CHAPTER III 

 THE WATER OF THE AIR 



20. Evaporation. It is a matter of common observation that 

 if water is left exposed to the air, it soon disappears. We say 

 that it has evaporated. That does not mean that it has gone 

 out of existence. It has only changed into the form in which 

 it is invisible and can pass off into the air. It has become a 

 gas. The same thing is true of the surface of every pond, lake, 

 river, sea, and ocean indeed, of any water that is exposed 

 to the air. There is, therefore, always considerable water in 

 the atmosphere, and so long as it remains in the form of a gas 

 (water vapor) it is invisible, just as is true of the remainder of 

 the atmosphere. 



21. Amount of water in the atmosphere. The amount of 

 water in the atmosphere in the form of vapor, or gas, varies 

 greatly at different times and places. These differences are 

 significant in many ways, for the amount of water in the air 

 is an important factor in such things as cloudy and clear days, 

 rain, dew, fog, snow, and the discomfort that we have upon 

 warm, muggy days in summer. 



In order to learn what controls the amount of water vapor 

 present in our atmosphere, let us use the following illustra- 

 tion : Suppose that a small quantity of water is placed in a 

 dish within an air-tight chamber. We know that the water 

 will immediately begin to evaporate, but it will not all evapo- 

 rate. The space in the chamber will soon contain all the 

 water vapor that it is able to contain under existing condi- 

 tions, and evaporation will therefore cease. The space within 

 the chamber is said to be saturated with water vapor when 

 it contains all that it is able to contain. If the chamber is 



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