THE MOISTURE OF THE AIR 259 



They are formed by ascending convection currents, and their level 

 bottoms seem to mark the level at which condensation takes place 

 as the air rises. They appear, especially in clear, hot weather, in 

 mid or late forenoon, after insolation has established convection 

 currents. They attain their greatest size at about the hour of 

 maximum heat. As evening approaches they commonly grow 

 smaller. They sometimes pass into other forms of cloud. 



Stratus clouds are horizontal sheets of lifted fog. 



Nimbus or rain clouds (Fig. 3, PI. XLV, p. 257) consist of thick 

 layers of dark clouds without definite shape and with ragged edges, 

 from which continued rain or snow generally falls. 



Cirrus clouds are delicate, fibrous, or feathery (Figs. 4 and 5, 

 PL XLV). They are generally white, and sometimes arranged in 

 belts. They are usually high and thin, and often of particles of 

 snow or ice. 



Between these types there are all sorts of gradations. 



Precipitation. The condensation of the water vapor of the air 

 leads to rain, snow, or hail, if the products of condensation fall. 

 Whether precipitation really takes place after the formation of 

 clouds depends on many conditions. To give rain or snow, the 

 particles of water or snow in the cloud must be heavy enough to 

 fall; and if they are to reach the bottom of the atmosphere, they 

 must not pass through air which is dry enough and warm enough 

 to evaporate them before they reach the bottom of the atmosphere. 

 In desert regions, water may sometimes be seen to be falling from 

 a high cloud, when not a drop reaches the ground. The falling 

 drops evaporate before they reach the land. 



Whether precipitation falls as rain or snow depends on the 

 temperature of condensation, and on the temperature of the air 

 where the precipitation takes place. Snow falling from a cloud 

 may become water before it reaches the bottom of the air. It often 

 snows on a mountain while it rains in the valley below. 



Since condensation follows cooling, and since precipitation often 

 follows condensation, sufficient cooling (below dew-point) of the 

 air may cause precipitation. It follows that there may be rain 

 (or snow) (l)when air is blown up a cold mountain-side; (2) when 

 it is blown poleward (or, in general, from a warmer to a cooler place) 



