40 SEVENTH REPORT. 



THE VARIATION OF LAND AND WATER TEMrERATURES. 



BY W. P. COOPER. 



An examination of the yearly mean isothermal lines for lower Michigan as given in the 

 Michigan Section of the Climate and Crop Service of the U. S. Weather Bureau in co-opera- 

 tion with the Michigan State Weather Service, affords some suggestive comparisons. 

 We have for instance the yearly mean isotherm of 46°, during the year 1903, 108 miles 

 farther north on the east side of Lake Michigan than where the same line passes through 

 the central part of St. Clair county, north of Detroit. The isotherm of 44° for the same 

 year passes just north of the line between Arenac and Iosco counties near the entrance 

 of Saginaw Bay, while on the other side of the lower peninsula the equivalent temperature 

 is 66 miles farther north in the region of Grand Traverse Bay. An examination of the 

 yearly mean isothermal chart for the ensuing year, 1904, shows that the isotherm for 44° 

 is 54 miles more northward on the western side of the lower peninsula than in Macomb 

 county adjacent to Lake St. Clair. It will be observed that this annual mean for 1904 

 is 12 miles less in latitude than for the preceding year. Also the isotherm for 43° is 30 

 miles higher in latitude on the east side of the Lake Michigan shore. Similar comparisons 

 of the isotherms for 42° and 41° show a difference of 42 and 78 miles respectively farther 

 north on the east shore of Lake Michigan as compared with the west shore of Lake Huron. 



As an exception to this general rule of higher isothermal lines on the western shore of 

 lower Michigan, the isotherm of 48° for 1903 is 8 miles farther north in Macomb county 

 adjacent to Lake St. Clair than where Van Buren county is washed by the waves of Lake 

 Michigan. Likewise during 1904 the line for 45° is 16 miles farther south in Berrien 

 county than in Macomb. Both these isothermals, however, show some deflection to the 

 northward on approaching the Lake Michigan shore. 



Comparing the isothermals on the east and west sides of Lake Michigan as given by 

 Alexander Winchell in Wallings atlas, we find that the mean isotherm for 47° swings, 

 from Grand Haven southwestward to Chicago, a distance of 78 miles of latitude. Similarly 

 the isotherm for 45° trends from about 20 miles south of the mouth of the Manistee to 

 Milwaukee or 60 miles to the southward. The isotherm for 44° runs from near Manitowoc 

 to Northport in Leelanau county, 72 miles farther north. Generally speaking isotherms 

 have about the same latitude on the west side of Lakes Michigan and Huron, rising to 

 the northward in crossing those bodies of water. The object of this paper is to present 

 some data showing the cause of this atmospheric variation. In obtaining this information 

 I used an H. J. Green thermometer number 7529. The readings were taken on the west 

 side of Saginaw Bay and east of Toliico Bay, northwest of Bay City. Less than five minutes 

 elapsed between the readings on the land and in the water. During the davtime the ther- 

 mometer was read during August 12, 13, 15, 16, 17, and 18, 1904, at 5:30, 7:30, 8:30, 9:30, 

 10:30, and 11:15 a. m. and hourly from 1:00 to 7:00 p. m. August 22 and 23 readings 

 were taken consectutively from 1:00 p. m. to 11:15 a. m. August 25 and 26 the ther- 

 mometer was read from 5:30 a. m. to 5:30 a. m. the following morning being consecutive 

 for 24 hours, ending on the morning of August 26. Judging from these last two series 

 of observations the maximum and minimum temperatures were very nearly obtained 

 by the readings taken from 5:30 a. m. to 7:00 p. m. We will now briefly summarize the 

 results of these observations. 



On August 12 the variation of land temperature amoimted to 28°; that of the water 

 very nearly 5.5° or 19 per cent of that of the land. From observations sent me by Mr. 

 C. F. Schneider, Director of the Weather Bureau, the amount of variation at West Saginaw 

 and Bay City was 32° and at Midlan^l 22° or 6° less than our land temperatures show. 

 The average of our land temperatures on the day in question was 71°, that of the water 

 72°. The maximum readings on the land near the bay was 1 ° less than for West Saginaw, 

 Bav Citv and Midland, that of the water 6.5° less. Minima readings on the land are. 3° 

 higher than at Bav Citv and Saginaw and 7° lower than for Midland. SimiLirly the 

 water observation is 21° higher than maximum observations at Saginaw and Bay City 

 and 11° higher than for Midland. 



August 13, the dav was cloudy until 9:30 a. m. From 5:30 to 11:15 a. m. the land 

 temperature increased 14.5°; that of the water 3° or 20.7 per cent. The average of 6 read- 

 ings taken during the forenoon was 71.6° for the land and 68.9° for the water. 



August 15, the day was clear. From 5:30 a. m. to 2:00 p. m. our land temperature 

 increased 22°, water temperature 7° or 31.8 per cent. ITnfortunately I have no data 



