Movements of the Atmosphere. 565 



air risirg or moving toward the poles is passing toward a 

 colder region. Lowering the temperature of the air, with- 

 out changing the amount of moisture in it makes it more 

 nearly saturated, while raising the temperature without 

 changing the amount of moisture makes the air dryer. 



Besides this, air is cooled by expansion and warmed hy 

 compression, and on these accounts ascending currents tend 

 to become damp and descending air more dry. 



719. Weather of the Wind Zones. It will be evident 

 from 718 that, so far as the world system of winds are not 

 interfered with by local conditions, they must give to the 

 countries over which they blow characteristic types of 

 weather. Under the tropical high pressure calm belts, 

 where the air is descending, and for a long distance to the 

 south and a shorter one to the north, there must be a region 

 of clear skies and dry weather, and it is under these two 

 zones that the deserts of the world are found. 



In the polar regions also the cloudiness and precipita- 

 tion are relatively small for the same reason. 



But at the equator, where large volumes of air are ris- 

 ing into the upper regions and after doing so pass toward 

 the poles, the air having become very moist before rising, 

 quickly becomes saturated and throws back to the earth 

 large amounts of rain. The heaviest rainfalls of the 

 world are under the equatorial calm belt of ascending cur- 

 rents. 



In the two temperate zones also, where the winds cool 

 as they move northward, frequent rains and showers and 

 much cloudy weather are the rule. 



There is thus a tendency for the systems of world winds 

 to develop three rainy or cloudy zones and four clear 

 weather or dry zones. The dry zones are under the tropics 

 and about tho poles ; the wet and cloudy zones are under the 

 equator and between the tropical and polar circles of both 

 hemispheres. 



720. Shifting of the Zones. Because the vertical rays 

 36 



