Kids Research Express

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. Topics: Formation and Effects Classification

These are clouds composed of ice particles, found at average levels of 8 km (5 mi) or more above the earth. The family contains three principal genera. Cirrus clouds are isolated, feathery, and threadlike, often with hooks or tufts, and are arranged in bands. Cirrostratus clouds appear as a fine, whitish veil; they occasionally exhibit a fibrous structure and, when situated between the observer and the moon, produce halo phenomena. Cirrocumulus clouds form small, white, fleecy balls and wisps, arranged in groups or rows. Cirrocumulus and cirrus clouds are popularly described by the phrase “mackerel scales and mares' tails.”

These are clouds composed of water droplets and ranging in altitude from about 3 to 6 km (about 2 to 4 mi) above the earth. Two principal genera are included in the family. Altostratus clouds appear as a thick, gray or bluish veil, through which the sun or moon may be seen only diffusely, as through a frosted glass. Altocumulus clouds have the appearance of dense, fleecy balls or puffs somewhat larger than cirrocumulus. The sun or moon shining through altocumulus clouds may produce a corona, or colored ring, markedly smaller in diameter than a halo.

These clouds, also composed of water droplets, are generally less than 1.6 km (1 mi) high. Three principal forms comprise this group. Stratocumulus clouds consist of large rolls of clouds, soft and gray looking, which frequently cover the entire sky. Because the cloud mass is usually not very thick, blue sky often appears between breaks in the cloud deck. Nimbostratus clouds are thick, dark, and shapeless. They are precipitation clouds from which, as a rule, rain or snow falls. Stratus clouds are sheets of high fog. They appear as flat, white blankets, usually less than 610 m (2000 ft) above the ground. When they are broken up by warm, rising air, the sky beyond usually appears clear and blue.

Clouds of this type range in height from less than 1.6 km (1 mi) to more than 13 km (8 mi) above the earth. Two main forms are included in this group. Cumulus clouds are dome-shaped, woolpack clouds most often seen during the middle and latter part of the day, when solar heating produces the vertical air currents necessary for their formation. These clouds usually have flat bases and rounded, cauliflowerlike tops. Cumulonimbus clouds are dark, heavy-looking clouds rising like mountains high into the atmosphere, often showing an anvil-shaped veil of ice clouds, false cirrus, at the top. Popularly known as thunderheads, cumulonimbus clouds are usually accompanied by heavy, abrupt showers. An anomalous, but exceptionally beautiful, group of clouds contains the nacreous, or mother-of-pearl, clouds, which are 19 to 29 km (12 to 18 mi) high, and the noctilucent clouds, 51 to 56 km (32 to 35 mi) high. These very thin clouds may be seen only between sunset and sunrise and are visible only in h...

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 i...