CLIMATIC INFLUENCES OF BODIES OF WATER 139 



156. Cause of the lake breeze. To account for the lake 

 breeze, the causes for the movement of air need to be recalled 

 (sect. 11). One cause of the movement is difference in tem- 

 perature. When air is expanded by heating, it becomes lighter ; 

 that is, a cubic foot of it weighs less, because the amount of 

 air that occupied a cubic foot of space before it was heated 

 has expanded and occupies more than a cubic foot of space, 

 and since only a part of the original amount will be contained 

 within a cubic foot, this part cannot weigh as much as the 

 whole. If, next to the warm air, there is a quantity of colder 

 and heavier air, this colder air will tend to flow in under the 



u 



FIG. 73. Daily variations in temperature 



Diagrams showing the daily variations in two cities on the thirteenth, fourteenth, 



fifteenth, sixteenth, seventeenth, eighteenth, and nineteenth of July, 1913. The 



solid line represents Peoria, and the broken line represents Chicago. Note the 



effect of Lake Michigan 



warm air and lift it up. This is what happens in the vicinity 

 of a stove or radiator where the cool air is continually flowing 

 toward the stove, where it is warmed and in turn displaced 

 by other cool air, thus making a continuous current. 



The same thing occurs along a lake shore. In summer the 

 land is much hotter than the waters of the lake, at least while 

 the sun is shining. Like the stove, the land heats the air 

 which is over it until this air becomes so light that the heavier 

 air of the lake begins to flow out over the land, crowding the 

 warm air upward (fig. 74). The air from the lake becomes 

 heated as it flows across the land, and rises as it is displaced 

 by more air from the lake. Thus there is a continuous flow of 



