WATER, AND ITS USES 93 



solid that has thus passed through both changes is a subli- 

 mate. Distillation is the vaporization of a liquid and the 

 condensation of this vapor back to a liquid state again. 



Air containing much moisture does not allow heat to pass 

 through it readily. When a cloud passes between an ob- 

 server and the sun on a hot day in summer the relief due to 

 absorption of the sun's heat within the cloud mass is very 

 noticeable. 



Not only does the total amount of water per cubic 

 foot in the air vary at different times, but the capacity of the 

 air for holding water vapor varies widely with changes in its 

 temperature. The ratio of the amount of water vapor pres- 

 ent in the air at any time to the amount that the air might 

 then hold is known as relative humidity. It tells what per 

 cent saturated the air is at that time. Air is saturated when 

 the humidity is 100 per cent. 



Grams of water vapor per 

 saturated cub' c meter. 



-J 5' 10 15 



Tempera ture Centigrade 



FIG. 33. Capacity of the air for moisture at different temperatures, j 



SUMMARY 



The invisible water vapor in the atmosphere when cooled sufficiently 

 is changed back into the liquid state. This cooling may result from 

 incoming cold air as a cold wind, or it may occur when the moisture- 

 laden air rises and mixes with air in the upper colder levels of the 

 atmosphere. Then, too, cooling may be the direct result of expansion 

 of air containing water vapor as it rises where the pressure upon it 

 becomes less and less. 



