CLOUDS AND CLOUD FORMS 
7 
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withdrawn at night by the decreased radiation 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 Crervs (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 
Day 
and night 
clouds. 
Classifica- 
tion of 
clouds. 
