102 OUTLINES OF FORESTRY. 



cause a heavier rainfall than a cold, moist air, be- 

 cause the warm air has a greater capacity for hold- 

 ing vapor. 



In general, the air of the equatorial zones of the 

 earth is both warmer and moister than that of the 

 temperate zones, and the air of the temperate zones 

 is both warmer and moister than that of the polar 

 zones. 



Consequently the rainfall is heaviest in the equa- 

 torial zones, and is greater in the temperate zones 

 than in the polar zones. 



The air near the coast of a continent or island is 

 moister than that over the interior. Consequently 

 the rainfall is heavier on the coasts than in the 

 interior. 



When the earth's surface is intensely heated, the 

 air over it becomes so hot that it rises far above the 

 surface. If sufficiently moist, the chilling so caused 

 produces a heavy rainfall. Much of the rain in 

 the tropical regions is caused in this manner. 



Mountains form excellent means for cooling the 

 air and causing its invisible water or vapor to fall 

 as rain. They act no matter from what direction 

 the wind may be blowing. 



"When the wind blows against the sides or slopes 

 of a mountain, it is forced by the pressure of the 



