Clouds are 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
freezing, 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 from 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 more 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.
(Adapted from Encarta Encyclopedia)
