144 



ELEMENTS OF GENERAL SCIENCE 



continue until about four o'clock in the afternoon or later. 

 Its effect is most pronounced near the water, and it does 

 not extend many miles inland. The same phenomena are 

 observed on ocean shores. 



160. Cause of the lake and sea breezes. To account for 

 these breezes the causes for the movement of air need to 

 be recalled (Chapter I). One cause is the difference in 

 temperature. 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 



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v^b. 



J L 



FIG. 73. Daily variations in temperature 



Diagram showing the daily variations in two cities from July 13 to July 19, 1913. 



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



the effect of Lake Michigan 



it was heated has expanded. 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 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 the other cool air, thus 

 making a continuous current. 



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

 is much hotter than the water, 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 above 

 the water begins to flow out over the land, crowding the 

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



