- 行业: Weather
- Number of terms: 60695
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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 rather shallow, almost horizontal layer in the atmosphere through which vertical temperature and moisture gradients produce a refractive index lapse rate greater than 157 N- units per kilometer. Conditions necessary for the formation of ducts are strongly increasing temperature and decreasing relative humidity with height. The resulting superstandard propagation causes the curvature of radio rays to be greater than that of the earth. Radio energy that originates within a duct and leaves the antenna at angles near the horizontal may thus be trapped within the layer. The effect is similar to that of a mirage (sometimes called “radio mirage”), and radar targets may be detected at phenomenally long ranges if both target and radar are in the duct. The greater the elevation angle between radar and target, the less the possibility of serious distortion due to transmission through ducts. Ducts may be surface-based or elevated, with typical thickness ranging from about 10 to 300 m. When elevated ducts occur, they are generally associated with subsidence or frontal inversions. Elevated ducts are rarely found above 5 km. See anomalous propagation, skip effect.
Industry:Weather
A rare and beautiful phenomenon in which snow is festooned from trees, fences, etc. , in the form of a rope of snow, several feet long and several inches in diameter, formed and sustained by surface tension acting in thin films of water bonding individual crystals. Such garlands form only when the surface temperature is close to the melting point, for only then will the requisite films of slightly supercooled water exist.
Industry:Weather
A radar designed to determine the approximate location of objects, usually aircraft or ships. The beams of such radars, called fan beams, are usually wider in the vertical than in the horizontal, making it possible to scan large volumes of space quickly. Compare tracking radar.
Industry:Weather
A radar display with coordinates of received signal amplitude versus range but differing from the A-display by starting from a range offset from zero.
Industry:Weather
A radar scanning procedure in which the antenna sweeps through an interval of azimuth less than the full 360° at a fixed elevation angle. Often the azimuth extent of a sector scan may be only a few degrees in order to concentrate the observations on a particular storm or region of a storm.
Industry:Weather
A radar system that infers wind speed by measuring the backscattering cross section (or normalized radar cross section).
Industry:Weather
A radar target of known radar cross section used to calibrate a radar or monitor its performance. Metal spheres or corner reflectors of known dimensions are examples.
Industry:Weather
A radar target that is small compared with the pulse volume, which is the cross- sectional area of the radar beam multiplied by half the length of the radar pulse. Typically, a point target is an object such as an aircraft, ship, or building that reflects the radar signal from relatively simple, discrete surfaces. See coherent target; Compare distributed target.
Industry:Weather
A psychrometer in which the wet- and dry-bulb thermometers are mounted upon a frame connected to a handle at one end by means of a bearing or length of chain. Thus, the psychrometer may be whirled by hand in order to provide the necessary ventilation.
Industry:Weather
