The Weather 



45 



temperature, or the water vapor exerts enough pressure upon 

 the water to stop evaporation. 



Absolute humidity. When the total amount of water vapor 

 in the atmosphere is considered with no reference to the tempera- 

 ture and the amount the air might hold, it is described as the 

 absolute humidity. The absolute humidity of the atmosphere 

 is measured by the hygrometer or the barometer, and is ex- 

 pressed in inches or fractions 'of an inch of mercury ; or it may 

 be expressed in ounces per cubic yard of air, as in the following 

 table. Only temperatures of common significance are given. 



FIG. 13. Capacity of air for water vapor at different temperatures. 



Exercise. Let us assume that a cubic yard of air is saturated at 

 50 F. How much water vapor or gas does it contain? If saturated 

 at 86 F., how much vapor does it contain? About how many times 

 as much vapor is required for saturation of air at 77 F. as at 50 F. ? 

 As at 68 F.? 



Consider the influence of changes of temperature upon satura- 

 tion of the atmosphere. Assume that air at 90 F. contains 

 .47 of an ounce of water vapor per cubic yard. To what 

 temperature must the air be cooled to bring about saturation? 

 Does evaporation occur when the air is saturated? If now 

 the temperature of the air be warmed to 90 F. and maintained 

 there with continued evaporation until the air is saturated, 



