CLOUDS AND CLOUD FOEMS 



75 



harbinger of fair weather. Both forms of this 

 cirro-cumulus are frost clouds. They drift at 

 an altitude of about twenty-two thousand feet, 

 and have a maximum velocity of about eighty 

 miles an hour. Their movements across the 

 sky seem to be systematic and orderly, though 

 of course the regularity of their driftings is 

 dependent entirely upon the steadiness of the 

 upper wind-currents. 



THE STRATUS (2) is a flat sheet cloud extend- 

 ing in long lines across the sky, at times bridg- 

 ing it, covering it from horizon to horizon. It 

 is the cloud, let us say, of the middle-air re- 

 gion, though every cloud that has a sheet-like 

 form or looks stratified is some kind of stratus. 

 It is usually formed when there is little wind 

 and only a mild radiation is going on. The air 

 as it rises gets gradually cooler until the dew- 

 point is reached, when this cloud forms and 

 extends itself across the sky in long, thin drifts 

 like the smoke from factory chimneys in calm 

 weather. In color it is a gray cloud, though 

 occasionally, when very thin and the sun or 

 moon is shining through it, it looks bluish in 

 tint. At times it has a concave or a convex ap- 

 pearance, and at other times it is wavy or un- 

 dulating. It is from ten to twenty thousand 



