CHAPTER XVI 

 MOISTURE IN THE AIR 



93. Variation in Amount. The air always contains some 

 moisture but the amount depends upon a variety of conditions. 

 It is greatest, of course, in the vicinity of large bodies of water 

 and least in deserts, but in any locality it may vary from day 

 to day, being affected by the prevailing winds, the amount of 

 sunshine, the elevation above sea level, and the temperature. 

 If we place a quantity of water in a bottle, and leave it un- 

 corked, it will soon escape into the air, but if the bottle be 

 corked, the limit to the evaporation is soon reached and the 

 rest of the water remains in the bottom of the bottle. From 

 this we discover that water cannot continue to evaporate into 

 a given space for an indefinite period. As soon as a space has 

 a certain amount of water vapor in it, no more can be taken 

 up and we say that it is saturated. If the temperature of the 

 space is then raised, its capacity for moisture is increased and 

 more will evaporate into it, but if the temperature be lowered, 

 its capacity is diminished and some of the moisture must be 

 dropped. The temperature point at which the water 

 vapor in a given space begins to turn back to liquid water 

 when the temperature is lowered is called the dew-point. The 

 dew-point is not a fixed point like the freezing and boiling 

 points, but varies from day to day or from hour to hour, ac- 

 cording to the amount of moisture in the space and its tem- 

 perature. When the temperature is high and the space nearly 

 saturated, a very slight drop in the temperature will cause some 

 of the moisture to condense and return to the liquid state. If 

 there is very little moisture in the given space, however, the 



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