Weather Glossary - B
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Back building thunderstormA thunderstorm in which new development takes place on the upwind side (usually the west or northwest side), such that the storm seems to remain stationary or propagate in a backward direction. Back-building thunderstorms can lead to severe flash flooding. |
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Back sheared anvil[Slang] A thunderstorm anvil which spreads upwind, against the upper-level flow. A back-sheared anvil often implies a very strong updraft and hence a possibly severe thunderstorm. |
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Backing windsWinds that shift in an anticlockwise direction with time at a given location (e.g. from southerly to south-easterly), or change direction in an anticlockwise sense with height (e.g. easterly at the surface and becoming more northerly aloft). The opposite of veering winds. |
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BackscatterBackscatter relates to radar signals being reflected off targets other than precipitation such as aeroplanes, topography (mountains and hills), dust (for example, radars in a desert locatality may suffer from dust echoes), birds and swarms of insects. In addition, occasionally the atmosphere is such that radar signals may be reflected off a broadscale temperature inversion such as is the case with a synoptic scale anticyclone. Some of the false echoes mentioned are transient (such as aeroplane reflections, dust and temperature inversions) while others may be a persistent feature relating to the radar site (eg. A mountain or hill). |
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Ball lightningA rare form of long-lived lightning that appears as a small, glowing ball. Ball lightning has been reported to pass through solid objects such as windows and walls without dissipating. The actual process by which ball lightning forms is still largely unknown. |
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BaroclinicA region in which a temperature (or density) gradient exists on a constant pressure surface. Baroclinic zones are favoured areas for strengthening and weakening synoptic scale weather systems. |
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BarometerA device used to measure atmospheric pressure. The two most common barometers are the mercury barometer and the aneroid barometer. The mercury barometer was initially developed by Evangelista Torricelli in 1644. |
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BarotropicA situation where temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavourable areas for severe thunderstorm development. See also baroclinic. |
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Beaufort ScaleOne of the first scales to estimate wind speeds and the effects was created by Britain's Admiral Sir Francis Beaufort (1774-1857). He developed the scale in 1805 to help sailors estimate the winds via visual observations. The scale starts with 0 and goes to a force of 12.
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Beaver tail[Slang] A particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or northeast to southwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or southwest. Its size and shape changes as the strength of the inflow changes. See also inflow stinger. |
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Black FrostBlack frost occurs when the dew point is too low for frost to form. That is the temperature is below zero but there is not enough moisture in the air to produce a visible frost. The reason it is called a black frost is that shortly after a black frost the leaves of frost susceptible plants will turn black and die. |
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BlizzardFalling and/or blowing snow with winds exceeding 30 knots, temperatures below -7°C and visibility less than 400m for more than 3 hours. |
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Blocking highAn intense or strong high pressure system that occasionally forms much further south than usual and remains almost stationary for days, weeks or even months. Blocking highs are most common in the southern Tasman Sea and tend to be paired with a strong cut-off low. |
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BoMAbbreviation for Bureau of Meteorology, the Australian government department responsible for the issuing of weather forecasts, warnings and taking observations across Australia. |
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BombAn extratropical cyclone (low) where the central pressure drops by at least 1hPa per hour for 24 hours. Bombs occur where there is an upper level trough and a strong low level moisture and temperature gradient. This often occurs between warm ocean water and colder adjacent landmass. These conditions can be found in the northwest Atlantic and on the east coast of Australia where they are known as an East Coast Low (ECL). Bombs obtain much of their energy from the release of latent heat and can sometimes exhibit features similar to tropical cyclones. |
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Boundary layerIn general, a layer of air adjacent to a bounding surface. |
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Bow EchoA radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or centre of a bow echo. Areas of circulation can also develop at either end of a bow echo, which sometimes can lead to tornado genesis - especially in the right (usually southern) end, where the circulation exhibits cyclonic rotation. |
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BrokenDescribes cloud cover when between 5/8ths to 7/8ths of the sky is obscured by cloud. |
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Bubble HighA mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air. |
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Bulk Richardson Number(BRN) A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favourable for supercell development. |
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BuoyancyIn meteorology the upward force exerted upon a parcel of air by virtue of the density difference between itself and the surrounding air. |
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Bust[Slang] An inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur. |
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Buys-Ballot's LawThe law describing the relationship between the wind direction and the pressure distribution. In the southern hemisphere, if one stands with their back to the wind, then the low pressure centre is approximately to the right-hand side and higher pressure on the left. Local topographic effects can affect this (sea breezes, valley winds and such). |
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BWERBounded Weak Echo Region (also known as a vault). A radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into and surrounded by higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. See WER. |
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Weather News
Thunderstorm hits Darwin as Ex-Tropical Cyclone Esther traverses the NT
10:55 EDT
The remnant low pressure system that was formerly Tropical Cyclone Esther is passing over the Northern Territory, causing heavy rain and severe thunderstorms.
St George flooding expected to affect nearly 20 homes when Balonne River peaks
10:07 EDT
Nearly 20 homes in low-lying areas of St George in Queensland's southern inland are at risk of flooding when the Balonne River reaches its expected peak later today.
Canberra's urban forest threatened by extreme heat, drought — and drivers parking on nature strips
09:55 EDT
If you park your car on the nature strip, beware — you're killing Canberra's street trees and the ACT Government will track you down.
