WEATHER AXD CLIMATE 151 



cloudy and a sunny day. The sun's rays falling all 

 day on the land and water warm them. This heat 

 then radiates outward and heats the air, which receives 

 very little heat directly. 



The surface of the earth receives most heat where 

 the sun's rays fall most nearly vertically. The rays 

 are then most concentrated and pass through less air. 

 We are all familiar with the fact that the air is 



Noon 



FIG. 56. Diagrani showing the difference in the angles at which 

 the sun's rays strike the earth at sunrise, noon, and sunset. 



warmer at noon when the sun's rays are nearly verti- 

 cal than it is in the evening or in the morning when 

 they are oblique. 



Day and night. In your study of geography you 

 learned that the earth rotates on its own axis once in 

 twenty-four hours, and also that it revolves about the 

 sun in a little more than three hundred and sixty- 

 five days. As a result of the rotation. of the earth on 

 its own axis, any given point on the earth is turned 

 during part of each twenty-four hours toward the 

 sun, producing day, and during the remainder of the 

 time away from the sun, producing night. The uni- 

 form succession of days and nights is due to the regu- 

 laritv of the rotation of the earth. 



