CLOUDS AND CLOUD FORMS 
V7 
far upward toward the blue in fantastic turrets, 
domes, and peaks. The bases of these clouds 
are usually dark in shadow, flat, and cut sharp ; 
while their tops are cast in wreaths and billows 
of vapor. They appear at times to be of great 
height, for though their bases are usually not 
more than five thousand feet up, their tops 
sometimes reach forty thousand feet from the 
ground. At such an altitude the crests look 
woolly, which probably indicates that the cloud 
has reached a cold region and has changed to 
frost-dust on its top. Usually these clouds 
appear to stand firmly and to be motionless, 
though they are always changing, their bottoms 
sinking away and their tops being continually 
renewed. Moreover, they are drifted by winds 
at the rate of about twenty-five miles an hour, 
though at other times they may scarcely move 
at all. After sunset they usually sink and dis- 
appear entirely. 
The heavy cumuli are summer clouds, and 
are not seen in cold climates nor upon cold 
days. The tropical region is their home, 
though they are native to the temperate zones 
in midsummer, and are often seen rising along 
the horizon like a range of snow-clad moun- 
tains, with hills and valleys running up or down 
Cumulus 
changes. 
Summer 
clouds. 
