Clouds. 193 



With the advancement of meteorological knowledge 

 much has been done, however, to bring about a de- 

 sirable change. Convenient names which ha\c a definite 

 application, more commonl}^ used to designate the different 

 kinds of clouds and it thus becomes possible to describe an 

 appearance of the sky in such a manner as to correctly represent 

 it to a person who has not seen it. In order to distinguish the 

 different classes of clouds it is necessary to consider how they 

 are formed. 



The atmosphere, like a sponge, can absorb a certain amount 

 of water. Thus the water evaporated from the streams and 

 pools, which dry up in summer, passes into the air, generally in 

 the form of invisible vapour. 



The atmosphere also can hold more water when warm than 

 at a colder temperature, and it becomes colder the farther it is 

 removed from the earth. 



Accordingly as the warm air rises from the earth, it becomes 

 cooler and the moisture that was before invisible is .seen in the 

 form of minute floating droplets, and a fog or cloud appears 

 according to the height at which it is developed. The degree of 

 temperature at which those appear is called the dew-point, and 

 the height at which this is reached is marked by the lower margin 

 of those clouds which have even base lines. 



If the clouds rise so high that the temperature falls below 

 the freezing point, the vapour is changed to snow or icy particles 

 which probably constitute the majority of clouds. 



The upward movement of the air, which it is necessary to 

 consider here, is, like the winds, caused primarily by differences 

 in the temperature of the air. This ascent of the warmer air, 

 which is known as convection, assumes a yorticulai or whirling 

 motion and is often very rapid. It may be observed on the eve 

 of a thunder storm when the cloud known as cumuless can be 

 seen rolling upwards with astonishing celerity. 



Having thus briefly treated of the causes of clouds, their 

 different classes may be taken up. These are distinguished 

 chiefly by their form but the altitude is also considered. 



