altocumulus


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Related to altocumulus: cirrocumulus, cirrostratus

cloud

cloud, aggregation of minute particles of water or ice suspended in the air.

Formation of Clouds

Clouds are formed when air containing water vapor is cooled below a critical temperature called the dew point and the resulting moisture condenses into droplets on microscopic dust particles (condensation nuclei) in the atmosphere. The air is normally cooled by expansion during its upward movement. Upward flow of air in the atmosphere may be caused by convection resulting from intense solar heating of the ground; by a cold wedge of air (cold front) near the ground causing a mass of warm air to be forced aloft; or by a mountain range at an angle to the wind. Clouds are occasionally produced by a reduction of pressure aloft or by the mixing of warmer and cooler air currents.

Classification of Clouds

A classification of cloud forms was first made (1801) by French naturalist Jean Lamarck. In 1803, Luke Howard, an English scientist, devised a classification that was adopted by the International Meteorological Commission (1929), designating three primary cloud types, cirrus, cumulus, and stratus, and their compound forms, which are still used today in modified form. Today's classification has four main divisions: high clouds, 20,000 to 40,000 ft (6,100–12,200 m); intermediate clouds, 6,500 to 20,000 ft (1,980–6,100 m); low clouds, near ground level to 6,500 ft (1,980 m); and clouds with vertical development, 1,600 ft to over 20,000 ft (490–6,100 m).

High cloud forms include cirrus, detached clouds of delicate and fibrous appearance, generally white in color, often resembling tufts or featherlike plumes, and composed entirely of ice crystals; cirrocumulus (mackerel sky), composed of small white flakes or very small globular masses, arranged in groups, lines, or ripples; and cirrostratus, a thin whitish veil, sometimes giving the entire sky a milky appearance, which does not blur the outline of the sun or moon but frequently produces a halo.

Intermediate clouds include altocumulus, patchy layer of flattened globular masses arranged in groups, lines, or waves, with individual clouds sometimes so close together that their edges join; and altostratus, resembling thick cirrostratus without halo phenomena, like a gray veil, through which the sun or the moon shows vaguely or is sometimes completely hidden.

Low clouds include stratocumulus, a cloud layer or patches composed of fairly large globular masses or flakes, soft and gray with darker parts, arranged in groups, lines, or rolls, often with the rolls so close together that their edges join; stratus, a uniform layer resembling fog but not resting on the ground; and nimbostratus, a nearly uniform, dark grey layer, amorphous in character and usually producing continuous rain or snow.

Clouds having vertical development include cumulus, a thick, detached cloud, generally associated with fair weather, usually with a horizontal base and a dome-shaped upper surface that frequently resembles a head of cauliflower and shows strong contrasts of light and shadow when the sun illuminates it from the side, and cumulonimbus, the thunderstorm cloud, heavy masses of great vertical development whose summits rise in the form of mountains or towers, the upper parts having a fibrous texture, often spreading out in the shape of an anvil, and sometimes reaching the stratosphere. Cumulonimbus generally produces showers of rain, snow, hailstorms, or thunderstorms.

Climatic Influence of Clouds

Cloudiness (or proportion of the sky covered by any form of cloud), measured in tenths, is one of the elements of climate. The cloudiness of the United States averages somewhat less than 50% (i.e., the country receives somewhat more than 50% of the possible sunshine); the Great Lakes region and the coast of Washington and Oregon have the greatest cloudiness (60%–70%), and the SW United States—Arizona and adjacent areas—are the least cloudy (10%–30%). Clouds have become an important focus in the study of global warming or cooling, including how the increase or decrease in cloud cover can effect the amount of radiation reflected from the earth back into space.

Bibliography

See R. S. Scorer, Clouds of the World (1972); R. Houze, Cloud Dynamics (1991).

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altocumulus (Ac)

altocumulus (Ac)
Elliptical globular cloud units occurring individually or in groups. They may have gray shading on their undersurface. Most individual clouds are frequently elongated elliptical or lenticular units and are distinguished from cumulus by their height and lack of vertical doming. Thin altocumulus clouds are represented on weather charts as image.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
References in periodicals archive ?
Thus, altocumulus (al-to-cu-mu-lus) is a puffy cloud at high altitude.
A key missing link is the absence of a middle cloud at Altocumulus level.
Friton, regional director of the American Meteor Society, saw many flashes of light through holes in an altocumulus doud deck "like white snowflakes in a minor snowstorm." For five hours Canadian observers under the supervision of Isabel K.
This is what produces the thick, opaque altocumulus and altostratus clouds at 10 to 15,000 feet where temperatures average 0 to -10 degrees F.
But as the week closed, tangible evidence of this pattern brought middle level altocumulus across the central and southerly parts of the country, ahead of and in conjunction with the initial trough.
The phenomenon is similar in appearance to the colorful atmospheric coronas that occasionally ring the Sun or Moon, when they are covered by altocumulus clouds.
This radar triad at Addu Atoll provided unprecedented observations of the entire tropical cloud population, including precipitating and nonprecipirating shallow cumulus clouds, midlayer altostratus and altocumulus, convective congestus, isolated deep convective clouds, upper-level anvil and cirrus clouds, mesoscale convective systems, and hydrometer types.
A major trough passed quickly south of the Cape, only to be succeeded even quicker by another system, while a northwesterly drift persisted aloft, as witnessed by several successive morning's turbulent altocumulus sheets.
The air along and just above the frontal surface is typically filled with broken to overcast stratocumulus, altocumulus, and cirriform layers.