From To Length From To Amount 1902 June 12 10:00 a.m. 11:00 a.m. 1 hr. 65 80 15 1907 March 26 3:15 p.m. 4:15 p.m. 1 hr. 45 65 20 April 24 3:00 p.m. 4:00 p.m. 1 hr. 52 70 18 1908 March 27 7:50 p.m. 8:50 p.m. 1 hr. 46 62 16 1909 April 6 10:30 a.m. 11:30 a.m. 1 hr. 45 62 17 1910 May 22 12:00 M. 1:00 p.m. 1 hr. 61 76 15 1911 May 10 8:50 a.m. 9:50 a.m. 1 hr. 56 74 18 May 10 8:50 a.m. 10:50 a.m. 2 hrs. 56 83 27 TABLE LX Temperature Falls of 20° or Over in 1 Hour, 1902-11 Date Time Temperature Fall (Degrees) From To Length From To Amount 1902 May 3 4:00 p.m. 5:00 p.m. 1 hr. 76 56 20 June 10 1:00 p.m. 2:00 p.m. 1 hr. 80 60 20 June 15 12:00 m. 1:00 p.m. 1 hr. 86 66 20 1904 May 26 12:00 M. 1:00 p.m. 1 hr. 65 45 20 1905 March 31 1:00 p.m. 2:00 p.m. 1 hr. 67 46 21 1906 May 26 7:45 a.m. 8:45 a.m. 1 hr. 71 46 25 June 29 2:30 p.m. 3:30 p.m. 1 hr. 89 67 22 1907 May 26 4:00 p.m. 5:00 p.m. 1 hr. 70 , 50 20 August 1 2:55 p.m. 3:05 p.m. 10 min. 82 63 19 1908 March 26 1:00 p.m. 2:00 p.m. 1 hr. 71 41 30 April 6 7:00 p.m. 8:00 p.m. 1 hr. 63 43 20 May 12 11:00 p.m. 12:00 MDT. 1 hr. 72 49 23 1910 April 11 11:00 a.m. 12:00 M. 1 hr. 78 50 28 1911 June 4 8:50 p.m. 9:50 p.m. 1 hr. 85 65 20 July 15 6:10 p.m. 7:10 p.m. 1 hr. 89 68 21 as will be seen from Table LX, are far more abrupt and frequent than the rises, there being a number of dates in the 10-year period when the fall exceeded 20° in one hour. The greatest change of this kind occurred on March 26, 1908, when the temperature fell 30° between 1 and 2 p.m., from 71° to 41°. The following data, giving further particulars of the days on which the most pronounced short-time changes occurred, will furnish the necessary information for a more extended study of the subject:TEMPERATURE 133 May 10, 1911. The greatest rise in temperature was 27° in two hours, from 8:50 to 10:50 a.m., rising from 56° to 83°. The rise thereafter was gradual, reaching 87° at 2 p.m. The wind had been easterly up to 8 a.m., but shifted to south during the following hour. The minimum temperature was 52° at 5 a.m., making the range for the day 35°. Mai^ch 26, 1908. The temperature fell 30° from 71° at 1 p.m. to 41° at 2 p.m., continuing until 37° was reached at 5 p.m., thus making a drop of 34° in 4 hours. The minimum was 35° at midnight, and the range for the day therefore 36°. The wind was high from the southwest during the morning, but changed to north shortly after 1 p.m., and continued with only slightly diminished velocity. April 11, 1910. The temperature fell 28° from 78° at 11 a.m. to 50° at 12 noon, continuing until 46° was reached at 3 p.m., thus making a drop of 32° in 4 hours. The minimum was 41° at midnight, and the range for the day therefore 37°. The wind was light from the south and west up to 11 a.m., when it changed to north and increased in velocity, reaching a maximum of 30 miles an hour at 11:50 a.m. These great rises and falls in temperature within a single hour, as shown in the tables, occur almost invariably in the spring and early summer. The changes in twenty-four hours accompanying the cold waves of the winter season are far greater than those of any other portion of the year, but the fall hour by hour is usually small as compared with the changes due to shifting winds in spring. For example, in February, 1900, the temperature fell 58° on the 8th-9th, from 62° at 8 a.m. of the 8th to 4° at 4 a.m. of the 9th, yet the greatest hourly change was but 13°, and that amount of fall occurred only once, the next largest being 9°, with none other greater than 4°. During the cold wave of November 11-12, 1911 (p. 128), the greatest of any of the hourly falls was but 12°. HOURLY TEMPERATURE CONDITIONS 1. Mean hourly temperatures by months.—The daily cycle of change from the highest temperature of the afternoon to the lowest * during the early morning hours, and back again, is familiar to everyone. Broken as is this cycle by the passage of storm areas, it is sufficiently distinct to impress even the most casual observer of weather conditions, and when sought in the average values of temperature shows a surprisingly even daily movement and regularity of oscillation through the months of the year, as will be apparent from a study of Table LXI and Fig. 24, which have been prepared from the hourly temperature records from 1890 to 1910, inclusive.CO Period January. . February. March... April___ May...... June..... July.... August... September October.. November December. Spring___ Summer.. Autumn.. Winter... Annual. .. TABLE LXI Mean Hourly Temperatures, 1890-1910 Hour Ending 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 23.9 23.7 23.3 23.0 22.8 22.7 22.8 22.7 23.2 24 2 22.5 22.0 21.6 21.3 21.0 20.8 20.9 21.1 21.9 23.0 33.9 33.4 33.0 32.4 32.3 32.1 32.4 33.0 34.0 35.4 44.6 44.1 43.6 43.0 42.7 42.5 43.4 44.4 45.5 46.5 54.1 53.5 53.1 52.5 52.1 52.2 53.7 55.3 56.3 57.4 64.1 63.6 63.0 62.5 62.2 62.7 64.4 65.8 67.2 68.1 69.8 69.2 68.6 68.0 67.7 68.2 69.7 71.2 72.7 73.7 69.2 68.5 67.9 67.4 66.9 67.0 68.3 70.0 71.5 72.5 63.4 62.7 62.1 61.4 60.8 60.5 61.3 62.8 64.6 66.3 51.7 51.1 50.5 49.9 49.4 49.1 49.4 50.5 52.2 54.1 38.1 37.6 37.2 36.7 36.5 36.3 36.4 36.7 37.7 39.3 27.1 26.8 26.6 26.3 26.1 25.9 26.1 26.1 26.6 27.7 44.2 43.7 43.2 42.6 42.4 42.3 43.2 44.2 45.3 46.4 67.7 67.1 66.5 66.0 65.6 65.8 67.5 69.0 70.5 71.4 51.1 50.3 49.9 49.1 48.9 48.6 49.0 50.0 51.5 53.2 24.5 24.0 23.8 23.5 23.3 23.1 23.3 23.3 23.9 25.0 46.9 46.4 45.9 45.4 45.0 45.0 45.8 46.6 47.8 49.0 11:00 25.3 24.2 36.0 47.4 58.2 68.9 74.5 73.4 67.6 55.5 40.5 28.9 47.2 72.3 54.5 26.1 50.0 Noon 26.3 25.2 36.7 48.0 58.6 69.4 74.9 74.0 68.4 56.4 41.5 29.8 47.8 72.8 55.4 27.1 50.8 1:00 27.0 26.0 37.1 48.4 59.3 70.0 75.4 74.4 68.9 57.2 42.3 30.5 48.3 73.3 56.1 27.8 51.4 2:00 27.6 26.7 37.9 49.0 59.8 70.3 75.7 74.8 69.4 57.8 42.8 31.1 48.9 73/6 56.7 28.5 51.9 3:00 27.9 27.2 38.3 49.3 60.1 70.2 76.0 75.2 69.7 58.2 43.2 31.3 49.2 73.8 57.0 28.8 52.1 Mdt. 2 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 27.8 27.5 26.9 26.5 25.9 25.5 25.1 24.7 24.3 25.0 27.4 27.0 26.4 25.9 25.2 24.7 24.2 23.7 23.1 23.9 38.5 38.2 37.5 37.0 36.4 36.0 35.5 35.0 34.2 35.3 49.3 48.8 48.0 47.2 46.7 46.4 46.1 45.7 45.0 46.1 59.9 59.5 58.5 57.8 56.9 56.3 55.8 55.3 54.6 56.3 70.0 69.6 68.8 67.8 66.9 66.4 65.8 65.2 64.8 66.6 76.0 75.8 75.1 74.2 73.0 72.2 71.6 71.0 70.4 72.3 75.1 74.6 73.9 72.8 72.0 71.5 70.9 70.3 69.8 71.3 69.7 69.2 68.2 67.3 66.5 65.8 65.0 64.4 63.9 65.4 58.0 57.3 56.3 55.5 54.7 54.0 53.3 52.7 52.3 53.6 42.9 42.3 41.5 41.0 40.3 39.7 39.2 38.7 38.5 39.4 31.1 30.6 30.1 29.7 29.1 28.6 28.2 27.8 27.5 28.3 49.2 49.0 48.0 47.3 46.7 46.2 45.8 45.3 44.6 45.9 73.4 73.3 72.6 71.6 70.6 70.0 69.4 68.8 68.3 70.1 56.9 56.3 55.3 54.6 53.8 53.2 52.5 51.9 51.6 52.8 28.8 28.4 27.8 27.4 26.7 26.3 25.8 25.4 25.0 25.7 52.1 51.7 50.9 50.2 49.5 49.8 48.4 47.9 47.4 48.6 daily by readines of flip in * a empera ure ^ mon s’ seasons, and for the year, 1890 1910. These readings are taken from traces of a Richard thermograph, corre readings of the maxmmm and minimum temperature and also by the observed readings at 7:00 a.m, and 7:00 p.m. The hourly values are shown graphically in Fig. 24. THE WEATHER AND CLIMATE OF CHICAGOTEMPERATURE 135 In the discussion of maximum and minimum temperatures (p. 26), it was shown that the highest temperatures of the year occur on July 16, and the lowest on February 1, the mean temperature, of course, occurring once on either side of these two dates. The present table and graph indicate the time of day at which the highest and lowest temperatures usually occur for each of the months of the year, and also the hours on either side at which the mean hourly temperature of the month is experienced. The warmest hours of the year are 3 and 4 p.m. in July, with an average temperature of 76°, and the coldest is 6 a.m. in February, with an average of 20?8. The mean hourly temperature of the month, shown in the right-hand Fig. 24 shows the average distribution of temperature for the day and year, based upon hourly thermograph readings for a period of 21 years, 1890-1910 (see Table LXI). The line inclosing the area of deepest shading shows the time of occurrence of the highest temperatures for the day and year, the shading decreasing with the decrease of temperature, and the area of lightest shading indicating the time of occurrence of the lowest temperatures for the day and year. The dotted lines, S.R. and S.S., show the time of sunrise and sunset. column in the table, is also the mean temperature for the month for the period from 1890 to 1910. The values, however, do not coincide with the means shown in Table I (p. 8) for two reasons: the period of observations on which the values in Table LXI are based is only about one-half the length of that used in Table I; and again, the values of the former are calculated from the hourly temperatures, while those of the latter are calculated from the maximum and minimum readings, and there is a slight difference in result between the two methods (p. 3). Fig. 24 will be found very useful in locating definitely the portions of the day in which any given temperature is likely to occur. For instance, the temperature averages 75° or136 THE WEATHER AND CLIMATE OF CHICAGO over from 1 to 6 p.m. in July, and from 3 to 4 p.m in August, as shown by the 75°-line in the drawing, and an average temperature of between 20° and 25° prevails in January and February from two or three hours before midnight until about noon. The temperatures average more than 60° at all hours of the day throughout June, July, August, and September. In similar manner the daily and yearly prevalence of any temperature can be determined. While the hourly annual and seasonal values have been added to the table, their use in comparisons is limited, especially in the case of spring and autumn, when temperatures are changing rapidly through a wide range. 2. Temperature phases.—It is evident that the hourly temperatures in their movement through the diurnal cycle referred to in th previous paragraph must touch the important values of the day in the following order: minimum, mean, maximum, mean. It is therefore of much consequence that the times throughout the months of the year at which these readings ordinarily occur be determined with greater accuracy than is done in Table LXI. The number of times the mean hourly values in question have occurred at the different hours of the day are set forth in Table LXII, and the average times of occurrence have been equated therefrom, as shown. For the year as a whole, the minimum temperature of the day is reached at 6 a.m., and the temperature rising thereafter passes the daily mean at 9:55 a.m., and continues to the maximum of the day at 3:25 p.m. Thereafter there is a fall past the daily mean at 9:40 p.m. to the minimum* of the following day. It will be seen, however, that throughout the various months these successive phases seldom occur at the mean yearly time, although there is comparatively slight variation in the time of occurrence of the maximum temperature. The. minimum, on the other hand, occurs as early as 4:50 a.m. in June, and as late as 6:35 a.m. in January, varying regularly with the rising of the sun. The apparent discrepancy in December is caused by the occurrence of the mean minimum once at 1 a.m. and once at 2 a.m., which advances the average time considerably, as computed from the data. The variation of the time of occurrence of the mean temperature of the day is also well marked, being much earlier in the summer than in the winter months. Under normal conditions, that is, when not subject to storm and cold wave conditions or otherwise, and when not subject to direct insolation, any given portion of the surface of the earth will coolTABLE LXII Temperature Phases, 1890-1910 Minimum Frequency A.M. 3:00 4:00 5:00 6:00 7:00 8:00 Aver- age Time First Mean Frequency A.M. 8:00 9:00 10:00 11:00 5 17 4 18 1 2 12 8 10 13 2 18 2 8 12 1 7 14 3 15 3 13 8 4 16 1 15 6 11 10 2 8 9 4 Aver- age Time Maximum Frequency P.M. 1:00 2:00 3:00 4:00 5:00 1 13 12 1 6 18 8 16 6 2 7 12 2 4 8 10 4f 6 5 5 3 3 7 13 2 1 2 9 11 3 15 8 2 1 15 8 1 19 4 1 1 4 17 6 1 2 11 10 2 Aver- age Time Second Mean Frequency - Aver- P.M. MDT. age Time 7:00 8:00 9:00 10:00 11:00 12:00 4 12 7 3 10:20 9 9 2$ 10-45 1 4 8 6 4§ 10:30 2t 4 4 9 6 9:25 6 11 5 2 9:15 2 8 7 3 1 8:40 7 11 3 8:50 3 10 10 9:20 13 7 1 9:25 8 13 1 1 9-55 2 11 4 3 2 9:40 2 8 9 4 9:40 0 3 8 8 3 1 9:40 Total Am- pli- tude Jan... Feb.. Mar. April. May.. June.. July. Aug.. Sept. Oct.. Nov. Dec.. 6 3 1 9 15 17 21 13 7 10 16 15 7 1 2 12 21 19 16 12 6:35 6:10 6:00 5:40 5:15 4.50 5:05 5:30 6:00 6:05 6:15 5.35 10:45 10:50 10:15 9:35 9:00 8:40 8:40 9:00 9:25 9:50 10:20 10:30 3:30 3:45 4:00 3:30 3.15 2:45 3:35 3:30 3:30 3:20 3:10 3:00 8:55 9:35 10:00 9:50 10:00 9:55 10:30 10:00 9:30 9:15 8:55 9:25 Year. 12 6:00 9:55 3:25 9:25 * 1 at 1:00 A.M. and 1 at 2:00 a.m. 11 at 12:00 noon. 11 at 1:00 a.m. §1 at 6:00 p.m. Table LXII shows the average time of occurrence of the minimum and maximum temperature of the day for each month and year; the hours in the morning and afternoon when the average temperature of the day is most likely to occur; the frequency of occurrence of these phases at given hours; and the average number of hours between the times of occurrence of the minimum and the maximum temperatures of the day. These values are based upon hourly observations from 1890 to 1910 (see Fig. 24). CO TEMPERATURE138 THE WEATHER AND CLIMATE OF CHICAGO off at a more or less rapid rate, depending upon cloudiness, convection, etc. This loss of heat, represented by the lowering of the temperature, goes on all the time, although the rate may vary considerably, and ordinarily increases rapidly with rising temperature. Normally a minimum temperature will be reached at sunrise or shortly thereafter, the mercury rising when the surface of the ground begins to receive heat by radiation. The amount received is small at first, but increases rapidly as the sun attains altitude, and although the loss of heat proceeds at the usual or even more rapid rate, nevertheless the temperature rises steadily because the amount of heat received exceeds that lost. When the sun reaches the meridian insolation is at a maximum, but thereafter diminishes with increasing rapidity as the sun declines. The temperature, however, continues to rise slowly, and attains its maximum some hours later, when the diminished intensity of insolation is then equal to the rapid losses by radiation, convection, etc. Actual cooling then sets in, notwithstanding that heat continues to be received up to sunset and in quantities that in the morning hours sufficed to cause a rapid rise in temperature. We thus find that although on the average the sun shines during 12 hours or more each day, yet its heating power is not adequate to maintain a rising temperature for more than about 9 hours. Not only is the heat received during the remaining 3 hours lost as rapidly as received, but in addition a portion of that previously stored is carried away by the convection and radiation process. This 3-hour period being added to the 12 hours of darkness during which heating from outside sources is inappreciable gives 15 hours of cooling for the entire day. After the minimum of the yearly temperature is passed, and as the sun advances in its northward course, the loss of temperature each day during the hours of cooling is a little less than the rise of temperature under the influence of the sunshine, so that little by little the daily mean rises, whereas the reverse occurs while the sun is returning southward and daily means are on the average steadily diminishing. 3. Hourly departures from mean hourly temperatures.—The discussion of hourly values in the preceding paragraphs is supplemented by the data contained in Table LXIII, which shows the difference between the mean temperature of any hour and the mean hourly temperature of the month in which it occurs. In this table the leastTABLE LXIII Hourly Departures from the Mean of the Hourly Temperatures, 1890-1910 Hour Ending Month January................ February............... March.................. April.................. May.................... June................... July................... August................. September.............. October................ November............... December............... 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. -1.1 -1.3 -1.7 -2.0 -2.2 -2.3 -2.2 -2.3 -1.8 -0.8 +0.3 +1.3 +2.0 +2.6 +2.9 +2.8 +2.5 +1.9 +1.5 +0.9 +0.5 +0.1 -0.3 -0.7 -1.4 -1.9 -2.3 -2.6 -2.9 -3.1 -3.0 -2.8 -2.0 —0.9 +0.3 +1.3 +2.1 +2.8 +3.3 +3.5 +3.1 +2.5 +2.0 +1.3 +0.8 +0.3 -0.2 -0.8 -1.4 -1.9 -2.3 -2.9 -3.0 -3.2 -2.9 -2.3 -1.3 +Ò. 1 +0.7 +1.4 +1.8 +2.6 +3.0 +3.2 +2.9 +2.2 +1.7 +1.1 +0.7 +0.2 -0.3 -1.1 -1.5 -2.0 -2.5 -3.1 -3.4 -3.6 -2.7 -1.7 -0.6 +0.4 +1.3 +1.9 +2.3 +2.9 +3.2 +3.2 +2.7 +1.9 +1.1 +0.6 +0.3 0.0 -0.4 -1.1 -2.2 -2.8 -3.2 -3.8 -4.2 -4.1 -2.6 -1.0 0.0 +1.1 +2.1 +2.3 +3.0 +3.5 +3.8 +3.6 +3.2 +2.3 +1.5 +0.6 0.0 -0.5 -1.0 -1.7 -2.5 -3.0 -3.6 -4.1 -4.4 -3.9 -2.2 -0.8 +0.6 +1.5 +2.3 +2.8 +3.4 +3.7 +3.6 +3.4 +3.0 +2.2 +1.8 +0.3 -0.2 -0.8 -1.4 -1.8 -2.5 -3.1 -3.7 -4.3 -4.6 -4.1 -2.6 -1.1 +0.4 +1.4 +2.2 +2.6 +3.1 +3.4 +3.7 +3.7 +3.5 +2.8 +1.9 +0.7 -0.1 -0.7 -1.3 -1.9 -2.1 -2.8 -3.4 -3.9 -4.4 -4.3 -3.0 -1.3 +0.2 +1.2 +2.1 +2.7 +3.1 +3.5 +3.9 +3.8 +3.3 +2.6 +1.5 +0.7 +0.2 -0.4 -1.0 -1.5 -2.0 -2.7 -3.3 -4.0 -4.6 -4.9 -4.1 -2.6 -0.8 +0.9 +2.2 +3.0 +3.5 +4.0 +4.3 +4.3 +3'.8 +2.8 +1.9 +1.1 +0.4 -0.4 -1.0 -1.5 -1.9 -2.5 -3.1 -3.7 -4.2 -4.5 -4.2 -3.1 -1.4 +0.5 +1.9 +2.8 +3.6 +4.2 +4.6 +4.4 +3.7 +2.7 +1.9 +1.1 +0.4 -0.3 -0.9 -1.3 -1.3 -1.8 -2.2 -2.7 -2.9 -3.1 -3.0 -2.7 -1.7 -0.1 +1.1 +2.1 +2.9 +3.4 +3.8 +3.5 +2.9 +2.1 +1.6 +0.9 +0.3 -0.2 -0.7 -0.9 -1.2 -1.5 -1.7 -2.0 -2.2 -2.4 -2.2 -2.2 -1.7 -0.6 +0.6 +1.5 +2.2 +2.8 +3.0 +2.8 +2.3 +1.8 +1.4 +0.8 +0.3 -0.1 -0.5 -0.8 Table LXIII contains the hourly departures from the mean of the hourly temperatures based upon Table LXI. CO CO TEMPERATUREO TABLE LXIV Mean Hourly Changes in Temperature, 1890-1910 Hour Ending MONTH 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. January -0.4 -0.2 -0.4 -0.3 -0.2 -0 1 +0.1 -0.1 +0.5 +1.0 +1.1 +1.0 +0.7 +0.6 +0.3 -0.1 -0.3 -0.4 -0.4 -0.6 -0.4 -0.4 —0 4 —0 4 r ebruary -0.6 —0.5 -0.4 -0.3 -0.3 -0.2 +0.1 +0.2 +0.8 +1.1 +1.2 +1.0 +0.8 +0.7 +0.5 +0.2 -0.4 -0.6 -0.5 -0.7 -0.5 -0.5 —0 5 —0 6 March -0.3 —0.5 -0.4 -0.6 -0.1 -0.2 +0.3 +0.6 +1.0 +1.4 +0.6 +0.7 +0.4 +0.8 +0.4 +0.2 -0.3 -0.7 -0.5 -0.6 -0.4 -0.5 -0 5 -0 8 April —0.4 —0.5 -0.5 -0.6 -0.3 -0.2 +0.9 +1.0 +1.1 +1.0 +0.9 +0.6 +0.4 +0.6 +0.3 0.0 -0.5 -0.8 -0.8 -0.5 -0.3 —0.3 —0 4 —0 7 May —0.5 -0.6 -0.4 -0.6 -0.4 +0.1 +1.5 +1.6 +1.0 +1.1 +0.8 +0.4 +0.7 +0.5 +0.3 -0.2 -0.4 -0.9 -0.8 -0.9 -0.6 -0.5 —0 5 —0 7 June —0.7 —0.5 -0.6 -0.5 -0.3 +0.5 +1.7 +1.4 +1.4 +0.9 +0.8 +0.5 +0.6 +0.3 -0.1 -0.2 -0.4 -0.8 -1.0 -0.9 -0.5 -0.6 —0 6 —0 4 July -0.6 -0.6 -0.6 -0.6 -0.3 +0.5 +1.5 +1.5 +1.5 +1.0 +0.8 +0.4 +0.5 +0.3 +0.3 0.0 -0.2 -0.7 -0.9 -1.2 -0.8 -0.6 —0 6 —0 6 August -0.6 -0.7 -0.6 -0.5 -0.5 +0.1 +1.3 +1.7 +1.5 +1.0 +0.9 +0.6 +0.4 +0.4 +0.4 -0.1 -0.5 -0.7 -1.1 -0.8 -0.5 -0 6 —0 6 —0 5 September —0.5 -0.7 -0.6 -0.7 -0.6 -0.3 +0.8 +1.5 +1.8 +1.7 +1.3 +0.8 +0.5 +0.5 +0.3 0.0 -0.5 -1.0 -0.9 -0.8 —0.7 -0 8 —0 6 —0 5 October -0.6 -0.6 -0.6 -0.6 -0.5 -0.3 +0.3 +1.1 +1.7 +1.9 +1.4 +0.9 +0.8 +0.6 +0.4 -0.2 -0.7 -1.0 -0.8 -0.8 -0.7 -0.7 —0 6 —0 4 November —0.4 —0.5 -0.4 -0.5 -0.2 -0.2 +0.1 +0.3 +1.0 +1.6 +1.2 +1.0 +0.8 +0.5 +0.4 -0.3 -0.6 -0.8 -0.5 -0.7 -0.6 -0.5 —0 5 —0 2 December —0.4 -0.3 -0.2 -0.3 -0.2 -0.2 +0.2 0.0 +0.5 +1.1 +1.2 +0.9 +0.7 +0.6 +0.2 -0.2 -0.5 -0.5 -0.4 -0.6 -0.5 -0.4 -0.4 —Ò.3 Annual -0.5 -0.5 -0.5 -0.5 -0.3 0.0 +0.7 +0.9 +1.2 +1.2 +1.0 +0.7 +0.6 +0.5 +0.3 -0.1 -0.4 -0.7 -0.7 -0.8 -0.5 -0.5 -0.5 -0.5 Table LXIV, showing the hourly rate of change of temperature, is based upon Table LXI, and the changes are shown graphically in Fig. 25. THE WEATHER AND CLIMATE OF CHICAGOTEMPERATURE 141 departures indicate the occurrence of the mean hourly temperature, and the greatest positive and negative departures, the maximum and minimum, respectively. The same change in time from month to month is apparent here as in Tables LXI and LXII. 4. Mean hourly change in temperature.—The change in mean temperature from day to day throughout the year (p. 30) is somewhat similar to the progress from hour to hour in the various months. Table LXIV is based upon Table LXI, and the rate of change is shown graphically in Fig. 25. The rate of increase is most rapid in about 3 hours after sunrise and continues so for a space of from 2 to 3 hours, while the rate of fall is much more gradual, exceeding 1° per hour only once in the early evening during the period from eld t i 234- 5 6 7 8 9 IO ii No on 1 23 4 -S 6 18 9 /OH MeIt. 1 . S R O.S /jO ÌO OS S S. OS' OS .1 ri M J : 0 N t> SR. OS i.o ¡0 OS + «-/■ Mlill Jiiiiii Awmwwmmw iiiiiiii riliijiipi s.6. as os os os 05 r ft A n j j A oSs O H D Fig. 25.—Mean hourly change of temperature, 1890-1910. June to October, inclusive. The greatest rate of fall occurs generally between 7 and 8 p.m., except in the months of September and October, when it is about an hour earlier. The data in the table are, of course, average values, and individual cases are affected by the occurrence of warm periods, cold waves, sunshine, and storm; so that, as is the case with all figures involving means, the changes shown cannot ordinarily be expected for any one particular day. Yet they afford a valuable basis in the comparison of the average day in one season with that of another. ILLUSTRATIONS OF SECONDARY CONTROLS OF TEMPERATURE From the energy of the sun is received all but an infinitesimal portion of the heat necessary to our existence. His shifting northward and southward with the revolution of the earth brings the142 THE WEATHER AND CLIMATE OF CHICAGO change of seasons, while the rotation of the planet upon its axis compels the diurnal swing of temperature from the cool of the morning to the heat of the afternoon. These are the primary factors which govern the course of temperature year in and year out, but in every locality there are secondary controls which exercise a greater or lesser influence upon the actual conditions of any particular time. Among these at Chicago the most important are the winds of spring and summer as distinguished between land and lake breezes; the winds throughout the year as distinguished by their different directions; the hot wave and cold wave areas which approach generally from the southwest and the northwest, respectively; and the conditions of cloudiness and precipitation. Certain of these factors or their effects have already been described at some length (pp. 37, 72, 76), but further illustration of their action in this connection will prove interesting. 1. Effect of lake and land winds.—The influence of the shift of the wind from land to lake and from lake to land in spring and summer is shown forcibly in Figs. 26 and 27, containing the temperature curves of days on which such winds were the potent factors of control. In the first of the two figures is shown the curve for July 21, 1901, which has previously been cited as the day of the highest maximum temperature on record at Chicago, 103°, during the prevalence of one of the severest hot waves that the interior of the country has experienced. Until 2 p.m. the wind was a land breeze from the southwest, under the influence of which the temperature rose steadily to 102°. At that time, however, the wind shifted suddenly through north to northeast, with a velocity of about 15 miles an hour, and the resulting importation of cool air from the lake brought the temperature down from 102° to 84° in the space of a single hour. By 3 o’clock the wind had diminished greatly in velocity and shifted still farther to easterly, becoming southeast by 3:10 and south by 3:25 p.m., when it was again a land breeze and began to freshen. The action of a very light lake wind in lowering the temperature is weak (p. 44), and the mercury began to climb again soon after the shift to easterly, and as the direction became southwest by 6 p.m., rose to a point higher by 1° than before. The fall after 6 o’clock was simply the diurnal change. Fig. 27 gives two instances of temperature curves of two days’ length each on which the effect of lake and land winds is well marked, the temperature responding closely to the change in wind direction.TEMPERATURE 143 On May 24, 1904, shown in the upper portion of the figure, the temperature rose until 9 a.m., when it was 63°, the wind being light and therefore of little consequence. With an increase in velocity, however, and its shifting from southeast to east and northeast, the temperature fell considerably until about 5 p.m., reaching 49° at that hour, when the wind veered to southerly, and notwithstanding the diurnal influence it rose steadily until after 10 p.m. During the LAKE INFLUENCE* Pig. 26.—Temperature fall in hot wave during shift of wind to lake, July 21, 1901. Fig. 26 shows the effect of a lake and land wind on temperature on a warm day, July 21, 1901. The upper figures show humidity; the lower figures show wind movement. The heavy line indicates temperature curve; the arrows show wind direction. remainder of the night the temperature fell only 2°, reaching the minimum at 6 a.m. of the 25th. Thereafter, under the steady southwest wind it rose to a maximum of 87° at 3 p.m., the fall after that hour being the regular diurnal change. The curve of April 20 and 21, 1907, illustrates the action of a steady lake wind in maintaining a fairly even temperature during the daytime. As a result, in this instance there was a range of only 5° throughout the entire 24 hours. The wind shifted to southwest144 THE WEATHER AND CLIMATE OF CHICAGO during the night, however, and on the 21st the temperature rose continuously, reaching its maximum, 57°, at about 4 p.m., and giving a range of 24° for the day. The conditions of sunshine and cloud were Fig. 27 shows the effect of land and lake winds on the temperature on selected days, May 24-25, 1904, and April 20-21, 1907. The arrows indicate the wind direction. Below the wind direction in each case is given the ¡state of the weather. Where the space is filled in, rainy weather is indicated; and this varies through cloudy and partly cloudy to clear weather, where the space is left free. practically the same on both days, so that the effect of the lake and land breezes is brought out clearly. The character of the weather is shown in both graphs directly under the arrows of wind direction, asTEMPERATURE 145 follows: black line, rain; heavily shaded strip, cloudy; lightly shaded strip, partly cloudy; clear strip, clear sky. The direction of the wind is ordinarily regulated by the distribution of atmospheric pressure, the air moving in a general course from the region of high pressure to that of low pressure. An easterly wind therefore prevails at Chicago when the barometer is high over the lakes and low directly to the west or southwest. There are, however, other times in this locality when lake winds occur under different conditions of pressure, and blow during a portion of the day, changing to land winds at night, in a manner quite similar to the land and sea breezes of our coasts. Such is likely to be the case when the pressure varies but slightly over a large surrounding region, and the resulting winds are consequently light. It never occurs if the pressure conditions are such as to produce strong land winds during the night. During a day of lake and land breezes, as on July 21, 1901, described above, the isobaric surfaces, or planes of equal air pressure, are depressed by the contraction of the cooling air over the waters of the lake. The land being much warmer, the opposite is the case, and the isobaric surfaces over the land are bulged up, as it were, by the heated and expanding air. This is a gravitational effect that expresses itself first of all in the convective upward current over the land, disarranging the isobaric surfaces and resulting in an overflow of the upper air from land to lake down the now inclined surfaces, increasing the pressure there and forcing a tower return current from lake to land. The exchange continues until an equilibrium of pressure is re-established by the cooling of the air over the land; and in the evening or at night, when its temperature has fallen below that of the air over the lake, the pressure conditions of the day are reversed and the land wind sets in. This is the cause of the lake breeze, to which Chicago is indebted for its equable summer temperatures. 2. Temperatures of cold days.—Fig. 28 shows the variation in hourly temperature during two cold days, February 8 and 9,1899, the minimum on the second day being —21° and the maximum only —8°. The wind direction and velocity, and the pressure curve as well, are given, but the main purpose of the graph is to show the hourly variation of temperature during a severe cold wave. The cold was ushered in by brisk west and northwest winds on February 8, and the movement continued brisk through the following day. 3. General effect of wind direction on temperature.—We have already instanced the effect of certain winds upon the temperature146 THE WEATHER AND CLIMATE OF CHICAGO at Chicago. In individual cases, however, other influences are likely to be at work, so that the result looked for may not always be easy to perceive. If the average of a large number of cases is taken, in which the factor under study is present, the effect of other disturbing influences will be minimized. In endeavoring to determine the effect of general wind direction upon temperature, the hourly temperatures for the days upon which certain winds prevailed in the period from 1902 to 1908, inclusive, have been averaged, and the results are shown in Fig. 29, the months of January, April, July, and October being chosen as representatives of the different seasons. FEBRUARY 8,1099. FEBRUARY 9,1899. r"""—■ "" 1 ■'■■■— ■ * 1 ■ Mdt 2 4 6 8 to Noon 2 4 6 8 10 Mdt. 2 4- 6 8 10 Noon 2 4 6 8 10 Mdt. FiG. 28.—Examples of cold days. Fig. 28 shows the temperature by hours during two cold days, February 8-9,1899; also the barometer trace, and the wind direction and velocity at each hour. Naturally, in all months the lowest temperatures occur with northerly winds, and the highest with southerly winds. The variation, however, is not nearly so marked in January as it is in April, and in this month the influence of the lake wind in reducing the diurnal range in temperature is plainly to be seen. It will be noted also that the variance for the different directions in the graphs is uniformly least at the time of daily minimum temperature, and greatest at the time of maximum. 4. Effect of cloudiness and rainfall upon temperature.—Fig. 30 has been prepared in a manner similar to that of the preceding graph, and for the same months and period of time. It will readily be seenTEMPERATURE 147 January April. Mdit. 2 4-68/0 Noon 24-68 io Mdt Mdt 2 4- 6 8 io Noon 2468/0 Mdt Fig. 29.—Effect of wind direction upon temperature. Fig. 29 shows the effect of wind direction upon the temperature by hours for January, April, July, and October, based upon observations from 1902 to 1908. JANUARY Mdt. 2 A- 6 8 to Noon 2 4 6 8 10 Mdt Clear * ■ Cloudy Rainy Apriu Mdt 2 4 6 8 to Noon 2 4 & 8 IO Mdt October Fig. 30.—Effect of the character of the day on the temperature. Fig. 30 shows the effect of the character of the weather upon the temperature by hours during January, April, July, and October, based upon observations from 1902 to 1908.148 THE WEATHER AND CLIMATE OF CHICAGO from the figure that the temperature fluctuates most on days of clear sky, because at such times the sun’s rays during the daytime and the process of radiation at night have their greatest effectiveness. In the cold waves of winter, however, the weather is usually clear, and this accounts for the comparatively small amplitude in the month of January, although even then it is considerably greater than it is in cloudy and rainy weather. The greatest diurnal amplitude occurs in clear weather in the month of October, and in every case the least fluctuation is experienced during rainy days. In general, clear weather in July brings the highest temperatures, and this is also the case during the afternoon in April and October. In January, however, cold waves lower the temperature of clear days much bel jw that on days of cloudiness and precipitation.PART II PRECIPITATIONPRECIPITATION Precipitation, which includes rain, snow, sleet, hail, and appreciable amounts of deposited dew and fog, is measured in inches and fractions thereof in depth of fall. Rainfall and melted snowfall, sleet, and hail are measured in inches and hundredths; but snowfall, sleet, and hail are also measured unmelted when practicable, and the depths recorded in inches and tenths. Measureable amounts of fog and dew occur but rarely in Chicago, and are recorded in the same manner as is rainfall. Mean precipitation is the average of all the individual amounts occurring within the period in question, daily, monthly, or annual, as the case may be, covering the entire time of observation, and the term is applied to rainfall and snowfall separately and to all the forms of precipitation combined. The term “normal” (pp. 3-4) is used in the comparison of precipitation records in a way similar to that in which it was used in the comparison of temperature conditions. There are no continuous records of precipitation for Chicago prior to 1867, when a series of observations was begun under the direction of the Smithsonian Institution, and continued until the date of the establishment of the local Weather Bureau office in 1870. Before the beginning of the Smithsonian Institution observations, however, records of a more or less intermittent character were being made at various places in northern Illinois, some of them overlapping one another, and being maintained through a portion of the period of official observations. These have been compared with the Chicago records by Professor Hazen in his Climate of Chicago (1893), in which he publishes, in the absence of official data, est'mated precipitation for the city as far back as 1843, obtaining his figures by interpolation, as he did in the case of temperature (p. 4). Such records must, of course, lack much in exactness, because near-by stations often vary considerably in precipitation, especially in summer thunderstorm periods. The estimates, however, were most, carefully made, and are the best data available on the subject. It should therefore be borne in mind, in the study of the records presented in this volume, that previous to the year 1867 the precipitation values are largely estimated, and that from 1867 to 1870, inclusive, they are the records of voluntary observers of the Smith- 151152 THE WEATHER AND CLIMATE OF CHICAGO sonian Institution, while since 1870 they are comprised in the official records of the Weather Bureau. ANNUAL, SEASONAL, AND MONTHLY PRECIPITATION Fig. 31 shows the annual amounts of precipitation for the entire period from 1843 to 1910, inclusive; Table LXV the annual and monthly, and Table LXVI the seasonal' precipitation from 1843 to 1913. The mean annual precipitation to 1910 is 33.99 inches, but there is a great variation from year to year; while the mean as obtained from the official record beginning with 1871 is 33.52 inches. From the graph it can readily be seen that much of the deficiency which caused this falling off of 0.47 inch in the mean occurred between the years 1885 and 1905, and covers very 1850 I nches 50 1670 1880 1890 1900 1910 Inches SO 40 30 Mean 20 20 10 Fig. 31.—Variations in the annual amount of precipitation, 1843-1910. nearly the time during which the Weather Bureau occupied quarters in the Chicago Opera House Block and the Auditorium Tower. The cause of this variation is believed to be due to the character of the exposure of the rain gage, as that at the latter location is known to have been somewhat unsuited to the purpose, and any changes in a city exposure are sure to affect the resulting record. The year of greatest precipitation for the entire time occurred in the period of the old records, 47.10 inches in 1858. The highest annual average for any decade, however, was entirely within the official period, 36.48 inches from 1881 to 1890; and this is true also of the wettest 5-year period, which covered the first half of this decade, and averaged 42.07 inches annually. Three years of the period last named approached the record of 1858: 1881 with 44.18 inches; 1883 with 45.86 inches, the largest annual amount of the official period; andPRECIPITATION 153 TABLE LXV Monthly and Annual Precipitation, Inches, 1843-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Normal, 1871-1906 2.00 2.16 2.55 2.88 3.37 3.66 3.64 2.88 3.02 2.55 2.50 2.07 33.28 1843 2.0 1.9 3.0 4.5 4.0 4.6 1.4 2.4 3.0 1.2 5.1 2.4 35.50 1844 2.8 1.3 1.76 2.76 6.0 5.5 5.0 4.2 0.8 1.6 0.7 0.7 33.12 1845 2.0 0.5 2.2 6.5 2.0 3.8 3.5 1.2 4.4 1.4 3.3 1.5 32.30 1846........ 4.9 1.8 2.6 7.8 2.4 4.3 2.9 1.0 5.0 1 0.8 1.5 5.0 40.00 1847 2.3 3.5 1.5 2.1 3.3 1.5 2.8 2.0 3.2 4.6 4.8 1.2 32.80 1848 1.6 2.4 4.5 3.3 3.8 4.4 3.4 5.1 2.2 3.2 2.1 8.4 44.40 1849 5.5 1.0 4.7 1.8 3.8 3.6 2.3 3.5 2.6 2.8 1.4 1.2 34.20 1850 2.1 0.7 2.0 3.8 1.9 3.1 2.1 6.4 1.4 2.0 3.2 1.7 30.40 1851 1.5 3.8 0.8 4.2 6.7 5.3 3.8 3.1 3.2 2.3 2.3 1.6 38.60 1852 2.0 1.2 5.5 4.8 2.4 2.5 3.4 0.6 2.3 6.8 4.0 3.3 38.80 1853 1.4 2.2 1.8 2.8 4.4 4.9 6.1 2.2 4.2 2.0 2.1 2.3 36.40 1854 1.3 1.9 2.1 2.1 3.1 2.6 3.0 0.9 1.6 3.6 1.0 1.4 24.60 1855 8.0 0.6 2.6 1.4 2.5 3.7 5.8 3.2 2.3 2.0 2.2 2.0 36.30 1856 0.9 1.8 0.6 2.6 4.8 2.5 2.1 1.3 2.5 2.1 3.98 3.86 29.04 1857 1.09 5.43 2.55 2.19 6.33 4.14 3.0 5.0 2.2 4.0 2.7 1.2 39.83 1858 1.3 1.4 3.0 3.1 7.8 6.3 5.9 3.2 4.0 4.6 4.5 2.0 47.10 1859 1.4 1.8 5.2 3.4 3.6 1.7 0.9 0.4 2.2 4.1 2.8 1.8 29.30 I860.. 1.6 1.6 1.0 2.8 4.6 3.5 5.3 2.6 2.8 4.1 2.3 4.2 36.40 1861 1.4 3.0 3.4 4.7 3.7 2.1 4.3 2.4 3.4 7.5 1.5 1.9 39.30 1862 4.0 0.7 2.0 5.2 4.3 2.9 6.7 3.6 5.6 2.92 1.2 1.3 40.42 1863 2.8 2.6 2.1 2.1 5.1 1.3 2.3 4.2 1.6 4.0 1.9 3.6 33.60 1864. 1.6 0.4 2.1 3.2 1.9 2.1 6.4 1.1 2.1 1.9 3.1 2.5 28.40 1865 0.4 3.1 3.1 3.8 1.5 5.1 6.1 7.2 4.8 4.0 0.5 0.6 40.20 1866 2.8 1.6 2.2 2.8 2.0 4.4 4.7 4.2 4.6 2.8 0.8 3.4 36.30 1867 1.93 2.22 1.58 1.70 4.42 1.86 1.52 2.33 0.57 1.28 1.89 1.11 22.41 1868 1.28 0.92 5.24 3.00 3.74 3.11 2.87 3.55 7.08 1.69 2.60 1.40 36.48 1869 1.97 2.23 1.33 4.30 5.69 5.03 3.26 1.32 0.89 1.10 2.42 2.03 31.57 1870 1.95 0.86 1.81 1.15 0.80 1.70 3.71 2.07 2.82 2.43 1.16 2.46 22.92 1871 4.13 1.45 2.66 3.70 3.90 5.56 2.52 2.01 0.74 1.88 3.62 3.44 35.61 1872 0.68 0.84 3.79 3.03 3.24 3.45 3.09 2.59 6.43 0.65 1.06 P.22 29.07 1873 2.56 0.47 0.89 6.22 7.20 1.44 4.04 1.58 3.53 2.43 1.61 4.44 36.41 1874 3.47 1.51 2.15 2.67 2.08 3.25 0.58 3.15 3.76 2.55 2.83 0.63 28.63 1875 0.96 1.99 1.43 2.32 3.64 5.17 7.18 3.29 4.39 4.32 0.75 2.62 38.06 1876 3.22 3.90 4.04 2.07 1.85 5.96 3.11 3.66 3.74 1.20 3.25 0.48 36.48 1877 1.91 0.06 5.37 2.42 1.81 6.04 2.98 3.06 2.02 6.51 6.08 2.75 41.01 1878 1.31 2.12 4.39 5.57 5.22 3.02 6.09 3.66 1.99 5.17 0.83 2.58 41.95 1879 0.54 1.47 2.37 1.93 3.89 3.18 5.58 0.45 1.18 2.72 4.93 2.47 30.71 1880 3.53 2.91 2.25 5.20 4.97 3.50 3.07 4.47 2.25 3.19 0.87 1.11 37.32 1881 0.87 5.98 2.99 1.84 1.85 5.93 4.31 0.54 4.34 6.89 5.97 2.67 44.18 1882 1.55 2.24 3.43 6.72 5.52 5.71 3.43 4.96 0.91 3.40 1.48 1.99 41.34 1883 1.74 4.74 0.42 3.72 7.32 5.61 5.53 1.21 1.36 7.36 5.26 1.59 45.86 1884 1.39 3.27 5.16 3.05 1.53 2.11 3.71 2.50 2.29 3.59 1.80 4.21 34.61 1885 3.18 2.01 0.57 4.00 3.17 5.20 2.44 11.28 2.97 3.87 2.33 3.35 44.37 1886 3.56 1.51 1.79 1.29 1.00 0.94 1.53 3.38 6.93 1.42 1.66 1.76 26.77 1887 3.13 5.10 0.89 0.46 1.38 1.63 1.05 3.35 4.03 2.03 2.41 3.67 29.13 1888 1.56 1.51 2.99 2.13 6.22 1.66 3.93 2.10 0.98 2.95 2.89 1.94 30.86 1889 1.64 1.31 1.43 2.35 5.38 2.93 9.56 0.39 2.75 1.82 3.49 1.90 34.95 ' 1890 2.98 2.47 2.10 3.23 5.13 3.25 2.57 2.58 1.39 4.20 1.59 1.25 32.69 1891 1.99 1.95 2.13 3.14 2.09 2.42 2.47 4.52 0.32 0.36 3.83 1.32 26.54 1892 1.99 .1.57 2.21 2.17 6.77 10.58 2.23 1.85 1.34 1.54 2.68 1.63 36.56 1893 2.08 2.44 1.69 4.16 1.93 3.59 3.08 0.18 1.98 1.75 2.45 2.14 27.47 1894 1.55 2.13 2.66 2.65 3.35 1.96 0.60 0.60 8.28 0.84 1.18 1.66 27.46 1895 2.15 1.60 1.32 0.86 1.99 1.79 2.42 6.49 0.89 0.51 5.60 6.76 32.38 1896 1.12 3.48 1.26 2.79 4.16 2.82 3.61 3.52 6.70 1.36 2.16 0.16 33.14. 1897 4.53 2.22 3.56 2.23 0.84 3.60 1.47 1.70 0.84 0.18 3.06 1.62 25.85 1898 3.54 2.59 4.60 0.76 2.23 5.30 1.94 3.03 3.16 3.26 2.25 1.11 33.77 1899 0.58 1.60 2.11 0.14 4.35 2.71 6.66 0.91 2.39 2.09 1.14 1.81 26.49 1900 1.21 3.52 1.58 1.02 3.59 2.06 4.64 4.24 1.56 1.35 3.30 0.58 28.65 1901 1.15 2.05 3.38 0.33 2.18 2.42 4.25 2.00 2.92 1.29 0.85 1.70 24.52 1902 0.66 1.53 4.16 2.26 -5.08 6.45 5.78 1.44 4.83 1.45 2.03 1.90 37.57 1903 1.09 3.03 1.67 3.77 0.93 1.62 4.78 3.49 4.00 1.09 0.34 2.28 28.09 1904 2.25 1.71 4.57 3.01 1.54 0.55 2.76 4.00 2.65 1.58 0.31 1.21 26.14 1905 1.33 1.95 2.43 3.03 5.14 3.27 5.02 4.46 4.18 1.82 2.05 0.68 35.36 1906 1.67 2.37 1.61 1.86 2.09 1.87 4.84 1.43 5.54 2.05 3.08 2.46 30.87 1907 4.21 1.00 2.94 2.37 3.50 3.64 3.15 4.22 4.49 0.93 1.92 2.73 35.10 1908 2.05 3.72 3.48 2.81 6.74 1.48 1.45 6.35 2.09 0.81 2.67 1.18 34.83 1909 1.96 3.84 1.63 7.73 2.18 5.09 1.77 6.20 3.60 1.20 3.84 4.18 43.22 1910 3.07 0.89 0.29 3.84 4.67 0.91 1.79 3.08 3.90 1.79 1.31 1.32 26.86 1911* 1.17 2.27 1.45 3.03 3.37 2.54 2.65 3.72 4.03 3.79 3.27 2.54 33.83 1912*. 0.84 1.57 2.20 2.55 3.97 1.78 3.86 3.59 3.26 3.52 1.45 1.08 29.67 1913* 1.33 1.97 3.44 1.91 4.38 1.08 3.30 4.06 1.49 2.23 1.47 0.45 27.11 * Not included in means.154 THE WEATHER AND CLIMATE OF CHICAGO TABLE LXY—Continued Mean Precipitation by Decades, Inches Decades Jan. Feb. Mar. April May June July Aag. Sept. Oct. Nov. Dec. Annual 1843-50 2.90 1.64 2.78 4.07 3.40. 3.85 2.92 3.22 2.82 2.20 2.76 2.76 35.35 1851-60 2.05 2.17 2.46 2.94 4.62 3.71 3.93 2.25 2.73 3.56 2.79 2.36 35.64 1861-70 2.01 1.76 2.49 3.20 3.32 2.96 4.19 3.20 3.35 2.96 1.71 2.03 33.16 1871-80 2.23 1.67 2.93 3.51 3.78 4.26 3.82 2.79 3.00 3.06 2.58 2.07 35.52 1881-90 2.16 3.01 2.18 2.88 3.85 3.50 3.81 3.23 2.80 3.75 2.89 2.43 36.48 1891-1900 2.07 2.31 2.31 1.99 3.13 3.68 2.91 2.70 2.75 1.32 2.76 1.88 29.83 1901-10 1.94 2.21 2.62 3.10 3.40 2.73 3.56 3.67 3.82 1.40 1.84 1.96 32.26 Mean 1871-1910.. 2.10 2.30 2.51 2.87 3.54 3.49 3.53 3.10 3.09 2.38 2.52 2.09 33.52 Mean 1843-1910.. 2.18 2.13 2.53 3.07 3.65 3.52 3.61 3.00 3.04 2.62 2.47 2.20 33.99 I Greatest .. 4.53 5.98 5.37 7.73 7.32 10.58 9.56 11.28 8.28 7.36 6.08 6.76 44.37 1871—J Year.... 1897 1881 1877 1909 1883 1892 1889 1885 1894 1883 1877 1895 1885 19131 Least 0.54 0.06 0.29 0.14 0.84 0.55 0.58 0.18 0.32 0.18 0.31 0.16 24.52 1 Year.... 1879 1877 1910 1899 1897 1904 1874 1893 1891 1897 1904 1896 1901 Table LXV shows the total monthly and annual precipitation for Chicago for the years from 1843 to 1913. Prior to 1867 measurements of precipitation were intermittent, and the amounts not actually observed (given to tenths in the table) were interpolated from the records of adjoining stations. From 1867 to 1871 the measurements were taken by voluntary observers reporting to the Smithsonian Institution. From 1871 to 1913 the observations have been taken by the United States Weather Bureau. The means of the monthly and annual amounts from 1871 to 1906 have been adopted as the monthly and annual normals, appearing at the top of the table. 1885 with 44.37 inches. The least yearly precipitation on record also occurred in the old period, 22.41 inches in 1867, while the least amount in the official period was 24.52 inches in 1901. Years with less than 30 inches, however, were very few prior to 1886, and but three such years have occurred since the removal of the Weather Bureau office to the Federal Building in 1905. Between these dates, out of 19 years, 11 show less than 30 inches, but, as indicated above, the record is thought to have been affected by the poor exposures of the rain gage. Taking the precipitation by seasons from 1843, the greatest amount in winter, as indicated in Table LXVI, is 14.90 inches in 1848-49, while the next greatest amount occurred in 1895-96, 11.36 inches. In spring, the greatest amount is 15.67 inches, in 1882. In summer, the greatest amount is 18.92 inches, in 1885; and in autumn, 17.20 inches, in 1881. The least amount in winter is 2.45 inches, in 1876-77; in spring, 2.73 inches, in 1887; in summer, 3.00 inches, in 1859, old record, and 3.16 inches, in 1894, new record; in autumn, 3.10 inches, in 1844, old record, and 4.08 inches, in 1897, new record. By seasons the means of the official period increase from a winter minimum of 6.48 inches, through spring with an average of 8.92 inches, to a summer maximum of 10.12 inches, falling off thereafter to an autumn mean of 8.00 inches. For the entirePRECIPITATION 155 period of record from 1843, the seasonal means are slightly higher than these, with the exception of that of summer, which is practically the same as that of the official reading. The same variation in seasonal amounts can be noted in the table as was seen to be the case with the annual values, and there are a number of marked instances in all of the seasons in which the TABLE LXVI Total Seasonal Precipitation, Inches, 1843-1913 (See explanation under Table LXV) Year Winter Spring Summer Autumn 1843 11.5 8.4 9.3 1843-44 6.5 10.52 14.7 3.1 1844-45 3.2 10.7 8.5 9.1 1845-46 8.2 12.8 8.2 7.3 1846-47 10.8 6.9 6.3 12.6 1847-48 5.2 11.6 12.9 7.5 1848-49 14.9 10.3 9.4 6.8 1849-50 4.0 7.7 11.6 6.6 1850-51 7.0 11.7 12.2 7.8 1851-52 4.8 12.7 6.5 13.1 1852-53 6.9 9.0 13.2 8.3 1853-54 5.5 7.3 6.5 6.2 1854-55 10.0 6.5 12.7 6.5 1855-56....... 4.7 8.0 5.9 8.58 1856-57 10.38 11.07 12.14 8.9 1857-58 3.9 13.9 15.4 , 13.1 1858-59 5.2 12.2 3.0 9.1 1859-60 5.0 8.4 11.4 9.2 1860-61 8.6 11.8 8.8 12.4 1861-62 6.6 11.5 13.2 9.72 1862-63 6.7 9.3 7.8 7.5 1863-64 5.6 7.2 9.6 7.1 1864-65 6.0 8.4 18.4 9.3 1865-66 5.0 7.0 13.3 8.2 1866-67 7.55 7.70 5.71 3.74 1867-68 3.31 11.98 9.53 11.37 1868-69 5.60 11.32 9.61 4.41 1869-70 4.84 3.76 7.48 6.41 1870-71 8.04 10.26 10.09 6.24 1871-72 4.96 10.06 9.13 8.14 1872-73 3.25 14.31 7.06 7.57 1873-74 9.42 6.90 6.98 9.14 1874-75 3.58 7.39 15.64 9.46. 1875-76 9.74 7.96 12.73 8.19 1876-77 2.45 9.60 12.08 14.61 1877-78 6.18 15.18 12.77 7.99 1878-79 4.59 8.19 9.21 8.83 1879-80 8.91 12.42 11.04 6.31 1880-81 7.96 6.68 10.78 17.20 1881-82 :. 6.46 15.67 14.10 5.79 1882-83 8.47 11.46 12.35 13.98 Year Winter Spring Summer Autumn 1883-84 6.25 9.74 8.32 7.68 1884-85 9.40 7.74 18.92 9.17 1885-86. 8.42 4.08 5.85 10.01 1886-87 9.99 2.73 6.03 8.47 1887-88 6.74 11.34 7.69 6.82 1888-89 4.89 9.16 12.88 8.06 1889-90...... 7.35 10.46 8.40 7.18 1890-91 5.19 7.36 9.41 4.51 1891-92 4.88 11.15 14.66 5.56 1892-93. 6.15 7.78 6.85 6.18 1893-94 5.82 8.66 3.16 10.30 1894-95 5.41 4.17 10.70 7.00 1895-96 11.36 8.21 9.95 10.22 1896-97 6.91 6.63 6.77 4.08 1897-98. 7.75 Y.59 10.27 8.67 1898-99 3.29 6.60 10.28 5.62 1899-1900 .... 6.54 6.19 10.94 6.21 1900-1901 .... 3.78 5.89 8.67 5.06 1901-2 3.89 11.50 13.67 8.31 1902-3 6.02 6.37 9.89 5.43 1903-4 6.24 9.12 7.31 4.54 1904-5 4.49 10.60 12.75 8.05 1905-6 4.72 5.56 8.14 10.67 1906-7 7.67 8.81 11.01 7.34 1907-8 8.50 13.03 9.28 5.57 1908-9 6.98 11.54 13.06 8.64 1909-10 8.14 8.80 5.78 7.00 1910-11* 4.76 , 7.85 8.91 11.09 1911-12* 4.95 8.72 9.23 8.23 1912-13* 4.38 9.73 8.44 5.19 1843-1910 means 6.52 9.26 10.11 8.14 1871-1910 means 6.48 8.92 10.12 8.00 Greatest 14.9 15.67 18.92 17.20 Year 1848-49 1882 1885 1881 Least 2.45 2.73 3.0 3.1 Year 1876-77 1887 1859 1844 * Not included in means. precipitation was much above or below the normal. These seasons are given in Tables LXVII and LXVIII. The summers of 1865 and 1885 were very wet, having almost twice the normal amount of rainfall for the season; while those of 1859 and 1894 were of quite the opposite character, averaging only about 1 inch for each month of the time. In only two cases, however, has more than twice the normal156 THE WEATHER AND CLIMATE OF CHICAGO amount of precipitation actually occurred in a season, the winter of 1848-49 with 14.9 inches, exceeding its normal by 8.67 inches, and the autumn of 1881 with 17.20 inches, exceeding its normal by 9.13 inches. On the other hand, the winter of 1844-45, the summers of 1859 and 1894, and the autumns of 1844 and 1867 all received approximately one-half, or less, of their normal amounts. TABLE LXVII Seasons with Excessive Precipitation Winter Normal 6.23 Spring Normal 8.80 1846-47 +4.57 +8.67 +4.15 +5.13 1858 +5.10 +5.51 +6.38 +6,87 +4.23 1848-49 1873 1856-57 1878 1895-96 1882 1908 Summer Normal 10.18 Autumn Normal 8.07 1844 +4.52 +5.22 +8.22 +5.46 +8.74 +4.48 1847 +4.53 +5.03 +5.03 +4.33 +6.54 +9.13 +5.91 1858 1852 1865 1858 1875. 1861 1885 1877 1892 1881 1883....... Table LXVII contains a list of seasons with an excess in precipitation of 4.00 inches and over, 1843-1913. TABLE LXVIII Seasons with Deficient Precipitation Winter Normal 6.23 Spring Normal 8.80 1844-45 -3.03 1870. -5.04 1876-77 -3.78 1886 -4.72 1887 -6.07 1895 -4.63 Summer Normal 10.18 Autumn Normal 8.07 1847 -3.88 1844 —4.97 1852 -3.68 1867 —4.33 1854 -3.68 1869 —3.66 1856 -4.28 1891 —3.56 1859 -7.18 1897 -3.99 1867 -4.47 1901 -3.01 1873 -3.12 1904 -3.53 1874 -3.20 1886 -4.33 1887 -4.15 1893 -3.33 1894 -7.02 1897 -3.41 1910 -4.40 Table LXVIII contains a list of seasons with a deficiency in precipitation of 3.00 inches and over, 1843- 1913. In Plate VIII are given the departures of monthly, seasonal, and annual precipitation from the normal, from 1843 to 1910. The figures show clearly the great irregularity in the amounts of precipi tation from month to month, and from year to year, and yet the average annual falls for the decades of the period, shown in Table LXV, differ but little, and would doubtless differ still less were the entire record obtained from a gage exposed in the same location and with the same surroundings. There is a tendency for the annual1849 /Q$3 *863 1873 /8S3 /S93 1903 WINTER <¿>.23 SPRING 8.80 SUMMER /o./8 AUTUMN 807 YEAR 33.28 /843 I8S3 /363 *873 *8Q 3 !Q9Z *903 Plate VIII.—Monthly, seasonal, and annual departures from the normal precipitation from 1843 to 1910. Normals are shown in figures under names of month, season, and year. 'InchesPRECIPITATION 157 precipitation to average up in the long run, as is shown in the plate by the departure line crossing and recrossing the normal precipitation line. The period from 1875 to 1885 was apparently one of abnormally heavy precipitation, and the period from 1893 to 1904 one of more than ordinary dryness. Taking the plate as a whole, however, there is little or no uniformity in the annual, seasonal, or monthly variations. The statements made (p. 23) relative to uniformity in the succession of years and seasons with regard to temperature, may appropriately be applied also in the case of precipitation. A careful study of Tables LXV, LXVI, LXVII, and LXVIII, and Plate VIII shows clearly that in no instance does the precipitation of any year or season afford a clue to that of any succeeding year or season. Instances are apparent of several years together with extraordinary precipitation, such as 1880-83, and there are times of deficient precipitation for several years, as 1899-1901* but these intervals do not appear to recur with any regularity. Again, some very wet years have been followed by years of deficient rainfall, as in 1885 (44.37 inches) and 1886 (26.77 inches). The reverse is also true, as in 1874 (28.63 inches) and 1875 (38.06 inches). Both wet and dry years have been followed by years of normal precipitation, and vice versa, and in none of the instances does there appear to be regularity of succession. The same is true also of the seasons. From the means given at the bottom of the table of monthly values, shown graphically in Fig. 32, for the official period, it will be seen that the average monthly amounts of precipitation increase gradually from the winter season to May, then remain nearly the same until the end of July, decreasing irregularly thereafter until November. August and September have about the same average rainfall, and the fall of December about equals that of January. It is apparent that the greatest amount of precipitation throughout the year is received during the growing season, when moisture is most needed; but at all times there is, on the average, a sufficient fall, so that there is no division in the nature of a wet and dry season. There is a great difference between the highest and lowest precipitation records for the various months of the year. The comparison is shown in Fig. 33, in which the line of mean precipitation is also drawn for the purpose of indicating the excesses and deficiencies, as well. The variation is much more pronounced in the summer158 THE WEATHER AND CLIMATE OF CHICAGO season, but even in the winter months there is a difference of from 4 to 6 inches between the greatest and least values. The exact amounts for the months of extreme record, and the year in which each occurred, are shown at the bottom of Table LXY, the greatest monthly amount being 11.28 inches in August, 1885, and the least, 0.06 inch in February, 1877. It will be seen that in no instance do any two successive months in one year hold the record of highest Vi Fig. 39.—Average number of days with precipitation. a—precipitation (0.01 or more), observed but 9 days in 20 years; b—precipitation (0.01 or more), observed but 6 days in 20 years; c=no precipitation (0.01 or more) in 20 years; d=precipitation (0.01 or more), observed but 19 days in 40 years. Fig. 39 shows the average monthly and annual number of days with precipitation (0.01 or more) at 15 selected stations, including Chicago, taken across the northern, central, and southern portions of the United States from west to east. See Table LXXIII. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 173 of 16 days each, in February, 1898, March, 1906, and January, 1907, respectively, and although they are considerably longer than the average, they were not marked by heavy precipitation at any time. The average length of the longest period each year with a fall of 0.01 TABLE LXXIV Longest Period Each Month and Year op Consecutive Days with 0.01 or More op Precipitation, 1871-1913 Year Jan. Feb. Mar, April May June July Aug. Sept. Oct. Nov. Dec. Annual 1871 3 1 3 3 3 3 2 3 2 2 3 3 3 1872 2 2 2 4 2 3 4 3 3 2 3 6 6 1873 5 2 3 5 4 2 4 3 2 6 4 4 6 1874 7 b5 2 2 2 4 2 3 3 3 4 2 7 ' 1875 3 5 6 6 5 4 6 3 3 5 : 2 6 6 1876 4 3 6 4 4 10 5 3 7 b4 2 3 10 1877 3 2 6 4 3 4 4 3 3 5 5 7 7 ■’ 1878 b4 7 4 5 3 3 3 3 3 4 3 6 7 1879 2 4 3 b5 2 3 2 2 3 3 3 3 5 1880 4 5 3 4 3 6 2 «’ll 3 3 3 3 11 1881 2 6 4 3 4 4 3 2 6 8 3 3 8 1882. 2 4 5 5 4 7 4 4 b5 4 4 3 7 1883 4 3 3 3 4 5 4 2 3 6 3 4 6 1884 2 4 3 a3 2 2 4 2 2 2 3 5 5 1885 4 6 2 9 2 4 4 4 3 3 4 3 9 1886 4 2 a5 6 4 4 2 2 6 3 3 3 6 1887 b4 5 2 3 4 b4 3 3 b8 2 4 3 8 1888 4 2 4 3 3 3 3 a5 3 2 3 4 5 1889........ 2 3 3 3 b5 4 5 2 2 b4 6 3 6 1890 5 b5 4 5 6 3 1 2 2 4 4 5 6 1891 a4 5 3 4 2 4 2 3 2 2 4 2 5 1892 3 2 2 3 a5 8 3 2 7 b3 3 3 8 1893.. 3 a4 7 6 c4 3 2 1 3 b5 4 4 7 1894 2 3 4 a5 4 3 1 2 8 3 4 3 8 1895 5 2 2 3 4 3 2 4 2 2 4 5 5 1896 4 5 2 3 4 2 2 2 4 4 4 2 5 1897 5 3 4 5 2 5 3 1 1 1 3 6 6 1898 3 6 5 2 4 7 3 3 4 4 4 2 7 - 1899 2 3 2 1 4 3 5 2 3 3 3 6 6 1900 5 4 3 3 4 3 2 4 4 2 5 2 5 1901 2 3 5 4 5 3 2 2 2 5 3 4 5 1902 4 c9 4 2 3 4 8 1 3 2 2 4 9 1903 2 2 4 6 3 2 4 4 5 3 2 3 6 1904 3 3 3 5 5 2 4 4 1 4 3 5 5 1905 3 2 3 4 3 3 5 4 a4 3 2 3 5 1906........ 3 3 6 3 2 4 b4 4 3 2 6 2 6 1907 6 4 5 3 3 4 4 2 2 2 2 3 6 1908 2 2 3 6 5 3 2 3 2 5 4 3 6 1909 2 3 2 4 3 5 3 5 3 2 4 5 5 1910 3 3 2 6 4 1 2 2 4 4 4 2 6 1911* 5 5 5 b5 4 5 3 3 5 0 3 4 4 6 1912*.: 3 2 2 2 4 3 2 5 5 4 3 3 5 1913* 3 a4 4 4 3 3 3 2 a3 5 a3 2 6 Average (1871-1910) 3 4 4 4 4 4 3 3 3 3 3 4 6 * Not included in averages. Note.—(a), (b), (c), etc., indicate that period extends into either preceding or following month, 1, 2, 3, etc., days. inch or more is 6 consecutive days; that with a fall of “trace” or more, 10 consecutive days. The amount of precipitation during a wet spell is of quite as much importance as the duration of the fall. The two previous174 THE WEATHER AND CLIMATE OF CHICAGO tables give simply the longest periods each month and year of consecutive days of precipitation, and in many instances these periods cannot properly be regarded as wet spells. Table LXXVI, however, includes all periods of 10 consecutive days or less in which the amount TABLE LXXV Longest Period Each Month and Year op Consecutive Days with Trace or More op Precipitation, 1871-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1871 3 2 5 5 3 3 2 3 2 2 3 3 5 1872 2 2 2 4 a2 3 4 3 3 2 3 3 4 1873. 5 2 3 5 4 2 4 3 2 ' 6 4 4 6 1874 7 b6 2 3 3 4 2 3 3 3 4 3 7 1875 4 5 10 6 5 4 6 4 3 5 2 6 10 1876 5 3 6 6 6 10 5 3 7 b4 7 5 10 1877 4 3 6 5 3 5 4 3 3 6 6 7 7 1878 b7 8 4 5 3 3 3 3 3 4 3 6 8 1879 3 5 3 a5 3 3 2 2 3 3 5 3 5 1880 4 5 3 4 3 6 3 a3 *17 a4 3 5 17 1881 5 12 c8 ¿6 4 5 3 2 6 8 4 b6 12 1882 3 6 12 6 e7 9 4 bi2 b5 4 5 3 12 1883 4 3 4 4 4 5 4 2 3 6 3 5 6 1884 b4 6 4 5 2 2 4 2 2 a3 6 6 6 1885 4 6 2 9 c4 4 7 4 7 b9 12 8 12 1886 5 c5 b5 8 4 4 a3 2 7 3 b8 7 8 1887.. 6 8 6 5 b5 3 a6 4 b8 6 6 7 8 1888 6 3 a6 4 6 4 3 b6 3 2 3 5 6 1889 3 5 3 4 10 5 5 2 4 b4 6 4 10 1890 5 a8 4 5 6 7 2 3 4 4 4 a6 8 1891 «7 5 6 b8 3 4 2 3 2 3 10 3 10 1892 4 7 a3 3 7 15 3 2 8 2 4 5 15 1893 «8 10 8 e12 4 3 a3 2 7 b5 5 4 12 1894 5 4 4 c7 4 4 3 2 8 3 7 6 8 1895 6 3 3 5 5 3 4 4 2 2 5 6 6 1896 5 5 4 b4 4 4 3 3 6 4 5 5 6 1897 9 5 8 6 3 5 3 2 2 2 3 6 9 1898 5 16 5 3 5 7 3 3 4 6 4 7 16 1899 3 c7 10 3 4 5 6 2 10 4 3 6 10 1900. 9 7 7 6 4 3 3 5 7 2 7 5 9 1901. «13 3 b8 7 5 5 5 2 4 6 4 8 13 1902 7 a10 8 4 8 5 8 2 6 3 5 8 10 1903 7 4 8 6 b13 3 4 10 5 3 3 7 13 1904 «18 8 c5 8 10 a4 4 4 3 5 3 7 18 1905 6 5 5 6 7 4 7 5 a4 7 c7 3 7 1906 5 5 16 3 3 4 b4 4 3 3 6 4 16 1907 16 6 5 7 5 5 4 3 3 2 3 6 16 1908........ 3 5 b4 7 7 3 c7 3 2 5 4 5 7 1909 4 4 5 5 4 5 4 5 3 7 8 7 8 1910 4 5 2 6 4 1 4 3 5 4 5 a8 8 1911* 8 5 6 b6 6 3 3 5 6 4 7 5 8 1912* <*6 f4 3 2 4 3 a4 7 5 4 3 5 7 1913* 7 . e8 6 5 11 5 3 4 4 7 6 2 11 Average (1871-1910) 6 6 6 6 5 5 4 3 5 4 5 5 10 * Not included in averages. Note.—(a), (b), (c), etc., indicate that period extends into either preceding or following month, 1, 2, 3, etc., days. of precipitation equaled or exceeded the average for the month in which it occurred, or was proportional to it in longer periods, and indicates the limiting dates and the amount which fell during the time. These wet spells are confined to no particular season or month.PRECIPITATION 175 TABLE LXXVI Wet Spells, 1871-1911 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Number of Wet Spells 1871 ill ill H) [4.50J (11 [2.7lJ l“l [2.97] 5 2 3 1 1 3 3 2 1 2 3 2 4 3 3 1 3 0 2 0 1 1 0 1872 12 6 3.04 29 ’ 8 [3.28. 1873 til 11} f5.35j I”! [4.05] 1874 ill 1875 i 10 ] { 3 [ 3.58] 14 10 \ [3.6lj 1876 (ii 12.27] f 24 ) { 10 \ 12.87J 1877 I”1 ill IJJ 1878 13.92] f 26 ) 3 4.15 1 9 1 4 \ [4.34] 1879 1880 ill f 25 1 [3.82] 1881 in (3.52J (ii [4.74] ill 1882 9 8 3.19 26 5 [2.93 1883 f 24 I 1 11 \ 3.45 13 \ 10 \ ^2.49J (I! U.93J ill UJ 1884 f 28 I 11 4.62 I'M [2.37] 1885 Ml [3.88] 3 2 6.36 25 3 [3.93J 1886 l»i [5.82 J !"! [3.34] 1887 2 12 2.27 17 10 [3.25 1888 1889. i 19 7 13.89 1 30 ’ 5 4.50 1890 1891 ra [3.09] 1892 f 26 1 1 10 [8.39] 1893 176 THE WEATHER AND CLIMATE OF CHICAGO TABLE LXXVI—Continued Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Number of Wet Spells 1894 - H! (7.32J 1 4 2 0 2 0 0 0 1 0 1 1 0 1 1 3 1 1 1895 1M [2.55J HI [5.77J i 9 1 ] 4 2.66] 26 8 !> 12.75] HI 16.15] 1896 ill 13.85] 1897 1898 I’l [2.37] I”1 [2.98J 1899 1900 1901 1902 f“l (2.72J 1903 1904 HI (3.16J 1905... 111 [3.85 J 1906. 1907 |‘!| [2.47J 1908 f 17 1 7 \ 5.83 15 5 \ [4.80] 1909 m [3.20] IS! [2.88] 1910 .. IS! [2.I5J 1911 H) (3.24J Number of wet spells 7 6 4 7 3 5 4 8 8 4 4 5 65 Table LXXVI includes all periods of 10 consecutive days or less in which the amount of precipitation equaled or exceeded the average for that month, or was proportional to it in longer periods. The first figure indicates the last day of the wet spell, the second indicates the duration in days, and the third figure indicates the total amount of precipitation in inches. They have not, however, occurred in every year of the record, and the table shows a much smaller number, relatively, during the years in which the measurements were made at the Chicago Opera House Building and the Auditorium Tower (p. 152). The most pronounced wet spell was that of 10 days’ duration, from June 17 to 26, 1892, in which a total rainfall of 8.39 inches occurred. Other remarkable periods of the kind were those of September 3-10, 1894, with a total fall of 7.32 inches, and of August 2-3, 1885, with 6.36 inches. Later in this same August 3.93 inches fell within 3 successive days, and the total precipitation of the month, 11.28 inches, is the greatestPRECIPITATION 177 monthly fall on record for the city. As a result of these August rainfalls innumerable basements were flooded and the capacity of the sewers overtaxed, the condition resulting in the first agitation for the building of the Drainage Canal. In December, 1895, occurred a period of 9 days’ precipitation, all in the form of rain, totaling 6.15 inches. In August, 1908, there was a period of 7 days with a total precipitation of 5.83 inches, and of this amount 2.31 inches fell on the 11th, 2.03 inches on the 12th, and 1.23 inches on the 15th. ANNUAL NUMBER OF DAYS WITH PRECIPITATION OF STATED AMOUNTS The annual number of days with precipitation of “ trace” or more and of 0.01 inch or more has already been given. The latter data are repeated in Table LXXVII, which gives in addition the TABLE LXXVII Annual Number op Days with Precipitation op Stated Amounts, 1871-1913 Year Less than 0.01 Inch 0.01 Inch or More 0.04 Inch or More 0.25 Inch or More 1.00 Inch or More Year Less than 0.01 Inch 0.01 Inch or More 0.04 Inch or More 0.25 Inch or More 1.00 Inch or More 1871 14 951 81 37 10 1895 45 106 79 37 7 1872 ...... 3 108 71 36 4 1896 60 114 89 41 8 1873 7 134 101 42 8 1897 49 125 92 36 3 1874 12 113 84 34 5 1898 56 133 108 40 6 1875 12 147 106 40 8 1899 72 110 84 32 2 1876 17 136 98 41 9 1900 57 116 95 42 4 1877 20 128 99 57 7 1901 65 114 80 30 4 1878 9 166 115 47 9 1902 62 133 103 47 8 1879 24 117 91 30 8 1903 76 116 87 39 4 1880 ...... 36 144 107 53 6 1904 69 123 90 29 5 1881 35 137 110 53 10 1905 56 126 96 40 5 1882 ...... 42 150 107 48 10 1906 54 122 91 34 9 1883 28 139 105 49 10 1907 58 127 98 48 8 1884 29 135 107 39 6 1908 50 109 88 41 9 1885 52 141 94 41 9 1909 60 128 100 49 8 1886 53 124 91 33 3 1910 57 106 83 35 4 1887 78 118 88 43 5 1911* 52 144 111 46 4 1888 42 124 91 34 5 1912* 63 110 85 34 6 1889 45 117 87 41 8 1913* 58 120 80 30 9 38 47 mi AA « IoyU 1891 iuO 118 1U1 81 28 5 Mean 43.2 125.1 93.8 40.2 6.4 ai LAI 104 Aft A loy^ 1893 48 irci 122 llrr 91 37 3 Greatest... 78 166 115 57 10 1894 51 107 79 37 4 Least 3 106 71 28 2 * Not included in means, t For 11 months. number of days in each year with precipitation of 0.04 inch or more, 0.25 inch or more, and of 1.00 inch or more, and also the number of days with less than 0.01 inch precipitation. In the matter of “traces,” from the small number recorded in the early years of the record it will be apparent that as close attention to very light showers and snow flurries was not given by the observers as is the case in the178 THE WEATHER AND CLIMATE OF CHICAGO present day. The numbers vary greatly from year to year, but the average annual occurrences, beginning with the smallest amount, are 43.2, 125.1, 93.8, 40.2, and 6.4 days, respectively. With the exception of the first of these means, each includes the occurrence of all the classes following, but the individual averages can be secured by taking the difference between any mean and that immediately following; and the results are given in Auxiliary Table F. AUXILIARY TABLE F Average Number of Days with Precipitation of Certain Amounts Amount in Inches No. Days “Trace”................................................................. 43.2 0.01 to 0.03 inch....................................................... 31.3 0.04 to 0.24 inch....................................................... 53.6 0.25 to 0.99 inch....................................................... 33.8 1.00 inch or over........................................................ 6.4 From these figures it is apparent that precipitation of 0.04 to 0.24 inch occurs more frequently than that of any other class, while amounts so light as to be unmeasureable are next in order of occurrence. In no year of the record have there been less than 106 days of appreciable precipitation; nor have there been more than 10 days with 1.00 inch or more, although this number was recorded in three successive years, 1881, 1882, and 1883, the wettest 3-year period since the official observations began. EXCESSIVE PRECIPITATION In many kinds of building, street, sewer, and dredging work information as to the greatest amounts of precipitation that have occurred in short spaces of time is of prime importance. Tables containing such data for Chicago have been prepared so far as possible. The records of excessive precipitation in earlier years, however, are in many cases either fragmentary or wanting altogether, and it was not until after the installation of the automatic recorders in 1897 that more complete data became available. Excessive precipitation is distinguished in several different ways, and the various classes are discussed in the following paragraphs. 1. Precipitation of 1 inch dr more an hour.—Table LXXVIII gives the dates of rainfall when the amounts reached or exceeded 1 inch an hour, the total fall and the time being entered in each case. While the record covers the period from 1871 to 1913, inclusive, it is obviously incomplete. In the earliest days the observers were required to note such occurrences, but in the absence of automaticPRECIPITATION 179 registers, the record for a single hour cannot be depended upon unless the total fall of the storm happened within that time. Some exceptionally heavy rainfalls will be seen in the tabulation, among the most pronounced of which are those of June 23, 1871, 2.13 inches in 1 hour; May 25, 1896, 1.24 inches in 15 minutes; July 27, 1889, 1.18 inches in 54 minutes; May 11, 1905, 1.12 inches in 26 minutes; August 5, 1905, 1.18 inches in 23 minutes. TABLE LXXVIII Excessive Precipitation—1.00 Inch or More an Hour, 1871-1913 April May June July Year d d o 4 a> o3 Q 5s Cl g 4 <37 J EH 07 "c3 Q Ss -f* a g 4 07 a ÉH 07 I! Q F* c3 £ "S § 4 07 a H 07 "«3 Q 1896.. 1905.. /1.24 \1.34 1.12 0M5m 1 0 0 26 25 25 11 1871.. 1892.. fl .93 \2.13 11.60 \1.05 0h30m 1 00 1 00 1 00 19 23 23 18 1889.. 1890.. 1891.. 1906.. 1912.. fl .55 il.18 J1.00 11.04 (1.10 11.20 (1.17 [1.64 1.00 0h35m 0 54 0 34 1 00 0 43 1 00 0 20 1 00 0 41 18-19 27 14 14 0 7 15 28 7 August September October November Year "S g 4 07 a H 07 Is Q S3 07 >1 S 4 07 a EH 07 "cS Q U C3 0} -»a d d o a +3 if -4^> +=> 0 Ö Ö 0 0 0 0 0 0 0 0 0 0 0 S3 0 H> 'S 0 0) "cl 0 ‘5a Excessive Periods t-4 O -g ■0 .2 *0 "e3 -g o'.9f • rt o la g 02 d d C2 O X w ti-t o fl ' .2 03 ■§■§ 5 Min. ip Min. 15 Min. 20 Min. 25 Min. 30 Min. 35 Min. 40 Min. 45 Min. . 50 Min. 60 Min. 80 Min. 100 Min. a & Q o a Q Averages April 3 3h 22m in. 0.90 min. 23 0.13 0.23 0.38 0.50 0.56 0.59 0.62 0.66 0.74 0.76 M ay 4 2 27 1.06 26 0.12 0.34 0.46 0.59 0.79 0.94 1.09 1.04 1.08 Jimp. 5 2 31 0.84 22 0.28 0.47 0.58 0.60 0.64 0.72 0.58 July 16 2 6 0.88 25 0.12 0.26 0.43 0.59 0.69 0.82 0.85 0.90 0.98 1.04 1.18 August September On.tnher . . . 16 6 2 55 2 12 1.18 1.00 34 28 0.20 0.17 0.39 0.36 0.55 0.46 0.69 0.53 0.81 0.72 0.94 0.73 1.07 0.79 1.09 1.08 1.23 1.21 1.14 1.28 1.31 1.47 1.67 1.98 2 4 18 0.72 22 0.10 0.22 0.30 0.35 0.59 0.67 Average 2 50 0.94 26 0.16 0.32 0.45 0.55 0.69 0.77 0.83 0.95 0.75 1.06 1.32 1.67 1.98 Greatest April 6h 10m in. 1.53 Oh 44m 0.18 0.30 0.38 0.50 0.56 0.59 0.62 0.66 0.74 0.76 May 4 00 1.36 0 40 0.16 0.53 0.77 0.98 1.09 1.14 1.20 1.04 1.08 .Time 4 25 0.98 0 32 0.48 0.68 0.75 0.70 0.79 0.88 0.58 July 5 26 1.81 0 40 0.29 0.57 0.77 0.98 1.19 1.26 1.09 1.13 1.2Ô 1.32 1.61 August 6 09 3.44 1 30 0.60 0.86 1.02 1.15 1.23 1.34 1.46 1.55 1.60 1.41 1.53 1.70 1.98 September.. . . 3 30 1.56 1 03 0.25 0.47 , 0.60 0.65 0.82 0.89 0.97 1.08 1.21 1.28 1.47 Oeinher 5 15 0.97 0 29 0.12 0.26 0.36 0.35 0.59 0.67 Greatest 6 10 3.44 1 30 0.60 0.86 1.02 1.15 1.23 1.34 1.46 1.55 1.60 1.41 1.61 1.70 1.98 Year. . . 1908 1905 1910 1910 1910 1910 1910 1910 1910 1909 1906 1908 1908 Month August 15 August 5 August 23 August 23 August 23 August 23 August 23 August 23 August 23 August 14 July 28 August 11 August 11 Day........ Hour of be- tnmrniir.. . 6:55 p.m. 6:06 a.m. 2:06 a.m. 2:06 a.m. 2:06 a.m. 2:06 a.m. 2:06 a.m. 2:06 a.m. 2:06 a.m. 6:58 a.m. 11:20 a.m. 9:39 p.m. 9:39 p.m. Table LXXXIII shows the average rate of excessive precipitation and the greatest rate of excessive precipitation in cumulative 5-minute periods, April to October, inclusive, 1897-1911. It should be noted, however, in that part of the table containing the averages, that as the length of the 5-minute periods increases, the figures represent less and less the y—*- true average rates for the whole number of excessive rains, as shown in the column headed “number.” This is because all the excessive rains do not extend over the entire period ^ of 100 minutes, and for some of the longer cumulative periods the figures represent the averages for only a few excessive rains, while for the period of 100 minutes the 1.98 inches representing same is not an average at all, there being but one storm having excessive rain which continued for that length of time. Under that portion of the table headed “ Greatest” the various amounts represent the highest for that period without regard to whether all occurred in the same storm or not. PRECIPITATIONCO o TABLE LXXXIV Length op Precipitation Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Total Rainstorms • Snowstorms ■ Mixed storms— Rain and snow, etc. All storms Number of storms Average duration (hrs.) Number less than 1 hr Number over 12 hrs Longest (hrs.) Number of storms Average duration (hrs.) Number less than 1 hr Number over 12 hrs Longest (hrs.) Number of storms Average duration (hrs.) Number less than 1 hr Number over 12 hrs Longest (hrs.) f Number of storms Average duration (hrs.) Number less than 1 hr Number over 12 hrs Longest (hrs.) 60 4.8 21 5 54.5 131 7.9 14 28 35.2 28 11.2 1 7 52.7 219 7.5 36 40 54.5 58 4.0 13 6 25.1 106 8.6 13 24 35.2 24 16.1 2 14 71.7 188 8.1 28 44 71.7 116 5.1 26 16 33.9 55 8.9 4 14 39.0 35 8.9 5 6 50.7 206 6.8 35 36 50.7 208 4.0 54 15 38.5 17 8.1 1 3 29.3 26 7.6 5 5 43.1 251 4.7 60 23 43.1 315 3.0 95 13 33.0 2 11.7 0 1 13.0 5 5.3 0 0 7.7 322 3.1 95 14 33.0 281 2.1 142 8 22.7 0 0 0 0 0 7 1.6 2 0 2.6 288 2.1 144 8 22.7 229 2.2 93 3 17.2 0 0 0 0 0 2 1.2 1 0 2.1 231 2.2 94 3 17.2 191 2.5 69 4 22.5 0 0 0 0 0 3 1.6 1 0 2.8 194 2.5 70 4 22.5 234 3.3 74 14 23.7 0 0 0 0 0 0 0 0 0 0 234 3.3 74 14 23.7 164 3.9 43 10 52.6 2 0.4 2 0 0.8 5 5.1 1 0 10.1 171 3.9 46 10 52.6 143 5.1 30 16 29.7 34 6.3 3 4 16.0 27 10.7 1 10 31.3 204 6.0 34 30 31.3 79 6.9 13 15 30.5 94 8.0 8 22 28.1 36 11.0 1 13 45.3 209 8.1 22 50 45.3 2,078 3.9 673 125 54.5 441 7.5 45 96 39.0 198 7.3 20 55 71.7 2,717 4.9 738 276 71.7 Table LXXXIV includes only storms from 1891 to 1910, in which 0.01 inch or more of rain or (melted) snow, sleet, etc., fell continuously, and only those for which the times of beginning and ending were recorded definitely, or could be closely approximated from the official records. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 191 amount of precipitation is then greater than it is during the winter. In individual records, January shows a rainstorm lasting 54.5 hours, while the longest record for July is 17.2 hours. Snowstorms of between 35 and 40 hours have occurred in January, February, and March, and a mixed storm of 71.7 hours in length occurred in February. The mixed storms of the winter are really the longest of any kind or season, as they usually occur in connection with widespread disturbances which give during their passage sleet and rain on the eastern side, turning to snow as the center of the area passes the point of observation. Out of the total number of storms recorded in the table, 738, or about 25 per cent, lasted less than 1 hour, and 276, or about 10 per cent, lasted more than 12 hours; of the rainstorms, 673, or about 33 per cent, lasted less than 1 hour, and 125, or about 6 per cent, more than 12 hours; of the snowstorms, 45, or about 10 per cent, lasted less than 1 hour, and 96, or about 20 per cent, more than 12 hours; of mixed storms, 20, or about 10 per cent, lasted less than 1 hour, and 55, or about 25 per cent, more than 12 hours. It is apparent, therefore, that the average duration of both snowstorms and mixed storms is considerably greater than that of rainstorms. RAINSTORMS AND SNOWSTORMS OF LONG DURATION It quite often happens that during the passage of a general disturbance across the region in which Chicago is situated there occur a number of separate falls of rain or snow which must, nevertheless, be considered as a part of the whole storm, and they usually happen with only short intervals of time between. Table LXXXV contains a list of such rainstorms and snowstorms from 1881 to 1910. The dates and duration in days of the various disturbances are given, together with the number of hours of actual precipitation, excluding any intervals between the periods of fall. The data in the table are entirely apart from those previously given on wet spells and excessive precipitation, although in some cases the same dates will be found in all sets. In Table LXXXV the precipitation fell without interruption of more than an hour or two, while in the table of consecutive days with precipitation (p. 173, Table LXXIV), the interval may have been more than twenty-four hours. For instance, if rain on one day ended before noon, and on the following day began again after noon, both days would be included in a series of consecutive days with rainfall, and yet the precipitation recorded might be in192 THE WEATHER AND CLIMATE OF CHICAGO connection with two separate storms following closely the one after the other. Again, the amount of precipitation has no relation to the length gf the entire storm, and the period, however long, is not necessarily one with excessive precipitation, or a wet spell, as the TABLE LXXXV Examples of Rainstorms and Snowstorms of Long Duration, 1881-1910 Year Period Hours of Actual Precipitation Amount Inches 1881 October 16-18 47 0.95 December 19-22 58 1.44 1882 April 9-11 42 1.49 May 11-13 41 0.76 August 30-September 1 37 0.32 December 19-21 47 1.20 1883 January 15-17 46 0.64 1884 December 5-7 34 0.97 1885 September 7-10 52 1.91 December 12-14 42 0.88 1886 March 30-April 1 46 0.78 November 16—18 44 0.85 1887 February 5-7 43 0.86 December 9-11 50 1.12 Year Period Hours of Actual Precipitation Amount Inches 1893.... April 19-22 84 0.56 1895.... December 18-21 76 5.48 1897.. .. January 1-5 78 2.49 1899.. .. October 26-28 58 1.60 1900.. ,. January 17-19 53 0.90 March 28-30 39 0.51 1901.... March 13-15 63 0.26 March 20-21 34 0.33 December 3-5 42 0.12 1902.. .. February 27-March 2 78 1.11 1905.... April 25-27 40 0.55 1906.... February 4-6 50 0.20 1910. ... January 12-14 54 1.30 Table LXXXV contains some examples of rainstorms and snowstorms of long duration. term is used in Table LXXVI. The long period of 84 hours in April, 1893, was of only light precipitation, the total amount being but 0.56 inch; while in the period of 76 hours in December, 1895, the amount for the storm was 5.48 inches. PERIODS OF DROUTH In Tables LXXXVI and LXXXVII are listed all periods of 15 days or more in which an inappreciable amount of rain—that is, less than 0.01 inch—and in which no rain at all fell, respectively. The longest period without precipitation of any kind covered the first three weeks of February, 1877, and was 21 days in length. During the remainder of that month only 0.06 inch fell, which makes it the driest month in the history of Chicago’s weather. Only 8 other periods of 15 days or more without precipitation have occurred since the beginning of the official observations, while there have been only 26 periods of 15 days or more altogether in which not more than a utrace” of precipitation occurred. The longest and one of the second longest of these, however, fell close together in the summer of 1893, during the time of the Columbian Exposition in Chicago, in one of the most remarkable periods of drouth for this locality. FromPRECIPITATION 193 July 17 to August 10 of that year, a period of 25 days, only an immeasurably small amount of rain fell, and the same was true of the 26 days following from August 17 to September 11. Between these two periods 0.18 inch occurred, 0.02 inch on the 11th and 0.16 inch on the 16th, this amount being the total precipitation for that August. TABLE LXXXVI Periods op Drouth, 1871-1913 Days Time Year 26 August 17-September 11 July 17-August 10 August 13-September 6 December 15, 1899-January 8 June 26-July 19 October 7-29 1893 25 1893 25..... 1899 25 1900 24 1898 23 1872 22 March 29-April 19 October 16-November 5 1887 21 1895 21 December 30,1901-January 19 October 18-November 6 1902 20 1891 20 July 21-August 9 September 21-October 10 October 12-31 1894 20 1897 20 1897 Days . Time Year 20 February 26-March 18 1910 19 December 21, 1897-January 8 1898 18 August 14-31 1879 18 August 29-September 15 1897 17 November 3-19 1907 17 September 22-October 9 1909 16 September 28-October 13 1886 16 October 24-November 8 1887 16 October 27-November 11 1893 16 November 14-29 1894 16 February 4-19 1910 16 June 21-July 6 1912 15 February 1-15 1913 Table LXXXVI contains a list of dry periods of 15 days or more at Chicago, from 1871 to 1913, in which only a trace of rain fell, arranged in order of length. From the beginning date of this dry spell on July 17 to September 12, on which day 0.03 inch occurred, a period of 58 days, the total precipitation was but 0.21 inch, and the time is the most prolonged period of drouth ever experienced in Chicago. Moreover, the rain which fell on September 13 was light, being only 0.22 inch, and it TABLE LXXXVII Periods op Drouth, 1871-1913 Days Time Year 21 February 1-21 1877 19 October 12-30 1897 18 July 22-August 8 1908 16 February 4-19 1873 16 October 1-16 1896 Days Time Year 16 May 15-30 1881 16 August 7-September 1 1889 15 August 31-September 14 1871 15 October 7-21 1900 Table LXXXVII contains a list of dry periods of 15 days or more at Chicago, from 1871 to 1913, in which no rain whatever fell, arranged in order of length. was not until after the 17th that the situation was relieved by sufficient rainfall. Another period, almost as remarkable, was that of 57 days, from November 6, 1872, to January 1, 1873, during which time the total fall was only 0.36 inch. These periods, together with all others of 2 weeks or more in which 0.10 inch or less occurred for each 2 weeks of time, are shown in Table LXXXVIII, whichTABLE LXXXVIII Dry Spells op Two Weeks or Longer, 1871-1911 CD Year January February March April May June July August September October November December Year 1871 5-22, 31 .09-18, .00 -14, 17 -15, .11 -8* 25- 1871 -22 1872 -18 21 -6 7-29 6 1872 .15-25 .03 -15 T-23 .36-57 1873 -1 4-25 31 -21 14- 1873 .10-22 .13 -22 .05-21 1874 -3 10 -6, 8 -4 8-21 17- 1874 .20 -27, .15 -28 .05-14 .12-20 1875 -5 9-23 1875 .09-15 1876 14 -2 22 -12 1876 .06 -19 .09 -21 1877 17 -28 21 -6 6-24 1877 .17 -43 .07 -17 .03-19 1878 23- 1878 1879 -14 14 -2 14-31 18 -1 1879 .07-23 .01 -19 T-18 .06 -15 1880 21- 1880 1881 -12 15-30 23 -5, 19 -14, .05 -2 23- 1881 .09-23 .00-16 .03 -15 .04-15 1882 -6, 27 .09 -17 3-20 1882 -22 .04-18 1883 7-25 28 -26 27 -15 1883 .07-19 .16 -30 .10 -19 1884 14—31 9-22 1884 .10-18 .02-14 THE WEATHER AND CLIMATE OF CHICAGO1885 16 .05 -5 -18 1885 1886 15 -4, 17 -8 28 -13 1886 .07 -21, .09 -22 T -16 1887 7-26, 29 -19 10 -10 24 -8 1887 .08-20, T -22 .10 -32 T -16 1888 2-20 18 -7 1888 .03-19 .09 -21 1889 21 -9 9-28 14 -3 1889 .08 -20 .01-20 .05 -21 1890.. 25 -16 18 -1, 6-30 1890 .07 -23 .04 -14, 10-25 1891 3-20 19 -8 4 -3, 8 -6 1891 .04-18 .05 -21 .12 -30, .12 -30 1892 14 -14 1892 .14 -31 1893 25 -9 17 -12 31- 1893 .02 -16 .21-58 1894 -14 24 -14 1-19, 21 -9, 15 -2, 23 -18 14 -9 1894 .01-15 .02 -22 .07-19, T -20, .07 -19, .11 -26 .09 -26 1895.... 5-30 9 -4 17 -5, 12 -5 1895 .09-26 .08 -26 .05 -19, .01 -25 1896 3-16 29 -13 1-16 27 -31 1896 .04-14 .06 -15 .00-16 .16 -35 1897 15 -15, 21 -31 18- 1897 .13 -32, .18 -41 1898 -8 26 -19 1898 .02-22 T -24 1899 29 -20 31 -1 12 -6, 29 -14 15- 1899 .07 -23 .14-32 .04 -26, .04 -16 T-25 1900 -8, 20 -3 18 -1 25 -11 30 -21 8-29 1900 T -15 .02 -14 .03 -18 .07 -22 .10-22 PRECIPITATION« TABLE LXXXVIII—Continued CO 05 January February March April May June July August September October November December 1901 18 -7 1-14 3-17, 30 .08-15 -17, 23 -8, 16 -17, .01 -8, 14 -23, .08 -2 26- 1901 .02 -18 .09-14 .03-19, 04 -20 1902 -19 7-21 21 -8 1902 .01-25 .01-15 .05 -19 1903 1-21 17 -25 1903 .03-21 .15 -40 1904 5-19 26 -12 13 -7, 11 .01 -1 1904 .03-15 .03 -18 .10-26 -21 1905 26 -12 19 -9 4-19 1905 .01 -15 .02 -21 .01-16 1906 9-26 ,1906 .08-18 1907 16-31 3-19 1907 .02-16 T-17 1908 18 -10, 30 -24, .01 -18 9-22,- 28 T-14, .09 -21 1908 .05 -20 -25 1909 23 -9 1909 T -17 1910 3-19, 27 .01-17, T -18, 21 -3 5 -10, 23 -36 .12 -14 7-20 5-19 1910 -20, .05 -14 .14 -23 .03-14 .06-15 1911 14 -4, 26 -22 .04 -11 1911 .05 -14 Table LXXXVIII gives all periods from 1871 to 1911, inclusive, of two weeks or longer, during which the amount of precipitation was less than 0.10 inch for each two weeks. The limiting dates of each period are shown in light-faced type. The first bold-faced figures give the amount of precipitation in inches, and the second, the duration of the dry spell in days. Such spells occur only when dry weather prevails also over a large expanse of the surrounding country, and in most instances are either immediately preceded or followed by abundant precipitation; * No record from 8th to 15th, inclusive. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 197 gives the limiting dates, the number of days of each dry spell and the amount of precipitation, throughout the official period. Such periods are limited to no particular time of the year, but are most numerous in the late summer and autumn. Of the number listed in the table, 43 occurred or began during the first six months of the year, and the remainder, 78, during the last six months. No year has been entirely free from these periods of dryness, but there are a number of instances in which none occurred during the growing season. The years 1894 and 1901 were marked by repeated spells, especially after July 1, there being 7 in each of these years. Both were dry years throughout, 1901 being the year of least precipitation in the official record, and with the record-breaking temperature of 103° in July. A drouth period of more than ordinary interest to Chicagoans is that covering 22 days, from September 17 to October 8, 1871, just prior to the Great Fire. But 0.11 inch of rain fell during the 22 days mentioned, and, indeed, for more than two and a half months previous there had been comparatively little rainfall; and it is undoubtedly because of the extreme dryness of the buildings, due to the drouth, that the fire spread so rapidly. The daily amounts of rainfall from July 1, 1871, to the time of the fire, are given separately in Table LXXXIX. TABLE LXXXIX Drouth Preceding the Chicago Fire op 1871 (Precipitation, in inches, by days, with departures from the normals) Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 July 1.57 0.15 0.23 ).01 — August 0.73 T T 0.09 September 0.53 3.10 — October* * Month 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Total Depar- ture July 0.34 0.01 T 0.20 0.01 0.15 — 2.52 2.01 0.74 0H -1.14 -0.92 -2.23 -0.65* August 0.30 6.08 0.11 0.66 September October* * Data in October to and including the 8th day only. T indicates “trace” of precipitation. Table XC contains a statement of the number of periods of 10 days or more, by months, from 1871 to 1910, inclusive, with precipitation of less than 0.01 inch, the number totaling for the entire time 125, which would be an average occurrence of about 3 for each198 THE WEATHER AND CLIMATE OF CHICAGO year. The average duration is 13 days, and the longest on record entirely within a month was 23 days. The table was prepared TABLE XC Summary op Dry Spells in 40 Years, by Months, 1871-1910 Jan. Feb. Mar. April May June July» Aug. Sept. Oct. Nov. Dec. Period Total number Average duration 11 6 5 6 6 6 18 14 15 20 7 11 ,125 (days) Greatest duration 12 15 13 13 12 11 12 13 12 14 12 12 13 (days) 19 21 18 19 16 13 19 19 15 23 17 17 23 Table XC contains a summary of the dry spells in 40 years, by months, 1871-1910, and is based upon periods of 10 consecutive days or more in which less than 0.01 inch of precipitation fell. by months strictly, no overlapping dates being counted, and the data therefore cannot be compared with those of the preceding tables on this subject. FREQUENCY OF PRECIPITATION, DAILY The number of times on each day of the year .that precipitation of measurable amounts has occurred is shown in Table XCI, for the period 1871 to 1910, inclusive, the data furnishing in some measure the probability of occurrence on any particular date. Some notable irregularities are apparent, however. It can scarcely be considered strange that precipitation has occurred 24 times on March 19, inasmuch as March is the month of greatest frequency (p. 168), and there are two other dates in March with records of more than 20 times; but that it should have occurred on March 13 but 8 times and on March 29 but 7 times, while the days before and after in both instances show at least twice the frequency, is remarkable. Other dates which show very few instances of precipitation are October 21, with the lowest record of 5 times, July 15, August 7, 8, 31, September 1, and December 15, with 6 times each. Scanning the table from the beginning to the end of the year, there appear to be short periods of from 3 to 5 days each on which the occurrence of precipitation is high, followed by similar intervals of low frequency, but there is not sufficient regularity to establish any rule of recurrence of either class. However, the table appears to furnish interesting and conclusive disproval of the belief, somewhat widely held, that storms are much more frequent during and immediately after the times of the equinoxes. The average occurrence on each date of the 5-day intervals from March 21 to 25, and September 21 to 25, 13.6PRECIPITATION 199 and 11.2, respectively, are in each case lower—and in March, considerably lower—than the average occurrence for the months in which the periods fall. On the other hand, the 5-day intervals from the 8th to the 12th in each of these two months show average occur- TABLE XCI Frequency of Precipitation on Each Day of the Year, 1871-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 11 14 19 14 18 16 10 16 6 15 11 15 2 18 12 16 12 21 17 18 11 8 12 19 16 3 15 18 14 13 20 16 19 10 15 8 9 17 4 16 18 16 11 14 15 16 17 13 17 10 18 5 9 17 14 16 15 15 15 13 13 13 16 16 6 15 15 15 15 11 17 16 10 10 14 11 15 7 11 14 15 12 14 18 14 6 12 13 16 17 8 16 16 15 14 18 18 12 6 14 8 13 12 9 13 14 14 18 15 15 18 12 16 8 17 10 10 8 14 16 18 18 15 11 13 12 10 17 16 11 16 16 19 17 16 11 8 13 13 14 17 13 12... 18 15 16 16 10 12 10 12 15 9 10 15 13 19 14 8 19 18 17 8 11 12 13 11 18 14 13 15 17 14 15 15 15 11 10 12 15 17 15 21 15 17 21 14 18 6 15 12 11 13 6 16 15 12 12 11 10 13 12 12 14 13 17 14 17 11 14 11 12 16 10 13 7 7 11 17 18 18 15 14 21 20 17 15 13 8 14 12 13 12 19 13 16 24 17 17 9 11 13 12 11 14 14 20 18 20 19 10 13 21 12 8 9 11 10 18 21 18 10 13 16 15 16 13 13 6 5 13 18 22 17 18 10 15 17 14 8 9 10 10 14 14 23 20 16 13 12 15 14 7 13 14 8 11 15 24 11 17 15 15 10 11 10 12 12 7 12 16 25 12 16 17 17 15 17 16 13 14 14 15 19 26 14 15 22 18 13 13 13 11 12 19 21 13 27 15 11 14 18 14 ' 19 12 12 11 14 15 12 28 12 18 16 15 13 11 13 11 14 10 12 13 29 10 7 9 17 12 11 12 10 11 15 16 30 15 18 12 13 11 8 9 11 15 13 14 31 15 14 13 10 6 12 10 Annual Average 14.5 15.1 15.4 14.9 15.0 14.7 12.2 11.1 11.7 11.6 13.9 14.7 125.1 Greatest .... 21 20 24 21 21 21 19 17 16 19 21 19 24 Least 8 10 7 9 10 9 6 6 6 5 9 6 5 Average per- centage ... 36 38 38 37 38 37 30 28 29 29 35 37 31 Table XCI shows the number of times that rain or melted snow to the depth of 0.01 inch or more has fallen on each day of the year. * For 9 years. rences higher than those of the respective months; and in similar manner many other 5-day intervals throughout the year can be noted which have higher average occurrences than do the equinoctial periods. NORMAL PRECIPITATION, ANNUAL, MONTHLY, AND DAILY A table of mean daily precipitation for the 42 years of the official period would present irregularities even more pronounced than do the data in Table XCI. Such mean values for the city of Baltimore are given in the publications of the Maryland Weather Service,200 THE WEATHER AND CLIMATE OF CHICAGO Volume II, and the daily amounts are very irregular. For instance, the mean daily precipitation for August 26, 27, and 28 is 0.15, 1.03, and 0.14 inch, respectively, and other instances of wide variation are shown. During a period of from 30 to 40 years heavy rains may have occcurred several times on certain dates, while on the days both before and after the rainfalls may have been few and the amounts TABLE XCII Daily Normal Precipitation, in Inches, 1871-1906 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 0.06 0.05 0.08 0.08 0.10 0.13 0.12 0.12 0.09 0.10 0.09 0.06 2 .06 .06 .08 .08 .10 .14 .13 .10 .08 .09 .09 .06 3 .06 .07 .08 .08 .11 .13 .13 .08 .08 .08 .09 .06 4 .06 .08 .08 .08 .12 .13 .14 .08 .10 .08 .10 .06 5 .06 .08 .08 .08 .14 .13 .14 .08 .10 .08 .10 .06 6 .06 .09 .08 .09 .14 .12 .15 .07 .11 .08 .10 .07 7 .06 .09 .08 .09 .13 .12 .14 .06 .12 .08 .10 .07 8 .06 .09 .07 .10 .13 .12 .12 .06 .12 .08 .11 .08 9 .06 .09 .07 .10 .13 .12 .12 .06 .12 .08 .11 .08 10 .07 .09 .07 .10 .12 .12 .12 .06 .12 .08 .09 .07 11 .07 .09 .07 .10 .11 .12 .11 .06 .12 .08 .09 .07 12 .07 .09 .07 .11 .11 .11 .10 .07 .12 .08 .09 .07 13 .07 .09 .07 .11 .11 .11 .09 .07 .12 .08 .09 .07 14 .08 .08 .08 .11 .10 .11 .10 .08 .11 .09 .08 .08 15 .08 .08 .08 .11 .08 .11 .10 .08 .10 .09 .08 .08 16 .09 .08 .08 .10 .07 .12 .10 .09 .10 .09 .08 .07 17 .09 .07 .07 .09 .07 .12 .09 .09 .10 .08 .08 .07 18 .08 .07 .07 .09 .07 .13 .10 .10 .09 .08 .08 .07 19 .08 .07 .07 .10 .07 .13 .09 .12 .09 .08 .07 .07 20 .08 .07 .08 .10 .09 .13 .09 .12 .09 .08 .07 .07 21 .08 .07 .09 .10 .09 .13 .10 .12 .09 .08 .08 .07 22 .07 .07 .09 .10 .10 .13 .11 .12 .09 .08 .08 .07 23 .07 .07 .10 .10 .10 .12 .13 .13 .09 .08 .08 .06 24 .05 .07 .09 .09 .11 .12 .13 .13 .09 .07 .07 .06 25 .05 .07 .09 .09 .11 .12 .12 .13 .09 .07 .07 .06 26 .05 .07 .10 .10 .12 .11 .12 .12 .09 .08 :06 .06 27 .04 .08 .10 .10 .12 .11 .12 .11 .10 .08 .06 .06 28 .04 .08 .10 .10 .13 .12 .13 .10 .10 .09 .07 .06 29 .05 .10 .10 .13 .13 .13 .09 .10 .09 .07 .06 30 .05 .09 .10 .13 .12 .14 .09 .10 .08 .07 .06 31 05 .09 .13 .13 .09 .09 Sums 2.00 2.16 2.55 2.88 3.37 3.66 3.64 2.88 3.02 2.55 2.50 2.07 33.28 Table XCII contains the daily normal precipitation based upon records from 1871 to 1906. The daily values were obtained by using the means of eleven successive dates—that is, beginning with January 1-11, the mean precipitation was taken and entered against January 6; then taking January 2-12, the mean precipitation was entered against January 7; and in this way the normals were secured for each day of the year. This process gives values which approximate very closely the mean curve resulting from a much longer period of actual observations. light. Even the monthly precipitation shows a great variation, and the annual amounts indicate an irregularity from year to year. Yet it is important to have some means by which comparisons may be made, not only with regard to the precipitation of years and months, but with regard to the daily falls, as well. For this purpose the Weather Bureau has used the records of daily rainfall and snowfall from 1871 to 1906, inclusive, in calculating and smoothing out thePRECIPITATION 201 daily normals of precipitation throughout the year, and these normals, together with the monthly and annual values, are given in Table XCII. In order that the daily means of precipitation should approach even more closely these smoothed-out normals, the period of observations would necessarily have to extend over a series of years of perhaps twenty times the length of the official record, and the reason for the more or less arbitrary values is therefore obvious. EXAMPLES OF DEPARTURES FROM DAILY NORMAL PRECIPITATION, SELECTED YEARS Tables XCIII, XCIV, and XCV have been prepared in order to illustrate the use of the daily normals of precipitation in determining the departures throughout the year, from day to day, and from TABLE XCIII Daily Departure in Inches of Precipitation from Normal in a Wet Year, 1909 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 -0.06 -0.05 -0.08 -0.08 -0.02 -0.13 -0.12 -0.12 -0.09 — 0.10 +0.44 -0.05 2 — .06 - .06 - .07 - .07 — .02 + .32 + .74 - .10 + .14 — .09 — .09 + .07 3 — .06 - .07 - .08 - .07 — .11 - .13 - .13 - .07 + .21 — .08 — .09 - .06 4 — .06 - .08 - .08 - .08 — .12 + .73 - .11 - .07 - .09 — .08 — .10 + +4 5.. — .04 + .73 - .08 - .01 — .14 - .13 - .05 - .08 - .10 — .08 — .10 + .44 6 — .06 - .09 - .08 + .59 — .14 - .12 - .12 - .07 - .11 — .08 — .10 - .07 7 + .07 - .09 - .08 - .09 — .13 + .75 - .14 - .06 - .12 — .08 + .45 + .48 8 , .06 + -51 + .36 - ; 10 + .10 + .71 - .12 - .06 - .07 — .08 + .03 - .08 9 — .06 + .33 + .09 - .07 + .18 - .03 - .12 - .04 - .06 — .08 .11 - .08 10 — .07 - .09 - .07 - .10 .12 - .12 - .12 - .06 - .12 + .17 — .09 - .01 11 — .07 - .09 - .07 - .10 — .11 - .12 - .11 + .53 - .12 0 + .03 + .07 12 — .07 - .09 - .07 +1.05 — .08 + .04 - .10 + .16 - .12 — .08 .04 + .77 13 — .05 - .09 - .07 +0.04 — .09 - .11 - .09 + .02 - .12 — .08 — .07 + .52 14.. — .08 + .58 - .08 - .11 + .36 - .11 - .10 +3.41 + .99 — .09 — .03 - .06 15 + .08 + .32 - .05 - .01 .08 - .11 - .10 +0.32 - .10 — .09 — .08 - .08 16.. .09 + .06 - .06 + .29 — .07 - .12 - .10 - .09 - .10 — .09 + .88 - .03 17 — .09 - .07 - .07 - .09 — .07 - .11 - .09 - .09 - .10 — .08 .08 - .07 18 — .08 + .03 + .29 +1.16 — .07 - .13 + .03 - .10 - .09 — .08 — .08 - .07 19 — .08 + .02 - .07 -0.10 — .07 - .13 - .09 - .12 - .09 — .08 — .07 - .07 20 — .08 - .07 - .08 - .10 — .09 - .13 - .09 - .12 - .09 + .44 — .07 - .04 21 — .04 - .07 - .09 + .57 — .09 - .11 - .10 - .12 + .96 .08 + .08 - .03 22 + .16 + .03 - .09 - .10 — .10 + .67 - .11 - .12 + .73 + .09 +1.07 - .07 23 .07 + .44 - .10 - .10 — .10 - .02 - .13 - .13 - .09 + .10 -0.08 - .06 24 — .05 - .07 + .24 - .08 — .11 + .10 - .13 - .13 - .09 .07 — .06 + .18 25 — .05 - .07 - .09 - .09 + .56 + .50 - .12 + .01 - .09 — .07 — .07 + .44 26 — .05 - .07 + .19 + .09 + .13 - .11 - .10 - .12 - .09 — .08 — .06 + .09 27 — .04 - .07 - .10 - .01 .11 - .05 - .12 +1.11 - .10 — .08 — .06 - .05 28 + .67 - .08 - .10 + .07 — .13 - .12 - .07 -0.10 - .10 — .09 + .03 + .07 29 + .60 - .10 +2.65 .13 - .13 + .41 - .09 - .10 .09 .07 - .06 30 .05 - .09 -0.10 .13 - .12 - .14 - .09 - .10 — .08 — .07 - .06 31 .05 - .09 .09 - .13 - .09 .09 - .06 Monthly -0.04 +1.68 -0.92 +4.85 -1 .19 +1.43 -1.87 +3.32 +0.58 -1.35 +1.34 +2.11 Accumulated. .. * -0.04 +1.64 +0.72 +5.57 +4.38 +5.81 +3.94 +7.26 +7.84 +6.49 +7.83 +9.94 month to month. The years 1909, 1910, and 1911, wet, dry, and nearly normal, respectively, in their total amounts, have been selected, as they are quite typical of their classes. The accumulated departures at the bottom of the table, just beneath the monthly202 THE WEATHER AND CLIMATE OF CHICAGO departures, carry the excess or deficiency forward in each instance, and the last in each table indicates the departure for the year illustrated. In the wet year 1909, February, April, June, August, September, November, and December have each an excess of precipitation, while the remaining months have a deficiency. The accumulated departures, however, show that, beginning with February, there was an excess throughout the remainder of the year, the amount at the end of December being above the normal by 9.94 inches. TABLE XCIV Daily Departure in Inches op Precipitation from Normal in a Dry Year, 1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 -0.06 -0.05 -0.08 -0.08 +0.24 -0.13 -0.12 0 -0.09 -0.10 -0.01 -0.06 2 - .05 + . 13 - .08 - .08 -1.03 + .27 — .13 -0.10 — .08 - .09 - .08 - .06 3 - .06 - .06 - .08 - .08 -0 .11 - .13 — .13 - .08 + .47 + .88 - .08 + .02 4 + .64 - .08 - .08 + .35 — .12 + .28 — .14 - .08 + .95 + .06 - .08 - .06 5 - .06 - .08 - .08 + .13 — .14 - .13 — .14 - .08 + .31 + .08 - .10 - .05 6 - .06 - .09 - .08 - .08 — .14 - .07 — .13 - .07] + .06 + .15 - .10 - .07 7 - .06 - .09 - .08 - .09 — .34 - .12 — .14 - .06 — .12 - .08 - .10 - .06 8 0 - .09 - .07 - .10 — .04 - .12 — .12 - .06 0 - .08 - .11 - .08 9 - .06 - .09 - .07 - .10 — .13 - .12 — .09 - .06 — .12 - .08 - .10 - .03 10 - .07 - .09 - .07 - .10 — .04 - .12 — .12 - .06 — .12 - .08 - .04 + .16 11 - .07 - ,09 - .07 + .32 — .06 - .12 + .09 - .06 — .12 - .08 - .09 - .07 12 + .48 - .09 - .07 - .11 — .11 - .11 + .24 - .07 +1.16 - .08 - .09 - .07 13 + .52 - .09 - .07 - .11 — .11 - .11 — .09 - .07 -0.12 - .08 - .09 - .07 14 + .08 - .08 - .08 - .11 — .10 - .11 — .06 - .08 — .11 - .09 - .08 - .08 15 - .08 - .08 - .08 - .11 — .08 - .11 + .24 + .47 — .10 - .09 - .08 - .08 16 - .09 - .08 - .08 + .09 — .07 - .12 .10 + .06 — .10 - .09 - .08 - .07 17 + .61 - .07 - .07 - .06 + .18 - .12 + .30 - .09 __ .10 - .0^ - .08 - .07 18 - .05 - .07 - .07 - .04 .07 - .09 — .10 - .05 — .09 - .08 - .08 + .01 19 - .08 - .07 + .09 - .10 — .07 - .13 — .09 - .12 — .09 - .08 - .07 - .07 20 - .03 + .01 0 - .10 — .07 - .13 — .09 + .28 — .09 - .05 - .03 - .07 21 - .01 - .04 - .09 - .10 + .08 - .13 — .10 - .12 — .09 + .18 - .08 - .07 22 - .07 - .01 - .09 + .59 + .72 - .13 + .32 - .12 — .09 - .08 - .08 - .04 23 - .07 - .07 - .10 + -U + .53 - .12 — .13 +1.68 + .06 - .08 - .06 - .05 24 - .05 - .07 - .09 + -H .11 - .12 — .13 -0.13 + .02 - .07 + .05 - .06 25 - .05 - .07 - .09 + .43 — .11 - .12 — .12 - .13 — .09 - .07 - .07 - .03 26 + .01 + .45 - .10 + .12 — .12 - .11 — .12 - .12 — .03 - .08 + .10 - .06 27 - .03 - .08 - .10 - .07 — .12 - .11 — .12 - .11 — .10 - .07 + .62 - .06 28 - .04 - .08 - .10 - .10 + .15 - .12 — .13 - .10 — .10 - .09 - .07 + .65 29 - .05 - .10 - .10 + .21 - .13 — .13 - .09 — .10 - .09 - .07 + .02 30 + .03 - .04 + .53 .13 - .12 — .14 - .09 — .10 - .08 + .04 - .06 si - .05 - .09 .13 — .13 - .09 - .09 - .06 Monthly +1.07 -1.27 -2.26 +0.96 +1.30 -2.75 -1.85 +0.20 +0.88 -0.76 -1.19 -0.75 Accumulated +1.07 -0.20 -2.46 -1.50 -0.20 -2.95 -4.80 -4.60 -3.72 -4.48 -5.67 -6.42 In the dry year 1910, there was an excess of precipitation in January, April, May, August, and September, and in the remaining seven months a deficiency. Beginning with February the accumulated monthly departures indicate a deficiency which at the end of the year amounted to 6.42 inches. In 1911, the year which was nearly normal in total precipitation, there was a considerable deficiency throughout the first sevenPRECIPITATION 203 months, although there was a slight excess for the months of February and April, while May was exactly normal. Beginning TABLE XCY Daily Departure in Inches op Precipitation from Normal in a Nearly Normal Year, 1911 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 1 0 -0.05 -0.08 -0.08 +0.18 -0.13 -0.12 +0.14 -0.09 +1.12 -0.09 -0.03 2 — 0.06 - .06 - .08 + .16 - .09 - .05 - .13 - .10 - .08 -0.09 - .09 + .12 3 — .06 - .07 - .08 - .07 - .11 - .12 - .13 - .08 - .08 + .31 - .09 + .09 4 — .03 - .08 - .08 + .69 - .12 - .13 i- .14 - .08 - .10 - .08 - .10 - .06 5 + .07 + .63 - .08 - .08 - .14 - .13 - .14 - .07 + .07 - .07 0 - .06 6 .03 + .09 - .08 - .04 - .14 - .09 - .15 - .07 + .21 + .17 + .21 - .07 7 — .05 - .09 - .08 - .09 - .13 - .12 - .14 - .06 - .04 - .08 - .10 - .06 8 0 - .09 - .07 - .10 + .03 - .12 - .01 - .06 - .12 - .08 - .11 + .01 9 — .06 - .09 - .07 - .10 - .13 - .12 - .12 - .06 - .12 - .08 + .01 - .03 10 — .07 - .09 - .07 - .10 - .12 - .12 - .09 + .14 + .07 - .08 - .08 + .36 11 .. + .18 - .09 - .03 - .05 - .11 + .17 - .07 + .50 - .09 - .08 +1.39 - .07 12 — .07 - .09 + .19 - .07 - .11 - .11 - .10 + .75 - .09 - .08 -0.06 - .07 13 + .48 + .19 - .07 + .51 - .11 - .11 - .09 +1.36 - .04 - .08 - .09 - .07 14 — .08 - .08 - .08 - .11 - .10 - .10 - .10 +0.08 +1.04 + .01 - .02 - .08 15 — .08 - .08 - .08 - .11 - .08 - .11 + .31 - .08 -0.09 - .09 - .08 - .08 16 — .09 + .03 - .08 - .10 - .07 + .37 + .06 - .05 - .10 + .39 + .20 + .43 17 — .09 + .48 - .07 - .09 - .07 + .47 - .09 - .06 - .10 - .08 + .42 - .07 18 — .08 - .06 - .07 - .03 - .07 - .13 - .10 - .10 + .90 - .08 - .08 - .07 19 — .08 + .03 - .07 + .23 - .01 - .13 + .37 - .12 - .09 - .08 - .01 - .07 20 — .08 - .03 - .08 - .05 + .68 - .13 - .09 - .12 - .09 + .13 - .04 + .08 21 — .08 - .07 - .09 + .13 + .75 - .13 - .10 - .12 + .33 + .63 - .08 - .02 22 — .07 - .07 + .09 - .10 + .27 - .13 - .11 - .08 - .09 + .06 - .06 - .04 23... — .07 - .07 - .10 - .10 - .08 - .12 + .78 - .13 - .09 - .08 - .08 - .06 24 — .05 - .07 - .09 - .09 - .11 + .68 - .06 - .04 + .06 - .07 - .06 - .06 25 — .03 + .22 - .09 - .09 - .11 - .03 - .08 - .13 0 - .07 - .07 - .06 26 — .05 - .07 + .39 - .10 - .12 + .03 - .12 - .12 - .09 - .06 - .06 + .17 27 — .04 - .08 + .11 - .10 - .10 - .11 - .12 - .11 - .03 - .08 - .06 - .05 28 — .04 - .08 - .06 + .13 + .64 - .12 - .12 - .02 + .04 - .09 + .19 - .06 29 .05 + .07 + .21 - .10 - .13 - .13 - .09 - .10 - .09 - .07 - .05 30..... .05 - .03 - .06 - .13 - .12 - .14 - .09 + .01 + .14 - .07 + .51 31 .02 - .09 - .09 + .28 - .09 - .05 - .01 Monthly -0.83 +0.11 -1.10 +0.15 0 -1.12 -0.99 +0.84 +1.01 +1.24 +0.77 +0.47 Accumulated -0.83 -0.72 -1.82 -1.67 -1.67 -2.79 -3.78 -2.94 -1.93 -0.69 +0.08 +0.55 with August the precipitation was uniformly above the normal, so that by the end of the year the deficiency was made up, and the excess of 0.55 inch had been accumulated. HOURLY PRECIPITATION 1. Mean hourly rainfall.—The mean hourly rainfall, by months, from April to October, inclusive, is shown in Table XCVI. The 10-year period 1902-11 has been taken as a basis of calculation for the mean values given, as prior to 1902 the local hourly records were kept on Eastern Time, which differs ah hour from the standard of time in use in Chicago. The cold months could not be included because automatic records of precipitation are not possible in freezing weather. These values from hour to hour are apparently as irregular as the occurrence of precipitation from day to day (p. 198), andto o TABLE XCVI Mean Hourly Rainfall in Inches by Months, and Total Hourly Rainfall, April to October, 1902-11 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. April 0.11 0.11 0.10 0.12 0.12 0.13 0.10 0.09 0.14 0.12 0.11 0.06 0.06 0.06 0.11 0.22 0.15 0.17 0.21 0.12 0.23 0.31 0.19 0.08 May .15 .14 .16 .10 .12 .10 .19 .10 .12 .14 .16 .25 .06 .06 .12 .16 .16 .30 .14 .20 .24 .09 .12 .10 June .15 .14 .08 .13 .24 .16 .08 .06 .10 .07 .07 .06 .04 .06 .12 .19 .08 .08 .18 .10 .12 .16 .10 .13 July :. .07 .05 .04 .10 .16 .14 .14 .19 .06 .03 .01 .29 .31 .23 .16 .13 .11 .20 .10 .16 .36 11 14 07 August .23 .20 .28 .17 .08 .20 .25 .22 .06 .10 .10 .10 .06 .19 .14 .13 .11 .18 .27 .05 .02 22 17 16 September .12 .34 .19 .22 .11 .23 .17 .10 .09 .12 .07 .06 .06 .09 .12 .10 .14 .24 .15 .17 .31 26 15 21 October .10 .04 .04 .08 .06 .06 .05 .05 .05 .05 .03 .04 .02 .02 .04 .13 ,06 .08 .06 .12 .09 .09 .04 .04 Sums 0.93 1.02 0.89 0.92 0.89 1.02 0.98 0.81 0.62 0.63 0.55 0.86 0.61 0.71 0.81 1.06 0.81 1.25 1.11 0.92 1.37 1.24 0.91 0.79 Means 0.13 0.15 0.13 0.13 0.13 0.15 0.14 0.12 0.09 0.09 0.08 0.12 0.09 0.10 0.12 0.15 0.12 0.18 0.16 0.13 0.20 0.18 0.13 0.11 Table XCVI contains the average amount of rainfall which has occurred during each hour of the day of each month from April to October, inclusive, 1902-11. These months only are given, as the self-recording rain gage from which the records are secured is not exposed during the colder months. See Fig. 41. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 205 the table, together with Fig. 41, will at least be interesting in indicating these irregularities. The heaviest rainfall is shown to occur at about 1 and 9 p.m. in July, at about 2 a.m. and 9 p.m. in September, and at about 6 p.m. in May. The hours on either side of these periods are also marked by precipitation somewhat above the average, but it is probable that some of the heaviest records are affected by one or two heavy rains. However, while there appears to be no decided reason why greater rainfall occurs in some of the hours than in others, there is yet, in general, a relation to be seen between the times of greatest hourly rainfall and the times of occurrence of thunderstorms (p. 209). Mdt. I 234567 8 0 10 ll Noon I 2 3 4 5 6 ,7 S 9 10 li Mdt. Fig. 41.—Average hourly precipitation. Fig. 41 shows the average hourly precipitation in inches for each month from April to October, inclusive, 1902 to 1911. The lightest shading shows the time of the year and day when the precipitation averages the least, and the average depths increase with the shading (see Table XCVI). 2. Hourly frequency of precipitation.—-There is a much greater regularity in the mean hourly frequency of precipitation than was found in the mean hourly amounts of rainfall in the preceding paragraph. Table XCVII shows the number of times that precipitation of any amount or character has occurred in the respective hours for the period 1902-11, inclusive. The data in this table, however, are not strictly comparable, as the number of days differs from month to month, and the number of times in which precipitation is possible is less, therefore, in the shorter months of the year. Because of this fact, the values given have been reduced to the percentage basis, as shown in Table XCVIII, and the hourly frequency drawn in Fig. 42 from the latter values. This method affords proper means of comparison with regard to the monthly and annual percentages as well as to the hourly frequencies. Taking the day as a whole, theto o 05 TABLE XCVII Hourly Frequency of Precipitation (Number of Times), 1902-11 Mdt. 123456789 10 11 Noon 12345678 9 10 11 Mdt. Total No. Times Precipitation was Possible in Each Hour January 53 63 61 61 61 65 72 80 87 84 73 73 76 79 73 66 62 67 65 67 60 72 67 62 1,649 310 February 54 55 58 52 43 44 45 54 69 71 71 68 71 74 69 69 64 64 63 69 64 65 66 60 1,482 282 March 47 58 60 50 46 49 57 59 51 52 59 56 64 62 62 62 58 58 53 50 54 62 53 55 1,337 310 April 47 46 46 44 45 48 52 54 51 53 50 60 * 54 55 58 61 55 44 56 50 51 52 56 46 1,234 300 May 39 40 40 43 39 38 42 39 31 32 31 42 42 43 50 51 46 59 45 42 45 50 42 40 1,011 310 June 34 36 29 26 27 29 31 29 22 23 29 24 27 31 32 34 30 30 29 32 35 31 31 33 714 300 July 22 26 24 25 23 21 30 27 22 21 11 32 36 35 33 27 19 24 26 24 34 33 25 25 625 310 August 25 25 23 25 22 25 29 25 25 25 27 18 24 21 20 17 26 25 23 27 26 17 19 24 563 310 September 23 33 31 33 31 33 43 34 27 25 21 26 25 26 32 32 33 30 31 31 33 32 29 28 722 300 October 27 27 26 30 27 29 38 33 31 28 26 33 32 34 32 32 33 32 33 26 28 27 30 26 720 310 November 34 32 37 29 30 30 40 43 48 46 44 45 40 42 40 38 35 35 40 34 40 43 40 37 922 300 December 59 63 63 56 52 56 60 66 74 77 74 72 75 74 71 71 71 73 65 63 65 60 60 60 1,580 310 Total 464 504 498 474 446 467 539 543 538 537 516 549 566 576 572 560 532 541 529 515 535 544 518 496 12,559 Total for Period 87,648 Table XCYII shows the number of times precipitation of any amount and character has occurred within the respective hours for a period of 10 years, 1902-11. See Fig. 42. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 207 greatest frequency is in the colder months, because the monthly frequency is also then greatest (p. 168). But in comparing similar hours of the day throughout the different months of the year, it is apparent that precipitation occurs most frequently in the middle of the day in winter, while in the warmer season not much difference is to be noted from one hour to another. In the former period the percentage of frequency is from 20 to 28, which means that in January precipitation occurs between 8 a.m. and 4 p.m. on one day in about every four. The highest percentage, 28, is at 9 a.m. in this month. The least hourly frequency is 4 per cent, at 11 a.m. in July, and the 3-hour period ending at this time has the least frequency of the year, rain Md£ * £ 3 4■ ■ S 6 78 9 10 il Noon 1 2 3 4 S £ 789 /oil Mdt Fig. 42.—Hourly frequency of precipitation. Fig. 42 gives the hourly frequency of precipitation of any amount and character expressed in terms of percentage of possible frequency. The space without shading indicates the time of the year and day when precipitation occurs least, and the frequency increases with the depth of shading. The dotted lines, marked S.R. and S.S., show the time of sunrise and sunset, respectively (see Tables XCYII and XCVIII). \ occurring then on only one day in about sixteen. The frequency by hours gradually diminishes from January to July and August, and increases thereafter during the remainder of the year. The average hourly frequency for June, September, and October is 10 per cent, representing an occurrence of precipitation on one day in ten. The least is in July and August, when the percentage is 8, and the greatest is in January and February, 22 per cent. The annual hourly frequency is 14.3 per cent, giving an average occurrence for any hour of one in seven. In these data all precipitation has beep, included, whether more or less than 0.01 inch in amount. A fall of 0.01 inch may extend over five or six hours, or even many more, and yet in each hour the fall would be inappreciable.bO o 00 TABLE XCVIII Hourly Frequency of Precipitation (Percentage of Possible), 1902—11 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. Mean January 17 20 20 20 20 21 23 26 28 27 24 24 24 26 24 21 20 22 21 22 19 23 22 20 22 February 19 20 21 18 15 16 16 19 25 25 25 24 25 26 25 25 23 23 22 25 23 23 23 21 22 March 15 19 19 16 15 16 18 19 16 17 19 18 21 20 20 20 19 19 17 16 17 20 17 18 18 April 16 15 15 15 15 16 17 18 17 18 17 20 18 18 19 20 18 15 19 17 17 17 19 15 17 May 13 13 13 14 13 12 14 13 10 10 10 14 14 14 16 16 15 19 14 14 14 16 14 13 14 June 11 12 10 9 9 10 10 10 7 8 10 8 9 10 11 11 10 10 10 11 12 10 10 11 10 July 7 8 8 8 7 7 10 9 7 7 4 10 12 11 11 9 6 8 8 8 11 11 8 ‘ 8 8 August 8 8 7 8 • 7 8 9 8 8 8 9 6 8 7 6 6 8 8 7 9 8 6 6 8 8 September 8 11 10 11 10 11 14 11 9 8 7 9 8 9 11 11 11 10 10 10 11 11 10 9 10 October 9 9 ' 8 10 9 9 12 11 10 9 8 11 10 11 10 10 11 10 11 8 9 9 10 8 10 November 11 11 12 10 10 10 13 14 16 15 15 15 13 14 13 13 12 12 13 11 13 14 13 12 13 December 19 20 20 18 17 18 19 21 24 25 24 23 24 24 23 23 23 24 21 20 21 19 19 19 21 Annua 1 14.3 Table XCVIII shows the hourly frequency of precipitation of any amount and character in terms of the percentage of possible occurrence. This table is based upon Table XCVII and the values are graphically represented in Fig. 42. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 209 SUMMARY OF PRECIPITATION DATA The main features of the precipitation data treated in the foregoing pages and tables have been collected in a single page. Table XCIX will therefore furnish a summary of the more important values for reference. THUNDERSTORMS Thunderstorms usually occur in the southeast quadrant of a general storm area, and are often attended by severe squalls and hail. Damage by lightning seldom or never occurs in the business district of Chicago, because the discharges are carried away quietly into the ground by the steel framework of the numerous skyscrapers. Damage does, however, occur sometimes in the residence sections, where the means of protection mentioned is wanting. Even there thunderstorms are rarely of the severest type, such as those which occur in the states of the Great Plains, and it is probable that the cool waters of the lake exercise a decided weakening influence upon the activity of these disturbances. 1. Annual and monthly frequency.—Table C gives the number of days with thunderstorms each month and year from 1880 to 1913. The annual occurrence is graphically shown in Fig. 43, and the average monthly frequency in Fig. 44. The average annual number of such days is 33, but the figures for the various years indicate a wide range, from 51 in 1904 to 18 in 1893, one of the longest and most pronounced summer drouths occurring in the latter year (p. 192). June is the month of greatest frequency, with an average occurrence of 6.3, but each month from May to August averages 5 or more. The frequency is, of course, least in the winter season, averaging less than 1 each month from November to February, with the lowest average 0.1 in December. While June is the month of greatest frequency, and several instances of the occurrence of 10 or more days will be noted in that period, the highest individual record is 14, in May, 1902 and 1912. Two thunderstorms occurred in January, 1909, 1907, and 1906, and two in February, 1906, but in no other winter months have more than one been recorded. 2. Hourly frequency.—Table Cl and Fig. 45 show the average occurrence of thunderstorms by hours, based upon data from 1901 to 1910, inclusive. It is apparent that by far the larger proportion of these storms happen during the latter half, or heated portion, of the day. A count of those recorded in the table gives 303 of thefco I—i o TABLE XCIX Summary op Precipitation Data, 1871-1910 Means Monthly and Annual Amounts Number op Days with Precipitation* § £ ® Greatest in Twenty-pour Hours a> bO^ £ g Mean Departure Greatest Least g eSM p f Q So 01-1 r, H® B QÎ 1—1 1 i-t « r 05 < 1 a> S S ö O Ö Ö 03^ 03 Inches Per- centage Depth Year Percentage of Mean Depth Year Percentage of Mean Average Greatest Least Depth Year January 2.10 6 0.10 5 4.53 1897 216 0.54 1879 26 . 11 19 5 7.5 1.80 1907 February 2.30 7 0.14 6 5.98 1881 260 0.06 1877 3 11 17 2 8.1 1.94 f 1883 11876 March 2.51 7 0.04 2 5.37 1877 214 0.29 1910 12 12 19 3 6.8 3.26 1884 April 2.87 9 0.01 0 7.73 1909 269 0.14 1899 5 11 18 5 4.7 2.75 1909 May 3.54 11 0.17 5 7.32 1883 207 0.84 1897 24 12 17 6 3.1 2.82 1873 June 3.49 10 0.17 5 10.58 1892 303 0.55 1904 16 11 19 4 2.1 3.44 1885 July 3.53 11 0.11 3 9.56 1889 271 0.58 1874 16 10 17 2 2.2 4.14 1878 August 3.10 9 0.22 7 11.28 1885 364 0.18 1893 6 9 15 2 2.5 6.19 1885 September 3.09 9 0.07 2 8.28 1894 , 268 0.32 1891 10 9 14 2 3.3 3.44 1875 October 2.38 7 0.17 7 7.36 1883 310 0.18 1897 8 9 19 1 3.9 2.55 1877 November 2.52 8 0.02 1 6.08 1877 241 0.31 1904 12 10 17 4 6.0 3.39 1883 December 2.09 6 0.02 1 6.76 1895 323 0.16 1896 8 11 20 5 8.1 2.66 1895 Year 33.52 100 1.24 4 45.86 1883 137 24.52 1901 73 125 166 in 1878 106 in 1895, 1910 4.9 6.19 1885 * Omitting days with only a trace of rainfall or snowfall. Table XCIX contains a summary of the more important precipitation data. The first column contains the mean precipitation, the second column the same values expressed in percentages of the annual mean, the third column contains the average departure from the mean, either above or below, and the fourth column contains the same values expressed in percentages. The remaining columns are self-explanatory. THE WEATHER AND CLIMATE OF CHICAGOPRECIPITATION 211 total number of 438, or 68 per cent, as having occurred in that period. It is evident from the graph that the afternoon is divided into two shorter periods of comparatively great frequency, from 1 to 5 p.m., and from 8 to 11 p.m., while the individual hours at which thunderstorms recur oftenest are 1 p.m. and 5 p.m. in July, when the average TABLE C Number op Days with Thunderstorms Each Month and Year, 1880-1913 (See Figs. 43 and 44) is 9 at each. Taking the year as a whole the time of greatest frequency is 9 to 11 p.m., and of the least, from 2 to 3 a.m. The ocur-rence of comparatively many thunderstorms at about 4 a.m. in May will be noted. This feature is remarkable, and finds its explanation in the alternation of lake and land breezes, already described at some length (p. 142), the night land breeze being at its height at that hour (p. 306), and with the rapidly increasing warmth of spring bringing about conditions favorable for the formation of thunderstorms.212 THE WEATHER AND CLIMATE OF CHICAGO 1880 1885 J890 /895 19 00 1905 1910 50 40 30 20 10 M n ill j I I I ■ 111111 I I I 50 4 O 30 20 10 Fig. 43.—Annual frequency of days with thunderstorms from 1880 to 1910 (see Table C). JFMAMJ JASON D .Til ill ill Fig. 44.—Average monthly frequency of days with thunderstorms, 1880 to 1910 (see Table C).TABLE Cl Frequency and Time Distribution op Thunderstorms, 1901-10 Mdt. 1 2 3 4 5 6 7 8 9 10 11 Noon 1 2 3 4 5 6 7 8 9 10 11 Mdt Total January 1 1 1 1 1 2 7 February 1 1 1 1 1 5 March 1 2 1 2 1 1 1 3 2 2 4 3 2 3 28 April 2 1 1 1 1 1 1 1 1 2 2 6 2 1 3 2 6 2 36 May 3 1 6 3 3 1 2 3 4 5 5 4 4 3 6 3 5 5 3 g 75 June 4 4 2 3 1 2 1 4 1 5 4 6 5 5 3 5 3 4 5 7 A o July 2 2 1 3 1 1 1 2 3 4 9 6 4 4 9 5 i 6 8 7 1 August 2 3 1 1 2 2 1 1 2 1 3 5 3 5 3 2 6 2 6 2 3 0 September 3 2 5 1 2 1 1 1 1 3 1 5 2 2 3 2 4 1 2 1 October 1 1 1 1 1 1 2 3 1 2 1 15 November 1 1 1 1 1 1 1 2 q December 0 Total 16 15 7 14 14 7 11 8 8 11 8 16 26 28 26 32 27 26 24 24 35 23 21 11 438 Table Cl shows the frequency and time distribution of thunderstorms based upon a record of 438 such storms from 1901 to 1910. The numbers in the various columns indicate the number of thunderstorms beginning within the respective hours, without regard to the length of time the storm continued. When more than one thunderstorm occurred on the same day, each storm is included in the list above; whereas, in Table C, only the number of days on which thunderstorms occurred is used. See Fig. 45. to t—4 CO PRECIPITATION214 THE WEATHER AND CLIMATE OF CHICAGO As the thunderstorm is a relatively small, local disturbance within the area of a much larger, general storm, it occasionally happens that when the latter moves across the country with less than the average velocity a succession of thunderstorms will occur within a space of several days at one place. A period of this character prevailed at Chicago on June 22, 23, and 24, 1892, during the meeting of the Democratic convention which nominated Cleveland for President. There were two thunderstorms on the 22d, one in the afternoon and the other in the evening continuing into the early 12 34-5 67 8 9IOII NOON I Z 3 4 5 6 7 8 9 IO II «or. • • • • . • • 9 # • 9 • • 2. m ¡P • • • • ¡P p • • • 1 • • • • • • 4-% • % • In • • f • ^ • • • • m' 1 » • n p • p m • • gg • K • • É • • li « 9 « /Æp rip ÉP • Ip gp 1 m m m i gjp * If a w % Ü * 1 Ü ■ 1 I jj I À |p em % • ■ 1 w §L % P2 ^ 9 9 9 9 Fig. 45. Fig. 45 shows the frequency and time distribution of thunderstorms, based upon records of 438 thunderstorms from 1901 to 1910. The dots in squares show number of thunderstorms for the hours not included in the shading. The heaviest shading shows the time of year and hour of greatest frequency (see Table Cl). morning of the 23d. A third quite heavy storm occurred on the afternoon of the 23d, and still another on the morning of the 24th. All these storms were marked by heavy thunder, vivid lightning, excessive rainfall, and on the 22d and 23d by severe squalls. On the second day 1.60 inches of rain fell in 1 hour, from 4:10 to 6:10 p.m., and 4.77 inches fell in the whole series of storms during the three days. HAIL Table CII gives the occurrence of hailstorms from 1887 to 1913, inclusive. When hail occurs it is usually in connection with thunderstorms. During the period of 27 years shown in the table, hail hasPRECIPITATION 215 been noted 66 times. In 1897 the phenomenon occurred 7 times, while in three years, 1887, 1892, and 1913, it was absent altogether. It is most frequent in May, at the time of the most rapid change in temperature, and has occurred 16 times in that month during the period of record. The frequency of May is followed in order by June, with a total occurrence of 13, April with 11, and March with 10. It TABLE CII Frequency of Occurrence of Hail, 1887-1913 occurs much less frequently after warm weather has set in, July and August having 6 and 5 hailstorms, respectively, and is noted but rarely during the other portions of the year. It has never been observed in September, November, or January, and has occurred but once in December, once in February, and twice in October. Hail is entirely distinct from sleet, which falls often in the winter season. SNOW Previous to 1884 the Weather Bureau maintained no distinct records of depth of snow, the falls being measured merely as melted, and included in the precipitation data together with rain. The combined record is still kept, but since the winter of 1884-85 the actual depth of snow has also been entered, regardless of the amount216 THE WEATHER AND CLIMATE OF CHICAGO of moisture contained therein. The average snow, when melted, will produce one-tenth of its depth in water, but there is a considerable variation from this proportion. Very wet snow may contain one-third or even one-half of its depth in water, while, on the other hand? light, dry, feathery snow may not give when melted more than one-twelfth to one-fifteenth of its depth in water. A snow gage is maintained at the Weather Bureau office in the Federal Building, but the swirl of winds affects the catch of such a gage much more than is the case in the measurement of rainfall. In order to secure as high a degree of accuracy as is possible under city conditions, the actual depth of each snowfall is also measured in another manner. For this purpose, three separate measurements are made in Grant Park and the readings are averaged, the latter being used as the official record. At present a portion of the snow is also weighed and the moisture content found, but formerly the average content—one-tenth of the depth—was entered in all cases, as no suitable device for determining the content under all circumstances had been found. Any fall of snow of less than 0.1 inch in depth is regarded as inappreciable, and is recorded as “trace,” as is an amount of less than 0.01 inch of rainfall. SEASONAL AND MONTHLY SNOWFALL The total and average snowfall by months and winters is shown in Table CIII, and the fall by winters, together with the average monthly amounts, is illustrated in Fig. 46. While there is a very great variation in the fall of snow from month to month, and from season to season, the average fall through the winter months, as seen from both graph and table, increases in a fairly gradual manner from the earliest snows of autumn to late winter, February having the greatest mean, 11.3 inches. The decrease in the amount of snowfall after the February maximum is reached is much more rapid than is the increase up to that time, chiefly because mean temperatures show then a greater rate of change and reach the melting point early in March. Although February has the greatest mean monthly snowfall, that of January is only slightly less, 10.2 inches; and, as a matter of fact, the period of heaviest snow extends from the latter portion of January to about the middle of February, there being a decided falling off during the last decade of the second month. The main snow-bearing wind at Chicago is that from the northeast, because fit approaches the city after passing over the broad expansePRECIPITATION 217 of the lake and is laden with moisture evaporated from its surface. The chill received from the colder land surface in winter, when temperatures are below freezing, rapidly condenses and crystallizes this moisture into snow. And so, with the fall of mean temperatures in the winter season to the minimum about February 1 (p. 26), the amount of Snow gradually increases to its maximum, decreasing thereafter as the mean temperatures rise. In the spring and summer, TABLE CIII Monthly and Seasonal Snowfalls, in Inches and Tenths, 1884-1914 Season Oct. Nov.* Dec. Jan. Feb. Mar. April May Total for Season 1884-85.. T 0.5 8.8 20.2 19.0 3.6 1.9 0 54.0 1885-86 0 0.7 14.6 26.7 6.0 1.9 1.0 0 50.9 1886-87 0 2.6 9.8 17.7 4.2 6.2 T 0 40.5 1887-88 T 2.5 9.9 11.9 2.2 3.5 2.0 0.1 32.1 1888-89 T 0.5 3.2 6.0 7.9 5.1 T T 22.7 1889-90 T 1.3 T 2.7 8.3 9.4 T 0 21.7 1890-91 T T 7.1 3.5 1.1 7.7 2.2 T 21.6 1891-92 0 6.8 5.5 15.3 2.8 3.1 T 0 33.5 1892-93 0, 0.8 2.1 15.2 11.8 1.0 0.6 0 31.5 1893-94 0 7.5 12.1 6.5 12.9 5.4 T T 44.4 1894-95 0 2.5 10.1 15.4 14.0 5.2 0 T 47.2 1895-96 0 14.5 3.4 2.0 27.8 8,9 T 0 56.6 1896-97 T 4.2 1.3 13.1 14.6 10.9 0.9 0 45.0 1897-98 0 T 6.2 15.9 13.7 1.0 T 0 36.8 1898-99 T 1.8 2.7 2.6 3.5 7.4 T 0 18.0 1899-1900 0 T 3.5 0.3 22.6 6.8 3.6 0 36.8 1900-1901 0 1.1 3.6 9.2 21.1 5.9 T 0 40.9 1901-2 0 0.1 4.2 6.2 5.5 2.1 0.1 T 18.2 1902-3 0 0.5 5.3 5.0 19.5 0.7 3.2 0 34.2 1903-4 0 2.2 18.6 11.0 13.4 14.3 T 0 59.5 1904-5 T T 6.8 8.5 14.1 4.5 0.2 0 34.1 1905-6 T T 2.8 2.9 5.9 9.0 0 0 20.6 1906-7 T 2.8 0.3 10.9 10.0 2.1 1.9 1.3 29.3 1907-8 0 3.0 12.8 13.2 19.8 0.1 T 0 48.9 1908-9 0 0.9 2.7 8.6 10.1 3.8 0.3 T 26.4 1909-10 T T 19.1 14.9 2.8 T 6.9 0 43.7 1910-11 T 1.2 9.3 2.2 9.8 4.9 2.4 T 29.8 1911-12* T 1.2 7.8 7.1 9.9 13.5 0.1 0 39.6 1912-13* 0 0.4 T 6.1 4.7 7.9 T 0 19.1 1913-14* 1.9 T 2.7 14.8 7.2 1.2 Average T 2.2 6.8 10.2 11.3 5.0 1.0 0.1 36.2 Greatest T 14.5 19.1 26.7 27.8 14.3 6.9 1.3 59.5 Year 1895 1909 1886 1896 1904 1910 1907 1903-4 Least Of T| Tf 0.3 1.1 T Of Of 18.0 Year 1900 1891 1910 1898-99 * Not included in averages, f In more than one year. however, the lake wind usually brings fair skies and bright sunshine, because its temperature is largely increased and its relative humidity correspondingly lowered as it passes over the warmer land surface, so that condensation into clouds and rain is very infrequent at such times. The greatest snowfall in any one winter was that of 1903-4, when 59.5 inches were recorded, an excess of 23.3 inches over the218 THE WEATHER AND CLIMATE OF CHICAGO average. This large amount was due chiefly to extraordinary falls of snow during December and March, nearly three times the average measurements being received in each of those months. The latter, 14.3 inches, was the greatest March snowfall on record, as with one i885*86 /890-3I 1835-96 ¡900-01 1905-06 I9IÖ-M. MEAN 36 2L inches. Fig. 46.—Total snowfall in inches by winters (upper graph) ; average monthly snowfall in inches (lower graph). exception was that of December, 18.6 inches. Of these amounts, 11.3 inches fell in the single storm of December 12, and 8.7 inches in the storm of March 14-15. By months, the greatest individual records are: October, 1913, 1.9 inches; November, 1895, 14.5 inches; December, 1909, 19.1 inches; January, 1886, 26.7 inches;PRECIPITATION 219 February, 1896, 27.8 inches, this being the greatest amount of snowfall on record at Chicago for any month; March, 1904, 14.3 inches; April, 1910, 6.9 inches, of this amount 6.4 inches falling on the 22d to 26th; May, 1907, 1.3 inches in the single storm of the 3d. Snow rarely falls in October, nor does it occur often in May. No instances of measureable amounts have ever occcurred in the former month except the unprecedented fall of 1.9 inches in 1913, and there have been but two times that more than a “trace” has occurred in May. Once in June, on the 2d, in 1910, “trace” of snowfall was observed at the Weather Bureau office, in a thunderstorm condition which indicated an overlying blanket of very cold air, although the surface temperatures were in the 40’s. The smallest snowfall in any one winter was that of 1898-99, when but 18.0 inches were noted, but the record is closely followed by that of 1901-2, when only 18.2 inches fell. Remarkably small amounts were recorded in the following months: December, 1889, “trace”; January, 1900, 0.3 inch; February, 1891, 1.1 inches; March, 1910, “trace.” There are several Novembers in which only a “trace” fell, and several Aprils in which no snow at all occurred. There is no fixed relation between the mean temperature of a winter and the amopnt of its snowfall. The greatest seasonal snow-fall, 59.5 inches in 1903-4, occurred in the coldest winter on record, 18?3. The next greatest amount, which was but little less than that of 1903-4, was 56.6 inches in the winter of 1895-96, which season had a mean temperature of 27?7, slightly above the average. Large amounts of snow fell also in the winter of 1884-85, which was colder than the average, and in the winter of 1885-86, which was warmer than the average. On the other hand, the smallest winter snowfall on record, 18.0 inches in 1898-99, occurred during a very cold season, while in 1905-6, with a mean temperature as much above the normal as the former was below, but 20.6 inches were recorded. During the winter of 1901-2, when only 18.2 inches of snow fell, the season was colder than usual; while in 1890-91, with a snowfall of but 21.6 inches, the mean temperature was considerably higher than the average. Absence of snow during a very cold winter causes much damage in the outer portions of the city, as was the case during the latter part of the winter of 1898-99, above mentioned, when many water and gas mains were frozen because they were unprotected from severe temperatures by snow covering. On the other hand, much snow in a very cold winter results in many accidents and great expense220 THE WEATHER AND CLIMATE OF CHICAGO in the business sections, because of the difficulty in removing the drifts and in keeping the pavements free from incrustations of ice. Such a condition existed during December and January in the winter of 1909-10, many accidents occurring on the slippery streets, while business was much delayed by innumerable piles of frozen snow in the thoroughfares, and by the ice coatings which rendered heavy loading impossible. Extreme cold, however, should obviously be unfavorable for the occurrence of any considerable snowfall, as the moisture content of the atmosphere at such times is very small, yet low temperatures serve to keep the snow covering on the ground after the storm. Ordinarily, considering the storms in which our snows commonly occur, the heaviest falls are accompanied by temperatures ranging between 20° and 28°. At Chicago, when the temperature falls as low as 10°, the termination of the storm is soon at hand. TABLE CIV Annual Depth of Snowfall, in Inches, 1885-1913 (See Fig. 47) Year Depth in Inches Year Depth in Inches Year Depth in Inches Year Depth in Inches 1885 60.0 1893 48.2 1901 40.5 1909 41.9 1886 48.0 1894 37.4 1902 19.7 1910 35.1 1887 40.5 1895 52.5 1903 49.2 1911 28.3 ' 1888 23.4 1896 44.2 1904 45.5 1912* 31.0 1889 20.3 1897......... 45.7 1905 30.1 1913* 23.3 18Qft 27 5 1R9R 35 1 1QDfi 20 9 1891 26^8 1899 17.0 1907 42.0 Mean 36.2 1892 24.1 1900 38.0 1908 36.7 * Not included in mean. The amount of snowfall for each calendar year of the period of record is shown in Table CIV, and graphically in Fig. 47, for purposes of reference in comparisons including annual precipitation, but the principal interest in the matter of snowfall naturally centers in winters, so that no discussion of the annual values is deemed necessary. While the variation of snowfall from season to season, and from month to month, is great, a study of the tables and graphs presented leads to the conclusion that there has been no permanent change from greater to lesser amounts, as many people believe. Considering that the greatest Chicago snowfall of record occurred as recently as the winter of 1903-4, and that the amounts in the seasons of 1907-8 and 1909-10 were far above the average, it is obvious thatPRECIPITATION 221 the snows of the present are just as heavy as were those of a generation ago. Many Chicagoans were born and reared in the northern and central portions of Mew England and New York state, where the snowfall is ordinarily much greater than it is in the Middle West, and such persons are likely to contrast the snowstorms of their childhood days with those of this region, and hastily conclude that the latter region has undergone a remarkable change in climate. Then, too, the snows of the present remain on the ground as long as did those of former years, because the character of our winter temperatures has suffered no permanent change. In the winter of 1909-10 snow lay on the ground continuously from December 5 to March 10, and sleighing was possible in the outlying sections of the city during three months of that period. INCHES. 60 so 4-0 30 20 JO O Fig. 47.—Annual depth of snowfall, in inches, 1885-1910 (see Table CIV). In order to afford a comparison of the snowfall of the Chicago region with others of the country lying east of the Rocky 'Mountains, Fig. 48 has been prepared. There is an average annual snowfall over northern New England and northeastern New York amounting to 110 inches, while the fall exceeds 60 inches over the middle portions of those sections. The only portion of the country in the section shown in the figure, the annual snowfall of which exceeds that of New England, lies on the southern shore of Lake Superior, where northerly winds sweeping the lake bring an average of 130 inches, Until recent years no accurate measurements of snowfall have been222 THE WEATHER AND CLIMATE OF CHICAGO made in the western mountains, and late data are not yet available for publication, but the observations thus far obtained indicate that in certain localities extremely heavy falls occur, far surpassing those of the Lake Superior shore. HEAVY SNOWFALLS 1. Greatest snowfall in twenty-four consecutive hours, monthly.— The greatest snowfall occurring in any twenty-four consecutive hours for each month since 1891, when this particular record was begun, is shown in Table CV. From this table it is apparent that the heaviest snowstorms occur most frequently in February, there having been 5 in that month with a fall of more than 10 inches within twenty-four hours, while no other month has more than 2 such storms. There is but a single instance of a 24-hour fall of more than 12 inches, that being on February 18-19, 1908, when 12.7 inches were recorded. Remarkably heavy snows, entirely out of the season when such depths might be expected, were those of November, 25-26, 1895, 11.5 inches, and of March 23-24, 1897, 10.0 inches, and both readings are by far the greatest recorded for their respective months. The heaviest 24-hour fall in December was 11.3 inches, in 1903, andTABLE CV Greatest Snowfalls in Twenty-four Consecutive Hours, in Inches and Tenths, 1891-1914 Season 1891..... 1891- 92.. 1892- 93.. 1893- 94.. 1894- 95.. 1895- 96.. 1896- 97.. 1897- 98.. 1898- 99.. 1899- 1900 1900- 1901 1901- 2... 1902- 3... 1903- 4... 1904- 5... 1905- 6... 1906- 7... 1907- 8... 1908- 9. .. 1909- 10.. 1910- 11.. 1911- 12.. 1912- 13.. 1913- 14.. October November December January February March April May Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date 2.5 2 0.3 8 3 2 2 1 5 5 T 4 0 5.8 27 5.5 è 5.0 11 1.0 14 1.0 5 T 9 0 0 0.8 25 1.0 18 5.7 24 8.7 17/18 0.5 19 0.4 21 0 0 3.0 21 8.1 2/3 5.9 23/24 10.2 12/13 5.3 28 T 5* T 18 0 2.5 12 10.1 27 9.1 25/26 9.1 5/6 5 0 4 0 T 13 T 20*. 11.5 25/26 1.5 2 1.6 5/6 11.0 3 3.0 1* T 2 0 T 17 4.0 5 0.6 4 6.0 23/24 3.9 11/12 10.0 23/24 0.6 16 0 0 T 2* 3.1 17 6.5 22/23 4.0 19 0.5 2 T 4* o T 14* 1.2 25/26 1.5 22 1.0 Ì7 2.8 22 3.0 30 T 1* 0 0 T 2* 2.4 14 0.3 13 11.3 27/28 2.5 29 2 9 11/12 o 0 1.0 14/15 2.8 31 3.1 26 11.4 3/4 3 5 30/31 T 2* o 0 0.1 Î8 2.6 13/14 3.0 20/21 1.2 6 1.5 30 0.1 8 T 10 0 0.5 27 2.0 24 2.0 11 7.0 - 3/4 0 3 27 2 6 3 o 0 1.4 28 11.3 12 3.4 12 4 8 Ì8 8 4 14 T 2* o T 22 T 13* 2..8 11/12 2.1 25 4.6 6 4.4 14 0.2 ■ 7 0 T 11* T 7* 1.5 1/2 0.7 22 1.8 14 3.4 19 0 . 0 T 9* 1.4 12 0.2 21 2.1 16 7.0 5 1.6 10 1.2 13 1.3 3/4 0 3.0 30 8.1 14 7.4 12 12 7 18/19 0 1 18 T 2* o 0 0.7 14 2.3 6 5.3 29/30 5.4 15/16 2.0 8 0.3 9 T 1* T 11* T 17 6.0 24/25 4.8 13 1.1 20/21 T 7* 2.5 25 0 T 27* 1.1 30 4.0 28 1.3 5 8.4 5/6 1.8 27 2.4 2 T 1* T 26 0.6 14 3.3 2/3 2.5 11/12 4.2 25/26 6.4 14/15 0.1 18 0 0 0.4 24 T 26* 1.8 6/7 4 1 25/26 2 2 21 T 4 0 1.9 21 T 8/9/10 2.0 23 9.5 31 3.0 22 0.6 21/22 * Also on other dates. PRECIPITATION 223224 THE WEATHER AND CLIMATE OF CHICAGO the heaviest in January was 9.5 inches, in 1914. These measurements, as do all others of depth of snow in the official records, represent the snowfall on the level; that is, as it would lie were it evenly distributed over the ground. During heavy shows the wind frequently piles high drifts in some places, and sweeps other spaces bare. But these inequalities are avoided in the records by taking the average of three separate measurements at places carefully chosen (p. 216). The data in the table cover the fall of snow for the 24-hour period, although, of course, many of the storms ceased before the full space of twenty-four hours elapsed; and some, especially the heavier ones, lasted longer than the interval taken for the record, but in practically all such cases the additional amount of snowfall was but little. The actual duration and the total fall of snow are given in Auxiliary Table G for each of the record storms in Table CV. AUXILIARY TABLE G Heaviest Snowstorms on Record, by Months Date Duration Depth in Inches Hrs. Min. 24-Hr. Total November 25-26, 1895 24 40 11.5 12.0 December 12, 1903 18 38 11.3 11.3 January 31, 1914.. 12 27 9.5 9.5 February 18-19, 1908 29 56 12.7 12.8 March 23-24, 1897 24 50 10.0 10.0 April 11-12, 1900 32 13 2.9 3.1 The snowstorm of February 27-28, 1900, in which 11.3 inches fell, lasted 27 hours, with a total fall of 11.5 inches; while the storm of February 2-3, 1901, in which 11.4 inches fell, lasted only 10 hours. Both storms were accompanied by high winds, which resulted in much drifting and consequent interference with transportation and communication. 2. Heavy and damaging storms of snow, sleet, and ice.—It is desired in this connection to make some reference to the most damaging storms of snow, sleet, and ice. Sleet invariably falls in connection with snow or rain, or both, and ice storms are commonly confused with sleet storms. Sleet is frozen rain; ice storms are occasioned by rain freezing upon objects with which it comes in contact. In the former the freezing occurs before the drops strike the earth; in the latter, the cold surfaces upon which the rain falls freeze thePRECIPITATION 225 water into a coating of ice. Such storms cause greater damage to telegraph, telephone, and fire-alarm wires than do the heaviest snows, and as a consequence communication by such means is Sometimes paralyzed, and the city even cut off from intercourse with the outside world. November 25-26, 1895. This heavy snowstorm is listed in Table CV, but it may properly be classed as an ice storm as well, as the snow was mixed with rain freezing as it fell. Wires were coated with ice and in many portions of the city broken down, stopping communication by telephone and telegraph for an entire day, while electric-light service and street-car schedules suffered as badly. A high north to northeast wind prevailed from midnight of November 25-26 until daylight, reaching a maximum of 48 miles an hour, its force adding much to the destruction caused by weight of ice. The temperature, which was near the freezing point on the morning of the 26th, fell steadily during the day and reached 8° by the following midnight, the ensuing cold weather lengthening the period of damage by maintaining the ice coatings. February 3-4, 1900. This storm began with a light fall of sleet turning to rain and later to heavy snow accompanied by brisk to high northerly winds. As a consequence of the wind the snow drifted badly. Streets were made very slippery and dangerous, and the condition resulted in much interruption to transportation. The temperature rose from a minimum of 18° on the 3d to a maximum of 33° at 5 p.m., falling very slowly thereafter and reaching 17° at 10 p.m. on the following day. February 3, 1903. On this day rain, snow, and sleet began at 6 a.m., and continued until the morning of the 4th, with high northeast winds, coating all exposed objects with ice, and causing much interference to transportation and communication generally, as the storm covered a large area. The Weather Bureau anemometer was so covered with ice during this storm that an accurate record of wind velocity could not be secured. The temperature ranged from 22° to 37° on the 3d, falling steadily on the 4th and reaching a minimum of 17° by midnight. February 29-March 1, 1908. Rain with some sleet fell from 6 p.m. of the 29th to 7:40 a.m. of March 1, causing the same general conditions noted in the above-described storms. A thunderstorm occurred in connection with this disturbance early in the morning of March 1. February 14-15-16, 1909. Rain, snow, and sleet with a northeast gale began about midnight of the 13th, continuing through the day of the 14th, and the condition was followed by a considerable fall of snow on the 15th and 16th. The maximum velocity of the wind was 48 miles an hour on the 14th, and the storm was of more than ordinary severity, resulting in much damage. The temperature remained steadily below freezing on all three days, ranging226 THE WEATHER AND CLIMATE OF CHICAGO from 29° to 23° on the 14th, from 24° to 20° on the 15th, and from 26° to 18° on the 16th. December 11-12, 1909. Sleet, mixed with rain and snow, began at 7:33 p.m. of the 11th, and continued until 8:30 a.m. of the 12th, when it changed to rain with temperature a few degrees above the freezing point. As a result of the upward trend of the temperature the severity of the storm was somewhat modified, and the damage and delays were not as serious as would otherwise have been the case. Moreover, the wind velocity was not high at any time, which also tended to a minimum of damage. January 4-5, 1910. Sleet began falling at 4:35 p.m. of the 4th, and continued until nearly midnight, in the form of small, hard pellets almost as fine as sand. The fall of sleet was 2.2 inches, but it drifted badly under brisk to high east to southeast winds. Light snow had preceded the sleet during most of the morning and afternoon, the temperature rising steadily from 1° to 22° above zero during the course of the day. During the 5th the temperature rose still farther, reaching 30° in the early morning, and falling again thereafter to 4° by midnight. With the drop in temperature the surface of the sleet solidified into a crusted stratum almost strong enough to support the weight of a man, and where the accumulation was not at once removed travel afoot for both man and animal was most difficult. January 12-13-14, 1910. Sleet, snow, and rain fell during this storm, the rain freezing as it fell. The disturbance was in all probability the most damaging ice storm ever recorded by the local Weather Bureau office. The total snowfall was 10.2 inches. No high winds prevailed during the storm, and the temperature was moderate, ranging from 35° to 28° throughout the three days. The unusual features were due to the fact that the snow fell first and the rain afterward, saturating the former, which subsequently froze into practically solid ice. Teaming was very difficult, and freight traffic was nearly paralyzed. A coal famine was threatened, and not a milk train entered the city on the 14th, because of the impossibility of moving freight trains from their sidings in the country surrounding. Although but little snow fell during the month of February following, the conditions brought about by this storm were largely responsible for the snow covering remaining on the ground until March 10. February 21, 1912. The damage by this storm was caused by high winds badly drifting the snowfall, which amounted to but 3.4 inches on the level. The snow began in the early morning with an increasing northeast wind. The wind shifted to north at about 8 a. m., and reached a velocity of 53 miles an hour at 2:06 p.m., decreasing thereafter, although it was strong throughout the afternoon and evening, and shifted to northwest about 5 p.m. The snow drifted from 3 to 4 feet deep in many places in the streets, while in other places the pavements were swept bare. As the worst part of the storm occurred during the hours of greatest business activity, delays in traffic and communication were correspondingly great. Accidents due toPRECIPITATION 227 the high wind and blinding snow were numerous, and although the weather cleared rapidly after the storm passed, recovery from the disastrous effects was necessarily slow. February 26, 1912. This storm was similar to that described in the preceding paragraph, but was of somewhat lesser intensity. ' It, however, caused much delay to communication and transportation. The snowfall was only 4.2 inches for the whole storm, but high winds drifted it much in the streets, which became very slippery. The temperature in the morning rose to about freezing, ranging between 30° and 31° from 7 a.m. to 2 p.m., falling thereafter to a minimum of 17° at midnight. In connection with the above storms reference may be made to severe windstorms discussed on pp. 289-92. FREQUENCY OF SNOWFALL The relative frequency of days with snow is shown for each month of the winter season in Table CVI, in which the record for the official period is given for all falls of 0.1 inch or more. The figures do not, of course, show all days with snow, because there are usually many days in each winter on which only an inappreciable amount (“trace”) falls. The average number of days for the season is 29. The greatest number of days in any one season, 41, occurred in 1886-87, although the total depth of fall, 40.5 inches, only slightly exceeded the average. The fewest days of snow in any one season, 18, occurred in 1888-89, and the total snowfall was 22.7 inches. The actual depth of snowfall in January is exceeded by that of February (p. 216), but the frequency is somewhat less in the latter month, the averages being 8.0 days for January and 7.6 days for February. January, 1886, holds the record for the greatest number of days with snow in any one month, there being 19 days with a fall of 0.1 inch or more. In January, 1900, however, there was but 1 day on which an appreciable amount of snow fell, the amount then being only 0.3 inch, which was the total measurement for the month. In December, 1889, snow fell only in “ traces.” In April, 1910, snow occurred on 7 days, making a total fall of 6.9 inches, while in the warm March immediately preceding appreciable snow did not fall at any time. Fig. 49 illustrates the greatest frequency and the average monthly frequency of days with snow; also the greatest monthly amounts, the greatest fall in twenty-four consecutive hours, and the average snowfall, by months, the various data being drawn from Tables CVI, CIII, and CV.228 THE WEATHER AND CLIMATE OF CHICAGO DEI^TH OF SNOW ON THE GROUND Snow remains on the ground for a variable period. Sometimes continuous cold weather, by preventing much thawing, permits the accumulated falls to remain on the ground throughout most of the winter, while in other seasons warm spells bring about rapid melting, to be followed later by colder weather and additional snowfalls. TABLE CYI Monthly and Seasonal Number op Days with Snowfall op 0.1 Inch or More, 1885-1914 Season Oct. ' Nov. . Dec. Jan. Feb. Mar. April May • Season 1885-86 0 1 8 19 7 4 1 0 40 1886-87 0 4 10 12 10 5 0 0 41 1887-88 0 2 4 9 4 4 1 0 24 1888-89 0 1 4 2 8 3 0 0 18 1889-90 0 3 0 4 4 11 0 0 22 1890-91 0 0 6 7 7 7 2 0 29 1891-92 0 3 1 9 6 9 0 0 28 1892-93 0 1 8 10 9 4 2 0 34 1893-94 0 4 7 3 5 2 0 0 21 1894-95 0 1 1 11 3 3 0 0 19 1895-96 0 4 5 3 10 5 0 0 27 1896-97 0 2 4 12 8 4 2 0 32 1897-98 0 0 7 6 12 3 0 0 28 1898-99 0 4 5 6 5 6 0 0 26 1899-1900 0 0 5 1 12 8 3 0 29 1900-1901 0 2 3 5 7 8 0 0 25 1901-2 0 1 7 8 13 3 1 0 33 1902-3 0 1 10 7 7 3 2 0 30 1903-4 0 3 8 11 11 6 0 0 39 1904-5 0 0 10 11 8 2 1 0 32 1905-6 0 0 7 7 8 12 0 0 34 1906-7 0 5 2 11 5 3 3 1 30 1907-8 0 1 8 8 8 1 0 0 26 1908-9 0 2 2 6 7 3 1 0 21 1909-10 0 0 14 12 6 0 7 0 39 1910-11* 0 3 11 5 6 5 1 1 32 1911-12* 0 5 8 11 6 7 1 0 38 1912-13* 0 1 1 8 6 9 0 0 25 1913-14* 2 0 2 9 11 7 1 0 32 Means— 1886-95 2.0 4.9 8.6 6.3 5.2 0.6 0 28.1 1896-1905 1.7 6.4 7.0 9.3 4.8 0.9 0 30.1 1886-87—1909-10.... 1.8 5.8 8.0 7.6 4.8 1.0 0.04 29.0 Oct. Nov. -Dec. Jan. Feb. Mar. April May Season Average 1.8 5.8 8.0 7.6 4.8 1.0 29.0 Greatest 2 5 14 19 13 12 7 1 41 Least 0 0 0 1 3 0 0 0 18 * Not included in averages. According to Table CVII, the greatest depth of snow on the ground at the end of January was 14.8 inches in 1885; and at the end of February, 18.3 inches in the same year, the monthly falls being 20.2 and 19.0 inches, respectively. These months were also quite cold, having a number of days with zero temperatures. As a rule, however, the depth of snow on the ground at the end of thePRECIPITATION 229 month cannot be taken as an indication of the amount of the snowfall, as it is possible that greater or lesser amounts may have existed Oct Nov. Dec. J^n. Feb. Mrr. ¿\pr. Mry* Oct Nov Dec. J/=*n. Feb. Mar /Ipr. May' Fig. 49.—Monthly frequency and amount in inches of snowfall, 1884-85 to 1910-11. A = greatest monthly frequency of days with snowfall; B=average frequency; C=greatest monthly amounts of snowfall; D=greatest snowfall in twenty-four hours; E—average monthly amounts of snowfall.230 THE WEATHER AND CLIMATE OF CHICAGO just before or just after that date, and not show in the record. For instance, in the winter of greatest snowfall, 1903-4, which was on the TABLE CVII Amount of Snow on Ground at End of Month, in Inches, 1885-1914 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov.. Dec. 1885.... 14.8 18.3 1886.... 10.0 3.0 1.0 0.5 4.0 1887.... 4.5 T 1.0 1888.,.. 2.0 1889.... T 1890 2.0 T 1891.... 0.3 5.0 1892.... 0.2 T 0.4 1893 2.6 0.2 2.1 1894.... 3.7 5.7 1895 8.5 4.0 0.8 1896.... 1897 5.0 1.0 T 1898.... 7.7 3.0 0.6 0.2 1899 0.4 T T T 1900.... 11.5 T 2.8 1901.... 3.8 2.8 T T 1902.... 3.4 T 0.7 1903.... T 1.0 5.6 1904.... 2.4 T 1905.... 2.5 T T T 1906.... 1907.... 3.Î t 1.6 1908 .. 3 1 3.7 1909 3.8 T 9.2 1910..;. 5.2 0.4 1.0 0.8 1911.... T T 0.6 1912 3.0 3.7 T 1913 0.1 4.2 1914.... 9.3 0.5 whole very cold, so that the snow might be expected to remain continuously on the ground, the comparison of the monthly falls with the amounts on the ground at the end of the months shows the results given in Auxiliary Table H. AUXILIARY TABLE H November December January February March Total depth of snowfall, inches, 1903-4 .. 2.2 18.6 11.0 13.4 14.4 Depth at end of month 1.0 5.6 2.4 trace 0.0 Table CVIII furnishes additional interesting data, the greatest amount of snow on the ground at any time during the months being entered, although the record is available only since 1893. The deepest snow covering during the period of record was 20.4 inches on February 7, 1895, a month characterized by heavy snowfalls and low temperatures, as was the month immediately preceding. TheTABLE CYIII Greatest Depth of Snow on Ground, in Inches, and Date, 1893-1914 January February March April May October November December Annual Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date Amount Date 1893 9.6 24 9.0 17 0.2 14 T 21 2.1 30 10 2 3 4 10 2 1894 5.9 24 10.8 13 5.3 28 2.5 12 10 1 27 10 8 Feb 13 1895 9.5 26, 27 20.4 7 5.0 4 12 0 26 4 0 2 4 20 4 Feb 1 1896 2.2 6 12.0 13 3.0 11 T 2 2 5 5 0 4 23 12 0 Feb 13 1897 7.5 27, 28 3.5 1, 12 5.0 23 T 10,16 3 0 17 7 5 Jan, 97 28 1898 11.5 26 7.5 23 3.4 2 1 5 28 0 9 25 26 11 5 Jan 26 1899 0.5 23, 30 2.0 22 3.0 30 T 1 T 2 3 2 4 14 3 Ò Mar 30 1900 T / 1, 2,13, 111.5 28 10.8 1 1.8 11 0 8 15 2.8 31 11.5 Feb. 28 1 29, 30 j 1901 . 4.2 3Ó 16.0 9 3.0 30 T 27 2 4 14 16 0 Feb 9 1902 3.4 31 5.9 11 0.1 2 T 7 T 27 19 7 24 5 9 Feb. 11 1903 2.1 11 7.2 8 0.1 23, 24 0.7 3 1 3 28 10 0 12 13 10 0 Dec. 12 13 1904 8.0 3 6.8 21 8.0 14 T 2 4.1 13 s!o •Tan. 3 Mar. 14 1905 4.2 25 9.0 8. 4.4 14 0.2 7 T 29 30 1 8 3 9 0 Feb 8 1906 0.5 2 3.0 15 3.6 19 1 2 12 0 2 21 3 6 Mar 19 1907 3.2 16, 29 8.2 5 T 10,14,17 0.3 13 0.2 3 16 30 8 0 14 8 2 Feb. 5 1908 5.0 12,13 13.5 19 3.0 1 T 2 0 7 14 1 8 6 13 5 Feb 19 1909 4.2 30 7.4 16 T { 1, 8, 16, 17, 24 } T 9,10 T 1 T ÎH, 121 123, 24/ T 17, 22 10.0 29,30 10.0 Dec. 29,30 1910 11.8 14 4.8 1 0.2 1 1.8 23 T 27, 28 1 0 30 2 1 10 11 8 Jan.14 1911 1.4 5 7,4 6 0.7 27 2.0 2 T 26 0 4 14 3 2 3 7 4 Feb 6 1912 5.8 14,16 4.3 3,5 6.2 15 T 1,17 T 24 T 18 6 2 Mar. 15 1913 2.0 7,10 4.2 27, 28 4.9 1 T 4 1.9 21 T 8,9,10 1.7 23 4.9 Mar. 1 1914 9.5 31 8.4 1 0.6 1 to co PRECIPITATION232 THE WEATHER AND CLIMATE OF CHICAGO next deepest covering of snow was 16.0 inches, and was measured on February 9, 1901. This month was persistently cold, although no extremely low temperatures occurred. The deepest snow on the ground during any January was on the 14th in 1910, 11.8 inches, during the winter in which the snow covering was continuous from December 5 to March 10, and the snowfalls were heavy in both December and January. Snow covered the ground that winter on a total of 100 days, 96 of these being consecutive in the period mentioned above. In the winter of 1903-4, snow covered the ground on 110 days, but, although the greatest seasonal snowfall on record occurred at that time, the periods were somewhat broken. The longest period of consecutive days with snow on the ground in this winter was 86 days, from December 7 to March 1, but the amounts varied greatly, as shown in the preceding paragraph. In the winter of 1896-97, the longest period of continuous snow covering was but 14 days, with a total of 81 days, notwithstanding the fact that considerably more than the average amount for the season fell. In the winter of 1905-6, a winter of light snowfall and comparatively high temperature, the longest period was 17 consecutive days, with a total of 82 days. On 16 of these days the depths were very little, there sometimes being but “traces” of snow in patches here and there. During the very cold winter of 1898-99, in which the total snowfall was only 18.0 inches, there were but 75 days on which the snow lay on the ground. Of these, 18 were consecutive, but on 12 of the 18 less than 0.1 inch was the amount of depth. The greatest amount of snow on the ground occurs usually in the month of February because the heaviest snows of the year fall in the early portion of that month and the latter portion of the month preceding (p. 216). DATES OF FIRST SNOWFALL IN AUTUMN AND LAST IN SPRING The date of the first snowfall in autumn and the last in the following spring are given for the season of record in Table CIX. On the average the first snow of autumn occurs on October 31, and the last of spring on April 23. In 1905 and 1909 snow fell as early as October 11, and in 1902 not until November 26. The latest date in spring on which snow has fallen was June 2, 1910, when a “trace” fell during a hailstorm. The earliest date on which snow was last recorded in the spring was March 28, in 1906.PRECIPITATION 233 PROPORTION OF RAINFALL TO MELTED SNOWFALL During the period of record from 1885 to 1910, inclusive, the rainfall and snowfall of the winter season, expressed in percentages TABLE CIX Dates of First Snowfall in Autumn and Last in Spring, Including “Traces” of Snow, 1885-1914 Year First in Autumn Last in Spring Year 1885 November 1 April 1 1886 1886 November 17 April 24 • 1887 1887 October 21 April 20 1888 1888 October 18 May 26 1889 1889 October 30 April 9 1890 1890 October 29 May 4 1891 1891 November 13 April 9 1892 1892 November 7 April 22 1893 1893 November 15 May 18 1894 1894 November 7 May 13 1895 1895 October 20 April 2 1896 1896 October 17 April 16 1897 1897 November 2 April 5 1898 1898 October 14 April 15 1899 1899 November 2 April 13 1900 1900 November 6 April 21 1901 1901 November 3 May 10 1902 1902 November 26 April 30 1903 1903 - November 5 April 16 1904 1904 October 22 April 16 1905 1905 October 11 March 28 1906 1906 November 10 May 10 1907 1907 November 10 April 2 1908 1908 November 4 May 2 1909 1909 October 11 June 2 1910 1910 October 26 May 2 1911* 1911* October 26 April 18 1912* 1912* November 1 April 4 1913* 1913* October 20 April 20 1914* Earliest October 11, 1905 and 1909 March 28, 1906 Latest November 26, 1902 June 2, 1910 Average October 31 April 23 * Not included in average. of the total monthly and seasonal precipitation, has been as given in Auxiliary Table I. Of the total annual precipitation, 12 per cent, or about one-eighth, falls as snow. AUXILIARY TABLE I Percentage of Precipitation Occurring as Rain and Melted Snow Rainfall Snowfall November Percentage 91 Percentage 9 December 68 32 January 53 47 February 50 50 March 78 22 April .. 96 4 Season 72.7 27.3 234 THE WEATHER AND CLIMATE OF CHICAGO Fig. 50 shows the estimated snowfall in the city of Chicago on Thanksgiving Day, November 26, 1903, during a freak snowstorm in which 14 inches fell in South Chicago, near the lake, &nd practically none at all west of Halsted Street.PRECIPITATION 235 DISTRIBUTION OF SNOWFALL IN THE CITY While the measurement of snowfall is made at the Weather Office and in Grant Park, near the lake, as has been indicated, for some time it has been the practice of the official observers to make notes of the depths of snow in the respective localities in which they reside. Ordinarily there is no great variation in the amount of snowfall throughout the city, but occasionally, because of peculiar local conditions, wide differences have been observed. One of the most remarkable occurrences of this character happened in connection TABLE CX Summary op Snowfall Data Means Greatest Monthly Amounts Number op Days with Snow Greatest Snowfall in Twenty-pour Hours Mean Snowfall, 1884-1910 As percentage of Precipitation 1884-1913 1 1885-1910 1891- -1914 Omitting Traces Including Traces Amount Year Percentage of Mean Average Greatest Least Average Greatest Least Amount Year October... T 1.9 1913 0 0 0 0.9 4 0 1.9 1913 November 2.2 9 14.5 1895 659 1.8 5 0 5.7 13 2 11.5 1895 December.. 6.8 32 19.1 1909 281 5.8 14 0 12.6 20 3 11.3 1903 January... 10.2 47 26.7 1886 262 8.0 19 1 13.3 20 4 9.5 1914 February.. 11.3 50 27.8 1896 246 7.6 13 3 12.6 20 8 12.7 1908 March 5.0 22 14.3 1904 286 4.8 12 0 10.3 19 2 10.0 1897 April 1.0 4 ' 6.9 1910 690 1.0 7 0 3.0 8 0 2.9 1900 May 0.1 1.3 1907 0 1 0 , 0.4 3 0 1.3 1907 Seasonal .. 36.2 59.5 1903-4 163 29.0 41 18 58.8 84 40 12.7 1908 in in in in in 1886-87 1888-89 1903-4 1889-90 Feb. The various records on which the above table is based were begun at different times, do not cover exactly the same periods. Hence the data with the snowstorm of Thanksgiving Day, November 26, 1903, and Fig. 50 has been prepared to show the distribution of snowfall throughout the city at that time. There was practically no snowfall west of Halsted Street or in the sections lying north of the river. The amount increased from the south branch of the river eastward to the lake, and southward through the limits of the city, reaching a maximum depth of 14 inches at South Chicago. While snow was falling over the South Side the sun was shining brightly throughout the western sections. The general distribution of atmospheric pressure was such as to cause a brisk north wind at the southern end236 THE WEATHER AND CLIMATE OF CHICAGO of Lake Michigan. The day was cold for the season, with temperature ranging from 13° to 20°, but the temperature of the water surface was, of course, considerably above the freezing point. A north wind at Chicago skirts the shore of the northern portions of the city, but, as the shore to the southward trends farther and farther to the southeast, a wind from the north produces more and more of the lake effect (p. 216). In this case the temperature over the land was such that the moisture carried on shore by the wind was soon chilled to the point of condensation, and heavy snow resulted in the southern sections. In Grant Park the fall was but 0.6 inch, while at Jackson Park 10 inches were measured, the depth increasing thence southward as above indicated. SUMMARY OF SNOWFALL DATA Table CX presents the main features of the data on snowfall in a manner similar to that used for the summary of precipitation data (Table XC1X), and will be found valuable for purposes of reference.PART III ATMOSPHERIC MOISTUREATMOSPHERIC MOISTURE The term humidity has* reference to the quantity of moisture present in the air at all times in the state of invisible vapor. The air is said to be dry when but little is present, and humid when the quantity is relatively considerable. If the quantity of moisture is measured as weight per unit of volume, as, for example, grains per cubic foot, the numerical value is designated the absolute humidity. If, however, as is most common in statistics relating to weather and climate, the measurement is expressed as a percentage of the quantity of vapor that can possibly exist at the temperature in question, then the numerical value is called the relative humidity. The conditions of humidity have at times fully as much to do with comfort and salubrity as do those of temperature, sunshine, and wind. Paradoxical as it may seem, a high degree of humidity makes a hot wave sensibly hotter, and a cold wave colder, than is the case when the amount of moisture in the air is relatively low. High humidity in warm weather, by materially retarding the evaporation of perspiration from the pores of the body, prevents the cooling produced by this process in other heated periods. On the other hand, during times of cold weather, by penetrating the clothing and communicating dampness to it, an atmosphere with high humidity increases the conductive qualities of the fabric and permits a more rapid escape of the body’s heat. The disagreeable features of damp climates, whether warm or cold, and the comparative pleasantness of regions in which the atmosphere has a low percentage of moisture are well known. Residents of the foothills along the eastern sides of the Rockies, and those of the dry sections of the interior Northwest, experience temperatures of zero and below with less discomfort than even much higher winter temperatures bring to localities of greater relative humidity; and the heat of many arid regions is rendered less oppressive by the extreme dryness of the air, while very moist climates are enervating at temperatures but little above the average. It is common to speak of the moisture content of the atmosphere, or of the capacity of the air for water vapor; but as a matter of fact, the presence of air has little to do with the volume of water in the form of vapor within any given space. This vapor exists practically 239240 THE WEATHER AND CLIMATE OF CHICAGO independent of the other gases of the air, and its mass per volume depends upon the temperature. Whenever, therefore, the expressions “capacity of the air for moisture” and “moisture content of the air” are used herein, the distinction noted must be kept in mind. This so-called capacity of the air for moisture in the form of vapor increases rapidly with rising temperature. A cubic foot of air at 32° will, if saturated, contain about 2 grains of aqueous vapor; at 50° the same volume will have a capacity of about 4 grains; at 70°, about 8 grains, and at 100°, about 20 grains. If the quantity of water vapor'in any free volume of air at 32° is actually 2 grains to the cubic foot, the relative humidity is 100 per cent. Now if, as may be the case in nature, the temperature rises while the actual amount of moisture to the cubic foot remains the same, the relative humidity at 50° becomes only 50 per cent, because 4 grains is the capacity at that temperature; at 70° it has fallen to 25 per cent, and at 100° the relative humidity would be about 10 per cent. In the first instance the air would be saturated with water vapor and no more could be taken up; in the last, it would be extremely dry, as it sometimes becomes in the desert regions of the arid Southwest, and any water surface exposed to such an atmosphere would evaporate rapidly. RELATIVE HUMIDITY, MONTHLY AND ANNUAL The average relative humidity, based upon readings of the psy-chrometer made at 7 o’ clock each morning and evening, is shown for the period of record in Table CXI. Prior to 1889 the observations were made at different hours, and those readings are therefore not comparable with the figures in the table, which are all that are available for any period of sufficient length to afford means of any special significance. The mean annual relative humidity at Chicago is 75.3 per cent, and the yearly averages have ranged from 80.2 per cent in 1900 to 68.5 per cent in 1895. As a rule, relative humidity is highest during cloudy and rainy weather, and lowest during clear weather with bright sunshine and strong southwest winds. The lake wind is always laden with moisture, but in the summer and late spring its relative humidity is not so high as in other seasons, because its capacity is increasing with the rise in temperature of its current as it sweeps over the warmer land. The conditions just pointed out, however, are not always apparent in the average readings. ForATMOSPHERIC MOISTURE 241 instance, in .1900, the year of highest average relative humidity, the total precipitation was but 28.65 inches, considerably below the normal; in 1895, the year of lowest relative humidity, while the total precipitation, 32.38 inches, was below the normal, it was greater than that of 1900 by more than 3 inches. The means for the various months, shown at the bottom of the table, give considerably higher relative humidity for the winter months than for the summer season, ranging from 82.3 per cent in January to 70.4 per cent in July. These figures indicate a closer TABLE CXI Mean Monthly Relative Humidity, Per Cent, 1889-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1889 82 82 76 76 71 79 72' 66 69 70 83 76 75.2 1890 81 82 74 72 71 73 64 74 76 82 73 76 74.8 1891 82 80 84 75 66 81 70 74 66 66 79 77 75.0 1892 82 85 76 68 80 84 72 72 70 66 78 83 76.3 1893 88 84 80 77 74 74 72 66 66 70 76 83 75.8 1894 80 80 74 72 72 60 56 64 70 71 72 78 70.8 1895 81 81 68 71 64 60 62 62 65 58 72 78 68.5 1896 80 76 70.. 67 62 68 68 66 72 69 80 81 71.6 1897 83 80 82 77 66 72 72 70 62 66 74 84 74.0 1898 86 84 77 70 76 72 67 73 72 82 78 86 76.9 1899 82 80 81 68 73 73 81 84 71 74 81 78 77.2 1900 84 83 84 74 70 77 82 82 82 84 80 80 80.2 1901 83 85 86 76 83 78 68 76 75 72 76 85 78.6 1902 77 85 80 69 74 78 79 78 78 75 80 86 78.2 1903 84 84 85 75 72 70 70 76 69 70 71 84 75.8 1904. 85 83 82 72 69 72 68 69 78 72 70 79 74.9 1905 82 84 82 77 78 77 74 73 72 69 76 78 76.8 1906 82 80 80 76 72 72 72 76 74 75 76 83 76.5 1907 86 77 78 72 73 74 73 74 76 77 71 82 76.1 1908 80 79 74 74 81 70 75 67 74 73 80 74 75.1 1909 81 80 74 74 74 77 65 75 78 71 73 84 75.5 1910 79 74 66 74 74 66 67 69 78 76 75 76 72.5 1911* 79 77 69 73 62 67 59 70 78 76 76 78 72.0 1912* 72 74 75 70 70 65 73 78 72 69 68 68 71.2 1913* 75 67 71 64 68 62 67 72 72 76 73 76 70.2 Means 82.3 81.3 77.9 73.0 72.5 73,1 70.4 72.1 72.4 72.2 76.1 80.5 75.3 * Not included in means. relation between the frequency of precipitation and relative humidity than exists between the latter and the actual amount of rainfall and snowfall, as expressed in average values. Precipitation is most frequent in the winter season (p. 168), and relative humidity is on the average highest at that time; precipitation is heaviest during the summer (p. 157), at which time relative humidity is on the average at its lowest. The high values of the averages for the colder portion of the year are not due to greater amounts of moisture in the air. As a matter of fact, the absolute humidity for the winter is much less than for the summer, but in periods of low temperature a turn242 THE WEATHER AND CLIMATE OF CHICAGO to slightly colder weather raises the relative humidity greatly, because of the very much smaller capacity of the air for moisture at that season. The average values for mean relative humidity shown in Table CXI, based as they are upon observations taken at 7 a.m! and 7 p.m., are somewhat higher than the mean observations taken at each of the twenty-four hours of the day would give, and also slightly higher than the mean obtained from the maximum and minimum relative humidity of the successive days. This excess is due to the fact that both observations of the day come at a time when the daytime effect of the sun’s heat in lowering relative humidity is felt but little in the summer season, and not at all in the winter months. The first observation is near the time of maximum relative humidity, and the second long after the time of minimum, so that the latter observation is much too high to produce a true mean when combined with the former. Since May, 1911, bi-hourly values of relative humidity, as recorded by a hygrograph, have been secured for each day, and these records for the year following, given in Table CXII, will illustrate the truth of the foregoing statements. The average of the bi-hourly readings (a) for each month will be found in each case to be lower than the averages of the 7 a.m. and 7 p.m. observations (6), the difference for the year being 1.6 per cent. The highest hourly average for the period given, 76.5 per cent, occurred at 6 a.m., and the lowest, 62.5 per cent, at 2 p.m,. the average of these two readings being 69.5 per cent, only three-tenths of 1 per cent below the average of the bi-hourly values. Fig. 51 shows graphically the mean relative humidity and the mean temperature of the months from May, 1911, to April, 1912, inclusive, based upon observations at the bi-hourly times shown in Table CXII. The general condition of higher relative humidity during the season of low temperatures (p. 241) is plainly to be seen in the graph, but the average temperature of any particular month can in nowise be taken as a certain index of the percentage of atmospheric moisture. For instance, January, 1912, was the coldest January of the official record, and we might therefore expect the relative humidity to be correspondingly high. Its mean, however, was about 10 per cent lower than the average for that month of the year and, while higher than that of the previous summer season, was considerably below the readings of December, 1911, and February, 1912, each of which was marked by higher temperature* The relation of temperatureATMOSPHERIC MOISTURE 243 and relative humidity is closest for short periods of change, and little dependence can be placed upon the relation of average values for individual months. The relative humidity of January, 1912, lower TABLE CXII Relative Humidity and Temperature, May, 1911, to April, 1912 Humidity, Per Cent 2:00 a.m. 4:00 a.m. i j 6.00 A.M. 8:00 a.m. 10:00 a.m. Noon 2.00 p.m. 4:00 p.m. 6:00 p.m. | 8:00 p.m. 10:00 p.m. Midnight Average (a) Average (6) [May 66.0 68.4 69.0 64.1 54.9 53.3 50.4 52.3 54.8 59.5 62.1 64.7 59.1 62.4 June 73.3 75.0 73.3 68.6 64.4 59.7 50.4 56.5 59.8 65.6 68.4 70.7 65.5 66.8 July 64.8 67.5 66.8 59.5 51.6 51.4 51.0 51.4 52.4 58.9 60.9 63.6 58.3 59.4 1 m 1) August 73.2 75.8 77.4 72.8 66.6 63.9 61.5 62.5 63.2 66.8 69.0 70.2 68.6 70.2 lyii^ September 79.3 80.2 82.5 78.2 72.9 69.6 68.2 71.6 74.5 76.2 76.9 79.3 75.8 78.0 October 77.9 78.2 79.4 77.4 73.0 69.3 67.8 69.6 72.7 74.6 74.5 75.5 74.2 76.1 November 77.3 77.5 79.0 78.4 73.3 68.1 67.0 67.5 70.8 73.3 75.8 78.2 73.8 75.5 December 81.0 80.6 81.8 81.6 77.8 75.2 70.5 70.2 72.5 76.2 .78.5 >80.1 77.2 77.6 January 69.7 73.2 77.4 75.6 72.2 68.7 65.8 67.5 69.3 69.4 69.4 69.8 70.7 72.0 1010 February 76.9 76.7 77.7 77.9 75.0 71.8 67.9 67.4 70.0 72.3 73.1 74.6 73.4 74.4 Ivl&s March 74.2 75.3 77.4 76.3 71.6 69.9 69.0 67.5 70.7 73.4 74.2 73.9 72.8 74.6 April 72.0 74.8 76.0 71.7 67.2 63.9 60.8 60.0 62.3 66.1 68.4 71.2 67.9 69.6 Average 73.8 75.3 76.5 73.5 68.4 65.4 62.5 63.7 66.1 69.4 70.9 72.6 69.8 71.4 Temperature, Degrees [May. 61.6 59.8 60.1 64.5 69.1 70.8 71.6 70.9 68.4 65.5 64.4 63.1 65.8 64.2 June 68.3 67.0 66.9 70.4 73.3 76.4 77.1 77.1 76.0 72.7 71.2 70.5 72.2 71.8 July 71.6 70.3 70.1 74.5 78.4 80.0 80.1 80.3 80.0 77.2 75.3 73.6 76.0 75.6 1911 August 69.0 67.4 66.6 69.6 72.3 73.7 74.6 74.7 74.5 72.3 71.2 70.1 71.3 70.8 September 64.8 63.8 62.9 64.6 67.4 69.1 69.9 69.4 68.6 67.8 67.0 65.6 66.7 65.8 October 51.3 50.4 50.0 50.8 53.8 55.7 56.9 56.5 54.9 53.7 52.4 51.5 53.2 52.4 November...... 33.7 33.2 32.6 33.0 35.4 37.7 38.9 38.6 37.5 36.2 35.0 34.1 35.5 34.8 December 33.5 33.3 32.9 32.6 34.3 35.7 37.3 37.8 37.1 35.8 34.7 33.8 34.9 34.7 [January 9.3 9.2 9.0 9.5 11.5 14.1 16.4 16.6 14.9 13.5 12.0 11.5 12.3 12.1 1912 February 20.4 19.8 19.0 19.1 21.3 23.2 24.7 24.8 23.8 22.7 21.6 20.6 21.8 21.4 March 26.3 25.5 25.1 26.7 29.2 30.5 31.9 32.1 30.8 29.6 28.7 28.0 28.7 28.2 April 44.9 44.3 44.0 46.5 49.8 51.9 53.6 54.0 52.0 49.8 47.6 46.6 48.8 48.0 Average 46.2 45.3 44.9 46.8 49.6 51.6 52.8 52.7 51.5 49.7 48.4 47.4 48.9 48.3 Table CXII contains a bi-hourly record of the relative humidity and temperature for a single year from May, 1911, to April, 1912. The humidity readings are taken from the hygrograph and the temperature readings from a thermograph. Average (a) is the average of the bi-hourly readings, while average (b) is that of the two regular observation hours, 7:00 a.m. and 7:00 p.m., at which time the relative humidity is taken from the whirling psychrometer, and the temperature from the official thermometers (see Fig. 52). than that of the preceding or the following month, was the result of the extraordinarily large number of bright, sunshiny days, which are common to extended periods of very cold weather. RELATIVE HUMIDITY, HOURLY Unfortunately, no hourly relative humidity readings for Chicago are available, and the period of bi-hourly values shown in Table CXII is much too short to establish with any degree of accuracy244 THE WEATHER AND CLIMATE OF CHICAGO Average relative humidity by months, from May, 1911, to April, 1912, inclusive. Average temperature by months, from May, 1911, to April, 1912, inclusive. (See Table CXII)ATMOSPHERIC MOISTURE 245 mean relative humidities for the various hours. Temporary irregularities due to storm movement are too numerous to warrant many generalities in this connection. For instance, the bi-hourly averages of July, 1911, and of July, 1912, show great differences (Auxiliary Table J). AUXILIARY TABLE J Comparison of Bi-hourly Relative Humidity, July, 1911, and July, 1912 2 4 6 8 10 12 2 4 6 8 10 12 A.M. A.M. A.M. A.M. A.M. M. P.M. P.M. P.M. P.M. P.M. MDT. July, 1911 65 68 67 60 52 51 51 51 52 60 61 64 July, 1912 78 79 80 75 70 68 66 64 68 71 74 76 Monthly averages: July, 1911, 58.3 per cent; July, 1912, 72.4 per cent. Fig. 52.—Relative humidity (bi-hourly) from May, 1911, to April, 1912, inclusive; temperature (bi-hourly) from May, 1911, to April, 1912, inclusive. However, the relation in short periods of temperature and relative humidity, and the average cycle of changes during the hours of the day, are apparent from the means at the bottom of each section of the table. The relation and change is nicely illustrated in Fig. 52. For any short period of time the actual amount of water vapor in the246 THE WEATHER AND CLIMATE OF CHICAGO air may be considered as fairly constant, and the effect of rising temperature in increasing the capacity of the air for moisture is therefore to lower the relative humidity; and the converse is equally true. As a result, the air is most humid at about the time of minimum temperature and driest at or just before the occurrence of the maximum temperature; that is, at about 6 a.m. and 3 p.m., respectively (p. 136). On individual days relative humidity follows closely the changes in temperature, cloudiness, and shift of wind, and usually rises decidedly upon the approach of rain or snow. On May 7, 1911, relative humidity was remarkably low throughout the day, the bi-hourly readings, beginning with 2 a.m., being 48, 49, 49, 43, 34, 27, 24, 23, 24, 30, 36, and 39 per cent, respectively. This day was clear during the entire twenty-four hours, with bright sunshine from sunrise to sunset, and with a moderate southwesterly wind. The temperature rose rapidly from 55° at 5 a.m. to the maximum for the day, 77°, at 3 p.m. On August 13, 1911, the bi-hourly relative humidity, beginning with 2 a.m., was 93, 94, 95, 89, 90, 96, 92, 93, 96, 94, 86, and 87 per cent. The temperature was quite uniform, ranging only 6°, from 68° in the early morning to 74° at the following midnight. Rain began shortly after 12 a.m., ending at 6:13 a.m., but two short showers occurred between 11 a.m. and 1 p.m. The weather was cloudy during the morning and early afternoon, the clouds breaking somewhat thereafter. Between 8 and 11 o’clock in the morning the sun shone through rifts in the clouds, which explains the temporary drop in relative humidity at that time, and there was in like manner sunshine during the afternoon. Owing to the nearly stationary temperature, however, and the evaporation from moist surfaces, the sunshine was not as effective in reducing the relative humidity as would otherwise have been the case. Relative humidity of 100 per cent, or complete saturation, is not uncommon in cloudy, wet weather, especially if foggy conditions prevail. There is no instance on record, however, of such state continuing throughout the entire period of any one day. Occasionally extremely low relative humidity is experienced. On October 21, 1871, with a temperature of 71°, the relative humidity at 3 p.m. was 20 per cent; and on April 25, 1872, with a temperature of 81°, the relative humidity at the same hour was 19 per cent. As far as known, the latter is the lowest relative humidity reading ever occurring at Chicago.ATMOSPHERIC MOISTURE 247 COMPARISON OF RELATIVE HUMIDITY AT CHICAGO WITH THAT OF OTHER PORTIONS OF THE UNITED STATES Table CXIII and Fig. 53 contain the average monthly and annual relative humidity for the same cities in the United States that were used in the comparison of monthly and annual precipitation (p. 165, Fig. 36). The highest relative humidity is, as we should expect, along the Pacific coast, where the prevailing winds are from the ocean, and along the southern coasts from Texas eastward, where the warm air is laden with moisture from the Gulf Stream. San Francisco, Cal., has the same annual percentage as Jacksonville, Fla., 80 per cent, although the precipitation of the former is but about TABLE CXIII Mean Monthly and Annual Relative Humidity in Per Cent for 15 Selected Cities, 1889-1910 (See Fig. 53) Station Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Portland, Ore 88 86 86 85 85 84 84 86 88 90 89 88 87 San Francisco, Cal.... 81 79 77 75 79 83 87 86 80 78 78 78 80 Yuma, Ariz 44 45 42 38 38 38 43 48 45 44 44 46 43 Havre, Mont 80 82 76 62 63 62 56 56 62 68 75 79 69 Denver, Colo 56 56 53 50 54 50 48 50 47 49 53 53 51 El Paso, Tex 47 40 30 26 24 29 46 48 48 46 47 48 40 Moorhead, Minn 88 87 85 75 68 73 74 74 75 77 85 88 80 Omaha, Neb 76 76 70 62 64 68 68 70 69 66 70 76 70 Galveston, Tex 84 85 86 83 80 79 78 78 77 76 81 82 81 Marquette, Mich 82 81 79 73 71 71 72 75 77 77 80 81 77 Chicago, 111 82 81 80 73 72 73 70 72 72 72 76 80 75 New Orleans, La 78 78 78 76 76 78 79 79 79 76 78 79 78 Northfield, Vt 80 76 76 71 72 76 78 83 88 81 80 81 78 New York, N.Y 74 72 71 68 71 73 73 75 76 73 73 73 73 Jacksonville, Fla 81 79 78 74 76 78 80 82 84 82 82 81 80 two-fifths of that of the southern city. Yuma, Ariz., in the midst of the arid region with its cloudless skies, has an annual percentage of 43,. while El Paso, Tex., in the foothills farther to the east and with three times the rainfall, has an average of but 40 per cent. At El Paso, however, the spring months are exceptionally dry; while at Yuma, although daytime temperatures cause extremely low humidity during the afternoons, clear skies and still atmosphere at night permit radiation to bring the temperature relatively low, and so raise somewhat the percentage of moisture. Denver, Colo., at an altitude of about 5,000 feet, has a low annual average,, because the crests of the Rockies to the west rob the prevailing westerlies of moisture before reaching the city. In regard to the other cities shown, theFig. 53.—Average monthly and annual relative humidity in per cent for 15 selected cities, 1889-1910. These values are also shown in Table CXIII. THE WEATHER AND CLIMATE OF CHICAGOATMOSPHERIC MOISTURE 249 values also depend largely upon location, prevailing wind direction, temperature, and proximity to water surfaces. DEW POINT Whenever the relative humidity is 100 per cent, that is, when the air is saturated with vapor of water, condensation must begin with any lowering of the temperature. Whenever the relative humidity is less than 100 per cent, the temperature to which the air must fall TABLE CXIV Mean Monthly and Annual Dew-Point, Degrees, 1889-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1889 24 14 29 37 45 54 60 57 51 39 33 33 40 1890 26 26 21 35 42 60 58 57 52 45 32 23 40 1891 24 22 25 38 41 58 56 59 55 39 27 28 39 1892 14 26 23 33 46 59 51 61 52 41 28 19 39 1893 10 18 27 36 44 58 64 57 50 42 28 22 38 1894 23 18 33 37 46 55 55 56 54 42 26 26 39 1895 13 12 22 36 46 54 56 57 55 31 28 24 36 1896 22 20 22 41 51 55 60 60 50 38 32 27 40 1897 18 24 29 37 42 54 64 58 54 45 31 21 40 1898. 25 23 33 34 47 59 61 62 56 45 29 20 41 1899 19 12 24 39 48 60 65 68 51 48 38 21 41 1900 23 16 24 37 47 55 65 69 59 54 32 24 42 1901 21 13 29 37 47 61 64 62 54 44 30 20 40 1902 19 17 32 35 49 55 65 60 52 45 40 23 41 1903 20 21 35 38 48 50 > 60 59 52 42 27 16 39 1904 14 13 29 31 45 53 58 56 55 42 32 22 38 1905 14 13 33 37 48 56 61 63 56 41 33 26 40 1906 28 22 24 41 49 57 61 65 60 44 34 28 43 1907 24 20 35 31 42 56 63 60 55 44 31 28 41 1908 23 21 31 39 52 58 65 60 60 45 36 24 43 1909 24 27 28 36 46 57 59 65 56 40 41 18 •41 1910 20 18 36 40 44 55 62 61 58 48 28 20 41 1911* 23 26 28 37 50 59 60 60 58 45 28 29 42 1912*....... 5 15 22 38 48 . 52 62 62 56 44 33 24 38 1913* 23 15 26 36 46 56 62 63 55 44 38 30 41 Means 20 19 28 36 46 56 61 60 54 43 32 23 40 * Not included in means. to produce saturation and condensation is called the “dewpoint.” The monthly and annual values of the dew point at Chicago are entered in Table CXIV. No discussion of the subject is necessary beyond calling attention to the fact that, during the period from April to October, inclusive, the mean dew point is below the mean temperature by about 10°; in February, March, November, and December, by about 6° to 7°, while the depression is least in January, averaging then about 4° (Table I).PART IV CLOUDINESS AND SUNSHINECLOUDINESS AND SUNSHINE AVERAGE CLOUDINESS, MONTHLY AND ANNUAL Table CXV shows the average cloudiness by months and years from 1890 to 1913, inclusive. These figures are based upon observations made between sunrise and sunset only, and are determined upon a scale of 0 to 10, 0 representing total absence of clouds, and 10 the condition of cloudiness which would obscure every portion of the TABLE CXV / Mean Monthly and Annual Cloudiness, 1890-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1890 5.9 6.5 5.1 4.6 6.1 5.1 3.5 4.7 5.1 7.0 5.3 5.3 5.4 1891 7.1 5.5 7.7 5.2 4.1 5.6 3.9 4.7 2.2 4.3 7.2 5.2 5.2 1892 5.6 7.3 5.8 6.3 6.8 7.2 4.4 4.5 4.0 4.3 6.6 6.6 5.8 1893 6.9 6.1 6.9 6.9 5.5 4.6 3.1 3.1 4.4 4.1 5.2 6.1 5.2 1894 5.8 5.0 5.2 4.9 5.0 4.2 1.9 4.6 4.0 5.9 6.5 5.6 4.9 1895 5.8 4.1 4.0 4.4 3.8 3.0 3.7 3.3 2.9 3.5 6.9 7.4 4.4 1896........ 7.0 5.3 4.6 4.6 3.4 3.7 4.2 3.1 5.5 4.5 6.4 7.2 5.0 1897 6.3 7.1 5.4 5.4 4.3 5.3 4.1 4.5 2.9 2.9 6.9 6.9 5.2 1898 :. 5.2 5.7 4.4 4.6 4.4 5.0 2.3 3.7 4.2 7.4 5.6 5.3 4.8 1899 5.5 6.0 7.4 4.2 5.1 3.7 4.5 3.4 5.4 5.3 6.5 6.3 5.3 1900 6.4 5.8 6.2 4.7 5.0 4.5 4.5 4.5 4.8 4.3 6.1 6.4 5.3 1901 6.5 3.8 7.0 4.5 5.9 3.7 3.2 4.9 4.0 3.6 5.6 6.7 5.0 1902 5.0 4.7 6.1 5.2 4.4 5.4 4.7 4.0 5.8 4.4 6.1 7.0 5.2 1903 6.3 5.5 5.7 5.7 5.6 5.6 4.5 6.0 4.7 4.0 5.2 6.1 5.4 1904 6.5 7.1 7.5 5.7 5.9 4.8 5.1 4.3 6.1 5.0 5.4 7.3 5.9 1905 6.4 5.4 6.5 5.8 5.4 5.0 6.2 5.3 5.5 5.5 5.4 5.2 5.6 1906 6.6 5.8 6.8 4.8 5.2 4.8 4.4 4.9 4.3 6.4 6.7 7.7 5.7 1907 7.6 5.7 6.5 6.6 6.0 4.9 5.8 4.6 6.1 5.3 4.6 7.4 5.9 1908 5.2 6.9 6.3 5.6 6.2 3.9 4.5 3.6 2.8 4.4 5.5 6.3 5.1 1909 7.7 7.8 6.7 6.6 4.8 6.2 5.5 4.0 4.9 5.2 6.8 6.8 6.1 1910 7.5 6.6 3.2 5.4 5.5 3.2 4.2 5.0 5.1 3.9 6.7 6.2 5.2 1911*....... 7.9 6.3 5.2 5.9 4.2 4.4 3.8 4.5 5.5 6.8 6.5 6.1 5.6 1912* 6.2 6.0 6.1 4.9 5.4 3.9 5.2 5.5 4.6 3.9 5.0 5.5 5.2 1913* 7.1 5.0 6.4 5.0 5.7 3.6 3.6 4.7 4.6 5.3 6.4 6.6 5.3 Means 6.3 5.9 6.0 4.9 5.2 4.7 4.2 4.3 4.5 4.8 6.1 6.4 5.3 * Not included in means. Table CXV contains the average cloudiness by months and years from 1890 to 1913, based upon observations made between sunrise and sunset, on a scale of from 0 to 10, 0 representing total absence of clouds, and 10 representing total cloudiness, that is, when the sky is entirely overcast. sky above the horizon. The average for the entire period is 5.3. This may be understood in two ways: that on the average the sky is totally clouded for a little more than half the time, or that somewhat more than half the sky is clouded during the whole time from sunrise to sunset. The proportions vary from month to month and from year to year, the greatest annual cloudiness being in 1909, a wet year, with an average of 6.1; and the least in 1895, a year of 253254 THE WEATHER AND CLIMATE OF CHICAGO nearly normal precipitation and of low humidity, with an average of 4.4. Cloudiness is considerably greater in the winter than in the summer, especially in November, December, and January, when the trend of temperature is downward and a comparatively little amount of moisture is sufficient for saturation. The least cloudiness occurs in the warm months of July and August, when the moisture capacity of the air is greatest, and the changes in temperature from day to day are relatively slight. The greatest cloudiness in a single month occurred in January, 1911, 7.9, a month of precipitation below the normal; while the least, 1.9, occurred in July, 1894, the month of lowest humidity, when there were only 2 days with appreciable precipitation. There is ordinarily a direct relation between cloudiness and humidity (relative), and between cloudiness and frequency of precipitation, but the relations are not strongly indicated in the average values. Then, too, clouds form and dissolve in strata of atmosphere entirely different from that in which the humidity observations are made; and, further, several types of cloud structure are recognized as accompanying fair-weather conditions. As a result, the relationships mentioned are interrupted to such an extent that it is impracticable from the present data to fix the amount of dependence of the one upon the others. CLEAR, PARTLY CLOUDY, AND CLOUDY DAYS The Weather Bureau classes each day as clear, partly cloudy, or cloudy, according to its position in the scale of cloudiness from 0 to 10: viz., clear, 0 to 3; partly cloudy, 4 to 7; cloudy, 8 to 10. Table CXVI contains the number of days of each class for every year since 1873. The averages show an almost even division, with a slight excess in favor of partly cloudy days, 115.6 clear, 135.6 partly cloudy, and 113.9 cloudy days. There is, nevertheless, a great variation in the division from year to year. Ordinarily, the year which has a very small number of clear days is characterized by a considerable excess of cloudy days, but this relation is not always the case. A notable exception was the year 1885, in which occurred only 99 clear days and 93 cloudy days, with the greatest number of the intermediate character on record, 173. In 1895 the greatest number of clear days occurred, 162, while the number of cloudy days, 73, was the least in occurrence in the whole period for any of the three classes. This was the year of least cloudiness, as shown in the previous paragraph. The least number of clear days occurred inCLOUDINESS AND SUNSHINE 255 1873, when there were but 86, and in that year there were 132 cloudy days, although there are several years in which the latter number was TABLE CXVI Annual Number op Clear, Partly Cloudy, and Cloudy Days, 1873-1913 (See Fig. 54) Year Clear Partly Cloudy Cloudy Year Clear Partly Cloudy Cloudy 1873 86 147 132 1895 162 130 73 1874 122 142 101 1896 136 136 94 1875 103 143 119 1897 137 114 114 1876 108 126 132 1898.. 153 106 106 1877 114 143 108 1899 127 123 115 1878 93 158 114 1900 120 140 105 1879 101 163 101 1901 154 106 105 1880 120 147 99 1902 120 145 100 1881 111 141 113 1903 117 132 116 1882 97 159 109 1904 94 139 133 1883 118 147 100 1905 101 144 120 1884 106 153 106 1906 102 128 135 1885 99 173 93 1907 93 130 142 1886 119 155 91 1908 133 1Î6 117 1887 115 135 115 1909 89 121 155 1888 110 128 128 1910 129 121 115 1889 104 128 133 1911* 114 114 137 1890 111 133 121 1912* 133 119 114 1891 119 122 124 1913* 132 97 136 18Q9I Q7 136 133 1893 130 124 111 Means 115.6 135.6 113.9 1894 144 121 110 * Not included in means. OAYS tQTS 1880 1885 1890 1895 (900 19OS. 1910 365 i l I 1 i""T" !■ i -r-'-T1" I- I — — —i—-|—i I — — — — — — i l I I "'i—r— — Fig. 54.—Relative frequency of clear, partly cloudy, and cloudy days. Fig. 54 shows the relative number and frequency of clear, partly cloudy, and cloudy days each year from 1873 to 1910. The actual number of these days may be found in Table CXVI. exceeded. The greatest number of cloudy days occurred in 1909, there being 155, and the year as a whole had the highest average256 THE WEATHER AND CLIMATE OF CHICAGO cloudiness. The years 1873 and 1905 had precipitation slightly above normal, and 1895 nearly normal. The division of clear, partly cloudy, and cloudy days for the official period is graphically illustrated in Fig. 54. TABLE CXVII Average Monthly and Annual Number op Clear, Partly Cloudy, and Cloudy Days, 1873-1910 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Clear days 7.7 7.9 7.8 9.2 10.4 9.3 12.8 12.7 11.9 11.1 7.7 7.3 115.6 Partly cloudy days ... 10.3 9.6 11.3 11.2 12.4 13.9 13.7 13.1 10.7 10.4 9.5 9.5 135.6 Cloudy days 13.1 10.7 11.9 9.6 8.2 6.8 4.5 5.2 7.4 9.5 12.8 14.2 113.9 The average number of such days in each month is shown in Table CXVII. The average values only are given, as too much space would be required for the data in detail, which are of only minor importance. The greatest number of clear days occurs in summer, while the greatest number of cloudy days prevails in TABLE CXVIII Sunrise and Sunset, Length op Twilight, Possible Hours of Sunshine, and Equation op Time, Chicago, III., 90th Meridian Time Sunrise Sunset Length op Twilight Possible Hours op Sunshine Equation op Time (Approximate) Add to Apparent Time Subtract from Apparent Time Hours Minutes Minutes Seconds Minutes Seconds January 20 7:18 4:50 1 43.1 9.6 1 22 February 20 6:39 5:29 1 23.3 10.8 4 31 March 20 5:53 6:02 1 38.8 12.2 1 52 April 20 5:02 6:36 1 47.6 13.6 10 35 May 20 4:25 7:08 2 6.2 14.7 13 11 June 20 4:14 7:28 2 20.6 15.2 8 16 July 20 4:32 7:20 2 9.0 14.8 3 24 August 20 5:03 6:44 1 49.0 13.7 12 50 September 20 5:35 5:52 1 38.7 12.3 16 03 Ontober 20 6:08 5:01 1 37.5 10.9 24 39 November 20 6:46 4:25 1 42.2 9.6 23 51 December 20 7:14 4:21 1 45.5 9.1 11 53 Table CXVIII gives the times of sunrise and sunset, length of twilight, and possible hours of sunshine on the 20th of each month; also the equation of time, approximately, Chicago, 111. winter. The relatively high frequency of clear days in May is due to the prevailing lake wind during that month, which, with its temperature increasing as it blows over the warmer land, dispels the clouds and reveals the deep azure of the unveiled sky far into the interior (p. 217). A few of the records of the individual months are worthy of note in this connection. The greatest number of clearCLOUDINESS AND SUNSHINE 257 days in any one month was 25, in July, 1894, and in this month there were only 2 cloudy days. Not a single cloudy day occurred in July, 1895, July, 1898, or in August, 1896. Except in the second of these, when there were 22 clear days, most of the record is made up of days with intermediate cloudiness. There were 22 cloudy days in November, 1876, December, 1907, and January, 1909, when there were but 2, 6, and 4 clear days, respectively. TIME OF SUNRISE AND SUNSET, LENGTH OF TWILIGHT, AND EQUATION OF TIME FOR CHICAGO Table CXVIII gives the time of sunrise and sunset for the 20th day of each month, also the length of twilight on that day, the number of hours of sunshine possible, and the correction which must be applied to mean sun time to give 90th meridian time, the standard Mdt. ) Z 34-S> ‘3 o "© > tí O tí v a> P © H P ’3 o > tí .2 tí O » '© _o "© > tí .2 tí o » ’S o > tí .2 tí o (V Sm P © "S P ‘3 -2 > tí .2 ’-¡3 © © .U P © 1s P ‘3 S "3 > tí .2 ‘■tí o f-i P © 1s P >> ■© o ■© > tí .2 tí O tí .2 *tí O > rS ’o o "3 > tí .2 *tí o P O tí P >> 15 ’o o ^3 > tí .2 tí O > +=> ‘3 o "3 > tí •2 tí O CD *-i P © 1s P >> -4-S o _o "© > tí .2 ‘-3 o © .Ü P O tí P ¡>> '© "© > tí .2 tí O O f-t p © 1s P 1872.. 1873.. 1874.. 1875.. 1876.. 1877.. 1878.. 1879.. 1880.. 1881.. 1882.. 1883.. 1884.. 1885.. 1886.. 1887.. 1888.. 1889.. 1890.. 1891.. 1892.. 1893.. 1894.. 1895.. 1896.. 1897.. 1898.. 1899.. 1900.. 1901.. 1902.. 1903.. 1904.. 1905.; 1906.. 1907.. 1908.. 1909.. 1910.. 1911.. 1912.. 1913.. 27 48 30 25 40 26 23 26 29 26 30 27 26 22 28 33 37 44 42 54 60 49 51 64 44 62 66 52 48 53 48 46 54 46 56 60 44 48 48 62 41 37 NW 13 37 38 36 40 22 28 28 22 32 26 27 24 26 24 29 42 45 32 48 60 55 60 84 51 48 51 48 48 70 58 50 50 48 47 54 50 54 48 48 48 53 46 W 13 34 32 30 38 35 32 35 25 27 35 32 28 28 28 20 35 37 40 68 56 57 56 61 50 57 49 62 58 53 56 57 52 52 54 49 48 54 38 50 48 43 52 SW 18 46 32 30 30 30 36 36 26 36 21 28 33 36 24 30 38 37 36 56 50 65 72 52 40 60 67 48 60 46 56 70 53 53 58 40 48 52 60 40 46 51 37 NE 15 2 8 29 5 1 10 10 15 1 9 22 27 2 6 23 13 24 12 20 14 20 30 11 18 18 5 28 8 21 25 2 24 29 13 24 25 7 22 15 26 3 25 36 25 30 45 26 25 25 28 24 27 32 25 30 22 44 44 48 59 51 60 50 62 50 62 49 48 56 43 48 54 72 42 48 48 40 68 50 50 38 43 44 SW 26 22 3 8 6 20 2 31 9 14 6 18 27 8 6 2 4 31 10 9 3 22 18 13 25 2 19 28 8 24 25 25 22 14 12 12 28 15 2 19 12 14 50 35 30 35 20 26 28 28 23 27 21 21 24 25 22 36 27 28 49 40 40 60 48 53 42 39 41 47 72 48 52 51 46 38 46 40 39 36 35 59 32 32 24 17 9 28 10 25 16 15 24 11 16 17 8 18 27 25 17 19 28 10 29 22 4 30 26 22 28 11 15 8 12 25 1 29 24 30 14 4 18 25 20 31 35 35 32 28 33 27 20 30 23 27 21 35 24 37 44 34 32 50 46 58 60 48 56 62 47 63 53 48 50 58 50 53 48 50 41 36 46 46 44 40 36 30 31 32 35 25 25 32 26 26 28 28 24 26 22 24 32 36 38 37 45 51 55 51 60 53 53 55 76 48 50 50 52 69 47 52 59 52 46 48 39 48 40 42 13 32 40 40 26 34 24 27 25 32 26 28 26 29 26 27 41 36 54 57 50 53 60 54 47 56 47 56 52 48 58 52 72 46 39 42 52 52 37 47 45 38 14 5 12 16 15 3 10 5 26 4 27 31 5 14 26 29 13 1 7 15 16 11 25 16 26 12 23 13 2 27 27 29 31 14 30 5 31 10 6 7 50 48 40 45 45 50 36 28 36 35 32 33 36 30 37 44 45 48 68 60 72 72 84 64 62 72 76 60 72 58 70 72 72 58 59 60 68 60 60 62 53 52 Sept. 24 Jan. .. Dec. .. Aug. 5 May 6 June 25 April 10 Sept. 16 April 15 Mar. 19 Mar. 21 April 22 April 27 May 8 Oct. 14 Oct. 3 Feb. 20 May 31 Mar. 27 Feb. 24 June 13 April 20 Feb. 12 Jan. 21 May 25 July 5 Nov. 7 April 28 Sept. 11 Feb. 3 April 25 May 25 Dec. 27 April 29 Nov. 21 Jan. 20 May 28 April 7 Aug. 23 Jan. 8 Feb. 21 Mar. 24 30 26 25 36 50 25 26 36 32 24 25 23 24 20 34 30 28 53 45 72 50 50 40 50 55 50 48 44 54 45 35 50 41 56 42 45 48 40 34 41 38 20 30 30 24 25 25 20 24 20 21 21 28 26 23 30 39 36 52 50 45 40 50 50 36 72 48 37 49 56 69 70 44 40 42 43 41 42 42 48 33 47 18 25 45 20 22 22 20 24 20 16 20 20 27 24 30 40 25 45 48 40 50 43 45 36 56 72 54 45 44 47 56 42 43 41 44 49 32 60 36 32 35 s SW SW s NE SW SW SW SW N NW W NW SW s NE S NE SE S NW NE SW NW SW S s SW SW w w SW SW NE NE NW SW SE 5 31 8 12 2 30 12 20 25 2 16 11 2 8 8 23 8 28 11 23 11 1 16 11 15 2 20 5 9 5 9 6 12 31 23 14 24 8 NW N NE NE SW W E SW SW w SW w NW SW SW NW NE NW SW S SW s w NE W SW s s w NE W SW w NW NW W SW w NE 8 1 15 31 24 9 31 26 13 30 26 16 28 1 17 8 2 1 8 20 21 11 17 22 26 24 20 17 30 2 15 3 20 26 29 26 8 9 7 SW w s N NE NW W N NE SW W NE SW W SW SW w SW SW SW NE SW w E NE SE SW NE W S w SW SW NW W NE W E N W 23 3 5 23 9 1 28 27 20 16 19 8 25 26 20 19 5 24 7 28 12 20 10 22 15 26 8 3 4 22 2 27 3 2 5 14 15 5 21 22 SW SW NE N S SW NE NE W NE SW SW SW N W N NE SW SW SW NW NE S E W SW SW SE W SW w s NW W w w w w NE SW 7 15 28 8 5 24 12 19 21 18 11 9 8 2 21 18 27 15 9 24 10 3 21 31 28 12 6 13 16 24 24 28 21 29 6 16 6 27 20 24 N N W SW s SW SW NW NE NE W N NW SW SW SW SW SE N NE SW E S S NE S NE NE SW S SW w s SW w w NW W s SW NE SW SW N W N SW w NE S NW SW NW SW SW N NE S SW s E N S N W S NE NE NE SW SW SW SW SW w SW NE S N NE W NW NE NW S N SW SW SW w NW N NW W SW NW SW E NW NE SW NW SW S SW NE SW s • NE E ■ SW NE NW W sw SW w SW w N 12 23 23 25 2 2 ■ 7 29 18 19 9 13 7 18 13 19 5 6 13 1 26 25 7 2 24 28 10 5 29 2 4 26 29 22 8 22 18 4 4 7 NE NE NW N E W SW NW SW SW NE N NE W NE SW NE NE SW S SW w NE W N NE SW SE SW S SE SW NW W SW NW W w SW NE 24 15 5 5 26 7 25 24 27 16 23 28 7 17 12 27 4 2 2 28 12 18 15 5 19 16 15 18 17 4 27 4 28 25 6 22 12 24 13 8 N W NE SW NW S SW NE N W S NW E N NW NW SW SW SE S S N NE E W SW W SW s SW NW N NW W W SE SW W w SW SW NE SW NW SW SW SW E N NE SW W SW NE SW NE NW SW SW SW S SW SE S NE S S S SW w NW W •SW w s w w NE 25 10 19 16 2 16 15 30 25 8 19 14 3 19 25 13 26 28 6 3 18 29 14 17 10 16 11 13 7 4 19 27 9 16 12 3 4 12 27 W NE SW N SW S W SE NE W NW W SW SW SW w SW s SW SW SW- s w SW s NE NE S SW s SE SW SW SW w w w s w NW 23 13 6 9 2 24 11 11 2 11 23 13 18 18 5 28 5 19 7 17 20 27 27 10 7 2 24 6 23 12 28 24 21 20 26 16 23 11 8 9 W w SW w SW w SW w NW SW NW NW S NW SW SE SW SW SW NE S SW s SW SW s NE SW SW w SW NE W SW s s SW NW s " sw NW s NW SW NE W NE W SW sw w sw N NE SW NW NE NE SW s w s s SW NE SW SW SW w .SW w w w NE SW N SW Highest.. Direction 66 NE 84 NE 68 NE 72 NE 72 SW 72 NW 72 W 72 ’ -SW 72 SW 63> SE 76 S 72 SW 84 NE Year.. Day.. 1898 22 1894 12 1890 27 1893 20 1903 25 1892 13 1897 5 1898 16 1900 11 1898 17 1898 7 1904 27 1894 Feb. 12 to CO co Table CXXXI shows the maximum wind velocity for each month and year from 1872 to 1913, together with direction and date. By reference to Table CXXIX, the length of the record at the different locations may be seen. WIND DIRECTION AND VELOCITY294 THE WEATHER AND CLIMATE OF CHICAGO seen that the number of gales of 40 miles an hour or over has been much smaller since the removal to the present location, and this is TABLE CXXXII Monthly and Annual Frequency of Gales, 1890-1913 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1890 5 5 14 11 7 3 2 5 5 5 4 66 1891 3 9 9 8 5 2 6 1 1 8 10 13 75 1892 7 2 11 11 6 4 2 1 1 3 8 3 59 1893 4 6 10 14 8 3 1 3 5 7 7 11 79 1894 6 6 13 6 11 4 2 1 1 1 5 5 61 1895 4 3 8 3 3 1 6 1 7 5 7 5 53 1896 3 8 8 10 7 2 0 0 5 4 7 2 56 1897 4 7 8 12 6 4 2 4 0 1 8 10 66 1898 7 7 9 6 5 3 2 2 2 6 10 5 64 1899 5 3 7 8 5 4 0 2 4 3 3 6 50 1900... 3 6 6 4 2 1 6 4 4 5 9 7 57 1901 6 3 11 4 4 8 4 1 3 5 3 2 54 1902 5 2 6 6 7 5 5 1 4 8 5 9 63 1903 4 3 6 11 5 0 2 2 2 3 4 3 45 1904 5 7 5 7 2 2 5 2 5 4 5 11 60 1905. 5 4 5 5 5 1 1 1 0 3 7 1 38 1906 5 3 3 2 5 3 1 1 3 3 3 0 32 1907 3 2 3 4 1 2 2 3 1 1 2 1 25 1908 2 8 3 5 4 2 1 3 0 0 4 1 33 1909 3 4 0 9 6 1 1 0 0 2 6 2 34 1910 2 1 3 2 3 1 1 2 0 2 0 0 17 1911* 4 4 4 1 0 0 3 0 1 5 5 3 30 1912*....... 1 3 1 3 3 1 0 0 0 1 1 2 16 1913* 0 1 4 0 2 0 3 0 0 0 2 0 12 Average 4 5 7 7 5 3 3 2 3 4 6 5 52 * Not included in averages. Table CXXXII gives the number of times each month and year during which the wind reached or exceeded a velocity of 40 miles an hour. In the study of this table, allowance should be made for the change in exposure of the anemometer as noted in Table CXXX (see Fig. 61). <890 less 1900 190S 1910 Fig. 61.—The frequency of storm winds. Fig. 61 shows graphically the number of times each year during which the wind has reached or exceeded a velocity of 40 miles an hour (see Table CXXXII). The decrease since the change to the Federal Building in 1905 is striking.WIND DIRECTION AND VELOCITY 295 more because of the change in the position of the wind-recording instrument than because of any lessening in the frequency of storms. While the definition of “gale” as a wind of 40 miles an hour or over has remained the same in the language of the Weather Bureau, it has been found necessary to lower the limit which in the Auditorium Tower records marked the occurrence of a storm, so as to equalize the issuance of storm warnings in anticipation of storm winds as recorded at the Federal Building. The verifying velocity at the former location was even above that defined as a “gale,” and was fixed at 46 miles an hour. Since the removal, however, it has been successively lowered to 40 and to 36 miles an hour, the latter being the present verifying velocity for storm winds at Chicago. The table shows 1893 to be the stormiest year of the record, there having been 79 gales in that year. Of the months, 14 gales occurred in April, 1890, and in the same month in 1893, and such winds are more frequent in March and April, and least frequent in August. Storm winds do not occur with the same frequency from all points of the compass, and there are several reasons underlying the unequal division. Auxiliary Table N, which was prepared a number of years AUXILIARY TABLE N Percentage of Storm Winds from Each Direction for Certain Stations on the Upper Lakes, from 1893 to 1897, Inclusive (From Cox, Bulletin No. 24, United States Weather Bureau, p. 157) Station v N NE E SE S SW W NW Duluth, Minn 1 30 0 0 0 16 0 53 Marquette, Mich 4 0 0 11 32 29 4 20 Sault Ste. Marie, Mich 0 3 2 18 0 6 4 67 Green Bay, Wis 30 3 0 2 4 24 21 16 Milwaukee, Wis 6 7 4 12 4 30 21 . 16 Grand Haven, Mich 2 1 13 6 7 6 15 50 Chicago, 111 6 11 3 13 22 29 10 5 Alpena, Mich 2 1 9 17 1 17 19 34 Port Huron, Mich 8 5 2 4 13 33 18 17 Mean 7 7 4 9 9 21 12 31 ago in a study of this phase of the subject, will illustrate the comparative frequency of storm winds from certain directions and the absence of such from others. The verifying velocities at the various stations are arbitrarily fixed, and depend entirely upon the exposure of the anemometer, there being a wide variation between the different limits.296 THE WEATHER AND CLIMATE OF CHICAGO Thus it will be seen that, while for the upper lakes as a whole storm winds are most frequent from southwest to northwest, these directions including 64 per cent of the total number occurring, the figures for Chicago indicate a greater frequency from southeast to southwest, with the same percentage, 64; because, being located at the southern limit of the Lake region, a larger percentage of storm centers pass north of the city than is the case with the other stations in the table, so increasing the frequency of southerly winds. The barometric gradient following in the rear of these southerly storms is relatively not so great at Chicago as in the northern Lake region, and, as a consequence, storm winds from the northwest are less prevalent here. The direction east yields the least number of storm winds for practically all stations, and their small number is in a measure the result of actually lower velocities from this quarter (p. 299). Again, onshore winds are usually of higher velocity than offshore winds, because of the much less friction offered by the surface of the water, and this tends to increase the frequency of storm winds from the direction of the lake, as is shown in the table by the relatively higher percentages from northeast at Chicago and Duluth, and from northwest at Sault Ste. Marie and Grand Haven, than occur on either side of these directions. A most important factor is the configuration of the country where the station is located, and the topography may be such that from certain directions storm winds never occur. At Duluth, for instance, the obstructions are such to the east, southeast, and south that winds have never reached verifying velocities from these directions; while at Sault Ste. Marie storm'winds are confined almost entirely to the direction of the valley of the St. Mary’s River, high winds seldom passing across the valley. There is another factor of much importance in some locations, which might be termed artificial: that is, the character of the exposure of the anemometer which indicates the occurrence of storm velocities, with reference to buildings. This factor has already been discussed in detail for Chicago (pp. 282-89) and it remains only to call attention to the fact that lake winds have a much higher velocity as they approach the shore than is the case after being retarded and deflected upward over the numerous structures in their course to the wind register at the Weather Bureau office. For this reason some onshore winds of storm velocity are not recorded as such at the Federal Building, and the records can be harmonized only by reducing the verifying velocities for onshore storms so as to allow for the retardation.WIND DIRECTION AND VELOCITY 297 MEAN HOURLY WIND VELOCITY Table CXXXIII and Fig. 62 show the average hourly velocity of the wind by months from 1890 to 1910, inclusive. The mean velocity for the year is 16.3 miles an hour, ranging from an average of 18.2 miles in March to 13.3 miles in August. The highest wind velocities occur in the spring, because at that time of year the equilibrium of the atmosphere is disturbed most by the rapid changes in temperature accompanying the rise from the winter’s cold to the heat of summer. Areas of general disturbance are more frequent at this time, and move rapidly across the country, often accompanied by severe gales. In August, the month of least average velocity, there is but little barometric gradient, and the weather from day to day is more or less settled. Throughout the year the average velocities of the winter months are considerably higher than those of the summer season. On the average, the highest velocities of the day occur between 2 and 3 p.m., at which time the mean for the year is 17.7 miles; while the period of least wind is that between 6 and 7 a.m., 15.4 miles. It will thus be seen that the day is nearest calm just after the occurrence of minimum temperature, and windiest just before the time of maximum in the afternoon (p. 136). For the year the period of least wind is during the night and early morning in July, while the highest velocities are experienced on the average at about 2 o’clock in the afternoon in the month of April. At the latter time the land temperatures are rising rapidly, causing and maintaining the lake wind from the northeast with its clear sweep for many miles over the smooth surface (p. 142). In both table and graph the effect of change of location is not material, as the values of both the Auditorium Tower and the Federal Building have been combined. PREVALENCE AND VELOCITY OF WINDS FROM DIFFERENT DIRECTIONS As previously stated, the observations of wind direction are confined to the eight principal points of the compass, and no record has been made of intermediate directions. During the period of occupancy of the Auditorium Tower the number of miles of movement and the time of the winds from each of these eight directions were recorded, so that it is possible to calculate the relative prevalence and the average velocity of these different winds. The data are given in Table CXXXIV for the years 1891-1904, inclusive. On the average the velocity of the wind varies greatly with its change of direction,to CO 00 TABLE CXXXIII Mean Hourly Wind Velocity, 1890-1910 Hour Ending Mean 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. January 16.7 16.5 16.3 16.1 16.0 16.1 15.7 16.1 16.5 16.7 17.0 17.8 17.5 17.8 17.8 17.7 17.3 17.5 17.2 17.2 17.2 17.0 17.1 17.3 16.9 February 17.3 17.1 17.1 16.9 16.7 17.0 17.0 17.4 17.6 17.9 18.2 18.9 18.9 19.0 19.2 19.1 18.9 18.4 18.2 18.1 18.1 18.0 17.9 17.7 17.9 March 17.4 17.4 17.4 17.3 17.3 17.5 17.4 17.7 18.0 18.3 18.4 19.1 19.1 19.5 19.8 19.7 19.4 18.9 17.9 17.8 17.8 17.8 17.5 17.7 18.2 April 17.0 17.1 17.2 17.3 17.4 17.4 17.3 17.7 18.2 18.5 18.7 19.4 19.4 20.0 19.8 19.8 19.2 18.6 17.7 17.6 17.4 17.3 17.2 17.3 18.1 May 15.8 15.8 15.8 16.0 16.0 15.7 15.6 15.9 16.4 17.0 17.4 18.0 18.2 18.4 18.6 18.5 18.1 17.3 16.0 15.5 15.6 15.6 15.6 15.6 16.6 June 13.6 13.5 13.2 13.1 13.3 13.3 13.0 13.2 13.5 13.9 14.4 15.0 15.2 15.4 15.7 15.6 15.3 14.6 13.6 12.8 12.8 13.0 12.9 13.3 13.9 July 12.6 12.8 12.8 12.8 12.7 12.7 12.3 12.5 13.0 13.4 13.8 14.5 15.0 15.3 15.4 15.3 14.8 14.3 13.1 12.4 12.4 12.5 12.5 12.8 13.4 August 13.0 13.0 12.8 12.8 12.6 12.6 12.2 12.2 12.5 12.9 13.4 14.0 14.3 14.8 15.2 15.0 14.7 14.0 13.1 12.9 12.8 12.8 12.9 13.1 13.3 September 15.6 15.4 15.2 15.2 14.9 14.9 14.6 14.6 14.7 15.1 15.4 16.1 16.3 16.6 16.7 16.4 15.6 14.8 14.2 14.6 15.0 15.6 15.6 15.7 15.4 October 16.4 16.2 16.1 15.8 15.6 15.8 15.7 15.7 16.0 16.0 16.2 17.0 17.2 17.4 17.4 16.9 15.9 15.6 15.7 16.1 16.4 16.4 16.4 16.4 16.3 November 16.8 16.7 16.7 16.7 16.8 17.0 16.8 17.2 17.6 17.8 17.9 18.3 18.3 18.4 18.4 17.9 17.3 17.4 17.1 17.2 17.3 17.4 17.1 17.1 17.4 December 17.3 17.3 17.2 17.1 17.0 17.1 17.2 17.3 17.6 17.7 17.9 18.5 18.4 18.6 18.5 18.2 18.1 18.2 18.1 18.4 18.4 18.2 17.9 17.7 17.8 Annual 15.9 15.7 15.6 15.6 15.5 15.6 15.4 15.6 16.0 16.3 16.6 17.2 17.3 17.6 17.7 17.5 17.0 16.6 16.0 15.9 15.9 16.0 15.9 16.0 16.3 Table CXXXIII shows the mean wind velocity for each hour of the day and for each month of the year, based upon records from 1890 to 1910, 15j years at the Auditorium and 5§ years at the Federal Building. The above values are graphically shown in Fig. 62. THE WEATHER AND CLIMATE OF CHICAGOWIND DIRECTION AND VELOCITY 299 influenced as it may be by obstructions, character of the surface passed over, rising or falling temperature, and the translation of the storm center around which the wind tends to blow. The strength and frequency of the higher winds from the southwest and south are due mainly to the last two of these influences (p. 292). The next higher velocities are from the northeast, largely because of the great lake surface which extends in that direction and allows the flow of currents over it with but little friction. Even a southwest wind blowing over the city may in some measure be accelerated by the lake surface. As a portion passes out over the lake and is released from the strong friction of the land with its innumerable structures' Fig. 62.—Mean hourly wind velocity. Fig. 62 shows the mean hourly wind velocity for each hour of the day and each month of the year based upon records from 1890 to 1910, The heaviest shading shows thé time when the greatest average hourly wind velocity occurs and the lightest shading the time when the average velocity is the least. The dotted lines marked S.R. and S.S. indicate the varying time of sunrise and sunset during the year (see Table CXXXIII). it tends to outrun the portions yet over the city, and so diminishes the pressure slightly along the shore, thus increasing the movement over the land. Due easterly winds at Chicago are much less prevalent than those of any other class, because they usually occur only as shifting currents. This is in turn due to the movements of the great areas of high and low barometric pressure across the Middle States, such rarely approaching or passing directly from the west. Winds from the east, moreover, have the least velocity of any direction, as the cyclonic areas of which they are a part are themselves moving from the west, so that the motion of the latter lessens that of the east wind with reference to stationary objects.co o o TABLE CXXXIV Average Number of Miles of Wind and Time of Blowing from Each Direction, Monthly and Annual, 1891-1904 Month N NE E SE S SW W NW Calm Miles d. h. Miles d. h. Miles d. h. Miles d. h. Miles d. h. Miles d. h. Miles d. h. Miles d. h. Miles d. h. January 864 2—10 972 2— 4 577 1—11 970 2— 8 1807 4— 2 2693 5—13 2868 6—16 2454 6— 4 4 0—2 February 932 2— 9 1503 3— 2 640 1—14 1033 2—10 1473 3— 0 2045 3—21 2775 6—11 2127 5— 9 1 0—1 March 1449 3—10 2484 5— 6 1025 2—12 1550 3—11 1861 3—21 2444 4— 9 1695 3—23 1544 3—22 1 0—1 April 1676 3—18 3425 7—11 979 2—13 1547 3—16 1995 4— 0 1838 3—12 1218 2—21 890 2— 9 3 0—2 May 1336 3— 6 3572 7—17 . 646 2— 1 1338 3—16 1819 4— 6 2372 4—22 1327 3— 7 623\ 1—18 5 0—2 June 816 2—15 2627 7—14 804 3— 2 1169 3—21 1349 3—14 1983 4—14 1071 2—23 616 1—22 7 0—4 July 776 2— 9 2404 7— 4 638 2—19 1035 3—16 1205 3— 7 2538 5—22 1287 3—16 539 1—20 10 0—6 August 827 2—14 2602 7— 9 906 3— 9 1239 4— 4 1178 3—12 2112 5— 3 924 2—17 599 2— 0 10 0—5 September 888 2—12 1788 4— 7 980 2— 3 1348 3—15 2351 5—10 2633 5—19 1233 3— 8 987 2—17 7 0—5 October 975 2—19 1552 3—21 610 1—20 1386 3— 8 2520 5—13 2774 5—14 1745 4—11 1234 3—11 3 0—2 November 932 2—15 1573 3— 7 270 0—20 877 2— 3 2541 5— 6 2959 5—15 2099 5— 5 1940 5— 7 1 0—1 December 832 2— 5 1060 2— 5 480 1— 1 847 1—21 2719 4—11 3840 7— 4 2700 6—16 2014 5— 9 2 0—3 Annual mean 1025 2—18 2130 5— 3 713 2— 2 1195 3— 4 1902 4— 5 2519 5— 4 1745 4— 8 1297 3—12 4 0—3 Average hourly velocity. 15.6 17.3 14.2 15.6 18.9 22.8 16.6 15.4 Table CXXXIV shows the total number of miles and the length of time in days and hours the wind was observed to blow from the above eight directions for each month from 1891 to 1904; also the length of time there was no wind or a calm prevailed. The average hourly velocity for each direction is also given. These values are graphically represented in Fig. 63. The direction prevailing during this period, southwest, as shown above, is different from that indicated for the same period in Table CXXVIII, as the method of calculation is somewhat different. In Table CXXXIV the averages are determined from the actual number of hours during which the wind blew from each direction. In Table CXXVIII, the prevailing direction of each year is determined from the prevailing direction of the months in that year. THE WEATHER AND CLIMATE OF CHICAGOWIND DIRECTION AND VELOCITY 301 The average hourly velocities for the various directions are: north, 15.6; northeast, 17.3; east, 14.2; southeast, 15.6; south, 18.9; southwest, 22.8; west, 16.6, and northwest, 15.4 miles an hour. Differences of exposure, however, influence the velocities from all directions, as has already been pointed out (pp. 282-89). The graphic values in Fig. 63 have been drawn from the data in Table CXXXIV. The degree of prevalence of each direction is shown for the months of the year by the length of line, while the average velocity of the direction is indicated by the breadth of line. It is easily to be seen that north winds prevail but little, and east winds least of all. Beginning with January, west and northwest winds diminish, and north and northeast winds increase. The north wind reaches its maximum prevalence in April, but the northeast wind continues to blow for longer periods until May, and remains the prominent wind until the beginning of autumn, when the south-to-west directions assume dominancy. At no time of the year is the north, east, or southeast wind the prevailing direction in the cycle of changes. The little occurrence of the second has already been explained. The other two more or less parallel the shore of the lake, and the lake and land influence tends to divert them either to the one side or the other. Taking the seasons into consideration, the preponderance of westerly winds in winter is scarcely more marked than that of the easterly winds of the summer and spring. The average times shown for each direction in the table give the best basis for determining the prevailing wind direction during the period covered, and this is shown to be southwest, with an average duration of 5 days 4 hours each month, against 5 days 3 hours each month for the direction northeast, the next most persistent wind. However, as calculated by the method in common use (p. 278)—that is, from the prevailing directions of the various months—the prevailing wind direction of the period in the table under discussion, 1891 to 1904, inclusive, is strongly northeast (see Table CXXVIII). During this time the prevailing direction as calculated was northeast for 60 months, and southwest for only 36 months; while, by an actual count of the hours and minutes each blew, the southwest wind averaged 1 hour longer for each month of the entire period. Such differences in recording the prevailing wind direction are, however, of rare occurrence, and probably only happen when certain wind directions are controlled by strong but non-periodic influences, such as govern the spring and summer lake wind at Chicago (p. 142).in the aggregate the wind has blown from that direction longer than from any other. The prevailing direction involves the factor of time. Resultant wind direction involves the factor of movement. Fig. 63.—Average duration and velocity of wind. Fig. 63 graphically represents the average duration and velocity of the wind for each direction of the compass for each month of the year, based upon records from 1891 to 1904, covering the greater portion of the record of the Weather Bureau while at the Auditorium. The average hourly velocities are indicated by figures at the end of the columns and the average duration is shown by the figures along the side of the columns. The width andjength of the columns are proportional to the velocity and duration of the wind from each direction. O 5° H % P. ►ö ri <1 go .CD tp y—1 • tr- p fT w m ^ S' p” S p a. m C* tr ^ 3 S- i S’ P {3 o cr CD m O P ' / / s y / / y / s y / / A ✓ / y / / y / / y / / / / / y / / / / y / / y / / s / y / / ✓ / y / / y / / y / / ✓ / / y M / / / / / / / / y / / / / / / y / y' / / / / y / / y / / / / y / / y s s y / / y / / y / / y / J. / / / / / ✓ / y / / y / / / / y / / / s y ✓ s s y / / / ✓ / / / / y s / y / s y / / y / / y t \ J. / / / / / / / / / s y / / y s / / / / y ✓ / y / / y / / y / / / y' / y' / / / / / / / / A. / / / / / / / / / / / y / / y / / / y / / / / y / / y / / y / / y / / y / / y / / / y / / y / s / / / / / / / / / / / / s / y / / y / / y / / / / y t \ t t t / / o. / / / / / / / / / / / / / / / / / y / / y t \ t / / \ t H. \ \ \ \ / \ ► / / / / — —► — — — — — \ \ \ D / \ — — / — / — — — — — — — — / — — / / J. F. M A. M J. J. A 5. 0. IN/. D. Fig. 65.—Prevailing hourly wind direction, Chicago. Fig. 65 shows the prevailing hourly wind direction by months based upon records from 1892 to 1910, contained in Table CXXXV (see also note to Table CXXXV). THE WEATHER AND CLIMATE OF CHICAGOTABLE CXXXVI Prevailing Hourly Wind Direction, Davenport, Iowa, 1892-1910 Month Mdt. to 1a.m. ® a a ◄ t-H o . ■*“ a a-*«' b- o . ^ a a < oo 9 a.m. to 10 a.m. 10 A.M. to 11 A.M. 11 A.M. to Noon Noon to I 1 P.M. 1 1 P.M. to 2 P.M. 2 P.M. to 3 P.M. 3 P.M. to 4 P.M. 4 P.M. to 5 P.M. 5 P.M. to 6 P.M. 6 P.M. to 7 P.M. 1 7 P.M. to 8 P.M. 1 8 P.M. to 9 P.M. 9 P.M. to 10 P.M. 10 P.M. to 11 P.M. 11 P.M. tO Mdt. Prevailing Direction l January NW NW w NW NW NW NW NW NW w w W SW SW NW NW, NW NW NW W NW NW NW NW NW February W W w NW NW NW W W W SW NW NW NW NW W W W W W W W NW W W W March NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW NW- April E E E E E E E E NW NW NW NW SW NW NW NW NW NW NW NW NW E NW NW NW May E E E NE E E NE NE SW SW E SW SW SW SW SW SW SW SW E E E . E E SW June E E E E E E E E E E SW SW SW SW SW SW SW SW SW SW E E E E E July SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW SW E SW SW SW August E SW SW SW E E E E SW SW SW SW SW SW SW SW SW SW SW SW E E E SW SW September SW SW S SW SW S SW SW SW SW SW SW SW SW SW SW SW SW s E E E E SW SW October SW NW SW SW NW SW SW E SW SW SW SW NW SW NW NW SE w NW NW NW NW W SW SW November NW w NW NW NW NW w NW NW NW NW NW NW SW SW NW NW w w W NW W NW NW NW December w SW SW w W w w W w w SW SW SW SW SW w w w w w W w W w w Annual E SW SW NW NW NW w E SW SW SW SW SW SW SW SW SW SW NW w NW E E SW SW Table CXXXVI shows the prevailing direction of wind by hours and months based upon records from 1892 to 1910. These values are graphically represented in Fig. 66. 00 o WIND DIRECTION AND VELOCITYMat. I Z34St>7&9 10 II NOON I 2. 3 4 J 6 7 g 9 JO < I Mdt. \ \ — \ \ \ \ \ \ — —► ■ — y y \ \ \ \ \ \ \ \ \ — — — \ \ \ — — — y \ \ \ \ — —« — -* — — \ — \ \ \ \ V \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ •*— \ \ \ \ y \ \ \ \ \ \ \ \ <— \ \ ✓ ✓ Jf •+■ — s / ✓ y y y y y y y y y y •a--' y y y y y y y y y y y y y y y y / y y y y y y y y y y y y y y <— y y < — y y y •< — <— y y y y y y y y y y y y — •6— y y t y y t / y y y y y y y y y y y t <— <— <— <— y y \ y y \ / <— y y y y \ y \ \ \ \ — \ \ \ \ — y \ — \ \ \ \ — V \ \ \ \ \ y y \ \ — — \ — \ \ — y y — — — — — — y y y y y — — — — — — — — — J F M A fl J J A S O N D Fig. 66.—Prevailing hourly wind direction, Davenport, Iowa. Fig. 66 shows the prevailing hourly wind direction by months at Davenport, Iowa, based upon records from 1892 to 1910, contained in Table CXXXVI. 308 THE WEATHER AND CLIMATE OF CHICAGOWIND DIRECTION AND VELOCITY 309 but there are several hours of prevailing easterly winds during the night and early morning. These instances are doubtless due to local causes, which it is impracticable to present in detail. While there is much difference between the prevailing wind directions of the two places during the spring and summer months, which serves to emphasize the influence of the lake at Chicago in determining its wind direction at that time, there is considerable uniformity during the winter, especially in the months of January and February. The prevalence of the northeast wind at Chicago is responsible for its low maximum temperatures and comparatively cool weather during the warm season (p. 37). With a normal summer temperature, such as prevailed in Chicago during the summer of 1910, there were only 9 days on which 90° was reached or exceeded, while at Davenport there were 28 such days, the lake wind being the cause of their infrequent occurrence in the larger city. 'COMPARISON OF WIND VELOCITY AT CHICAGO WITH THAT AT OTHER CITIES OF THE UNITED STATES While comparisons have been made in the previous pages between the values of temperature, precipitation, sunshine, and humidity for Chicago and other portions of the United States, as represented by selected cities, it is much more difficult to present properly the relative wind velocities of those places. The exposure of the anemometer, as has already been shown (p. 296), influences the record of wind movement to such a degree that it is almost an impossibility to secure comparable records in any one of the larger cities. The data for 1909, a typical year, on wind velocity for the various places used in former comparisons, however, are presented for what they may be worth, in Table CXXXVII and Fig. 67. Other things being equal, wind velocity increases with elevation, and the highest velocities occur in the regions lying in the paths of the most frequent storms. Chicago, being in the path of the southwest disturbances which cause our heaviest northeast winds, and on the southern side of those whose centers pass over the northern Lake region, has doubtless*as high a wind velocity as have the majority of northern cities. Wind velocity, on the average, is considerably lower in the southern sections of the country, because there the changes in weather and temperature are not«so abrupt, and the reason that this relation is not apparent in the table is doubtless due to differences in exposure of the anemometers as regards elevation,310 THE WEATHER AND CLIMATE OF CHICAGO configuration of the country, and surrounding buildings. If it were possible to secure similar exposures for all of the lake cities, we should probably find that the wind velocity does not differ much from place to place, when average conditions are compared. Of the cities shown in the table, Chicago leads in average wind velocity for the year, with 14.4 miles an hour, and New York is second with 13.0 miles; while the cities showing the lowest velocities are Portland, Ore., and Yuma, Ariz., with 6.5 miles each. TABLE CXXXVII Average Hourly Wind Velocity by Months for 15 Selected Cities for the Year 1909 (See Fig. 67) Station Anemometer above Ground Feet January February i March l April May | June July August September October November December Annual Portland, Ore 106 7.7 8.1 6.2 6.0 6.1 6.3 5.9 6.2 5.5 5.3 7.8 7.1 6.5 San Francisco, Cal.. 204 10.1 7.8 7.4 8.3 11.1 12.2 11.7 11.5 8.3 6.5 5.4 6.1 8.9 Yuma, Ariz 58 5.7 7.1 7.5 8.2 7.2 6.4 6.6 5.6 4.7 4.8 6.1 7.9 6.5 Havre, Mont...... 44 9.0 8.3 7.8 9.1 10.7 7.5 7.6 6.3 7.1 7.5 8.9 9.9 8.3 Denver, Colo 136 7.2 • 8.5 7.4 8.2 7.8 6.5 6.2 6.1 6.2 7.1 7.1 5.9 7.0 El Paso, Tex 133 10.2 13.2 14.2 15.0 14.6 10.4 10.8 10.5 10.6 9.2 10.4 11.8 11.7 Moorhead, Minn... 57 10.5 8.7 9.5 11.0 9.5 6.7 6.7 6.7 7.6 10.1 10.0 8.9 8.8 Omaha, Neb 121 10.6 10.4 10.5 12.2 9.2 7.0 6.4 6.6 6.2 8.6 9.4 9.5 8.9 Galveston, Tex 112 11.0 13.3 11.0 12.9 12.5 10.5 11.0 10.2 10.1 10.4 12.3 13.7 11.6 Marquette, Mich... 116 13.7 10.9 9.3 10.7 8.8 7.2 9.3 9.0 9.7 11.2 12.3 11.0 10.3 Chicago, 111 310 13.4 17.3 14.6 17.4 15.7 11.1 11.8 10.7 12.1 14.7 17.3 16.5 14.4 New Orleans, La... 121 8.3 11.1 10.8 10.8 9.2 7.0 7.2 7.4 7.9 7.2 7.5 9.9 8.7 Northfield, Vt 70 10.3 9.0 8.2 10.6 9.3 7.6 8.0 6.7 7.6 7.5 9.0 7.0 8.4 New York, N.Y 350 13.8 16.4 16.6 14.5 12.3 9.7 10.6 10.2 10.2 12.6 12.8 16.1 13.0 Jacksonville, Fla... 129 8.8 9.8 10.9 9.8 8.4 9.1 9.2 7.6 8.4 8.4 8.4 9.2 9.0 The maximum velocities for each month of the year 1909, recorded at the cities included in the previous table, are given in Table CXXXVIII. While the highest velocity at Chicago during that year was 60 miles an hour, several cities experienced more severe winds: El Paso, Tex., 67 miles; Omaha, Neb., 66 miles; Galveston, Tex., 68 miles; New Orleans, La., 66 miles, and New York City, 83 miles. COMPARISON OF WIND VELOCITY WITH OTHER CONDITIONS Following the discussion of temperature, precipitation, sunshine, and wind velocity, it will be interesting to compare the monthly values of these elements together. Fig. 68 illustrates in four curves these monthly mean values in successive order. The curves thatFig. 67.—Mean hourly wind velocity in miles. Upper line of figures=height of anemometer in feet above ground; lower line of figures=mean hourly velocity for 1909. 00 WIND DIRECTION AND VELOCITY312 THE WEATHER AND CLIMATE OF CHICAGO show the closest relation are, of course, those of temperature and sunshine, and while the line of precipitation gives an increase in the warm period, the changes are not at all regular. Still, the influence of the enlarged capacity of the atmosphere for carrying moisture when temperatures are high is easily detected. In general the velocity of the wind decreases as the values of the other three elements increase. The influence of sunshine and high temperature is to increase wind velocity (p. 292), and the lower velocities of summer must therefore be accounted for by the fact that active and general storm areas are much less frequent in that season, and high winds are in many cases due to brief squalls. TABLE CXXXVIII Monthly Maximum Wind Velocities for 15 Selected Cities for the Year 1909 Station Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Portland, Ore 36 32 32 25 24 25 18 26 24 24 39 33 39 San Francisco, Cal 48 34 34 32 33 36 34 34 36 29 28 33 48 Yuma, Ariz 29 36 37 37 26 23 28 27 32 28 30 32 37 Havre, Mont 36 36 35 34 42 39 36 46 38 36 46 43 46 Denver, Colo 45 38 36 44 37 31 33 28 30 34 35 32 45 El Paso, Tex 43 58 60 60 50 54 67 40 38 54 43 49 67 Moorhead, Minn 44 30 30 38 36 30 24 28 27 32 39 30 44 Omaha, Neb 66 39 37 48 38 27 38 23 35 33 30 32 66 Galveston, Tex 50 44 53 46 60 40 68 33 34 54 30 46 68 Marquette, Mich 46 47 33 ' 54 29 30 40 40 36 36 40 32 54 Chicago, 111 48 48 38 60 50 48 42 32 36 46 48 52 60 New Orleans, La 37 45 37 37 30 28 30 36 66 27 28 27 66 Northfield, Vt 41 36 31 42 37 30 36 36 29 31 36 33 42 New York, N.Y 57 73 60 83 48 43 46 40 45 46 48 58 83 Jacksonville, Fla 56 49 39 29 40 42 55 46 27 27 27 42 56 Fig. 69 indicates the influence of temperature upon wind velocity. Here the average hourly temperatures and the average hourly wind velocities are given for the period 1890 to 1910, inclusive, and the relation is seen to be direct. The lowest point in the temperature curve is at 6 a.m., and that in the velocity curve at about 6:40 a.m., while the peaks of both curves are reached at 3 p.m. After the maximum values are attained there is a steady decrease until evening, the temperature continuing the decrease until the time of minimum the following morning, when the cycle is begun over again. The wind velocity, on the whole, does not change much during the night, but there is, nevertheless, a downward tendency. The increase in wind velocity from minimum to maximum is due to the circulation of the lower strata of the atmosphere as a result of ascensional currents caused by the increase in temperature of the ground under theWIND DIRECTION AND VELOCITY 313 influence of insolation. The relation between the two elements, as illustrated, exists at all seasons of the year. Fig. 68.—Average monthly values. Fig. 68 contains graphs showing the average monthly temperature, average monthly precipitation, average monthly percentage of possible sunshine, and average monthly wind velocity, based upon all available records, the temperature and precipitation being for the same period, but the sunshine and wind for shorter periods.314 THE WEATHER AND CLIMATE OF CHICAGO SUMMAKY OF WIND DATA Table CXXXIX gives for reference a summary of the principal features relative to the average and extreme values of wind direction and velocity. 19 iS 17 /6 IS Fig. 69.—Average hourly values. Fig. 69 contains graphs showing the average hourly values of temperature and wind based upon records from 1890 to 1910. TOKNADOES Tornadoes, commonly misnamed cyclones, are very severe local storms, developing usually in the southeast quadrants of general storm areas. They occur most frequently in the Great Plains states, and but seldom in the Lake region. These storms are attendantWIND DIRECTION AND VELOCITY 315 upon severe thunderstorm conditions, and their energy may be likened to that of a great disturbance compressed into a very small area. The width of the tornado path seldom exceeds a mile, and it is often not more than a few hundred yards. Generally, the narrower the path of the tornado, the greater is its energy. A notable exception to this rule was the severe tornado which occurred at St. Louis, Mo., in May, 1896, with a path of destruction more than a mile across. Chicago and its vicinity, due in a large measure to the TABLE CXXXIX Summary of Wind Velocity and Direction Data Means Maxima Storm Winds Month Mean Highest t-< e3 6.6 14 1893 1910 May 16.6 18.8 1893 13.5 1910 53 72 1903 40 1907 4.9 11 1894 June 13.9 16.1 1899 10.6 1910 48 72 1892 35 1903 2.6 8 1901 July 13.4 15.5 1890 10.7 1906 48 72 1897 36 1896 2.5 6 1895 f August 13.3 15.2 1893 10.7 1909 47 72 1898 32 1909 1.8 4 1897t September.... 15.4 18.9 1893 10.6 1910 46 72 1900 35 1910 2.6 7 1895 October 16.3 18.8 1893 11.2 1910 51 63 1898 36 1908 3.8 8 1902 November.... 17.4 21.2 1898 13.0 1910 53 76 1898 39 1910 5.6 10 1898 December 17.8 22.0 1893 14.6 1910 52 72 1904 37 1910 4.6 11 1893$ Year 16.3 23.9 April 1893 10.6 June Sept. 1910 5! 84 1894 32 1909 51.8 79 1893 * Also 1908. f Also 1900. X Also 1904. Table CXXXIX contains a summary of the more important wind velocity data based upon records from • February 1; 1890, to February 1, 1910. counteracting influence of Lake Michigan (p. 209), have been particularly free from the occurrence of these remarkable and destructive phenomena. As far as known, but two tornadoes have ever occurred within the limits of Cook County, and only the first of these entered the city of Chicago. This tornado occurred two days before the St. Louis tornado, May 25, 1896, passing through that portion of the city known as Norwood Park; the second tornado referred to occurred on April 6, 1912. A thorough study of the former was made immediately after its occurrence, and its path is shown in Fig. 70.316 THE WEATHER AND CLIMATE OF CHICAGO This tornado occurred between 1 and 2 a.m., and at about the same time two other tornadoes occurred in northern Illinois and one in Iowa, while a fourth was reported in the southeastern portion of the lower peninsula of Michigan on the following afternoon. All were formed in the southeast quadrant of a storm or cyclone moving east from Manitoba with a trough of low barometric pressure extending thence southward. General thunderstorm conditions at the time prevailed over the western Lake region and the Middle West; and in the vicinity of the Weather Bureau office in Chicago there was an exceptionally severe thunderstorm, with vivid lightning, heavy squalls of wind, and excessive rainfall. The fall of rain from 2:08 s ig. 70 shows the path of a tornado which passed through Cook County, 111., on May 25,1896. The small arrows along the track indicate in what direction fallen trees lay after the passage of the storm. to 2:23 a.m. at the station amounted almost to a cloudburst, there being 1.24 inches in the 15 minutes. The highest wind velocity recorded at the station was 62 miles an hour at 1:50 a.m. The tornado first appeared over the Des Plaines River, close to the boundary fine separating the towns of Maine and Leyden in the northern part of Cook County. Generally, the track of the storm lay in an easterly direction, and the disturbance was most destructive in the section called Canfield-on-the-Hill, a portion of Edison Park, and the highest portion of the county, being 125 feet above the lake. The tornado moved thence southeastward, but turned slightly northeastward after reaching Norwood Park,WIND DIRECTION AND VELOCITY 317 thence moving eastward, skirting the dividing line between Niles and Jefferson. The path of the storm was clearly cut for over four and a half miles between the Des Plaines and the Chicago rivers, with a width varying from one-fourth to three-fourths of a mile. After reaching the Chicago River the track widened, indicating a decrease in energy, and by the time the tornado arrived at North Fortieth Avenue it had practically lost its identity. The sections over which the disturbance moved were for the most part but sparsely settled, and yet destruction was very great. Six houses were leveled to the ground, and about thirty others so badly damaged as to be almost beyond repair, and the total loss of property amounted to more than $100,000.— Cox, Bulletin No. 3, Geographic Society of Chicago.PART VI BAROMETRIC PRESSUREBAROMETRIC PRESSURE At any moderate elevation above sea level the pressure of the atmosphere is not a direct factor in either weather or climate, in so far as it affects the comfort and health of the individual. No extended space will therefore be devoted to a discussion of the subject, but the main features will be presented as briefly as practicable. Barometer readings are essential to the forecaster of weather conditions, and were observations of all other weather elements omitted it would still be possible to anticipate many changes of conditions from the barometric pressures alone. As far as the ordinary changes in pressure are concerned, however, they do not in the slightest degree affect the sensations of the person in health. Rheumatics, indeed, may often feel twinges when the barometer falls sharply, or drops considerably below the normal, because of the relatively higher pressure within the body; and such persons can sometimes foretell the coming of rains and snows, because precipitation occurs frequently after a falling barometer. The normal atmospheric pressure at the level of the sea is about 14.7 pounds to the square inch, but the pressure is usually measured in heights of a column of mercury which will exactly equal the pressure of the air. The normal pressure at sea level then becomes 30 inches, and this is sometimes termed the pressure of 1 atmosphere. Pressure diminishes with altitude above sea level, for short distances at the rate of about . 001 inch for each foot difference. In determining the average or normal pressure for any location, therefore, it is necessary to take into account the altitude, and any changes in the position of the barometer must be carefully allowed for. The level of Lake Michigan, on the shore of which Chicago is situated, is approximately 581 feet above sea level, but there is some variation in the altitude of some portions of the city above the surface of the lake. The records of the Weather Bureau with reference to air pressure, however, have all been reduced to the elevation of the station above sea level on January 1, 1900, so that the readings presented herein are comparable, notwithstanding the changes in the location of the office since its establishment in 1870. The station elevation is 823 feet above sea level, and is approximately 223 feet ' 321322 THE WEATHER AND CLIMATE OF CHICAGO above the average street level of the city. Therefore, when data for street level are desired, the Weather Bureau records of station air pressure should in each instance be increased by about .24 inch. TABLE CXL Mean Monthly and Annual Station Barometric Pressure, 1873-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual Departure in Thousandths i 1873 1.123 1.152 1.146 1.028 1.072 1.110 1.150 1.150 1.114 1.127 1.075 + 197 + 120 -12 1874 1.237 1.197 1.155 1.183 1.105 1.087 1.106 1.124 1.146 1.212 1.185 + 208 + 162 +30 1875 1.300 + 185 1.098 1.089 1.055 1.086 1.104 1.094 1.152 1.091 1.168 + 016 + 120 -12 1876 1.170 1.166 1.101 1.085 1.091 0.998 1.109 1.145 1.113 1.050 1.090 + 178 + 108 -24 1877 1.218 1.264 1.131 1.060 1.130 1.026 1.083 1.081 1.130 1.098 1.134 + 186 + 128 - 4 1878 1.135 1.042 1.029 0.894 1.041 1.048 1.095 1.018 1.149 1.098 1.123 + 143 + 068 -64 1879 1.201 1.176 1.164 1.106 1.135 1.101 1.076 1.088 1.188 1.225 1.162 + 168 + 149 +17 1880 1.115 1.136 1.169 1.034 1.089 1.068 1.095 1.130 1.153 1.158 1.260 + 186 + 133 + 1 1881 1.214 1.190 0.975 1.108 1.127 1.036 1.127 1.132 1.060 1.176 + 161 + 192 + 125 - 7 1882 1.213 1.118 1.136 1.130 1.086 0.997 1.128 1.099 1.188 1.122 + 230 + 176 + 135 + 3 1883 1.210 1.331 1.233 1.046 1.038 1.025 1.096 1.178 1.178 1.204 + 171 + 171 + 157 +25 1884 1.233 1.116 1.114 1.046 1.044 1.144 1.029 1.140 1.123 1.214 + 174 + 182 + 130 - 2 1885 1.205 1.085 1.156 1.109 1.023 1.123 1.075 1.089 1.139 1.085 + 067 + 133 + 107 -25 1886 1.136 1.154 1.059 1.147 1.067 1.084 1.089 1.086 1.152 1.275 + 108 + 249 + 134 + 2 1887 1.052 1.218 1.171 1.050 1.089 1.091 1.083 1.116 1.183 1.157 + 156 + 151 + 126 - 6 1888 1.317 1.152 1.208 1.247 1.032 1.044 1.149 1.154 1.179 1.078 + 232 + 168 + 163 +31 1889 1.091 1.215 1.110 1.119 1.065 1.091 1.096 1.190 1.131 1.241 + 168 + 154 + 139 + 7 1890 1.231 1.146 1.166 1.197 1.027 1.093 1.131 1.178 1.214 1.045 + 154 + 181 + 147 +15 1891 1.139 1.094 1.121 1.110 1.215 1.056 1.143 1.110 1.233 1.207 + 152 + 128 + 142 +10 1892 1.179 1.183 1.156 1.139 1.007 1.041 1.186 1.136 1.197 1.166 + 152 + 181 + 144 +12 1893 1.059 1.176 1.096 0.991 1.017 1.085 1.100 1.134 1.107 1.109 + 118 + 159 + 096 -36 1894 1.150 1.187 1.090 1.103 1.058 1.112 1.143 1.157 1.150 1.038 + 154 + 173 + 126 - 6 1895 1.081 1.200 1.119 1.108 1.114 1.156 1.123 1.065 1.120 1.162 + 191 + 079 + 126 - 6 1896 1.216 1.030 1.147 1.129 1.056 1.098 1.135 1.146 1.138 1.158 + 109 + 252 + 142 +10 1897 1.183 1.147 1.116 1.150 1.124 1.086 1.065 1.115 1.272 1.180 + 161 + 153 + 146 +14 1898 1.098 1.160 1.183 1.166 1.062 1.111 1.157 1.103 1.134 1.113 + 129 + 145 + 130 - 2 1899 1.181 1.143 1.059 1.129 1.133 1.162 1.114 1.112 1.187 1.237 + 160 + 143 + 147 +15 1900 1.151 1.102 1.152 1.154 1.089 1.087 1.098 1.152 1.183 1.207 + 174 + 160 + 142 +10 1901 1.133 1.163 0.940 1.215 1.013 1.073 1.096 1.143 1.161 1.226 + 236 + 145 + 129 - 3 1902 1.241 1.105 1.079 1.075 1.130 1.051 1.118 1.126 1.107 1.151 + 129 + 175 1.124 - 8 1903 1.060 1.179 1.251 1.032 1.159 1.097 1.120 1.112 1.203 1.210 + 204 + 145 + 148 +16 1904 1.171 1.271 1.106 1.147 1.075 1.123 1.136 1.184 1.193 1.192 + 172 + 119 + 157 +25 1905 1.307 1.299 1.164 1.037 1.061 1.092 1.062 1.108 1.151 1.205 + 142 + 179 + 151 +19 1906 1.127 1.325 1.208 1.151 1.092 1.030 1.104 1.117 1.185 1.137 1.244 1.244 1.164 +32 1907 1.266 1.215 1.140 1.093 1.104 1.047 1.064 1.148 1.115 1.221 + 174 + 095 + 140 + 8 1908 1.111 1.153 1.138 1.045 0.993 1.148 1.180 1.151 1.195 1.233 + 126 + 147 + 135 + 3 1909 1.231 1.016 1.034 1.059 1.029 1.122 1.108 1.168 1.230 1.187 1.218 + 089 1.124 - 8 1910 1.173 1.236 1.168 1.009 1.139 1.128 1.068 1.131 1.194 1.128 + 105 + 196 + 140 + 8 1911 1.222 1.222 1.141 1.189 1.148 1.086 1.130 1.152 1.175 1.178 + 105 + 204 + 163 +31 1912* 1.189 1.119 1.260 1.079 1.035 1.170 1.146 1.077 1.151 1.179 + 146 + 079 + 136 + 4 1913* 1.134 1.165 1.114 1.190 1.153 1.189 1.126 1.154 1.207 1.129 + 196 + 241 + 166 +34 Normal (1873- 99) 1.174 1.162 1.126 1.100 1.078 1.080 1.111 1.121 1.157 1.149 + 157 + 165 + 132 Departure (in thousandths).. +42 +30 -6 -32 -54 -52 -21 -11 +25 +17 +25 +33 Correction for sea-level, add. 0.921 0.913 0.899 0.895 0.882 0.875 0.869 0.874 0.883 0.891 0.908 0.915 0.893 Mean sea-level.. 2.095 2.075 2.025 1.995 1.960 1.955 1.980 1.995 2.040 2.040 2.065 2.080 2.025 * Not included in average departure. Table CXL contains values which added to a base of 28.000 inches equal the mean monthly and annual station barometric pressure. Elevation of barometer 823 ft. above sealevel. The last column contains for each year the departure in thousandths of an inch from the normal annual pressure, based upon records from 1873 to 1899. The corrections to be added to the station pressure for reduction to sealevel and the sea-level readings themselves are also given.BAROMETRIC PRESSURE 323 MEAN STATION PRESSURE, MONTHLY AND ANNUAL The actual pressure at any place in the temperate zone varies considerably in the course of a year, but the fluctuations are usually well within 1 inch above or below the normal. Table CXL gives the average station pressure—that is, the pressure at the elevation of 823 feet—for each month and year from 1873-1913, inclusive. In order to simplify the data as much as possible, only the excess of pressure over 28.000 inches has been entered, and that amount must be added to the values in order to secure the station pressure. For instance, the mean station pressure for January, 1873, was (28.000+ 1.123) 29.123 inches, while that for April, 1878, was (28.000+0.894) 28.894 Inches. According to the table, the mean pressure at the station elevation is 29.132 inches, which reduced to the level of the sea becomes 30.025 inches. ’ On the average the pressure is highest in January, 29.174 inches, and lowest in May, 29.078 inches. Reduced to sea level these values become 30.095 and 29.960 inches, respectively; but, owing to the influence of temperature, the lowest sea-level average occurs in June, 29.955 inches. Barometer normals were established by the Weather Bureau for the period 1873-99, inclusive. As these figures have continued in use up to the present time (1914), they only, being the means for that period, are given in the table, and are used in all cases in calculating departures. MEAN DEPARTURE FROM NORMAL PRESSURE, MONTHLY AND ANNUAL In calculating the mean departures from the station normal pressures, the normals for the various months have been used in each case instead of the normal pressure for the entire year, but the average departure from the annual normal is given across the bottom of Table CXLI. HIGHEST AND LOWEST PRESSURES AT CHICAGO, REDUCED TO SEA LEVEL Table CXLII contains the highest and lowest sea-level pressures observed at Chicago, by years. The highest pressure ordinarily occurs during the passage of a cold wave and the lowest during a severe disturbance with accompanying high winds. The mean of these highest pressures is 30.76 inches, and that of the lowest, 29.21 inches, making the mean annual range of 1.55 inches. The absolute324 THE WEATHER AND CLIMATE OF CHICAGO TABLE CXLI Monthly and Annual Departures from Normal Station Pressure, 1873-1913 Year Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Annual 1873 -.051 -.010 + .020 -.072 -.006 ~~;030 + .039 + .029 -.043 -.022 -.082 +.032 -.012 1874 + .063 + .126 -.004 + .035 + .029 + .083 + .027 + .007 -.005 + .003 -.011 + .063 + .028 +.043 + .030 1875 + .023 -.028 -.011 -.023 + .006 -.007 -.027 -.005 -.058 + .011 -.149 -.012 1876 + .004 + .102 -.025 -.015 + .013 -.082 -.002 + .024 -.044 -.099 -.067 + .013 -.024 1877........ + .048 -.035 + .005 -.040 + .052 -.054 -.028 -.040 -.027 -.051 -.023 + .021 -.004 1878 -.120 -.097 -.206 -.037 -.032 -.016 -.103 -.008 -.051 -.034 -.022 -.064 1879 + .027 -.059 + .014 + .038 -.006 + .057 + .021 -.035 -.033 + .031 + .076 + .005 + .003 + .017 1880 -.026 + .043 -.066 + .011 -.012 -.016 + .009 -.004 + .009 + .103 + .021 + .001 1881 + .040 + .039 + .036 + .059 + .031 -.038 + .028 -.151 -.008 + .049 -.044 + .016 + .011 + .003 + .027 + .004 +.027 -.007 1882 -.044 + .010 + .030 + .008 -.083 + .017 -.022 + .031 -.027 + .073 + .011 + .003 1883 + .169 + .107 -.054 -.040 -.055 -.015 + .057 + .021 + .055 + .014 +.006 + .025 1884 -.046 -.012 -.054 -.034 + .064 -.082 + .019 -.034 + .065 + .017 +.017 -.002 1885 -.077 + .030 + .009 -.055 + .043 -.036 -.032 -.018 -.064 -.090 -.032 -.025 1886 -.008 -.067 + .047 -.011 + .004 -.022 -.035 -.005 + .126 -.049 + .084 + .002 1887 -.122 + .056 -.010 + .045 -.050 + .011 + .011 -.028 -.005 + .026 + .008 -.001 -.014 -.006 1888 + .143 -.083 + .082 + .147 -.046 -.036 + .038 + .033 + .022 -.071 + .075 + .003 + .031 1889 + .053 -.016 + .019 -.013 + .011 -.015 + .069 -.026 + .092 + .011 -.011 + .007 1890 + .057 -.035 -.016 + .040 + .097 -.051 + .013 + .020 + .057 + .057 -.104 -.003 + .016 + .015 1891 -.068 -.005 + .010 + .137 -.024 + .032 -.011 + .076 + .058 -.005 -.037 + .010 1892 + .005 -.015 + .021 + .030 + .039 -.071 -.039 + .075 -.01*1 + .025 + .040 +.017 -.005 + .016 + .012 1893 + .014 -.030 -.109 -.061 + .005 + .023 -.050 -.040 -.039 -.006 -.036 1894 1895 -.024 + .007 + .042 + .009 -.076 + .025 + .038 -.036 -.007 + .003. + .008 -.020 + .036 + .032 + .076 + .032 + .012 + .036 -.056 -.007 -.037 -.111 + .013 -.003 + .034 + .008 -.086 -.006 -.006 1896 -.132 + .021 + .029 -.022 + .018 + .024 + .025 -.019 + .009 -.048 + .087 + .010 1897. -.015 -.010 + .050 + .046 + .006 -.046 -.006 + .115 + .031 + .004 -.012 + .014 1898 -.002 + .057 + .066 -.016 + .031 + .046 -.018 -.023 -.036 -.028 -.020 -.002 1899 + .007 -.023 -.019 -.067 + .029 + .055 + .082 + .003 -.009 + .030 + .088 + .003 -.022 + .015 1900 -.060 + .026 + .054 + .011 + .007 -.013 + .031 + .026 + .058 + .017 -.005 + .010 1901 -.041 + .001 -.186 + .115 -.065 -.007 -.015 + .022 + .004 + .077 + .069 -.020 -.003 1902 + .067 -.014 -.057 -.047 -.025 + .052 -.029 + .007 + .005 -.050 + .002 -.028 + .010 -.008 1903 + .017 + .125 -.068 +.081 + .017 + .009 -.009 + .046 + .061 + .047 -.020 + .016 1904 -.003 + .109 -.020 + .047 -.003 + .043 + .025 + .063 + .036 + .043 + .015 -.046 + .025 1905 + .133 -.047 + .137 + .038 -.063 -.017 + .012 -.049 -.013 -.006 + .056 -.015 + .014 + .019 1906 + .163 + .082 + .051 + .014 -.050 -.007 -.004 + .028 -.012 + .087 + .079 + .032 1907 + .092 -.063 + .053 + .014 -.007 + .026 -.033 -.047 + .027 -.042 + .072 + .017 -.070 + .008 1908 -.009 + .012 -.055 -.085 + .068 + .069 + .030 + .038 + .084 -.031 -.018 + .003 1909 + .057 -.001 -.146 -.092 -.041 -.049 + .042 -.003 + .047 + .073 + .038 + .061 -.076 -.008 1910 + .074 + .042 -.091 + .061 + .048 -.043 + .010 + .037 -.021 -.052 + .031 + .008 1911 + .048 + .015 -.040 + .060 + .015 + .089 + .070 +.006 + .019 + .031 + .018 + .029 -.052 + .039 + .031 1912* -.043 + .134 -.021 -.043 + .090 + .035 -.044 -.006 + .030 -.011 -.086 + .004 1913* + .003 -.012 + .090 + .075 + .109 + .015 + .033 + .050 -.020 + .039 + .076 + .034 Normal 1.174 1.162 1.126 1.100 1.078 1.080 1.111 1.121 1.157 1.149 1.157 1.165 1.132 Departure... + .042 + .030 -.006 -.032 -.054 -.052 -.021 -.011 + .025 + .017 + .025 + .033 * Not included in average departure. Table. CXLI contains the monthly and annual departures (in thousandths) of pressure from the normal values, based upon records from 1873 to 1899. To these normals must be added the base of 28.000 inches. In this table is also given the average monthly departure. See Table CXL. TABLE CXLII Highest and Lowest Observed Sea-Level Pressures, 1872-1913 Year Highest Lowest Year Highest Lowest Year Highest Lowest Year Highest Lowest 1872 30.76 29.33 1884.... 30.66 29.29 1896.... 30.71 29.37 1908.... 30.75 29.22 1873 30.75 29.31 1885.... 30.71 29.18 1897.... 30.74 29.29J 1909.... 30.63 29.04 1874 30.80 29.15 1886.... 30.66 29.09 1898.... 30.82 29.27 1910.... 30.76 29.26 1875 30.71 29.12 1887.... 30.81 29.03 1899.... 30.76 29.24 1911*... 30.77 29.27 1876 30.62 29.25 1888.... 30.93 29.28 1900.... 30.61 29.20 1912*... 30.72 29.32 1877 30.70 29.17 1889.... 30.86 29.31 1901.... 30.72 29.31 1913*... 30.67 29.31 1878 30.64 29.22 1890.... 30.74 29.29 1902.... 30.94 29.11 1879 30.66 29.24 1891.... 30.68 29.19 1903.... 30.70 29.13 Mean... 30.76 29.21 1880 30.79 29.01 1892.... 30.67 29.16 1904.... 30.74. 29.06 Extremes 30.94 28.98 1881 30.69 29.23 1893.... 30.94 28.98 1905.... 30.78 29.18 /1893\ 1893 1882 30.72 29.25 1894.... 30.85 29.26 1906.... 30.72 29.03 11902/ 1883 30.72 29.43 1895.... 30.65 29.20 1907.... 30.71 29.18 Range. ..1.96 * Not included in means.TABLE CXLIII Mean Hourly Barometric Pressure, by Months Month Hour Ending Means 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. January .148 .154 .153 .149 .147 .152 .162 .168 .176 .178 .169 .148 .133 .128 .132 .138 .143 .150 .154 .156 .156 .156 .153 .150 .152 February .157 .156 .153 .152 .155 .159 .169 .173 .176 .179 .178 .166 .151 .141 .141 .142 .146 .153 .157 .159 .161 .161 .160 .160 .159 March .125 .121 .116 .116 .119 .126 .135 .138 .142 .143 .141 .135 .123 .111 .107 .106 .109 .112 .117 .124 .128 .130 .130 .128 .124 April >... .120 .118 .117 .117 .123 .132 .143 .143 .147 .148 .144 .137 .130 .119 .111 .109 .108 .109 .113 .122 .128 .129 .128 .125 .126 May .092 .090 .091 .094 .102 .110 .119 .122 .122 .122 .119 .112 .105 .096 .088 .083 .080 .079 .080 .088 .096 .098 .098 .097 .099 June .096 .096 .097 .103 .111 .119 .128 .130 .129 .129 .127 .121 .112 .104 .096 .089 .084 .084 .084 .091 .099 .103 .104 .099 .106 July .128 .127 .128 .133 .139 .147 .155 .160 .160 .160 .158 .152 .144 .135 .126 .117 .111 .110 .111 .118 .126 .129 .131 .131 .135 August .132 .132 .132 .135 .141 .150 .158 .160 .163 .164 .159 .152 .144 .133 .125 .119 .115 .115 .119 .128 .133 .136 .137 .135 .138 September .173 .173 .174 .176 .182 .192 .200 .204 .207 .206 .202 .192 .180 .169 .161 .157 .156 .157 .161 .169 .173 .175 .176 .175 .179 October .174 .174 .173 .177 .181 .187 .196 .201 .202 .201 .196 .182 .168 .159 .156 .155 .158 .162 .165 .169 .172 .173 .171 .174 .176 November 172 .175 .172 .172 .175 .177 .186 .191 .193 .194 .186 .171 .160 .153 .154 .158 .164 .171 .174 .176 .177 .177 .176 .174 .174 December .157 .161 .160 .156 .155 .160 .169 .171 .179 .184 .174 .157 .144 .139 .142 .150 .155 .160 .164 .166 .166 .168 .168 .161 .161 Annual means... .140 .140 .139 .140 .144 .151 .160 ! .163 .166 .167 .163 .152 .141 .132 .128 .127 .127 .130 .133 .139 .143 .145 .144 .142 .144 Table CXLIII contains the values which added to a base of 29.000 inches will give the mean hourly barometric station pressure for each month; these means are taken from records from 1891 to 1904, and are graphically shown in Fig. 71. BAROMETRIC PRESSURE 325326 THE WEATHER AND CLIMATE OF CHICAGO extremes are 30.94 and 28.98 inches, respectively, and the absolute range, therefore, is 1.96 inches. The highest absolute pressure occurred on two occasions, February 4, 1893, during a well-marked cold wave in which the temperature fell to —8°, and on January 28, 1902, in a cold wave with a minimum temperature of —4°. The lowest reading, 28.98 inches, occurred on April 20, 1893, thus bringing the absolute extremes of pressure within three months of each Mdt I 234-5C789/OII NOON I 2 3 A- 5 6 7 8 9 <0 II MdC Fig. 71.—Mean hourly pressure. Fig. 71 shows the mean barometric station pressure for each hour of the day and each month of the year based upon records from 1891 to 1904. The areas of deepest shading indicate the times of greatest pressure and vice versa. The dotted lines, S.R. and S.S., indicate the varying times of sunrise and sunset. These values are taken from Table CXLIII. other. A severe storm prevailed on the last-named date, with rain turning to snow, and a very high wind, which reached a velocity of 72 miles an hour. The storm was followed by a sharp drop in temperature, from 48 on April 20 to 30° on the 21st, a very low reading for that period of the year. Considerable damage was done to shipping in the lakes by the high westerly gale, which continued during the 21st. MEAN HOURLY PRESSURE BY MONTHS Table CXLIII contains the average hourly pressure readings by months, and the values are well illustrated in Fig. 71. In the table are given the decimals only, and in reading the amounts each must be added to 29.000 inches. The pressures in both table and graph are for the station elevation of 823 feet. Primary and secondary maxima and minima are apparent from a study of the data, the primary maximum occurring at from 9 to 10 a.m., and the primaryBAROMETRIC PRESSURE 327 minimum from 2 to 3 p.m. in the colder months, and from 2 to 6 p.m. in the warm season. The secondary maximum follows the primary minimum in the evening, and the secondary minimum occurs in the early morning, before the primary maximum. While the highest average monthly pressure occurs in January, as shown in Table CXLII, the highest average hourly reading occurs at 9 a.m. in the month of September. The explanation of these variations in pressure is unimportant, so far as the main features of weather and climate at Chicago are concerned, but should further information be desired reference may be had to any standard textbook on meteorology. In Fig. 72 are given graphically the curves of pressure change during the course of the day, for the months of January, April, July, and October, and the year. MEAN HOURLY PRESSURE DEPARTURES In the swing of pressure through the diurnal period the pressure is first above the normal pressure for the day and then below, as indicated in Fig. 72, in the previous paragraph. Table CXLIV presents the hourly departures from the mean daily pressure for each month of the year. The period of greatest departure varies, and the changes are somewhat abrupt from the primary maximum in the morning to the primary minimum in the afternoon, especially in winter, when the latter occurs earlier than it does in other seasons. In the hours where the departures are zero (.000) the values just equal the mean daily pressures for the month. Whenever the zero departure does not occur in the table, the mean daily pressure is reached between the hours in which the departure changes from positive (+) to negative (—), or vice versa. PRESSURE PHASES The pressure phases contained in Table CXLV have been prepared along the same lines as the similar tables showing the temperature and sunshine phases (pp. 135, 263), for the purpose of determining more exactly than is possible with hourly values the time of occurrence of the maxima and minima in the different months of the year. RELATION BETWEEN PRESSURE AND OTHER CONDITIONS ILLUSTRATED In Fig. 73 is given a particular instance of the relation between pressure change and those of temperature and wind direction and velocity. Ordinarily, when the barometer falls the temperature rises,CO fco 00 TABLE CXLIV Mean Hourly Departures prom Mean Daily Pressures, by Months Hour Ending Month 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Noon 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 Mdt. Jan -.004 + .002 + .001 -.003 -.005 000 + .010 + .016 + .024 + .026 + .017 -.004 -.019 -.024 -.020 - .014 -.009 -.002 + .002 + .004 + .004 + .004 + .001 -.002 Feb -.002 -.003 -.006 -.007 -.004 000 + .010 + .014 + .017 + .020 + .019 + .007 -.008 -.018 -.018 -.017 -.013 -.006 -.002 000 + .002 + .002 + .001 + .001 Mar.... + .001 -.003 -.008 -.008 - .005 + .002 + .011 + .014 + .018 + .019 + .017 + .011 -.001 -.013 -.017 -.018 -.015 -.012 -.007 000 + .004 + .006 + .006 + .004 April.... -.006 -.008 -.009 -.009 -.003 + .006 + .017 + .017 + .021 + .022 + .018 + .011 + .004 -.007 -.015 -.017 -.018 -.017 -.013 -.004 + .002 + .003 + .002 -.001 May . . . -.007 -.009 -.008 -.005 + .003 + .011 + .020 + .023 + .023 + .023 + .020 + .013 + .006 -.003 -.011 -.016 -.019 -.020 -.019 -.011 -.003 -.001 -.001 -.002 June.. . . -.010 -.010 -.009 -.003 + .005 + .013 + .022 + .024 + .023 + .023 + .021 + .015 + .006 -.002 -.010 - ..017 -.022 -.022 -.022 -.015 -.007 -.003 -.002 -.007 July -.007 -.008 -.007 -.002 + .004 + .012 + .020 + .025 + .025 + .025 + .023 + .017 +.009 000 -.009 -.018 -.024 -.025 -.024 -.017 -.009 -.006 -.004 -.004 Aug -.006 -.006 -.006 -.003 + .003 + .012 + .020 + .022 + .025 + .026 + .021 + .014 + .006 -.005 -.013 - .019 -.023 -.023 -.019 -.010 -.005 -.002 -.001 -.003 Sept -.006 -.006 -.005 -.003 + .003 +:oi3 + .021 + .025 + .028 + .027 + .023 + .013 + .001 -.010 -.018 -.022 -.023 -.022 -.018 -.010 -.006 -.004 -.003 -.004 Oct -.002 -.002 -.003 + .001 + .005 + .011 + .020 + .025 + .026 + .025 + .020 + .006 -.008 -.017 -.020 -.021 -.018 -.014 -.011 -.007 -.004 -.003 -.005 - .002 Nov.... -.002 + .001 -.002 -.002 + .001 + .003 + .012 + .017 + .019 + .020 + .012 -.003 -.014 -.021 -.020 -.016 -.010 -.003 000 + .002 + .003 + .003 + .002 000 Dec -.004 000 -.001 -.005 -.006 -.001 + .008 + .010 + .018 + .023 + .013 -.004 -.017 -.022 -.019 -.011 -.006 -.001 + .003 + .005 + .005 + .007 + .007 000 Table CXLIV shows the hourly departures from the mean of the daily barometric pressure for each month, expressed in thousandths of an inch. THE WEATHER AND CLIMATE OF CHICAGOBAROMETRIC PRESSURE 329 and when the barometer rises the temperature falls. In the case in the graph, however, the fall shown up to 8 a.m. was unaccompanied by any considerable rise in temperature because of the snow and the heavily clouded condition. After 8 a.m. the pressure began to rise, and the wind, -which had been from the south and southwest, changed to west. The temperature fell rapidly, the amount between 8 and 10 a.m. being 20°. In advance of a cold wave, such as the one illustrated here, there is usually a considerable fall in pressure, JANUARY JULY rUt 2 4-6 8 10 Noon 2 4- 6 8 10 Mdt. APRIL. Mdt 2 4 6 8 10 Noon 2 4 6 8/o ndl Dotted Lines- Mean Monthly Pressure. OCTOBER Mdt 2 4 6 8/0 NoonS 460/0 Mdt. YEAR Mean Hourly Pressure record 1891- 1904- inCI- Dotted tine- Mean Yearly Pressure. Fig. 72. Fig. 72 contains the mean hourly pressure curves for the months of January, April, July, October, and for the year, based upon records from 1891 to 1904. followed by a rise accompanied by brisk to high winds shifting to west or northwest, the temperature beginning to fall with the change in direction. ’ The relation indicated above is not so close in the summer season. In the upper portion of Fig. 74 a period of two days is shown, July 20 and 21, 1907. The changes in temperature and pressure on the first day of the period were small, with east to north winds; but on the second day, with falling barometer the temperatureco co o TABLE CXLV Pressure Phases Morning Maximum Afternoon Minimum Night Maximum Night Miminum A.M. Average a.m. P.M. Average p.m. p.m. A.M. Average p.m. p.m. A.M. Average a.m. 7 8 9 10 11 12 Nn 1 2 3 4 5 6 7 7 8 9 10 11 12 1 2 11 12 1 2 3 4 5 6 January 3 11 9-45 1 11 2 2-10 1 3 3 2 2 3 2 900 2 1 fi 1 fi 0 -'AZ February 4 8 4 10:00 6 7 5 3-00 2 3 4 5 4 9*20 4 1 1 4 7 1 Q . J.* March 1 5 8 4 9:45 3 3 9 3 3:50 2 5 5 6 3 3 11-30 4 g g T 9.1* April 1 7 11 2 9-45 3 3 3 5 2 500 3 5 3 5 1 10*50 2 0 q K fi o- in May 2 4 9 1 9-15 2 3 8 4 5-50 2 3 4 6 11-30 3 K 1 9 ' on June 1 8 3 2 2 8-45 4 5 6 6-08 4 9 3 1 H oo 1 7* 4 Q 1 1 . A* July 6 5 8 2 9-15 3 8 5 6*08 2 5 8 2 11 - 35 3 t A g ft 1 • *n August 1 4 11 9:30 1 5 8 1 5:30 5 12 4 1 11-00 5 6 ft 1 September 4 8 5 1 9:10 4 6 8 5* 10 2 6 4 3 1 T 10 50 4 2 q g 4 1 * * * o.ne; October 5 8 5 9:00 6 10 3 3-50 1 4 6 2 4 2 2 10-50 7 q o q fi 1 • *£ November 1 4 3 8 9:10 1 11 5 2:15 2 4 2 5 4 10-20 4 2 3 1 1 5 2 ì 9-10 December 2 13 9:50 2 11 1 2:00 2 1 5 6 3 2 10:40 1 1 5 1 1 8 2 3:30 Sums 6 32 52 99 16 4 42 28 33 30 42 18 5 10 24 50 61 49 11 11 34 11 40 37 49 32 17 4 Annual average 0 3 4 8 1 9:30 0 4 2 3 2 4 2 4:20 0 1 2 4 5 4 1 1 10:50 3 1 3 3 4 3 2 0 2:35 Table CXLV shows the pressure phases, giving the number of times each month during the period from 1891 to 1904 that the morning nunrimimi, afternoon minimum, night maximum, and night minimum pressure readings have occurred at the hours mentioned, together with the average monthly and annual times of occurrence. THE WEATHER AND CLIMATE OF CHICAGOBAROMETRIC PRESSURE 331 rose to 90°, with south to southwest winds. Although the barometer rose slightly thereafter, the fall in temperature following was mainly that of the diurnal change. In the second period, July 1 and 2, 1911, shown at the bottom of the graph, the temperature rose on the first day to a maximum of 96° from a minimum of 75°, and on the second day to a maximum of 98° from a minimum of 79°. The changes December 24,1902. Mdt. 2 4 G 8 10 Noon 2 4 6 8 10 Mdt. Fig. 73.—Example of a cold wave. Fig. 73 shows the various conditions of weather immediately preceding and during the prevalence of a cold wave, midnight to midnight, December 24, 1902. in pressure on both days were slight, but the tendency was downward to the amount of a little more than the diurnal change. Fig. 75 illustrates all the weather conditions as recorded in Chicago on a single day. The figure is a copy of the daily local record for December 27, 1904, on which day a severe storm from the southwest moved directly across the Great Lakes. The map in the lower right-hand corner shows the storm center in the morning332 THE WEATHER AND CLIMATE OF CHICAGO close to Chicago. The heavy lines, or isobars, indicate a considerable pressure gradient, as they are crowded closely together from southwest to northwest. The temperature conditions over the country, shown by the dotted lines, indicate the presence of a cold wave in the northwest, and this followed in after the passage of the storm. The barometer curve shows the changes at Chicago, decreasing until about noon, and then increasing as the storm center passed the city. With Fig. 74.—Examples of warm days. Fig. 74 shows the temperature and pressure curves and direction of wind on July 20-21, 1907, and July 1-2, 1911, two periods of warm weather. the rise in barometer the wind, which had been from the northeast and east, shifted to south between 10 and 11 a.m., and to southwest by noon. The storm moved to the northeastward with its center passing west of Chicago. Had it passed to the south of the city the change in wind direction would have been the opposite of what actually occurred, and it would have veered through north to northwest. As it was, the center moved to the north so far that the wind after the shift held to southwest instead of going around to northwest, as isBAROMETRIC PRESSURE 333 usually the ease, and became high, the gale lasting from 11 a.m. until after midnight, with a velocity of 72 miles an hour occurring several times during the course of the afternoon. With the shift to southwest the temperature fell rapidly, from 43° at 11 a.m. to 11° DAILiY IiOCAIi RECORD. (Station)_C1.LC1.G0.-.U.L December 27th# 1904 (Date) Time (local standard), -9X1— mcr 12-1 1-2 2-3 3-4 4-5 5-6 1-2 2-3 3-4 4-5 5-6 6-7 7-8 Temperature at encj of hour.. Nature of precipitation and timo of beginning and ending .......................... Amount of precipitation. Sunshine (intenthaofhours)«..«. Wind direction............... Wind movement. Maximum velocities (exceeding 37 ¡37 D.flfe 36 ¡56 |S6 ¿t & oejeaeii -—ftuiftiing I I at! 36 36 .onr to ¿1 Ugh si low- 36 36 t r 43 Bin a-pth of srowf Sunrise: Time, Character, 7-: 28- Cloudy- WFl NK NE 14 15 17 ill . 6.2 HE 18 inch C i o 28 Pi a d 19 1:1 15 12 11 11 11 12 13 12 it i gnt show T 10 H. ecibitàtlin. IncJ meltec. ...LL....“........ 26. 59 51 M ÊL 72 ow, .24 1: Sunset: Time, Character, 4l34 ClOBtiy.. SW SW SW iSW 54 59 49 M. 46 52_ 45 52_ J5G 39 A.M. CEHTEA1 TIME WEATHER MAP Fig. 75. Fig. 75 is a copy of the Chicago Daily Local Record for December 27, 1904, showing all the conditions of weather prevailing on that day. A similar record is made out for each day of the year. at 5 p.m. The figure as a whole shows the marked response in the conditions of temperature, wind direction, and velocity to the changes of pressure as the storm passed west and north of the city. The general characteristics of the storm will be given more in detail in a later paragraph (p. 342).PART VII STORM TRACKSSTORM TRACKS As stated in the introduction Chicago is close to the paths of certain types of storms, the average routes of which are indicated in Fig. 76. The southwest storms pass very close to the city, as a rule, and their passage is preceded by northeast winds. Those whose centers cross the Southwest as far north as Oklahoma usually pass directly over Chicago or somewhat to the north of the city, and under such conditions the northeast wind shifts around through southeast Fig. 76.—Mean tracks and average daily movement of storms in the United States. and south to southwest and west (track 4). Occasionally, however, the centers of such disturbances pass up the Ohio Valley, or so far to the southward that Chicago is only on the edge of the storm, or is beyond it altogether, if it is not extensive (track 5). The most frequent storms are those which move from the far Northwest and pass with their centers over the northern Lake region (track 1). Chicago is generally on the southern edge of storms of this kind, or at least well within the storm area, and experiences south to southwest winds, shifting later to northwest as the disturbances move eastward. 337338 THE WEATHER AND CLIMATE OF CHICAGO Another track (track 2), but one less frequently followed, is that from the middle western plateau region northeastward, merging with track 1 over the upper Michigan peninsula, with Chicago on the southern edge. A still less frequented track is track 3, although the storms which follow it are likely to be severe. It extends from the middle Rocky Mountain region eastward across the southern Lake sections, and might appropriately be termed the “overland route.” Such storms are attended by southeast and south winds shifting to west and northwest. Taken as a class, storms usually pursue a path extending somewhat to the north of east, although those approaching from the far northwest, as a rule, turn southeastward over the Lakes. The areas of high pressure usually move in a general south-of-east direction, but both high- and low-pressure areas are carried eastward by the general circulation of the atmosphere, as an eddy is carried along in a stream of water. The exact direction of the individual area is controlled by the configuration of the surface of the ground, and by the distribution of pressure and temperature existing at the time. Areas of high pressure usually bring cool weather, and always accompany the cold waves of the winter season.. In the figure the heavy lines indicate the paths of high-pressure areas, and the light lines, those of the areas of low pressure. These paths are as originally charted by Bigelow, except that there have been added the “overland track,” and that from the south extending northeastward over the Ohio Valley (tracks 3 and 5), which were not given by Bigelow. The average daily movement of the various areas is shown by the broken lines successively spaced for I, 2, 3, and 4 days. The northeast blows of winter, due to the approach of storms from the southwest, are usually accompanied by snow (p. 216), especially after the temperature of the water of the lake has fallen to the freezing point, late in December. Other storms which pass over Chicago in the winter time are accompanied either by rain or snow, depending upon the temperature of the air and the exact position of the path of the storm center. The farther north the center, the greater is the likelihood of rain instead of snow. There are practically no storms moving from the southwest over Chicago in the summer time, the prevailing northeast winds of that season being due to an entirely different cause (p. 145). A track of many summer storms is close to that of the “overland route” of winter storms. The route across the Lake region in summer changesSTORM TRACKS 339 little from that of winter, but in the spring these storms move farther to the south. All the storm tracks over the Middle States, whether they pass across the northern Lake region, directly over Chicago, or up the Ohio Valley, merge into a single track in the upper valley of the St. Lawrence River, together with that which stretches north-north-eastward along the Atlantic coast. TRACKS OF SELECTED STORMS, COLD WAVES, AND HOT WAVES In the succeeding pages further reference will be made to the movement of storms which affect the weather conditions of Chicago, but the accompanying charts can be used to best advantage only by those who understand the weather map. The lines of equal air pressure, or isobars, are drawn for each 0.10 inch of pressure; the temperature lines, or isotherms, are drawn dotted for each 10° difference in temperature. Storm centers are marked LOW, while the centers of cold areas are marked HIGH, and indicate areas of low and high pressure, respectively. Areas of precipitation occurring during the previous twenty-four hours are shaded. The surface wind ordinarily blows from the high to the low pressures. That is, its direction is usually outward from the HIGH and inward toward the LOW. Figs. 77 and 78 show the passage of a storm on November 21 to 23, 1903, eastward and southeastward over the northern Lake region along the most northerly track of storms (No. 1, Fig. 76). As will be seen in the last of the two figures, the rainfall areas barely reached Chicago, and the weather conditions as a consequence of the storm’s movement were not important. The passage of a storm along the “overland route” is illustrated in Figs. 79 and 80, which show the movement of the storm of January 20 and 21,1895. This disturbance moved quite rapidly, passing from Colorado to Lake Michigan in twenty-four hours, deepening as it advanced. The increase in energy is indicated by the low reading of the barometer and the. closely crowded isobars around the storm center. As it approached, Chicago was in its southeast quadrant, and a thunderstorm occurred in the city early in the morning of January 21, although it was the middle of the winter. The center passed slightly to the northward with rain accompanying it, which turned to snow about noon of the 21st. The fall, however, was light, as is usual with disturbances of this character. A southeasterly shifting340 THE WEATHER AND CLIMATE OF CHICAGO Fig. 77. Fig. 77 shows a storm entering the United States over northwest Washington, which followed Track 1 (see Fig. 76). Fig. 78 shows the path of the storm referred to in Fig. 77. The location of this storm at the end of each twelve hours’ progress is shown by the small circles at intervals along the track.STORM TRACKS 341 Fig. 79. Fig. 79 shows a storm central over eastern Colorado which followed Track 3 (see Fig. 76). Fig. 80. Fig. 80 shows the rapid twenty-four-hour movement of the storm referred to in Fig. 79. The storm has increased greatly in energy.342 THE WEATHER AND CLIMATE OF CHICAGO through southwest to northwest gale prevailed, reaching a maximum velocity of 64 miles an hour from the southwest, and a marked cold wave followed. The passenger steamer “Chicora” left Milwaukee early in the morning of January 21, bound for St. Joseph, Michigan, but was never heard .of afterward, the vessel with its crew and few passengers being lost in the lake. Figs. 81 and 82, weather maps of December 26 and 27, 1904, show the movement of the storm referred to in the discussion of Fig. 75 (p. 331). The track as marked out indicates that the storm passed from the North Pacific coast on December 24 southeastward across the middle Rockies, reaching Texas on the 26th. Then recurving and moving northeastward, it developed great energy, and its center passed close to Chicago on the 27th with severe shifting gales and general precipitation, and was followed by a cold wave. . The precipitation was chiefly in the form of rain, as the lake wind blowing toward the disturbance in its approach raised the temperature considerably above the freezing point, where it remained until the wind shifted to southwest. The storm after reaching Texas followed the usual course (track 4, Fig. 76). Another storm which followed nearly the same path as that illustrated in the preceding paragraph was that of February 18 and 19, 1908, shown in Figs. 83 and 84. The point at which this storm entered the country on February 16 was even farther to the north, near Puget Sound, whence it moved southeastward, reaching northwestern Arkansas in 48 hours. It then recurved and moved northeastward, passing with its center southeast of Chicago on the morning of the 19th. This disturbance was accompanied locally by severe, shifting gales with a maximum velocity of 48 miles an hour from the east, and by the heaviest snowfall on record attending any single storm, a fall of 12.7 inches of snow falling within a space of twenty-four hours (p. 222). The winds drifted the snow badly, which resulted in serious interruptions of traffic. The track of the storm, after reaching Arkansas, lay about midway between tracks 4 and 5, shown in Fig. 76. Thé storm which wrecked the city of Galveston, Texas, on September 8, 1900, moved from its entry into the country at that place northeastward across the Lake region, its center passing somewhat north of Chicago, as indicated in Fig. 85. The chart shows the weather conditions prevailing at 8 p.m. on that day, but the track of the storm is marked from its first appearance in the West Indies onSTORM TRACKS 343 September 1 until it passed down the St. Lawrence Valley on the 12th. After its first appearance in the Windward 'Islands it moved slowly in a northeasterly direction. During September 3 and 4 it was south of Cuba, and by the morning of the 5th it had passed over that island. There it began to take a due northerly course, and continued in that direction directly across Key West, and by the evening of September 6 was centered to the south of Tampa, Florida. Instead of recurving to the northeast, as these storms usually do soon after having assumed a northerly direction, its course was changed to slightly north of west into the Gulf of Mexico, and here it was apparently lost for a couple of days, complete observations not being obtainable. The reason for this abnormal movement can be determined from a study of the weather maps of September 8 and the days previous. While its normal course after reaching Florida would have been to the northeast, this way was blocked by an area of high barometric pressure which covered the Atlantic coast, while the pressure was low over the interior of the United States. Hurricanes, as well as other storms, take the path of least resistance, and the disturbance was consequently shunted across the Gulf of Mexico to strike the Texas coast at Galveston. It is probable that the head of the storm, although lifted high enough to come under the sway of the prevailing westerlies above the neutral plane, as shown by its progress northward as far as Tampa, at this stage fell from the control of the upper winds, and drifted westward down the gradient toward the low area on the Great Plains, reaching Galveston before it again came well within the region of the prevailing westerlies in the upper atmosphere. The force of the storm was much intensified in its movement across the Gulf of Mexico, the barometer falling in the center to a minimum of 28.53 inches at Galveston at 8:10 p.m. The highest wind velocity recorded at Galveston was 84 miles an hour at 6:15 p.m., at which time the anemometer blew away, but it was estimated that a velocity of 120 miles an hour occurred later. Following the night of September 8 the storm lost force. It is usual for these hurricanes, after passing from a water surface to the land, to lose energy rapidly and fill up in the center, there being less moisture for development, and the land surface through friction diminishing the air currents in rotation around the center. During the 9th and 10th it moved to Oklahoma, still further diminishing in intensity, and following northward the trough of low barometric pressure. By the morning of the 11th it had joined forces with LOW which had been in the interior for several days, and it appeared again as a well-defined storm over Iowa, the barometer falling rapidly in its front, and the winds increasing within its area.—Cox, Bulletin 3, Geographic Society of Chicago. During the 11th it crossed the Great Lakes with its center directly north of Chicago, and caused winds of almost hurricane force, a344 THE WEATHER AND CLIMATE OF CHICAGO Fig. 81. Fig. 81 shows a storm central over western Texas which after this date followed Track 4 (see Fig. 76). Each twelve hours’ progress of the storm is indicated by the small circles along its path. Fig. 82. Fig. 82 shows the twenty-four-hour movement of the storm referred to in Fig. 81. Each twelve hours’ progress of the storm is indicated by the small circles along its path. This is the storm mentioned in connection with Fig. 75.STORM TRACKS 345 Fig. 83. Fig. 83 shows a storm central over northwestern Arkansas on the morning of February 18, 1908. The path of this storm lay about midway between Tracks 4 and 5 (see Fig. 76). Each twelve hours’ progress of the storm is indicated by the small circles along its path. * Fig. 84. Fig. 84 shows the location of the storm referred to in Fig. 83 on the morning of February 19, 1908. This storm caused the heaviest twenty-four-hour snowfall on record at Chicago (12.7 inches).346 THE WEATHER AND CLIMATE OF CHICAGO maximum velocity of 72 miles an hour from the southwest being recorded. The storm after reaching Oklahoma passed slightly north of track 4 in Fig. 76, and, although it originated in the West Indies, its direction of movement from Oklahoma was not unlike those of the storms of December 26-27, 1904, and February 18-19, 1908, which entered the country on the north Pacific coast. Fig. 85. Fig. 85 shows the general weather conditions over the United States at 8:00 p.m., September 8, 1900, with the track of the “Galveston Hurricane” marked from its beginning in the West Indies on September 1, until it passed down the St. Lawrence, September 12. This storm after reaching Oklahoma passed slightly north of Track 4 (see Fig. 76). Figs. 86, 87, and 88 show the development in spring of a high-pressure area over the Lake region. This development is due to the cold waters of the lakes lowering the temperature of the air over their surfaces, which contracts and permits more air to flow in above, finally causing a HIGH area of great magnitude covering the entire Lake region and the North-Central states. These areas in the spring are accompanied by fair and cool weather over the region of the Great Lakes for comparatively long periods, as they block the approach of storms in their normal movement from the westward. Figs. 89, 90, 91, and 92 represent the movement of a cold wave from the far Northwest, February 11 to 14, 1899. The sea-levelSTORM TRACKS 347 Fig. 86. Fig. 86 shows the beginning of the development of a high-pressure area in spring over the Great Lakes region (see Figs. 87 and 88). Fig. 87. Fig. 87 shows the continued development of a high-pressure area in spring over the Great Lakes region (see Figs. 86 and 88).348 THE WEATHER AND CLIMATE OF CHICAGO Fig. 88 shows the continued development of a high-pressure area in spring over the Great Lakes region (see Figs. 86 and 87). Fig. 89. Fig. 89 shows the beginning of a cold wave in the Canadian Northwest on February 11, 1899, and its steady movement southeastward is shown by Figs. 90, 91, and 92.STORM TRACKS 349 Fig. 90. Fig. 90 shows the twenty-four-hour movement of the cold wave first noted on Fig. 89, and its continued movement southeastward is shown by Figs. 91 and 92. Fig. 91. Fig. 91 shows the twenty-four-hour movement of the cold wave noted on Fig. 90 (see also Fig. 89), and its continued southeastward movement is shown by Fig. 92.350 THE WEATHER AND CLIMATE OF CHICAGO pressure of 31A inches, shown in the center of the HIGH in the first map of the series, is extraordinarily high, even for severe cold waves, and this area occasioned one of the coldest periods ever recorded at Chicago. The temperature on February 11 was already low over the Middle States, as well as in the Northwest. The high-pressure area in the middle Rockies shown in the third and fourth charts was rather a redevelopment, and it remained in that region for several days. By referring to the list of cold days (p. 96) it will be seen Fig. 92 shows the twenty-four-hour movement of the cold wave first noted on Fig. 89, arid its position over the southeastern states on February 14, 1899 (see Figs. 89, 90 and 91). that the temperature during this cold spell at Chicago fell below zero on 7 consecutive days, the lowest reading of the period being —21° on February 9. Figs. 93 and 94 illustrate the movement of another cold wave from the Northwest, as shown on the weather maps of February 7 and 8, 1895. On the first day the high-pressure area was centered over North Dakota with a reading of 31.1 inches, and it moved southeastward during the following twenty-four hours. The intensity of the cold, moreover, was heightened by the development of a severeSTORM TRACKS 351 Fig. 93 shows a well-defined cold wave which subsequently moved over Chicago, giving one of the coldest periods on record. Fig. 94. Fig. 94 shows the movement east and south of the cold wave referred to in Fig. 93. Note the development of the severe storm central off the New England coast.352 THE WEATHER AND CLIMATE OF CHICAGO Fig. 95. Fig. 95 shows the weather conditions prevailing over the United States at 8:00 p.m., July 21, 1901, during an intense hot wave. Note the 100° isotherms. Fig. 96. Fig. 96 shows the weather conditions prevailing over the United States at 8:00 p.m., July 22, 1901. Note the development over the Great Lakes region of an area of relatively high pressure, and the consequent lowering of the temperature in that section caused by lake winds. The hot wave still prevails in the interior.STORM TRACKS 353 storm which moved northeastward along the Atlantic coast, causing a well-marked gradient from the HIGH to the LOW, and consequent strong winds over the eastern half of the country. During this cold wave the temperature at Chicago reached —15°, and the thermometer fell to below zero on 3 consecutive days. In the two paragraphs, immediately preceding, the conditions usually attending well-marked cold waves which affect Chicago have been illustrated, and some reference to the conditions which bring about hot waves will here be appropriate. That of July, 1901, the temperature features of which have already been instanced in this volume in connection with other subjects, is an excellent example, and the general conditions are given in Figs. 95 and 96, weather maps of July 21 and 22. During the prevalence of a hot wave « the barometric pressure is usually high over the South Atlantic states and relatively low over the northern tier of states and in the middle sections of the country. This distribution of pressure gives southerly winds over that portion of the United States lying east of the Rocky Mountains, as is shown on the chart of the 21st. By the evening of the 22d the intensity of the heat was broken in the Lake region by the development there of a high-pressure area, as shown on the map. The period marked one of the severest hot waves ever experienced in this country, record-breaking high temperatures occurring at many stations, the maximum at Chicago on the 21st being 103°. Further details regarding the conditions of this hot wave will be found on pp. 61, 72, and 142.PART VIII CONCLUSIONCONCLUSION In the foregoing pages the weather and climate of Chicago have been discussed at length, and it is hoped that the work will prove of value for reference and to students of meteorology. Special emphasis has been laid upon extreme and unusual conditions and the changeableness of weather, and but little has been said regarding periods of settled conditions. This is because it is in exceptional incidents that people are most interested and they soon forget those of ordinary character. The conclusion should not' be drawn that the climate of Chicago is not a healthful one, for it is, on the contrary, as stated previously, one well adapted for a person in health. Its snowstorms and cold waves stir the blood to renewed activities, but the latter as well as its hot waves are seldom of long duration, tempered as they usually are by the modifying influence of the waters of Lake Michigan. Its rainfall is ample and well distributed. Its humidity is moderate and in the spring and early summer, when the lake winds are most prevalent, the temperature of the air is rising, thus preventing the humidity from reaching a high point. These lake winds bring to the city the purest air, and whether the winds blow from the lake or the land, they usually have a certain briskness tending to carry away impurities and to prevent the smoke from settling over the city. The very changeableness of the weather is one of Chicago’s greatest assets, as it is a potent factor in the activities of the people and in the upbuilding of the city. Many of its days in June are so perfect that one is reminded of Lowell’s words, “What is so rare as a day in June?” and there are frequent periods in July equally charming, often when the country in the interior away from the lakes is sweltering in intense heat. Moreover, the longest periods of sunshiny weather usually occur in the autumn, Indian Summer appearing at its best in Chicago and vicinity. Some cities boast of an equable temperature, others, of continuous sunshine, but where there is no changeableness the weather must become most monotonous. Because of marked variations in weather, fine, pleasant days are all the more appreciated, and the statistics show that Chicago has its full share of these. 357APPENDIXESAPPENDIX I WEATHER OF HOLIDAYS WEATHER OF NEW YEAR’S, FOURTH OF JULY, AND CHRISTMAS Unusual interest attaches to the more important of our holidays, and in order to furnish information as to the local character of these days in past years, the conditions have been sketched out in Figs. 97, 98, and 99 from the data given in Table GXLYI, covering the period from 1872-1913, inclusive. Thanksgiving Day, being a movable holiday, has not been included in the table or the graphs. It should be understood, of course, that the weather of these respective days in former years can in no wise be taken as a certain indication of their character in the future. New Year’s Day has been clear 11 times during the period shown, while Christmas Day has been clear but 6 times. The occurrence of cloudy weather on these days was 20 and 26 times, respectively, and the remainder were partially overcast. The Fourth of July, occurring in the height of the summer season, has experienced but few cloudy days, only 6 in all, while on 18 occasions the weather has been clear, and the remainder partly cloudy. Minimum temperatures have been below zero but twice on both New Year’s Day and Christmas Day, and no very low temperatures have occurred on either day. The warmest weather for these days was 65° for New Year’s Day, 1876, and 56° for Christmas Day, 1895. The greatest depth of snow on the ground at Christmas occurred in 1909, 8.0 inches, records extending back to the Christmas of 1893 only. Out of the 21 years shown in this record, there has been no snow on the ground on this date in 6 years, and in 4 of the 20 years only an unmeasurable amount, represented by “T” in the table, has been recorded. On the Fourth of July, the highest temperature was 102° in 1911; 95° occurred in 1901, and from 90° to 94° has been observed in 5 other years. On New Year’s Day there have been 12 days with 361TABLE CXLVI Weather op New Year’s Day, Fourth op July, and Christmas, 1872-1913 Year New Year’s Day (25°) July 4 (71°) Christmas (27°) Temperature a> -g a) oi s-s ,a> o3 .2 *-3 cd *o 2 Ph Wind Temperature U cd pPäs Ö .2 *3 cd 15 Igi o t-4 PL, Wind Temperature