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Backing

A term applied when a forecast wind direction changes in an anti-clockwise sense, i.e. from south back to northeast, via east. The opposite term is veering, thus " southwest 4 veering north 5 or 6 ", would imply a wind originally force 4 from the southwest, becoming a northerly force 5 or 6 by the end of the forecast period, passing via west.

Baroclinic

The temperature along a constant pressure surface (say 500 mbar) varies; a thickness gradient exists. The degree of baroclinicity is given by the product of the layer thermal wind (q.v.) and the Coriolis parameter. For practical purposes, the strength of the thermal wind alone is a good guide.

Baroclinic leaf

An elongated cloud pattern formed within the jet stream zone associated with marked baroclinicity (i.e. strong thermal contrast). The boundary (in satellite imagery) on the polar air-mass side of the development is well defined, and has the look of a 'stretched-out' "S" shape. The downstream/warm air-mass edge is less distinct. This feature represents the initial (or frontogenetic) stage of a system development, certainly in the mid-troposphere, and often (but not always) at the surface. Not all baroclinic leaves lead to marked cyclogenesis although they will be the first stage of such. (See also Dry Intrusion).

Baroclinic zone

Area where there is a marked contrast between cold and warm air masses. Can be determined on a thickness chart by a "packing together" of thickness (q.v.) contours. Usually associated on a msl chart with classical fronts, and therefore an area for potential cyclonic development.

Barotropic

A (theoretical) state in which surfaces of constant pressure and constant temperature coincide at all levels. The atmosphere cannot sustain development, and thickness (q.v.) gradients are zero. If thickness contours are widely spaced (the realistic state), the atmosphere is said to be quasi-barotropic.

BC

(abbr) Patches (as in BCFG in METAR coding, meaning patches of fog).

Beaufort wind scale

This scale was originally devised by Francis Beaufort (later Admiral Sir Francis, Hydrographer of the Royal Navy), who lived from 1774 to 1857. He had a very active naval career and was also interested from an early stage in meteorological observations afloat. In 1805 & 1806, he devised a scale for his own use, which was based upon the amount of canvas a sailing vessel could carry in the given conditions. The scale underwent various modifications and was not introduced into general RN use until the middle part of the 19th century but thereafter, it quickly gained world-wide acceptance. However, various versions developed , and in 1906, the UK Meteorological Office attempted to co-ordinate the usage, and at the same time provided the first definitive wind speed equivalents for each level of 'force', and since 1920, the scale has been used to define forecast wind conditions in the Shipping Forecasts for waters in the NE Atlantic/NW Europe continental shelf. For a description of the current scale in use, see http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm

BECMG

(abbr) "Becoming", used in aerodrome (TAF) issues and others. A permanent change in conditions.

BKN

(abbr) "Broken", 5 to 7 oktas (eighths) of cloud cover.

BL

(abbr) "Blowing", used in connection with snow, sand etc.

Blizzard

[for the U.K. Met Office only - other services will have different criteria and the definition has changed over time; it has not always been so strictly defined] "The simultaneous occurrence of moderate or heavy snowfall with winds of at least force 7, causing drifting snow and reduction of visibility to 200 m or less". (Moderate snow is said to occur when the visibility is 'substantially' impaired, and the snow cover increases in depth at a rate up to about 4 cm per hour. Heavy snow should reduce the visibility to a 'low value' (in the low hundreds of metres), and the snow cover increases at a rate exceeding 4 cm per hour. )

Blocked pattern

Large scale obstruction of the normal west to east progression of surface cyclones in the mid-latitudes. The upper flow changes from predominantly zonal (q.v.) to meridional (q.v.). In a 'meridional' block, the upper flow divides upwind of the block and flows around quasi-stationary vortices - one anticyclonic and the other cyclonic. In the 'omega' block case the strongest flow is diverted to lower latitudes, leaving a slow-moving anticyclonic vortex on the poleward flank of the displaced zonal flow.

Blocking anticyclone

Acellular pattern of high pressure in the mid-latitudes, which diverts or prevents the normal west-to-east motion of depressions (areas of low pressure).

Bomb

A name applied to mid-latitude depressions which deepen violently. The term was coined by Sanders & Gyakum (US Monthly Weather Review), in a 1980 paper dealing with such events, and requires a pressure fall in the depression centre of 24 hPa (or mbar) or more in 24 hours at latitude 60degN for the name to be applied. At latitude 45degN, the required value is 19hPa, and at 55degN, 23hPa. (see Explosive cyclogenesis.)

Boundary layer

In operational/synoptic meteorology this is usually taken to be the layer at the bottom of the atmosphere wherein surface friction is important. It can vary in depth from as little as 100 m or less on a still, cold night to upwards of 1 km or more in a windy, well-mixed situation. Also known variously as the 'mixed layer' or 'friction layer' and is a function of wind speed, vertical temperature profile (i.e. stability) and surface roughness. (N.B: micrometeorologists regard the boundary layer as the first few cm of the lower atmosphere and this can lead to confusion when reading some texts. )

BR

Mist (abbr. from French): vis >=1000m and not more than 5000m, where obscuration is caused by water droplets in suspension. Used in aviation reports, forecasts etc.

Bright band effect

As snow descends through the melting (or freezing) level, the melting snowflakes 'look like' huge raindrops causing radar reflectance to increase sharply, implying heavier precipitation than is actually occurring. Corrections can be applied, provided the calibration system has some knowledge of the vertical temperature profile. The effect is usually confined to a layer about 1000 ft (300 m) thick.

Buys Ballot's Law

As originally formulated, " if you stand with your back to the wind (in the northern hemisphere), then low pressure lies on your left-hand side ". [ For the southern hemisphere, then the low pressure is on the right-hand side. ] (Personally, I feel we should face the wind to see what is coming, so I would reverse these, i.e. face the wind, low pressure on right etc.)

[ Prof. C.H.D. Buys Ballot (1817-1890) was a famous Dutch Meteorologist who founded the Royal Netherlands Meteorological Institute (KNMI) in 1854 and played an important role in establishing the first international organisation for meteorology - after the inaugural meeting of the International Meteorological Congress in Vienna in 1873. However, he is now best known for the 'Law' as above; it is possible that the credit for a rigorous treatment of the physics might be credited to Alexander Buchan, long-serving (1860-1907) Secretary of the Scottish Meteorological Society. ]