Cloud, condensed form
of atmospheric moisture consisting of small water droplets or tiny
ice crystals. Clouds are the principal visible phenomena of the
atmosphere. They represent a transitory but vital step in the water
cycle, which includes evaporation of moisture from the surface of
the earth, carrying of this moisture into higher levels of the atmosphere,
condensation of water vapor into cloud masses, and final return
of water to the surface as precipitation.
The formation of clouds caused by cooling of the air results
in the condensation of invisible water vapor that produces
visible cloud droplets or ice particles.
Cloud particles range in size from about 5 to 75 micrometers (0.0005
to 0.008 cm/0.0002 to 0.003 in). The particles are so small that
light, vertical currents easily sustain them in the air. The different
cloud formations result partly from the temperature at which condensation
takes place. When condensation occurs at temperatures below freezin,
clouds are usually composed of ice crystals; those that form in
warmer air usually consist of water droplets. Occasionally, however,
supercooled clouds contain water droplets at subfreezing temperatures.
The air motion associated with cloud development also affects formation.
Clouds that develop in calm air tend to appear as sheets or stratified
formations; those that form under windy conditions or in air with
strong vertical currents have a towering appearance.
Clouds perform a very important function in modifying the distribution
of solar heat over the earth's surface and within the atmosphere.
In general, because reflection fro the tops of clouds is greater
than reflection from the surface of the earth, the amount of solar
energy reflected back to space is greater on cloudy days. Although
most solar radiation is reflected back by the upper layers of the
clouds, some radiation penetrates to the surface of the earth, which
absorbs this energy and reradiates it. The lower parts of clouds
are opaque to this long-wave earth radiation and reflect it back
toward earth. The result is that the lower atmosphere generally
absorbs mor radiative heat energy on a cloudy day because of the
presence of this trapped radiation. By contrast, on a clear day
more solar radiation is initially absorbed by the surface of the
earth, but when reradiated this energy is quickly dissipated because
of the absence of clouds. Disregarding related meteorological elements,
the atmosphere actually absorbs less radiation on clear days than
on cloudy days.
Cloudiness has considerable influence on human activities. Rainfall,
which is very important for agricultural activities, has its genesis
in the formation of clouds. The marked effect of clouds on visibility
at flight levels proved to be a major difficulty during the early
days of the airplane, a hazard that was alleviated with the development
of instrument flying, which permits the pilot to navigate even in
the midst of a thick cloud. The sharp increase in consumption of
electricity for lighting during cloudy days represents one of the
major scheduling problems faced by the electric-power industry.
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