140 



ELEMENTS OF GENERAL SCIENCE 



the air across the shore line from lake to land as long as the 

 land is hotter than the water, excepting when local breezes 

 are counteracted by stronger and more general movements of 

 the air. The breeze gets well started about the middle of the 

 forenoon, because it takes until that time to get the land suffi- 

 ciently heated. During the night the land may be cooler than 

 in the daytime. Sometimes the land is so much cooler than 

 the water that the air flows from the land to the lake, but 

 this land breeze is much less noticeable than the lake breeze. 



\ 



FIG. 74. Air movements from water to land 



When the land is warmer than the water, cool and heavy air from the water flows 



over the land, pushing the lighter, warm air upward. Such movement of the air 



is known as a lake breeze or sea hreeze 



The question why land and water equally exposed to the 

 rays of the sun should differ so much in temperature needs 

 further explanation. To understand this matter we shall study 

 some of the effects of heat on water and land. This study 

 should also give information that will enable us to see whether 

 the lake has influences on temperature other than those rep- 

 resented by the lake breeze. 



157. Causes of unequal heating of land and water. The more 

 rapid heating of the land is due, first, to the fact that it is 

 not transparent. All the heat of the sun is received on the 

 surface of the land, and the upper few inches are very rapidly 



