- 行业: Weather
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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, ...
The spread of frequencies of a spectral line as a consequence of the random motions of molecules. Because of the Doppler effect, the observed frequency of radiation emitted by a molecule depends on its motion relative to the observer. Even if a stationary molecule could emit radiation of only a single frequency, a gas of such molecules moving randomly in all directions with a distribution of speeds (see Maxwell–Boltzmann distribution) would be observed to emit radiation over a range of frequencies proportional to the mean molecular speed relative to the speed of light. This mean speed is proportional to the square root of absolute temperature and hence so is Doppler broadening. At normal temperatures and pressures Doppler broadening is dwarfed by collision broadening, but high in the atmosphere Doppler broadening may dominate and, indeed, provides a means of remotely inferring temperatures.
Industry:Weather
The statement that the volume integral of the divergence of a vector, such as the velocity V, over a volume V is equal to the surface integral of the normal component of V over the surface s of the volume (often called the “export” through the closed surface), provided that V and its derivatives are continuous and single-valued throughout V and s. This may be written
where n is a unit vector normal to the element of surface ds, and the symbol ∮ ∮S indicates that the integration is to be carried out over a closed surface. This theorem is sometimes called Green's theorem in the plane for the case of two-dimensional flow, and Green's theorem in space for the three-dimensional case described above. The divergence theorem is used extensively in manipulating the meteorological equations of motion.
Industry:Weather
A nautical term for the equatorial trough, with special reference to the light and variable nature of the winds. Compare horse latitudes; see equatorial air.
Industry:Weather
The period of greatest heat in summer. Although the name is popularly supposed to have been derived from the period when dogs are especially liable to go mad, it is actually taken from Sirius, the Dog Star. In ancient Greece and Rome the heliacal rising of Sirius was associated with, and believed to cause, the hot, dry, sultry season of summer. The loss of human energy and the wilting of vegetation caused by this weather led to a belief in the baleful effect of Sirius on all human affairs. No formal meteorological or climatological definition exists. In the United States dog days are considered to persist for four to six weeks between mid-July and early September; in western Europe they last from about the third of July to the eleventh of August, and also are associated with the period of greatest frequency of thunder.
Industry:Weather
1. A name for the harmattan on the Guinea coast of western Africa; because of its dryness, it brings relief from the hot humid onshore winds. 2. Colloquial name for the sea breeze in tropical and subtropical climates (such as the West Indies, South Africa, Jamaica, Australia) that is invigorating because of its relative coolness. See Cape doctor.
Industry:Weather
A photoelectric spectrophotometer that is used to determine the ozone content of the atmosphere. The instrument compares the solar energy at two wavelengths in the absorption band of ozone by permitting the radiation of each to fall alternately upon a photocell. The stronger radiation is then attenuated by an optical wedge until the photoelectric system of the photometer indicates that equality of radiation exists. In this manner the ratio of the radiation intensity is obtained. The ozone content of the atmosphere is computed from this value. See Knudsen's tables.
Industry:Weather
A unit used in the measure of the column abundance of ozone in the atmosphere. One Dobson unit is the equivalent of 2. 69 × 1016 molecules of ozone per square centimeter. Alternatively, 1 Dobson unit corresponds to a layer of ozone 10 μm thick, if the ozone were held at standard temperature and pressure (273 K, 1 atm pressure).
Industry:Weather
A streamline or flow boundary representing the height that separates stable flow approaching a hill or mountain into two regions: a lower region, where the flow is horizontal and splits around the obstacle (i.e., the flow is partially blocked), and an upper region, where the flow is three-dimensional and goes over the hill. This occurs when the relationship between the flow and the obstacle height H is characterized by a Froude number Fr less than 1. The height of the boundary Hc has been found to be Hc = H(1 − Fr ). Thus, it lies between the top and bottom of the hill and is lower for more stable flow. This concept has been used in air pollution studies to determine where pollution plumes are apt to impinge on a slope surface.
Industry:Weather
