AIR TEMPERATURES AND THE SEASONS 33 



pass through it with very little hindrance. This may be 

 illustrated better by the more familiar 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 substances 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 ab- 

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

 has been warmed, the lower layer of air 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 

 temperature of the air therefore depends principally upon the 

 temperature of the area over which it is moving, whether 

 this area is land or water. 



This explains why it is that in summer a wind which is 

 coming across a large body of water is cooler than one which 

 comes across the land, for in summer the water is cooler than 

 the land. In the winter the reverse is true. In the same way 

 a wind from the north is likely to be colder than one from 

 the south. 



When the sun is not shining directly upon a part of the 

 earth, as at night or on a cloudy day, the earth's surface is 

 continually losing heat to the surrounding space and there- 

 fore becoming cooler. That the heat which passes off from 

 objects upon the earth warms any transparent medium through 

 which it passes much more than it does when the sun's rays 

 pass through such a medium is well illustrated by a green- 

 house. Here the rays of the sun pass through the glass with 

 little loss, but the heat which is given off by the objects in 

 the greenhouse, after they have been warmed by the sun, 

 does not easily pass through the glass. The heat is therefore 

 retained in the greenhouse. 



