430 



Eighth Annual Report of the 



Thermal Influence of the Great Lakes. 



Referring again to the charts of sea-level temperature, it will 

 be observed that the Great Lakes cause a deflection of the 

 isotherms similar to that due to the ocean. The temperature of 

 the lake waters, and consequently of the air over them, follows 

 the seasonal changes more rapidly than does the ocean, owing to 

 the smaller mass of the former; but the situation of the lakes 

 upon the western side of 'the State gives them a greater effect 

 upon the prevailing winds, at least during the autumn and win- 

 ter, than is derived from the Atlantic. The absolute effects of 

 these inland seas upon the air temperature may be better studied 

 in the interior of the continent than in New York, where a va- 

 riety of other influences are also at work. For comparison, Moor- 

 head and Duluth, Minnesota and Marquette, Michigan, are 

 chosen; the three stationis having approximately the same lati- 

 tude and elevations. Moorhead represents a nea.Tly pure con- 

 tinental climate; Duluth, 220 miles to the east, is upon the west- 

 ern border of Lake iSuperior; and Marquette, still 230 miles east 

 of Duluth, is upon the southern shore of Lake Superior. The fol- 

 lowing table shows a modification of temperature which may be 

 attributed to the influence of the lake. 



The winds of summer are land winds for DulutTi and Marquette 

 and hence modify the temperature less than the lake winds of 

 "winter. ' 



The extremes and annual averages of monthly mean tempera- 

 ture for cities of both the upper and lower Lake Regions are given 

 in the following table; and those for eastern New York are also 

 added for comparison. 



