EXPERIMENT STATION BULLETINS. 



493 



usual conditions, and maximum temperatures above 90 degrees are rare 

 along the lake shore. It requires a combination of conditions, such as 

 southwestern breezes of just the proper force, a clear atmosphere con- 

 taining but little moisture and a distribution of atmosphere pressure 

 tending to prevent the shift of wind to "on shore" rather than "off 

 shore" breezes. Usually when the land areas adjoining large bodies of 

 water becomes excessively hot, convectional currents are established 

 with strong breezes from the lake during the afternoon, causing very 

 decided falls in temperature. These are called "land and sea breezes" 

 and are common throughout the Great Lakes region. 



It will be noted that temperatures have reached 100 degrees or 

 higher at a majority of places in the Upper Peninsula. This can be ac- 

 counted for from the fact that southwesterly currents which are usually 

 present during hot waves, have a clear sweep over much of the region, 

 without having crossed large bodies of water. When these currents do 

 encounter large bodies of water they are reduced in temperature as 

 illustrated at Manistique, on the north shore of Lake Michigan where the 

 extreme maximum is but 89 degrees. Another reason for the extremes 

 in temperature in the Upper Peninsula is the clearness of the air in that 

 region, and the fact that the daylight period is longer. The time be- 

 tween sunrise and sunset is 30 to 40 minutes longer in the Upper Penin- 

 sula of Michigan than in southern Michigan, during the summer season. 

 (See Table 4.) 



TABLE 4 — A TABLE SHOWING THE POSSIBLE NUMBER OF HOURS OF SUNSHINE 

 DURING THE GROWING SEASON AT SAULT STK, MARIE AND LANSING^, MICHIGAN 



Table 5 shows the lowest temperature ever recorded at various places 

 in the Upper Peninsula. The extreme lowest, — 49 degrees, was registered 

 in the elevated regions of Marquette county, at Humboldt, about 35 miles 

 from Lake Superior. At the city of Marquette, on the lake shore, the 

 lowest ever recorded was —27 degrees. In the Keweenaw Peninsula, al- 

 though much further north, —24 degrees is the lowest ever reached either 

 at Eagle Harbor or Calumet, clearly showing the temperating effect of 

 the lake. Cold waves come from the Canadian Northwest and are 

 ushered in by strong northwest winds. These must traverse the rela- 

 tively warm waters of the Great Lakes before striking the land, and their 

 frigidity is markedly reduced. Likewise in the fall season, when great 

 masses of cold air, originating in Northwestern Canada, tend to swoop 

 down over Michigan, the lake water, warmed through the previous sum- 

 mer, gives up its heat to the oncoming cold breezes and frosts are pre- 

 vented along the immediate lake shore. Further inland these breezes 

 loose their heat again and frosts occur very early in the season. This 

 is strikingly shown in Chart VI. 



