194 OUTLINES OF FORESTRY. 



2. By acting as a screen which both prevents the 

 earth's surface from being too rapidly heated on 

 exposure to the sun's rays, or too rapidly cooled 

 when deprived of such rays. 



Since air can hold more vapor when hot than 

 when cold, if the temperature of a mass of warm 

 moist air is sufficiently cooled, the moisture it can 

 no longer hold as vapor appears as rain or as some 

 other form of precipitation. 



The lowering of temperature required to pro- 

 duce rain is obtained : 



1. By warm moist air blowing along the earth's 

 surface towards colder regions. 



2. By warm moist air rising directly from the 

 earth's surface into the higher and colder regions 

 of the atmosphere. 



Rain is generally caused by warm moist air 

 blowing towards the polar regions of the earth. 

 Cold dry air blowing towards the equatorial re- 

 gions has its capacity for moisture increased, and 

 tends rather to cause droughts than rain. 



In tropical regions, a wind that has crossed an 

 ocean, and has thereby become saturated with 

 moisture, may bring rain on reaching the coast of 

 a continent or island, in no matter from what 

 direction it comes. 



