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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, ...
1. Those rare occasions when the moon takes on a markedly blue cast. Usually, when the moon (or sun) is seen low in the sky or even high in a polluted one, it is yellowish or reddish. This is a consequence of the greater extinction of short wavelength radiation by small particles and molecules. Yet, there are particle sizes for which extinction is greater for long wavelengths, and the moon seen through a sky populated by such particles is bluish. It is rare that a large population of the particles of the appropriate size (and only that size) are produced, so the blue moon is very rare: 1883 (Krakatoa), 1927 (a late monsoon in India), 1951 (forest fires in Alberta, Canada). Because of the rarity of blue moons, the phrase, “once in a blue moon,” has been used metaphorically since the midnineteenth century to denote a real, but rare event. 2. In recent times, often used incorrectly to mean the occurrence of a second full moon within a calendar month. This strange new use of the term, popularized by a board game in the mid-1980s, arose from a mistake made by the author of a magazine article written in the 1940s. Since two full moons in one month occur fairly regularly, it should not be considered a rare event.
Industry:Weather
1. Thin, new ice on freshwater or saltwater, appearing dark in color because of its transparency, which is a result of its columnar grain structure. On lakes, black ice is commonly overlain by white ice formed from refrozen snow or slush. 2. A mariner's term for a dreaded form of icing sometimes sufficiently heavy to capsize a small ship. 3. A popular alternative for glaze. A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below 0°C. It may also be formed when supercooled fog droplets are intercepted by buildings, fences, and vegetation.
Industry:Weather
1. The water mass at the deepest part of the water column. It is the densest water that is permitted to occupy that position by the regional topography. In the case of a basin, bottom water may be formed locally, or it may represent the densest water that has existed at sill depth in the recent past. 2. Water masses found at the bottom of ocean basins. The most important bottom waters of the World Ocean are Antarctic Bottom Water and Arctic Bottom Water. Baffin Bay Bottom Water has a salinity of 34. 49 and a temperature of −0. 4°C and is found in Baffin Bay below a depth of 1800 m; its low oxygen content of 3. 6 ml l−1 indicates slow water renewal. Japan Sea Bottom Water has a salinity of 34. 1 and a temperature of 0. 04°C; it is formed by winter convection in the northern Japan Sea and occupies the Japan Sea basins at depths below 2000 m.
Industry:Weather
1. The variation with depth of motions associated with variation of density with depth. The baroclinic component of the velocity is the total minus the barotropic component. In a baroclinic state, neutral surfaces are inclined to surfaces of constant pressure. The baroclinic torque vector is proportional to the vector cross product and is responsible for generating vertical shears associated with baroclinic flow. 2. Of, pertaining to, or characterized by baroclinity.
Industry:Weather
1. The sloping margin of a stream or river that confines flow to the natural channel during normal stages. The top of this channel margin may be exceeded during overbank flood flows. 2. A steep slope or face, usually developed in unconsolidated material such as sand or gravel. 3. A shallow area in the sea or other water body, consisting of shifting sediment, and designated by a qualifying word, such as “gravel bank. ”
Industry:Weather
1. The reference line in a measurement by triangulation. In meteorological observations it has several applications, for example, 1) the horizontal distance from the observation point to the location of a ceiling-light projector; 2) the horizontal distance between a ceilometer projector and detector; and 3) the bearing, distance, and slope of the line between the observational points in a double-theodolite observation. 2. In dual-Doppler analysis, the length and bearing of the line separating the radars.
Industry:Weather
1. The obstructing, on a large scale, of the normal west-to-east progress of migratory cyclones and anticyclones. A blocking situation is attended by pronounced meridional flow in the upper levels, often comprising one or more closed anticyclonic circulations at high latitudes and cyclonic circulations at low latitudes (cut-off highs and cut-off lows). This anomalous circulation pattern (the “block”) typically remains nearly stationary or moves slowly westward, and persists for a week or more. Prolonged blocking in the Northern Hemisphere occurs most frequently in the spring over the eastern North Atlantic and eastern North Pacific regions. Compare blocking high. 2. The retardation of stable, low-level, forced upslope flow on the windward side of a mountain or mountain barrier; Smith (1979) asserts that “this windward-side slowing is due to the difficulty that the heavy (cold) surface air has in running upslope. ”
The stable flow is characterized by having a Froude number much less than 1. In cases where a gapless mountain range is long enough to be a complete barrier to the flow, very stable air may be totally blocked or “dammed” (see damming), and the near-surface flow may be diverted back down the slope. If the blocking condition persists for more than a significant portion of a day, a barrier jet can form parallel to the mountain range. “In. . . Flow near an isolated mountain or a ridge with ends or gaps, absolute blocking of flow is not possible. The layer of dense air may pile up slightly ahead of the mountain, but this can be relieved by airflow around the mountain or through gaps in the ridge” (see also gap wind). In stable flow (Froude number less than 1) over an isolated peak, the flow in the lower portions is partially blocked and diverts around the peak, whereas in the upper portion the flow can pass upward over the peak. The boundary between the lower and upper regions has been called the dividing streamline.
Industry:Weather
1. The layer of fluid near a boundary that is affected by friction against that boundary surface, and possibly by transport of heat and other variables across that surface. In meteorology, this is the atmospheric boundary layer. 2. In a physical or mathematical system, a region over which some property or term in the equations varies rapidly, that is, over its full range; conversely, a region outside of which certain terms may be neglected.
Industry:Weather
1. That part of the velocity field that is either uniform with depth or has zero horizontal component of vorticity. In a purely barotropic flow, the pressure gradients and the density gradients are parallel so that their cross product, the baroclinic torque vector, is zero. Variations of fluid density are therefore not directly relevant to barotropic motion. 2. Of, pertaining to, or characterized by a condition of barotropy.
Industry:Weather
1. See river basin. 2. Any body of water not having horizontal communication with the open ocean at all depths. The maximum depth at which there is horizontal communication is the sill depth.
Industry:Weather
