CLOUDS AND CLOUD FORMS 
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 timesit 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 
The stra- 
tus. 
