AIK AND WATEE 23 



23. Amount of water in the atmosphere. It has been esti- 

 mated that the water in the atmosphere of the earth would, 

 if condensed into liquid water, make a layer at least an inch 

 deep over the whole surface of the earth. 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 producing such things as cloudy 

 and clear days, rain, dew, fog, snow, and the discomfort that 

 we have upon warm, muggy days in summer. 



An illustration will help to show what controls the amount 

 of water vapor present in the atmosphere. 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 evaporate. The space 

 in the chamber will soon contain all the water vapor that it 

 is able to contain under existing conditions, and evaporation 

 will then 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 warmed, however, an additional 

 amount of water will evaporate, while if it is cooled, some of 

 the water vapor will return to the liquid form and may be 

 seen upon the walls in the form of drops. 



It thus appears that the water evaporates into the adjacent 

 space until the space is saturated and that the amount of 

 water which is needed to saturate a given amount of space 

 depends upon the temperature. The exact amounts of water 

 in grams per cubic meter of saturated space are given for 

 reference in the table in section 25. The amount of air in 

 the space has no influence on the amount of water vapor 

 which the space is able to contain. 



24. Moist and dry air. The space about us is not often 

 saturated with water vapor. Among the reasons for this 

 may be mentioned the fact that in many parts of the world 

 there is not much water exposed and therefore not much 



