238 THE WEATHER 



densed in tiny droplets, and we have a fog. If a fog is 

 formed some distance above the earth, we call it a cloud. 

 Some clouds consist of ice particles. 



Clouds and fogs are not permanent, but may be ab- 

 sorbed again if the temperature of the air is raised above 

 the dew point, as it often is by the rising sun. But even 

 while a cloud lasts it is both losing water particles and 

 gaining them. Many persons think that clouds are lighter 

 than air, and float in the air. This is not so; cloud par- 

 ticles are heavier than air, and settle slowly toward the 

 earth. But as they reach a layer of unsaturated air, 

 they evaporate and disappear. This level, at which com- 

 plete evaporation takes place, is the bottom of the cloud. 

 Clouds also extend outward until their water particles 

 evaporate as rapidly as they are supplied. It is because 

 cloud material is constantly being formed at one place, 

 and is disappearing at another, that clouds change their 

 forms so readily. No one who has watched carefully the 

 forming of thunder clouds on a sultry afternoon can doubt 

 that they are really "springs" of water vapor, that rise 

 into the upper air, and then fall again in great, billowy 

 masses. 



269. Forms of Clouds. The four principal forms taken 

 by clouds are the following: 



(1) Cumulus clouds (Fig. 218); these are so called from the word 

 meaning "a heap," because they are rounded like heaps of wool. We 

 have the same meaning in the word " accumulate." Cumulus clouds 

 are formed by ascending air currents laden with moisture. They are 

 commonly seen on warm summer afternoons, at a height of one to five 

 miles, and often bring on thunderstorms and showers. 



(2) Cirrus clouds (Fig. 219). Cirrus is from a word meaning 



