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BACKING: A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is clockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the northeast to the north to the northwest. It is the opposite of veering.
BACKSCATTER: A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. Also the scattering of radiant energy into space before it reaches the earth's surface.
BALL LIGHTNING: A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air.; Related term: globe lightning
BAROCLINITY: The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. Also known as baroclinicity. An example is the tight temperature gradient along the East Coast of the United States during the winter that gives rise to intense cyclogenesis.
BAROGRAPH: An instrument that continuously records a barometer's reading of atmospheric pressure.; Related term: aneroid barometer
BAROMETER: An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.
BAROMETRIC PRESSURE: The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).; Related term: atmospheric pressure
BAROTROPY: The state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. It is considered zero baroclinity.
BARRIER WINDS: Refers to the westerly flow of air along the northern slope of the Brooks Range in northern Alaska that precedes the arrival of colder air from the north.
BATHYTHERMOGRAPH: A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referred to as a B.T.
BEAUFORT WIND SCALE: A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.
BELLOT WINDS: Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somerset Island and the Boothia Peninsula, connecting the Gulf of Boothia and Franklin Strait.
BERMUDA HIGH: A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores Islands.; Related term: Azores High
BERNOULLI'S THEOREM: A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. Theorized by Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist.
BIOSPHERE: The transition zone between the earth and the atmosphere within which most terrestrial life forms are found. It is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere.
BLACK BLIZZARD: A local term for a violent duststorm on the south-central Great Plains that darkens the sky and casts a pall over the land.; Related term: black roller
BLACK ICE: Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.
BLIZZARD: A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10Â°F, winds exceeding 45 mph, and visibility reduced by snow to near zero.
BLOCKING HIGH: The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. It prevents the movement of migratory cyclones across its latitudes.; Related terms: cut-off high and Omega block
BLOWING DUST: Dust that is raised by the wind to heights of six feet or greater. It is reported as "BLDU" in an observation and on the METAR.
BLOWING SAND: Sand that is raised by the wind to heights of six feet or greater. It is reported as "BLSA" in an observation and on the METAR.
BLOWING SNOW: Snow that is raised by the wind to heights of six feet or greater. It is reported as "BLSN" in an observation and on the METAR.
BLOWING SPRAY: Salt spray that is raised by the wind to heights of six feet or greater. It is reported as "BLPY" in an observation and on the METAR.
BLUE NORTHER: Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-black sky with strong wintery winds from the northwest or north and temperatures that may drop 20Â°F to 30Â°F in a few minutes.; Related term: Texas Norther
BOILING POINT: The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100Â°C or 212Â°F.
BOULDER WIND: A local name referring to an extremely strong downslope wind in the Front Range of the Rocky Mountains near Boulder, Colorado.
BOUNDARY LAYER: The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.; Related terms: surface boundary layer and friction layer
BOW ECHO: A radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. It is bent outward in a "bow" shape.
BOYLE'S LAW: States that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert Boyle (1627-1691), an Irish physicist and chemist and co-founder of the Royal Society.
BRIGHT BAND: A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest.
BROKEN: The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer.
BUBBLE HIGH: A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. Convergence along the leading edge of a bubble high may help form additional thunderstorms.; Related term: meso high
BUYS BALLOT'S LAW: Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere. This law was named after the Dutch meteorologist, Buys Ballot, who developed the formula in 1857.
BWER: Acronym for Bounded Weak Echo Region. Refers to radar echo signatures with low reflectivity in the center, surrounded by higher reflectivity. It is associated with strong updrafts and is found in the inflow region of a thunderstorm.; Related term: vault