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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • backing - 1. According to general internationally accepted usage, a change in wind direction in a counterclockwise sense (e.g., south to southeast to east) in either hemisphere of the earth; the opposite of veering. 2. According to widespread usage among U.S. meteorologists, a change in wind direction in a counterclockwise sense in the Northern Hemisphere, clockwise in the Southern Hemisphere; the opposite of veering.

  • band - 1. A range of wavelengths. 2. Frequency band. 3. Absorption band. 4. A range of radar frequencies, such as X band, S band.

  • bar - A unit of pressure equal to 106 dyne cm-2 (106 barye), 1000 millibars, 29.53 inches of mercury.

  • barrier - In polar terminology, an early term for ice shelf; first used by Sir James Clark Ross for the face of the Antarctic ice shelf later named for him, Ross Barrier.

  • basin - 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.

  • BL - Abbreviation for boundary layer.

  • blizzard - A severe weather condition characterized by high winds and reduced visibilities due to falling or blowing snow. The U.S. National Weather Service specifies a wind of 30 knots (35 miles per hour) or greater, sufficient snow in the air to reduce visibility to less than 400 m (0.25 miles). Earlier definitions also included a condition of low temperatures, on the order of -7°C (20°F) or lower, or -12°C (10°F) or lower (severe blizzard). The name originated in the United States but it is also used in other countries. In the Antarctic the name is given to violent autumnal winds off the ice cap. In southeastern France, the cold north wind with snow is termed blizzard (see also boulbie). Similar storms in Russian Asia are the buran and purga. In popular usage in the United States and in England, the term is often used for any heavy snowstorm accompanied by strong winds.

  • block - Same as blocking high.

  • blocking - 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.

  • blowing snow - Snow lifted from the surface of the earth by the wind to a height of 2 m (6 ft) or more above the surface (higher than drifting snow), and blown about in such quantities that horizontal visibility is reduced to less than 11 km (about 7 statute miles). As an obstruction to vision, it is encoded BS in a surface aviation weather observation and as BLSN as an obstruction to vision in a METAR or SPECI observation. Blowing snow can be falling snow or snow that already accumulated but is picked up and blown about by strong winds. It is one of the classic requirements for a blizzard.

  • boundary layer - 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.

  • break - 1. A sudden change in the weather; usually applied to the end of an extended period of unusually hot, cold, wet, or dry weather. 2. A hole or gap in a layer of clouds (see breaks in overcast). 3. See windbreak.

  • breeze - 1. In general, a light wind. In the Beaufort wind scale, this is a wind between 4 and 10 kt (4 and 12 mph). 2. In the Beaufort wind scale (Beaufort wind force numbers 2-6), a wind speed ranging from 4 to 27 kt (4 to 31 mph) and categorized as follows: light breeze, 4-6 kt; gentle breeze, 7-10 kt; moderate breeze, 11-16 kt; fresh breeze, 17-21 kt; and strong breeze, 22-27 kt.

  • broken - Descriptive of a sky cover of from 0.6 to 0.9 (to the nearest tenth). This is applied only when obscuring phenomena aloft are present, that is, not when the sky cover is composed entirely of surface-based obscuring phenomena. In aviation weather observations, a broken sky cover may be explicitly identified as thin (predominantly transparent); otherwise a predominantly opaque status is implicit. An opaque broken sky cover is the minimum requirement for a ceiling, and this is frequently termed broken ceiling.

  • buoyancy - 1. That property of an object that enables it to float on the surface of a liquid, or ascend through and remain freely suspended in a compressible fluid such as the atmosphere. Quantitatively, it may be expressed as the ratio of the specific weight of the fluid to the specific weight of the object; or, in another manner, by the weight of the fluid displaced minus the weight of the object. 2. (Or buoyant force, buoyancy force; also called Archimedean buoyant force.) The upward force exerted upon a parcel of fluid (or an object within the fluid) in a gravitational field by virtue of the density difference between the parcel (or object) and that of the surrounding fluid. The magnitude of the buoyancy force F per unit mass may be determined by Archimedes's principle as F = g where g is the acceleration of gravity, r[&rgr;] the density of the buoyed fluid parcel or object, and r[&rgr;]0 the density of the surrounding fluid. In the atmosphere, a buoyant force on an air parcel may be attributed directly to a local increase of temperature and may be written F = g where T and T0 are the temperatures of the heated air and that of the environment, respectively. The coefficient (T/T0 - 1) is sometimes called the buoyancy factor. The force F is sometimes called the reduced gravity. See free convection.

  • burn off - With reference to fog or low stratus cloud layers, to dissipate by heating from the sun, primarily during the early morning hours.

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