MOISTURE OF THE AIR 235 



reach levels beyond which they cannot go; they must spread out as 

 horizontal winds (cf. 263). When the cold, upper air falls to levels 

 having a greater density, the opposite effect is observed: the air is 

 compressed, and becomes warmer. Thus cold air that comes tumbling 

 down the slopes of the Alps into the Swiss valleys is often changed into 

 a hot wind (the/oe/m), because of its own compression. 



265. Moisture of the Air. As we have already learned 

 (cf. 89), water changes into steam (evaporates) at all 

 ordinary temperatures. In the open air the amount of 

 evaporation usually seems to have no limit; this is true 

 because drafts and winds constantly carry new portions 

 of air over the water, and remove the 

 air that has already taken up water. 

 But when we have an enclosed volume 

 of air over water, only a limited 

 evaporation of the water can take 

 place; this is the same as saying that, 

 at a given temperature, only a limited Fig. 216. 



- i . . Bell Jar of Air over Water. 



amount of steam can expand into a 

 given space or can mix itself with (diffuse into) a given 

 volume of air. Figure 216 shows such an enclosed 

 volume of air. At first water evaporates rapidly into the 

 air of the jar; but after a time the amount of moisture 

 (steam) becomes no greater, no matter how long the 

 apparatus is allowed to stand. The water does not cease 

 evaporating, but the steam is condensed to water as rapid- 

 ly as the water is changed to steam. The condition of the 

 air, or the space, in the jar is then like that of a sponge 

 filled with water; we say that the air is saturated with 

 water vapor. If we raise the temperature, the air be- 

 comes unsaturated, although it was saturated at the lower 



