GENERAL CIRCULATION OF THE ATMOSPHERE 279 



If, however, the air of the trades is forced up over mountains, 

 it is cooled, and some of its moisture may be condensed and fall as 

 rain or snow. The windward sides of high mountains in the trade^ 

 wind zone have heavy rainfall. An illustration is afforded by the 

 east side of the Andes Mountains in the trade-wind zone. The 

 rainfall is there heavy (Fig. 238) . Another illustration is afforded 

 by the volcanic cones of the Hawaiian Islands. The trade-winds 

 yield little rain to their lower slopes, but forced up over the 

 mountains they yield abundant moisture at higher levels. These 

 levels are readily seen by the change in vegetation at the level 

 where the rainfall becomes abundant. 



After the air of the trades passes over a mountain range, it 

 descends, and is warmed both by contact with the warm surface 

 and by compression. It therefore takes up moisture. The lee- 

 ward sides of mountains in the trade-wind zones are therefore 

 regions of little precipitation. The west slope of the Andes Moun- 

 tains in the zone of the trades is an example (Fig. 238). A high 

 mountain range on the east side of a continent in the zone of the 

 trades would tend to make all the low land to the west of it dry. 



Rainfall in the zones of the prevailing westerlies. The prin- 

 ciples which apply to the trade-wind zones apply also in the zones 

 of the westerly winds. These winds are, on the whole, blowing 

 from lower to higher latitudes, and so are being gradually cooled. 

 They might therefore yield some moisture, even at sea-level or on 

 low land, and especially on land in the winter season. The heat 

 of the land in summer often prevents condensation and precipita- 

 tion of the moisture in the westerly winds until the air has moved 

 far to poleward. But when such winds cross mountains they 

 yield moisture to their windward slopes and summits, and become 

 dry on the leeward slopes. A high mountain range on the west 

 side of a continent in the zones of westerly winds would tend to 

 make all the low land to the east of it dry, unless the moisture came 

 from some source other than the westerly winds. 



From these principles we may understand the rainfall of the 

 United States, so far as it depends on planetary winds. The pre- 

 vailing winds, for most of the country, are from the southwest. 

 Coming to the land from the Pacific Ocean, these winds find the 



