TEMPERATURE CHANGES AND THE SEASONS 19 



12 

 M 



8 10 



8 10 



of each month and to make a chart which shows the change 

 in length of day during the year. 



19. How the air is warmed. The air does not get much 

 warmth directly from the sun's rays, even though the rays 

 pass through it. The air is relatively transparent, and both 

 heat and light pass through it with very little hindrance. 

 This may be illustrated 

 better by the more fa- 

 miliar case of the sun's 

 rays passing through 

 glass. If the glass of 

 a window is perfectly 

 clean and transparent, 

 the sunshine will pass 

 through it and warm the 

 room within, but the 

 glass is not much 

 warmed. If it is covered 

 with dust and other sub- 

 stances which hinder the 

 passage of the sunshine, 

 the window is somewhat 

 warmed and the room is 

 heated and lighted less. 

 In the same way the 

 transparent air gets little 

 warmth from the sun's 

 rays which are passing through it, excepting as the heat is 

 absorbed by dust, clouds, and water vapor. When the earth 

 has been warmed, the air in contact with the earth is warmed 

 by the earth. Being lighter, it is replaced by cold air, the whole 

 body of air being warmed more or less in this way. The tem- 

 perature of the air, therefore, depends principally upon the 

 temperature of the area over which it is moving, whether this 

 area is land or water. 



FIG. 16. Yearly distribution of hours of 

 light and darkness 



Diagram showing the length of time the sun 



is ahove the horizon on the first and fifteenth 



of each month at latitude 40 N 



