cloud

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

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.