At one time, meteorologists believed that clouds
literally "floated" in the sky, and that their water was
___1___ the form of
bubbles rather than droplets. Actually clouds do not
float in the ordinary sense. The droplets of water or
crystals of ice ___2___
the cloud are generally falling or evaporating. The
overall shape of the cloud is preserved for a time
because other droplets or crystals keep replacing those
that have been lost. Upward currents of air also help
___3___ clouds by
keeping their water drops and ice crystal from falling
toward the surface. The clear spaces between the clouds
are generally formed by dry, descending currents of air.
Clouds disappear, or dissipate, when they lose their
water to evaporation, precipitation ( rain or snow ), or
dispersion in the wind. Clouds ___4___
their load of moisture as rain or snow when the droplets
form drops that are heavy enough to drop to the ground.
In addition to producing rain and snow, clouds exert an
effect on climate in several other ways. Very heavy
clouds can block off a good deal of light and warmth
from the Sun. However, clouds generally conserve more
heat than they block. In effect, the clouds act like a
warming blanket ___5___
the surface of Earth. |