CLOUDS AND CLOUD FORMS 



71 



ing air-currents feed it, and when these are 

 withdrawn at night by the decreased radiation 

 from the earth, the cloud sinks and disappears. 

 Hence it is that when radiation begins in the 

 morning with the warming rays of the sun, 

 clouds are formed, and when it ceases at even- 

 ing the lower clouds disappear and only the high 

 and comparatively dry ones remain. 



The meteorologists have established four 

 broad classes of clouds according to their differ- 

 ent forms, and the different heights at which 

 they are usually seen. The classification is 

 largely for the sake of convenience because, as 

 has been already intimated, clouds are substan- 

 tially the same thing whether high or low in the 

 air ; and the different forms run into each 

 other so closely that it is often difficult to tell 

 one from another. The four classes, beginning 

 with the highest and ending with the lowest, are 

 the cirrus, the stratus, the cumulus, and the 

 nimbus. There are some subdivisions which 

 may be recited in order, but the broad divisions 

 are given at first to avoid confusion. 



THE CIRRUS (1) is the frailest and the lightest 

 of all the cloud forms, and drifts at the great- 

 est altitude. It is sometimes seen fifty thou- 

 sand feet or more above the earth, though its 



