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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
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.
Industry:Weather
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.
Industry:Weather
1. A measure (or condition) of height, especially of great height, as a mountain top or aircraft flight level. In meteorology, altitude is used almost exclusively with respect to the height of an airborne object above the earth's surface, above a constant-pressure surface, or above mean sea level. The measurement of altitude is accomplished by altimeters in aeronautics, and the entire study is called altimetry. 2. In astronomy, same as elevation angle.
Industry:Weather
The form of the hydrostatic equation used to compute the altimeter setting, which is the pressure “reduced” to sea level using the temperature profile of the ICAO (International Civil Aeronautical Organization) standard atmosphere.
Industry:Weather
Value of the atmospheric pressure used to adjust the subscale of a pressure altimeter so that it indicates the height of an aircraft above a known reference surface.
Industry:Weather
A precision aneroid barometer calibrated to indicate directly the local sea level altimeter setting.
Industry:Weather
The measurement of altitude or height. In aeronautical science, altimetry is equivalent to hypsometry.
Industry:Weather
The period from about 5000–2500 B. C. , proposed by Ernst V. Antevs (1952), during which the summer temperature of western North America was 1°–2°C warmer than today and during which the lakes of the American Great Basin dried.
Industry:Weather
Corrections that must be made to the readings of a pressure altimeter to obtain true altitudes. There are several sources of corrections: 1) pressure corrections due to changes in sea level atmospheric pressure readings; 2) air temperature corrections resulting from the differences between the actual temperature of the column of air beneath the altimeter and a standard atmosphere temperature profile; and 3) corrections for errors in the calibration of the mechanical parts of the altimeter.
Industry:Weather
