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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 first-order rate coefficient (in units of inverse time) for the occurrence of a photochemical reaction. The j-value is calculated from the product of the absorption cross section of the molecule being photolyzed, the quantum yield for the process, and the actinic flux, all integrated over the wavelength region of interest. See actinometer.
Industry:Weather
A rainbow formed when the light source is the moon rather than the sun. Even though the optics of both is the same, the luminance of the lunar rainbow is much lower. As a result of the eye's reduced sensitivity to color at low light levels, the bow may exhibit little color. There are several reasons why the lunar rainbow is seen much less frequently than the solar bow. While the moon and sun spend equal time above the horizon, when they are both present, only a solar bow can be seen. This, by itself, means that the lunar rainbow could form only half as often. Further, the moon goes through phases, and so even at night may not contribute enough light to produce a discernible bow. Finally, the convective showers in which the rainbow is frequently seen are much less common at night.
Industry:Weather
A rainbow formed when the light source is the moon rather than the sun. Even though the optics of both is the same, the luminance of the lunar rainbow is much lower. As a result of the eye's reduced sensitivity to color at low light levels, the bow may exhibit little color. There are several reasons why the lunar rainbow is seen much less frequently than the solar bow. While the moon and sun spend equal time above the horizon, when they are both present, only a solar bow can be seen. This, by itself, means that the lunar rainbow could form only half as often. Further, the moon goes through phases, and so even at night may not contribute enough light to produce a discernible bow. Finally, the convective showers in which the rainbow is frequently seen are much less common at night.
Industry:Weather
The synoptic-scale development of an atmospheric cyclonic circulation on the downwind side of a mountain range. The “lee” side is relative to the mean background airflow. Weak development can occur due to a redistribution of uniform vorticity as large-scale flow passes over a mountain barrier (see lee trough). Stronger cases of lee cyclogenesis occur when the mountain range interacts with a developing baroclinic wave. In this instance the mountain acts to position the cyclone or generate a secondary cyclone in the lee. Often this leaves a weaker parent cyclone that is typically poleward of the development and far from the mountains. Lee cyclogenesis is a multistage process involving a phase of rapid deepening followed by a transition to slower baroclinic deepening. The mountain barrier disrupts the orderly advection of low-level cold air that would occur behind the cyclone over flatter terrain and induces a quasigeostrphic imbalance. As it tends to restore this imbalance (see geostrophic adjustment), the atmosphere produces the beginnings of a cyclonic system near the surface. The lack of cool air at low levels and the descending upper-level air make the mountain lee environment statically less stable than the surroundings. This enhances the possibility for vertical coupling of potential vorticity maxima associated with the approaching upper-level trough and the incipient lee disturbance at lower levels. Lee cyclogenesis is common on the leeward side of the major mountain ranges of the world including the Alps, the Himalayas, the Rockies (both east and west), and the Andes. Many minor ranges support lee cyclogenic activity. See also Genoa cyclone, Colorado low, Alberta clipper, pampero.
Industry:Weather
A mildly sloping tunnel, with the upper end below the water table of an alluvial aquifer, that is used to collect and transmit water to a lower-elevation surface outlet.
Industry:Weather
The separation of electric charges accompanying the aerodynamic breakup of water drops, first studied systematically by the German physicist P. Lenard (1892). Experiments have shown that the degree of charge separation in spray processes depends upon the drop temperature, presence of dissolved impurities, speed of the impinging air blast, and contact with foreign surfaces. The largest fragments of the broken drops are observed to carry positive charges and the fine spray of drops carried off in the impinging air current carries a net negative charge. Distilled water drops of 4-mm diameter, broken after a 5-cm free fall into an updraft of 1 m s−1, were found by Chapman (1953) to yield about 10−10 C of separated charge per drop. The Lenard effect was incorporated by Simpson (1927) into his breaking-drop theory of thunderstorm charge generation, but many critical details are but poorly understood.
Industry:Weather
An ion of relatively large mass and low mobility that is produced by the attachment of a small ion to an Aitken nucleus. Large ions were discovered by P. Langevin and are sometimes referred to as “Langevin ions. ” Large ions have ion mobilities of the order of 10−8 m s−1 per volt m−1, or some 10 000 times lower than those of small ions. As a result these atmospheric ions contribute practically nothing to the conductivity of the air, except in rare cases where small ions are nearly absent. Typically, they bear only a single electronic charge, as is true of small ions. Large ions move so slowly that they are not destroyed by being neutralized by still other large ions of paired signs, for such collisions are too infrequent. Instead, they are neutralized by union with a small ion of opposite sign. Their mean lifetimes are of the order of 15–20 minutes over the oceans, but may approach 1 h in very polluted air. The ion density of large ions varies widely depending upon the degree of atmospheric pollution. Representative low-altitude values might be 109 m−3 in clean country air, 1010 m−3 in an industrial area, and 108 m−3 over the oceans.
Industry:Weather
A flow regime in which fluid motion is smooth and orderly, and in which adjacent layers or laminas slip past each other with little mixing between them. Exchange of material across laminar layers occurs by molecular diffusion, a process about 106 times less effective than turbulence. Laminar flow can be easily predicted as velocity increases at a steady rate from a boundary. This contrasts with the chaotic and random nature of turbulent flow. Laminar flow is not a common occurrence in the statically neutral and unstable atmosphere and is confined to a very thin layer (1 mm) adjacent to very smooth surfaces such as snow and ice. However, in strongly statically stable regions such as the the nocturnal boundary layer, the Richardson number can be large enough that turbulence is suppressed, and the flow is laminar over a layer many tens of meters thick.
Industry:Weather
Atmospheric coherent structures in the form of persistent organized counterrotating roll vortices that are approximately aligned with the mean wind and span the depth of the planetary boundary layer. Longitudinal rolls are frequently present in the atmospheric boundary layer in near-neutral to moderately unstable stratification. They are believed to be the result of nonlinear equilibration of mixed convective–dynamic normal mode instabilities of the mean boundary flow. Longitudinal rolls produce a nonlocal transport not only of momentum, but also of scalar quantities that mix the boundary layer more efficiently than local turbulent diffusion. The quasi-two-dimensional longitudinal rolls generate a mean secondary circulation that organizes the smaller-scale three- dimensional turbulent eddies into linear patterns. The existence of longitudinal rolls significantly changes the fluxes within the boundary layer and at the surface. Flux profiles also differ between the updraft and downdraft regions of longitudinal rolls. In favorable thermodynamic conditions cloud streets (linear boundary layer cloud patterns) form in the updraft regions between the rolls. See coherent structures, Langmuir circulation, two-dimensional eddies, horizontal convective rolls.
Industry:Weather
The synoptic-scale development of an atmospheric cyclonic circulation on the downwind side of a mountain range. The “lee” side is relative to the mean background airflow. Weak development can occur due to a redistribution of uniform vorticity as large-scale flow passes over a mountain barrier (see lee trough). Stronger cases of lee cyclogenesis occur when the mountain range interacts with a developing baroclinic wave. In this instance the mountain acts to position the cyclone or generate a secondary cyclone in the lee. Often this leaves a weaker parent cyclone that is typically poleward of the development and far from the mountains. Lee cyclogenesis is a multistage process involving a phase of rapid deepening followed by a transition to slower baroclinic deepening. The mountain barrier disrupts the orderly advection of low-level cold air that would occur behind the cyclone over flatter terrain and induces a quasigeostrphic imbalance. As it tends to restore this imbalance (see geostrophic adjustment), the atmosphere produces the beginnings of a cyclonic system near the surface. The lack of cool air at low levels and the descending upper-level air make the mountain lee environment statically less stable than the surroundings. This enhances the possibility for vertical coupling of potential vorticity maxima associated with the approaching upper-level trough and the incipient lee disturbance at lower levels. Lee cyclogenesis is common on the leeward side of the major mountain ranges of the world including the Alps, the Himalayas, the Rockies (both east and west), and the Andes. Many minor ranges support lee cyclogenic activity. See also Genoa cyclone, Colorado low, Alberta clipper, pampero.
Industry:Weather
