Clouds
At any one time, about half the planet is shrouded by clouds. In the estimation of
many, they are beautiful and, at times, awesome. Everyone appreciates the impor-
tance of clouds as bringers of precipitation and providing shade from the sun. Yet,
clouds also play vital roles in the transport of energy in latent form and in mainte-
nance of the planet’s greenhouse effect. Clouds are visible indicators of ongoing
physical processes in the atmosphere and harbingers of weather to come. Through
observation of clouds, it is possible to predict weather a day or more in advance at
an individual location.
Clouds are composed of suspended water droplets and ice crystals that are indi-
vidually almost too small to see. Although each piece literally floats on air, collec-
tively they can represent considerable mass. A cumulus cloud on a summer day
might hold as much as 50 tons of water. As entire clouds, these liquid and crystal
pieces are quite efficient in reflection and refraction ofsolar energy. Moreover, clouds
represent phase changes in water with accompanying gains and losses of energy. Cru-
cially, clouds absorb longwave energy fromEarth’ssurface and then reradiate it—the64 Clouds
Clouds are visible evidence of processes in the atmosphere. Here, stratus and cumulus clouds hug
the mountains along College Fjord, Alaska. (Photo courtesy of Steve Stadler)