100 OUTLINES OF FORESTRY. 



X. RAIN. 



THE vapor which rises from the surface of the 

 ocean, and, indeed, from all water surfaces, mixes 

 or diffuses through the air, and is carried by the 

 winds to different parts of the earth. The air over 

 parts of the earth at considerable distances from 

 any large body of water may therefore contain 

 much vapor. 



The quantity of water the air can hold, in an 

 invisible state as vapor, increases rapidly with an 

 increase in the temperature. Consequently, when 

 air containing vapor is considerably chilled, it can 

 no longer hold as much as it formerly did, and a 

 part appears as rain, or as some other form of pre- 

 cipitation, such as dew, snow, hail, fog, cloud, etc. 



The amount of water that falls, or is precipitated 

 from the air, depends not only on the quantity of 

 air that is chilled, and on the extent of this chill- 

 ing, but also on the quantity of moisture the air 

 contained before it was chilled. 



The lowering of temperature necessary to pro- 

 duce rain may be caused in the following ways : 



