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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 dewdew,
thin film of water that has condensed on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of overlying air in contact with them, causing the condensation of some water vapor.
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 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 stratospherestratosphere
, second lowest layer of the earth's atmosphere. The level from which it extends outward varies with latitude; it begins c.5 1-2 mi (9 km) above the poles, c.6 or 7 mi (c.10 or 11 km) in the middle latitudes, and c.10 mi (16 km) at the equator, and extends outward c.
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. 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 warmingglobal warming,
the gradual increase of the temperature of the earth's lower atmosphere as a result of the increase in greenhouse gases since the Industrial Revolution. Global warming and its effects, such as more intense summer and winter storms, are also referred to as climate
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 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).



the accumulation in the atmosphere of the products of condensation of water vapor in the form of a vast number of minute water droplets or ice crystals, or both. A similar accumulation near the ground is called fog. Clouds significantly affect weather; for example, they determine the formation regime of precipitation and affect the heat regime of the atmosphere and the earth. On the average, clouds cover about half the sky and contain up to 109 tons of water in a suspended state. They are an important link in the hydrologic cycle of the earth and can travel thousands of kilometers, carrying and thus redistributing enormous quantities of water.

Since water vapor is present mainly in the lower part of the atmosphere—the troposphere—nearly all clouds are concentrated in the troposphere at various altitudes. However, cirrus and cumulonimbus often extend into the stratosphere, with cumulonimbus sometimes reaching an altitude of 16 km or more. Nacreous clouds also can form in the stratosphere (at an altitude of about 25 km), and noctilucent clouds can form in the mesosphère (altitude of about 80 km).

Clouds are classified on the basis of height into low-level, middle-level, and high-level clouds (see Table 1). Low-level clouds include stratus, stratocumulus, and nimbostratus. Stratus is uniform, lacks an ordered structure, and forms a comparatively thin layer. Stratocumulus forms a layer with a clearly defined structure consisting of waves, banks, or large sheets. Nimbostratus forms a continuous, thick, gray cover that produces widespread rainfall or snowfall. Middle-level clouds include altostratus and altocumulus. Altostratus is a grayish or slightly bluish cloud veil; altocumulus resembles stratocumulus but is thinner. High-level clouds include cirrus, cirrostratus, and cirrocumulus. Cirrus consists of diffuse, often transparent, clouds in the form of parallel or tangled filaments. Cirrostratus is a white or blue, quite uniform, cloud veil. Cirrocumulus consists of thin, transparent clouds having a wavy pattern or a form of clusters of flakes.

In addition, there is a fourth cloud type, called clouds with vertical development. These clouds have comparatively level bases and dome-shaped peaks, often of fantastic outline; examples are cumulus, cumulus congestus, and cumulonimbus. There are numerous varieties of the cloud types described above.

Clouds form in atmospheric regions with high relative humidity. The presence in the atmosphere of an enormous number of minute particles that act as condensation nuclei makes possible the appearance of nuclear droplets once saturation is reached. Saturation conditions are created by the cooling of air, which may be due to the expansion of air, for example, during the ordered rise in atmospheric fronts (Ns clouds and Ns-As-Ac systems are formed in this manner), during disordered turbulent mixing or wave motions (St, Sc, Ac), during convective ascent (Cu, Cu cong, Cb), and during flow over mountain barriers (Ac). Further cooling of the air leads to excess vapor, which is absorbed by the growing droplets. Thus, the droplets initially grow primarily as a result of the condensation of water vapor. Later, as the droplets grow larger, the processes of collision and coalescence of droplets (coagulation of the cloud elements) play an increasing role. Coagulation is the primary growth mechanism for cloud droplets with a radius greater than 30 microns (μ).

At negative temperatures, clouds may be one of the following: water (supercooled) clouds, ice-crystal clouds, or mixed clouds, that is, a mixture of droplets and crystals. The smallness of cloud droplets enables them to remain for a long time in liquid form even at negative temperatures. At —10°C, for example, 50 percent of all clouds are water clouds, 30 percent are mixed clouds, and only 20 percent are ice-crystal clouds. Supercooled droplets are encountered in clouds at temperatures as low as —40°C. Supersaturation over crystals is much greater than over droplets, since the saturation pressure of water vapor over ice is less than it is over water. As a result, in mixed clouds, crystals grow much faster than droplets, promoting precipitation.

Most droplets in clouds measure thousandths and hundredths of a millimeter, and the droplet concentration is hundreds per cubic centimeter. Crystals usually have dimensions that are tens of times greater, but the crystal concentration is thousands or tens of thousands of times smaller, as low as 100 crystals per liter. The shape of the crystals depends primarily on the temperature of formation and is extremely varied—needles, columns, clusters of columns, thick and thin plates, and, finally, irregularly shaped particles. “Superlarge” droplets that reach tenths of a millimeter with a concentration equal to or less than one droplet per liter are also generally present in clouds. Such particles are the nuclei for precipitation and make the main contribution to the radar signal from clouds of water droplets.

The mass of condensed water per unit volume of a cloud is called the liquid-water content of the cloud and usually ranges from tenths of a gram to a few grams per cubic meter in water-droplet clouds and from thousandths to tenths of a gram per cubic meter in ice-crystal clouds. Data on the physical structure of clouds have been obtained primarily by means of aircraft outfitted with special equipment. Diffraction and refraction of light by cloud particles cause various optical effects, such as glories, halos, and coronas, by which the presence of droplets or crystals in clouds can be assessed. Radar methods of investigating clouds are used extensively, and satellite and laser methods are being developed.

The physical processes that control the development of clouds are diverse and complex. After forming on condensation nuclei, cloud droplets grow, move within the cloud, are carried out of the cloud, and evaporate. The lifetime of cloud particles can be many times shorter than the lifetime of the cloud as a whole. The life cycle of a cloud ends when the cloud evaporates. Precipitation contributes to the removal of water and speeds up the process of cloud disintegration. The prolonged existence of clouds is due to the low velocities of descent of particles (droplets with a radius of 1–10 μ fall at a velocity of 0.05–1.2 cm/sec). It is also due to the presence of rising air currents, which not only support cloud particles but, together with turbulent motions, ensure an influx of water vapor and contribute to the production of new particles.

Certain processes in clouds can be controlled by artificially altering the phase state and microstructure in the cloud. The greatest successes have been achieved in the dispersing of supercooled clouds and fog and in moderating hail-producing clouds in order to prevent hailstorms. To disperse supercooled clouds and fog, cooling agents (for example, dry ice particles) or particles of ice-nucleating substances, such as silver iodide or lead iodide, are introduced by airplane or by means of special generators on the ground. These procedures promote the formation in clouds of a sufficient number of ice crystals, which later grow and fall from the clouds. In the process, the water vapor pressure in

Table 1. Principal forms and characteristics of clouds
Name(abbreviation)Size of cloud thickness (km)Preferential phase structureLifetime of cloudMaximum vertical velocitiesTypes of precipitation at ground level
altitude of lower boundary (km)thickness (km)horizontal length (km)
Stratiform clouds
Stratus (St)..................... 01−0.70.1−1.010−103water droplets1 day or moretens of cm/secnone or drizzle
Stratocumulus (Se) ................ 0 4−2.00.1−1.010−103water droplets1 day or moretens of cm/secnone or drizzle
Altocumulus (Ac) ................. 2−60.1−0.810−102water droplets, mixed1 day or moretens of cm/secnone
Clrrocumulus (Cc)................. 6−90.2−1.010−102ice crystals1 day or moretens of cm/secnone
Nimbostratus (Ns)................. 01−1.01−10102−103mixed1 day or moretens of cm/secrain, snow
Altostratus (As) .................. 3−60.5−3102−103mixed, ice crystals1 day or moretens of cm/secnone
Cirrostratus (Cs).................. 5−90.5−5102−103ice crystals1 day or moretens of cm/secnone
Cirrus (Ci) ..................... 6−100.2−3102−103ice crystals1 day or moretens of cm/secnone
Cumuliform clouds
Cumulus (Cu) ................... 0 8−2.00.3−31−5water dropletstens of minutes1 m/secnone
Cumulus congestus (Cu cong).......... 0.8−2.03−52−10water dropletstens of minutes10 m/secnone
Cumulonimbus (Cb)................ 0.4−1.55−125−50mixedtens of minutes15−20 m/seccloud burst, hail

the cloud decreases, the droplets evaporate, and cloud (fog) dispersion sets in. Fog and low clouds over airport runways are dispersed by this method. The time and place for introducing the reagent are determined by means of special meteorological radar stations. Clouds can be generated artificially by means of thermal convection sources or by the introduction of additional moisture. For example, the combustion of 1 kg of kerosene produces about 1.2 kg of water vapor. This is usually sufficient to form condensation trails behind airplanes flying at an altitude of 8–12 km. The lifetime of such trails depends on the atmospheric humidity.

An active search is under way for methods of artificially controlling and redistributing precipitation. The large natural variability in the amount of natural precipitation greatly complicates the problem of determining the actual effectiveness of the methods used. The development of such methods is drawing increasing attention to the economic, legal, and social aspects of the problem of artificial weather control.


Atlas oblakov. Edited by A. Kh. Khrgian. Leningrad, 1957.
Fizika oblakov. Edited by A. Kh. Khrgian. Leningrad, 1961.
Shmeter, S. M. Fizika konvektivnykh oblakov. Leningrad, 1972.
Trudy VIII Vsesoiuznoi konferentsiipo fizike oblakov i aktivnym vozdeistviiam. Leningrad, 1970.
Izmenenie pogody chelovekom. Edited by I. P. Mazin. Moscow, 1972. (Translated from English.)
Mason, B. J. The Physics of Clouds. Oxford, 1957.
Proceedings of the International Conference on Cloud Physics, Toronto, August, 1968. Toronto, 1968.



Suspensions of minute water droplets or of ice crystals produced by the condensation of water vapor.
(nuclear physics)
The nucleons that are in the nucleus of an atom but not in closed shells.
(science and technology)
Any suspension of particulate matter, such as dust or smoke, dense enough to be seen.


A visible accumulation of minute water droplets and/or ice particles in the atmosphere above the earth's surface. Cloud differs from ground fog, fog, or ice fog only in that the latter are in contact with the earth's surface.


1. a mass of water or ice particles visible in the sky, usually white or grey, from which rain or snow falls when the particles coagulate
2. a large number of insects or other small animals in flight


A communications network. The word "cloud" often refers to the Internet, and more precisely to some datacenter full of servers that is connected to the Internet. However, the term "cloud computing" refers to the software and services that have enabled the Internet cloud to become so prominent in everyday life (see cloud computing). See private cloud and personal cloud.

A Cloud May Refer to Any Network
A cloud can be a wide area network (WAN) like the public Internet or a private, national or global network. The term can also refer to a local area network (LAN) within an organization.

For decades, network diagrams have used a cloud-like symbol to reduce the entire infrastructure of a network into simple entry and exit points when the specific network architecture is not material to the illustration. Inside the cloud, there may be any number of cables, routers and switches that handle the forwarding of data from one point to another. The cloud diagram may also include the servers that perform the required data processing.

Network Cloud
For decades, the cloud symbol has represented a network without divulging technical details. The symbol is used when only the points of entry and exit need to be identified.

Today's Cloud
Quite often, the cloud is the Internet, and cloud computing on the Internet has several advantages for developers, content publishers and users. See cloud computing.


Clouds are made of air and water, two of the four basic elements. Some would say that the soul is represented by air and spirit by water. Your positive energy and idealism could be represented by white clouds and your more negative personal characteristics, and private thoughts, by dark storm clouds.