CLOUD

any visible mass of water droplets, ice crystals, or a mixture of both that is suspended in the air, usually at a considerable height. A shallow layer of cloud at or near ground level is designated as fog. Clouds are formed when relatively moist air rises. As a mass of air ascends, the lower pressures prevailing at higher levels allow it to expand. In expanding, the air cools adiabatically (i.e., without heat exchange with the surrounding air) until its temperature falls below the dew point, upon which the air becomes supersaturated. The excess water vapour that it contains then condenses onto microscopic dust or smoke particles called condensation nuclei. This process rapidly gives rise to droplets on the order of 0.01 mm (0.0004 inch) in diameter. These droplets, usually present in concentrations of a few hundred per cubic centimetre, constitute a nonprecipitating water cloud. Clouds generally consist of a mixture of water droplets and ice crystals, with crystals predominating in the colder, upper regions of the cloud. Because clouds are created and sustained by upward-moving air currents, water droplets must reach a size sufficient to overcome the lifting effect of the currents before they can fall to the Earth as drizzle or rain. Condensation alone cannot generate droplets of sufficient size to produce even drizzle. Drizzle particles and raindrops are formed either by the coalescence of cloud droplets or by the production of ice crystals and their subsequent melting as they descend through the warmer regions of the cloud. 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 from 13 to 5 km (45,000 to 16,500 feet), are, from highest to lowest, cirrus, cirrocumulus, and cirrostratus. Middle clouds, 7 to 2 km (23,000 to 6,500 feet), are altocumulus, altostratus, and nimbostratus. Low clouds, 2 to 0 km (6,500 to 0 feet), are stratocumulus, stratus, cumulus, and cumulonimbus. Clouds may also be classified according to the air motions that produce them. In this method of classification there are four major types: layer clouds formed by the widespread regular ascent of air; layer clouds produced by widespread irregular stirring or turbulence; cumuliform clouds resulting from penetrative convection; and orographic clouds generated by the ascent of air over hills or mountains. Precipitation in significant amounts generally falls only from altostratus, nimbostratus, and cumulonimbus clouds. Steady rain that lasts for an entire day or longer is usually produced by the altostratus or nimbostratus variety. In most cases, cumulonimbus clouds are accompanied by showers of rain, snow, or hail, often with a thunderstorm or a tornado. Cirrus, cirrostratus, and cirrocumulus, high clouds composed exclusively of ice crystals, may produce snow, but the snow generally evaporates before reaching the ground if there are no lower clouds present. When thick clouds of waterdrops are present at lower levels, however, the snow from above may become heavier and reach the ground as snow or rain, depending on the temperature. Stratus clouds may produce drizzle, whereas the cumulus type sometimes yields showers, particularly in the tropics. Clouds are valuable aids in weather forecasting. If the sky, for example, is becoming overcast with cirrus and cirrostratus clouds, thickening and lowering to altostratus clouds, precipitation is imminent, probably from an approaching warm front in temperate latitudes or perhaps a hurricane in the tropics. New active thunderstorms are indicated by cumulus growing rapidly into cumulonimbus on a humid day, even though the Sun may be shining brightly between the clouds. The presence of cloudiness marks smaller diurnal temperature variations. In the evening a low overcast acts like a blanket, preventing the temperature from dropping much at night. A clear evening sky, on the other hand, indicates rapid cooling, which leads to dew, frost, or even fog the following morning.