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.


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

Content of the region between the stars, including vast, diffuse clouds of gases and minute solid particles. Such tenuous matter in the Milky Way Galaxy accounts for about 5percnt of its total mass. By no means a complete vacuum, the interstellar medium contains mainly hydrogen gas, with a smaller amount of helium and sizable quantities of dust particles of uncertain composition. Primary cosmic rays also travel through interstellar space, and magnetic fields extend across much of it. Most interstellar matter occurs in cloudlike concentrations, which can condense to form stars. Stars, in turn, continually lose mass through stellar winds (see solar wind). Supernovas and planetary nebulae also feed mass back to the interstellar medium, where it mixes with matter that has not yet formed stars (see Populations I and II).

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Radiation detector developed by C.T.R. Wilson. Its detecting medium is a supersaturated vapour (see saturation) that condenses around ions produced by the passage of energetic charged particles, such as alpha particles, beta particles, or protons. In a Wilson cloud chamber, supersaturation is caused by the cooling induced by a sudden expansion of the saturated vapour by the motion of a piston or an elastic membrane. In a diffusion chamber, the saturated vapour is cooled to supersaturation as it diffuses into a region kept cold by a coolant such as solid carbon dioxide or liquid helium.

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Any visible mass of water droplets, ice crystals, or a mixture of the two that is suspended in the air, usually at a considerable height. Clouds are usually created and sustained by upward-moving air currents. Meteorologists classify clouds primarily by their appearance. The 10 main cloud families are divided into three groups on the basis of altitude. High clouds, which are found at mean heights of 45,000–16,500 ft (13–5 km), are, from highest to lowest, cirrus, cirrocumulus, and cirrostratus. Middle clouds, at 23,000–6,500 ft (7–2 km), are altocumulus, altostratus, and nimbostratus. Low clouds, at 6,500–0 ft (2–0 km), are stratocumulus, stratus, cumulus, and cumulonimbus. A shallow layer of cloud at or near ground level is called fog.

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orig. Mahpiua Luta

(born 1822, on the Platte River, Nebraska Territory, U.S.—died Dec. 10, 1909, Pine Ridge Agency, S.D.) American Indian leader. The principal chief of the Oglala Teton Dakota (Sioux), he led the opposition of both the Sioux and the Cheyenne to the U.S. government's development of the Bozeman Trail to goldfields in the Montana Territory (1865–67). Relentlessly attacking workers along the route from Fort Laramie (in modern Wyoming) to Montana, he refused offers to negotiate until the U.S. agreed to halt the project, whereupon he laid down his arms and allowed himself to be settled on the Red Cloud Agency in Nebraska.

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Vast spherical cloud of small, icy bodies orbiting the Sun at distances ranging from about 0.3 light-year to one light-year or more that is probably the source of most long-period comets. In 1950 the Dutch astronomer Jan Hendrik Oort (1900–92) noted that no comets have orbits that would indicate an interstellar origin. He proposed that the Sun is surrounded by billions of these objects, which are only occasionally detectable when they enter the inner solar system. The Oort cloud is believed to be composed of primordial bodies dating from the formation of the solar system (see solar nebula). Whether the Oort cloud merges, in its inner region, into the disk-shaped Kuiper belt is not known.

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A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity (clouds can also occur as masses of material in interstellar space, where they are called interstellar clouds and nebulae.) The branch of meteorology in which clouds are studied is nephology or cloud physics.

On Earth the hunter condensing substance is typically water vapor, which forms small droplets of ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance of the clouds at their base. Thin clouds may appear to have acquired the color of their environment or background, and clouds illuminated by non-white light, such as during sunrise or sunset, may be colored accordingly. In the near-infrared range, clouds would appear darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those wavelengths.

Classification of clouds

Clouds are divided into two general categories: layered and convective. These are named stratus clouds (or stratiform, the Latin stratus means "layer") and cumulus clouds (or cumuliform; cumulus means "piled up"), respectively. These two cloud types are divided into four more groups that distinguish the cloud's altitude. Clouds are classified by the cloud base height, not the cloud top. This system was proposed by Luke Howard in 1802 in a presentation to the Askesian Society.

High clouds (Family A)

Middle clouds (Family B)

Low clouds (Family C)

These are found up to 2,000 m (6,500 feet) and include the stratus (dense and grey). When stratus clouds contact the ground, they are called fog.

Clouds in Family C include:

Vertical clouds (Family D)

These clouds can have strong up-currents, rise far above their bases and form at many heights.

Clouds in Family D include:

Other clouds

A few clouds can be found above the troposphere; these include noctilucent and polar stratospheric clouds (or nacreous clouds), which occur in the mesosphere and stratosphere respectively.

Some clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically-specific cloud in the world is Morning Glory, a rolling cylindrical cloud which appears unpredictably over the Gulf of Carpentaria in Northern Australia. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in unpowered glider aircraft.

Cloud fields

A cloud field is simply a group of clouds but sometimes cloud fields can take on certain shapes that have their own characteristics and are specially classified. Stratocumulus clouds can often be found in the following forms:

  • Actinoform, which resembles a leaf or a spoked wheel.
  • Closed cell, which is cloudy in the center and clear on the edges, similar to a filled honeycomb.
  • Open cell, which resembles a honeycomb, with clouds around the edges and clear, open space in the middle.


The color of a cloud, as seen from the Earth, tells much about what is going on inside the cloud. Clouds form when relatively warm air containing water vapor is lighter than its surrounding air and this causes it to rise. As it rises it cools and the vapor condenses out of the air as micro-droplets. These tiny particles of water are relatively densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color. As a cloud matures, the droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall as rain. In this process of accumulation, the space between droplets becomes larger and larger, permitting light to penetrate much farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, it may be that a percentage of the light which enters the cloud is not reflected back out before it is absorbed (Think of how much farther one can see in a heavy rain as opposed to how far one can see in a heavy fog). This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.

Other colours occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-sized droplets in the cloud.

A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud which shows green is an imminent sign of heavy rain, hail, strong winds and possible tornadoes.

Yellowish clouds are rare but may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of smoke.

Red, orange and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. The clouds are not that color; they are reflecting the long (and unscattered) rays of sunlight which are predominant at those hours. The effect is much the same as if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads this can produce blood-red clouds.

Global dimming

The recently recognized phenomenon of global dimming is thought to be caused by changes to the reflectivity of clouds due to the increased presence of aerosols and other particulates in the atmosphere.

Global brightening

New research From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth's Surface by Martin Wild et al. (Science 6 May 2005; 308: 847-850) indicates global brightening trend.

Global brightening is caused by decreased amounts of particulate matter in the atmosphere. With less particulate matter there is less surface area for condensation to occur. Since there's less condensation in the atmosphere and increased evaporation caused by increasing amounts of sunlight striking the water's surface there is more moisture, causing fewer but thicker clouds.

Clouds on other planets

Within our solar system, any planet or moon with an atmosphere also has clouds. Venus' clouds are composed entirely of sulfuric acid droplets. Mars has high, thin clouds of water ice. Both Jupiter and Saturn have an outer cloud deck composed of ammonia clouds, an intermediate deck of ammonium hydrosulfide clouds and an inner deck of water clouds. Uranus and Neptune have atmospheres dominated by methane clouds.

Saturn's moon Titan has clouds which are believed to be composed largely of droplets of liquid methane. The Cassini-Huygens Saturn mission has uncovered evidence of a fluid cycle on Titan, including lakes near the poles and fluvial channels on the surface of the moon.

See also

Further reading



  • Hamblyn, Richard The Invention of Clouds — How an Amateur Meteorologist Forged the Language of the Skies Picador; Reprint edition (August 3, 2002). ISBN 0312420013
  • Could Reducing Global Dimming Mean a Hotter, Dryer World?

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