THE MOISTURE OF THE AIR 255 



Amount of water vapor in the air. The amount of water vapor 

 in the air varies greatly from place to place, and from time to 

 time at the same place. Attempts have been made to estimate 

 the amount in the air at one time, but the results are far apart. 

 It is probably enough so that if it all fell as rain at once it would 

 make a layer of water at least an inch thick. Some idea of the 

 amount of water vapor in the air is gained in another way. A 

 cubic foot of space at F. is capable of containing J/2 grain of water 

 vapor; at 60 F., 5 grains; and at 80 F., 11 grains. The weight 

 of air in a room 40 x 40 x 15 feet, at a temperature of 60 F., and 

 under ordinary pressure, is about 1,800 pounds. The weight of 

 water vapor which this space is capable of containing is nearly 20 

 pounds. 



Moisture and movements. Since water vapor makes the air 

 lighter, and since movements result when the air of one place is 

 lighter tnan that of another, it follows that differences in the amount 

 of moisture in the air in different places are a cause of atmospheric 

 movements. 



Saturation. When there is as much water vapor in the air as 

 there can be, it is said to be saturated. Though we say the air is 

 saturated, yet it is, in reality, not the air, but the space which the 

 air occupies which is saturated. The amount of water vapor neces- 

 sary to saturate a given space depends on the temperature, and is 

 nearly the same whether air is present or not. 



Humidity. The amount of moisture which the air contains is 

 its absolute humidity. The percentage of moisture which air con- 

 tains at any temperature, in comparison with what it might con- 

 tain at that temperature, is known as its relative humidity. When 

 it contains all the water vapor it can hold, its relative humidity 

 is 100; when it contains half as much as it might, its relative 

 humidity is 50. Air is commonly said to be "dry" when its rela- 

 tive humidity is low, and " moist" when its relative humidity is 

 high. Fig. 220 shows the average relative humidity for the United 

 States, the range being from 80 along the coasts to less than 40 

 in some parts of the southwest. The area where the relative hu- 

 midity is 35 or less is essentially desert, and the area where it is 

 less than 50 is distinctly dry. In Death Valley, California, the 



