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 quantity required for saturation. A 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, therefore, 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 belt 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 portion of both Utah and 
Nevada. The amount of annual relative humidity 
in the remaining portion of the elevated district, 
