Middle clouds, 7 to 2 km (23,000 to 6,500 feet), are altocumulus and altostratus. High clouds, which are found at mean heights above the ground of 13 to 5 km (42,500 to 16,500 feet), are cirrus, cirrocumulus, and cirrostratus. The eight main cloud families are divided into three groups on the basis of altitude. Meteorologists classify clouds primarily by their appearance. Raindrops are formed either by the coalescence of cloud droplets or by the production of snowflakes, graupel (amalgamations of frozen water droplets), or hail and their subsequent melting as they descend through the warmer regions of the cloud. Condensation alone can only generate droplets of sufficient size to produce drizzle. See all videos for this articleīecause 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. Created and produced by QA International. Thus, mixed clouds last for only short times before becoming all ice crystals.ĭifferent types of clouds form at different altitudes. In such clouds the ice crystals will grow preferentially with respect to the water droplets as water vapour evaporates from the droplets and deposits onto the ice crystals. Mixed clouds occur only when the cloud temperature is below 0 ☌ (32 ☏). Clouds consist entirely of water droplets, entirely of ice crystals, or of a mixture of water droplets and ice crystals, with crystals predominating in the colder, upper regions of the troposphere. These droplets, usually present in concentrations of a few hundred per cubic centimetre, constitute a nonprecipitating water cloud. This process rapidly gives rise to droplets on the order of 0.01 mm (0.0004 inch) in diameter. Condensation nuclei are composed of microscopic particles in the air.
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 such that water vapour condenses onto cloud condensation nuclei or tiny water droplets (or deposits onto ice nuclei or tiny ice crystals). As a mass of air ascends, the lower pressures prevailing at higher levels allow it to expand. Fog is a shallow layer of cloud at or near ground level.Ĭlouds are formed when relatively moist air rises.
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