| Section S | S index | 1241-1249 of 1376 terms |
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supercell—An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min). Most rotating updrafts are characterized by cyclonic vorticity (see mesocyclone). The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells often propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, large hail, and strong, long-lived tornadoes. See also convective storm, thunderstorm, splitting convective storm, cell, bulk Richardson number.
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supercooled fog—Fog containing water droplets in liquid form that exists at temperatures colder than 0°C; a supercooled cloud immediately above the surface.
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supercooled rain—Liquid precipitation at temperatures below freezing. At midlatitudes, supercooled rain often forms first as an ice crystal or snow in the clouds, which then melts as it falls through an elevated layer of air warmer than freezing before reaching the thick bottom layer of cold air that cools the drop below freezing. A supercooled drop freezes instantly on contact with surfaces such as electrical power lines, trees, and roads during an ice storm. See glaze.
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supercooling—(Also called subcooling or undercooling; see note below.) The reduction of temperature of any liquid below the melting point of that substance's solid phase; that is, cooling beyond its nominal freezing point. A liquid may be supercooled to varying degrees, depending upon the relative lack of freezing nuclei or solid boundary irregularities within its environment, and freedom from agitation. This supercoolability of a substance with a crystalline solid form (as opposed to amorphous matter) stems from the unique energy transformations necessary for the formation of the first crystal nucleus, whereafter all adjacent liquid immediately becomes solid unless or until the latent heat released elevates the system's temperature sufficiently to arrest the process. It should be noted that the reverse process is not possible; a crystalline solid cannot be “superheated”; therefore, a substance's melting point is very conservative, only slightly dependent upon pressure. Supercooled clouds are quite common. In extreme cases, they have been observed at temperatures as low as −40°C (−40°F). The smaller and purer the water droplets, the more likely is supercooling. Compare supersaturation; see nucleation. (Note: The choice of terminology for this concept has been variable and controversial for many years, largely based on conceptual views of the prefixes super, sub, and under. Supercooling remains, however, the most frequently utilized term.)
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