THE WATER VAPOR OF THE AIR 



227 



FIG. 156. Cirrus clouds. Light feathery clouds that float at an elevation 

 of 4 or 5 miles above the earth's surface. When in the form of plumes with 

 frayed and torn edges increasing cloudiness and rain or snow are usually 

 indicated, especially if the clouds are moving rapidly. Cirrus moving very 

 slowly seldom indicate an approaching storm. In temperate latitudes 

 cirrus nearly always move from a westerly direction. 



FIG. 157. Cirrus, merging into cirro-stratus clouds. A form often seen 

 when rain or snow is approaching. The cloud layer seems to thicken 

 gradually until the sky is obscured. This thickening is sometimes partly 

 due to the growth of the clouds themselves; usually it is caused mainly by 

 the coming of denser masses as the earlier clouds pass on. 



