THE WEATHER 45 



westerly winds have much heavier rainfall on the western than 

 on the eastern side, because the winds, as they ascend the west- 

 ern side, are cooled and some of the moisture condenses into 

 clouds and rain, while on the eastern side the air is warming 

 as it descends and becomes relatively dry. A good example of 

 this will be found by comparing the rainfall on the west slope 

 of the Cascade Mountains in Washington and Oregon with the 

 rainfall on the east side of the same mountains (fig. 32). 



In the tropical regions we have the belt of trade winds on 

 each side of the equator. The great heat of the equatorial re- 

 gions causes the air to expand and thus become less dense, and 

 it is pushed upward in the region of greatest heat, the heat 

 equator, which is near the true equator. In this region of 

 rising air there is little wind at the surface of the earth, and 

 it is known as the belt of calms. To the north and south of 

 the belt of calms the air is rushing southward (or northward) 

 toward the heat equator, but by the rotation of the earth these 

 winds are turned from the direct course they would otherwise 

 pursue, so that in the northern hemisphere they become north- 

 east winds and in the southern hemisphere they blow from the 

 southeast. These winds blow very steadily and are known as 

 trade winds. 



We have, then, three important wind regions on the earth's 

 surface besides the winds of the polar regions, which are not 

 well known and will not be discussed here. The three belts of 

 which we have spoken are the belt of calms, the trade winds 

 (northern and southern hemispheres), and the westerlies (in 

 both the northern and the southern hemisphere). 



42. Rainfall in the United States. Rainfall is very unequally 

 distributed in the United States (fig. 32). It ranges in amount 

 from 100 inches per year in northwestern Washington to 

 2 inches per year in southeastern California. While there are 

 no very large regions in the United States which deserve to 

 be called deserts, yet much more than a third of the country 

 is arid and cannot be farmed successfully without irrigation 



