| Section A | A index | 451-459 of 917 terms |
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altocumulus—(Abbreviated Ac.) A principal cloud type (cloud genus), white and/or gray in color, that occurs as a layer or patch with a waved aspect, the elements of which appear as laminae, rounded masses, rolls, etc. These elements usually are sharply outlined, but they may become partly fibrous or diffuse; they may or may not be merged; they generally have shadowed parts; and, by convention, when observed at an angle of more than 30° above the horizon, an altocumulus element subtends an angle between 1° and 5°. Small liquid water droplets invariably compose the major part of the composition of altocumulus. This results in sharpness of outline, small internal visibility (both common cumuliform characteristics), and in the occurrence of coronae and irisation (colored diffraction phenomena). With sufficiently low temperatures, ice crystals may appear in all forms of altocumulus, but mainly in the species castellanus or floccus, each unit of which may produce an individual snow shower. The crystals that fall from altocumulus sometimes produce parhelia, or a moon or sun pillar, any of which indicates the presence of tabular crystals. Should the composition become entirely ice crystals, the cloud would lose its characteristic sharpness of outline. Altocumulus often forms directly in clear air. It may be produced by an increase in size or thickening of the elements of an entire layer or patch of cirrocumulus (Ac cirrocumulomutatus); by subdivision of a layer of stratocumulus (Ac stratocumulomutatus); by transformation of altostratus (Ac altostratomutatus) or nimbostratus (Ac nimbostratomutatus); or by the spreading of cumulus or cumulonimbus (Ac cumulogenitus or Ac cumulonimbogenitus). Altocumulus frequently occurs in a given sky at different levels; also, it often is associated with clouds of other genera. Virga may appear with most of the species of altocumulus. This supplementary feature, however, should not be confused with the very white trails of ice crystals that frequently are formed with the dissipation of altocumulus floccus. When detached, the ice crystal trails are cirrus. Sometimes mamma occur with altocumulus. Cirrocumulus and stratocumulus are the clouds most easily confused with altocumulus. The elements of cirrocumulus never have shadows of their own, and nearly always are smaller. Stratocumulus elements are larger than those of altocumulus. The rolls or cells that are associated with altocumulus are thought to be a result of the absorption of terrestrial radiation and/or the presence of wind shear, which drives Rayleigh–Bénard convection or Kelvin–Helmholtz shear instability. See cloud classification.
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altostratus opacus—See opacus.
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altostratus— (Abbreviated As.) A principal cloud type (cloud genus) in the form of a gray or bluish (never white) sheet or layer of striated, fibrous, or uniform appearance. Altostratus very often totally covers the sky and may, in fact, cover an area of several thousand square miles. The layer has parts thin enough to reveal the position of the sun, and if gaps and rifts appear, they are irregularly shaped and spaced. Within the rather large vertical extent of altostratus (from several hundred to thousands of feet) a very heterogeneous particulate composition may exist. In this most complete case, there may be distinguished 1) an upper part, mostly or entirely ice crystals; 2) a middle part, a mixture of ice crystals and/or snowflakes and supercooled water droplets; and 3) a lower part, mostly or entirely supercooled or ordinary water droplets. A number of partial combinations of these composition types may occur, but never an entire cloud like 3) above. The particles are widely enough dispersed so as not to obscure the sun except by its thickest parts, but rather to impose a “ground-glass” effect upon the sun's image, and to prevent sharply outlined shadows from being cast by terrestrial objects. Halo phenomena do not occur. Altostratus is a precipitating cloud (praecipitatio) and therefore often is accompanied by virga and mamma. Rain, snow, ice pellets, etc., are present in the cloud and under its base, frequently rendering the base quite indistinct, particularly when the precipitation does not reach the ground. When precipitation reaches the ground, it is usually very light and of a relatively continuous nature. Altostratus may be formed by the thickening of cirrostratus (As cirrostratomutatus), or by the thinning of nimbostratus (As nimbostratomutatus). If widespread precipitation develops in altocumulus, altostratus may result (As altocumulogenitus). Sometimes, particularly in the tropics, altostratus may be produced by the spreading of the middle or upper portion of cumulonimbus (As cumulonimbogenitus). Cirrostratus and nimbostratus are the two other forms most easily confused with altostratus. In the first case, it should be remembered that cirrostratus does allow terrestrial shadows and frequently produces halo phenomena. Nimbostratus is darker colored, hides the sun, is more uniform in optical thickness, and always produces precipitation. At night, if precipitation does not reach the ground, it is conventional to call the doubtful layer altostratus. Any stratiform (layered) cloud necessarily forms because further vertical development is inhibited by the presence of a temperature inversion. Rolls and cells in altostratus are thought to be of similar origin to those in altocumulus. See cloud classification.
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aluminum oxide humidity element—A humidity-sensing element consisting of a base of aluminium, an oxide made by anodizing the base material, and an evaporated conductive coating of metal. The base material acts as one electrode and the evaporated metal as the other, or outer electrode. An increase in relative humidity causes the impedance between the electrodes to drop. The reverse is true when the relative humidity decreases.
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ambient air—1. Background, environmental, or surrounding air. When studying the dynamic and thermodynamic processes acting on an individual element such as an air parcel, cloud, smoke plume, raindrop, or ice crystal, ambient air represents the atmosphere outside of that element. The ambient air is often assumed to be static and of relatively large domain, within which the element resides. 2. The air that surrounds us, within which we live. When air pollutants of high concentration from exhaust or stack gases are emitted into cleaner air, the resulting polluted mixture is called the ambient air. National Ambient Air Quality Standards (NAAQS) apply to this final mixture, not to the undiluted emission gases.
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