Clouds consist of extremely tiny droplets of water, 0.0008 to 0.0024 in (0.02 to 0.06 mm) in diameter, or minute crystals of ice. These are sustained by the slightest upward movements of air. In order for cloud droplets to form it is necessary that microscopic dust particles serve as centers, or nuclei, of condensation. Dusts with a high affinity for water are abundant throughout the atmosphere.
Where the air temperature is well below freezing, clouds may form of tiny ice crystals. However, water in such minute quantities can remain liquid far below normal freezing temperatures; the liquid is said to be supercooled. Thus, water droplets exist at temperatures down to 10° F (-12°C); a mix-tore of water droplets and ice crystals from 10° to -20°F (-12 to -30°C) or even lower; and predominantly ice crystals below -20°F (-30°C). Below -40°F (-40°C) all of the cloud is ice. Clouds appear white when thin or when the sun is shining upon the outer surface. When dense and thick, clouds appear gray or black underneath simply because this is the shaded side.
Cloud types may be classified on the basis of two characteristics: general form and altitude. On the basis of form there are two major groups: stratiform or layered types, and cumuliform or massive, globular types.
The stratiform clouds are blanket like, often covering vast areas, but are fairly thin in comparison to horizontal dimensions. Stratiform clouds are subdivided according to the level of elevation at which they lie. The highest type is the cirrus cloud and its related forms, cirrostratus and cirrocumulus. These are roughly within the altitude range of 20,000 to 40,000 ft (6000 to 12,000 m) and are composed of ice crystals.
Cirrus is a delicate, wispy cloud, often forming streaks or stringers across the sky. It does not interfere with the passage of sunlight or moonlight and appears to the ground observer to. be moving very slowly, if at all. Cirrus bands often indicate the presence of the jet stream aloft. It is possible for the observer on the ground to estimate direction of upper airflow by means of fibrous cirrus formations. Cirrostratus is a more complete layer of cloud, producing a halo about the sun or moon. Where the layer consists of closely packed globular pieces of cloud, arranged in groups or lines, the name cirrocumulus is given. This is the mackerel sky of popular description.
At intermediate height range, from 6500 to 20,000 ft (2000 to 6000 in), are the altostratus and altocumulus clouds. Altostratus is a blanket layer, often smoothly distributed over the entire sky. It is grayish in appearance, usually has a smooth underside, and will often show the sun as a bright spot in the cloud. Altocumulus is a layer of individual cloud masses, fitted closely together in geometric pattern. The masses appear white, or somewhat gray on the shaded sides, and blue sky is seen between individual patches or rows. Altostratus is commonly associated with the development of bad weather, whereas altocumulus is usually characteristic of generally fair conditions.
In the low cloud group, from ground level to 6500 ft (2000 in), are stratus, nimbostratus, and stratocumulus clouds. Stratus is a dense, low-lying dark-gray layer. If rain or snow is falling from this cloud, it is termed nimbostratus, the prefix nimbo merely meaning that precipitation is coming from the cloud. Stratocumulus is a low-lying cloud later consisting of distinct grayish masses of cloud between which is open sky. The individual masses often take on the form of long rolls of cloud, oriented at right angles to the direction of wind and cloud motion. Stratocumulus is generally associated with fair or clearing weather, but sometimes rain or snow flurries issue from individual cloud masses.
Fog is simply a form of stratus cloud lying very close to the ground. One type, known as a radiation fog, is formed at night when temperature of the basal air falls below the dew point. Another type, advection fog, results from the movement of warm, moist air over a cold or snow-covered ground surface. Losing heat to the ground, the air layer undergoes a drop of temperature below the dew point, and condensation sets in. A similar type of advection fog is formed over oceans where air from over a warm current blows across the cold surface of an adjacent cold current. Fogs of the Grand Banks off Newfoundland are largely of this origin because here the cold Labrador Current comes in contact with warm waters of Gulf Stream origin.
The cumuliform clouds tend to display a height as great as, or greater than, their horizontal dimensions. Cumulus is a white, woolpack cloud mass, often showing a flat base and bumpy upper surface somewhat resembling a head of cauliflower. These clouds look pure white on the side illuminated by the sun, but may be gray or black on the shaded or underneath side. Small cumulus clouds are associated with fair weather. Under different conditions, discussed below, individual masses grow into cumulonimbus, the thunderstorm cloud mass of enormous size, which brings heavy rainfall, thunder and lightning, and gusty winds. A large cumulonimbus cloud may extend from a height of 1000 to 2000 ft (300 to 600 m) at the base up to 30,000 or 40,000 ft (9000 to 12,000 m). When seen from a great distance, the top of the cumulonimbus cloud is pure white, but to observers beneath, the sky may be darkened to almost nighttime blackness.
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