Understanding the Weather [ 437 



When cumulus clouds are glistening white they are an indica- 

 tion of good weather; but on a summer afternoon they may grad- 

 ually darken and become an unmistakable threat of a storm- 

 often accompanied by thunder and lightning. 



Artists are fond of ornamenting their landscapes with cumulus 

 clouds, but the cloud "portraits" they produce are often decidedly 

 incorrect. They show the clouds as rounded masses at both top and 

 bottom whereas the base of a cumulus cloud is always flattened. 

 The base forms at the level where rising warm air cools enough 

 to cause its water vapor to condense. Then, if the current of rising 

 air is strong, the cloud grows upward with its rounded head mark- 

 ing the top of the rising air column. 



CIRRUS CLOUDS "MARES' TAILS" 



The white feathery wisps that you are likely to see on a fine 

 summer day belong to a second cloud group. These are cirrus 

 clouds (from the Latin word meaning "curl") . "Cirrus" sounds a 

 little like "icy," and this helps us remember that cirrus clouds are 

 made up of tiny particles of ice not merely moisture. They are 

 the highest of all clouds, and may range from two to seven miles 

 aloft. As cirrus clouds suggest long wisps of hair, they are often 

 called "mares' tails." 



If cirrus clouds are moving from the southwest, the temperature 

 is apt to fall. If they are coming from the north, it is probably 

 going to be fair and warm. 



STRATUS CLOUDS 



Thin flat clouds make up the third group, well named 

 "stratus," for this is the Latin word for "spreading out." Stratus 

 clouds do spread out across the sky, sometimes as far as we can see. 

 To remember this name, think of the similarity of "stratus" and 

 "straight." Most often the stratus clouds appear as low, gray sheets. 

 They may merge with rain clouds and precede a storm, or they 

 may clear away like lifting fog. 



