THE WATER VAPOR OF THE AIR 233 



the lower air becomes considerably warmer, and, therefore, 

 lighter, than the air above. The heavier air, then, is above, 

 and rests upon a mass of warmer and lighter air next to the 

 earth. Soon the warm air breaks through and ascends in 

 broad streams here and there, while the cooler, heavier air 

 settles downward between the streams of warm air (Fig. 165). 

 The ascending streams continue to rise as long as their tem- 

 perature is higher than the air through which they are ascend- 

 ing. They cool as they rise, and when they become as cool 

 as the surrounding air their upward movement stops. 1 Some- 

 times the current stops before it has risen high enough to form 

 clouds (A, Fig. 165), then nothing is seen of it. But when 



FIG. 165. Rising air currents, due to warming of earth's surface. 

 At "A" no cloud is formed. 

 At "B" a small cloud results. 

 At "C"' a stronger current builds a much higher cloud. 



any current ascends high enough to cool below the dew-point 

 of its rising air, cloud immediately begins to form. 2 This 

 point marks the base of the cumulus, and is at the same level 

 for all cumuli (plural of cumulus) forming at that time and in 

 that neighborhood. 



258. The Size of a Cumulus Cloud. (1) If the rising air 

 at the cloud base is only a little warmer than the surrounding 

 air, it will rise only a little higher and will form a low-topped 

 cloud (Fig. 158). (2) If the rising air at the cloud base is con- 



1 Effect of momentum is disregarded here. 



2 Special conditions such as supersaturation are not entered into in this 

 discussion. 



