MOISTURE IN THE AIR 45 



Relative humidity 



At a definite temperature, the atmosphere can hold 

 only a certain amount of water vapor. When the 

 air can hold no more, it is said to be saturated. 

 When it is not saturated, the amount of water vapor 

 actually held by the air is expressed in percentages of 

 the cjuantity required for saturation. X relative hu- 

 midity of 100 per cent means that the air is saturated ; 

 of 50 per cent, that it is only one half saturated. The 

 drier the air is, the more rapidly does the water evapo- 

 rate into it. To the dry-farmer, therefoi'e, the relative 

 humidity or degree of dryness of the air is of very 

 great importance. According to Professor Henry, 

 the chief characteristics of the geographic distribu- 

 tion of relative humidity in the United States are 

 as follows : — 



(1) Along the coasts there is a l)elt of high humid- 

 ity at all seasons, the percentage of saturation 

 ranging from 75 to 80 per cent. 



(2) Inland, from about the 70th meridian east- 

 ward to the Atlantic coast, the amount varies be- 

 tween 70 and 75 per cent. 



(3) The dry region is in the Southwest, where the 

 average annual value is not over 50 per cent. In this 

 region are included Arizona, New Mexico, western 

 Colorado, and the greater jxjrtion of both Utah and 

 Nevada. The amount of annual relative humidity 

 in the remaining portion of the elevated district. 



