CLOUDS AND CLOUD FORMS 



77 



far upward toward the blue in fantastic turrets, 

 domes, and peaks. The bases of these clouds 

 are usually dark in shadow, flat, and cut sharp ; 

 while their tops are cast in wreaths and billows 

 of vapor. They appear at times to be of great 

 height, for though their bases are usually not 

 more than five thousand feet up, their tops 

 sometimes reach forty thousand feet from the 

 ground. At such an altitude the crests look 

 woolly, which probably indicates that the cloud 

 has reached a cold region and has changed to 

 frost-dust on its top. Usually these clouds 

 appear to stand firmly and to be motionless, 

 though they are always changing, their bottoms 

 sinking away and their tops being continually 

 renewed. Moreover, they are drifted by winds 

 at the rate of about twenty-five miles an hour, 

 though at other times they may scarcely move 

 at all. After sunset they usually sink and dis- 

 appear entirely. 



The heavy cumuli are summer clouds, and 

 are not seen in cold climates nor upon cold 

 days. The tropical region is their home, 

 though they are native to the temperate zones 

 in midsummer, and are often seen rising along 

 the horizon like a range of snow-clad moun- 

 tains, with hills and valleys running up or down 



