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Weather Glossary - B


Back building thunderstorm

A 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.

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.

Backing winds

Winds 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.

In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or south-easterly direction. See also Veering winds.


Backscatter 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).

Ball lightning

A 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.


A 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.

Baroclinic instability is a situation that results from the tight thermal gradients when air parcels are unstable to slantwise forcing. That is - forced motion in certain directions both vertically and horizontally will result in an acceleration of this motion. An air parcel in this situation will usually move along an isentropic surface but can also be subject to other forms of forcing dependent on the situation. Baroclinic instability is a key ingredient in mid-latitude cyclogenesis.Baroclinic zones are also important driving mechanisms of many of the oceans currents.


A 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.


A 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.

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere and winds do not change direction with height.

As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cut-off lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.

Beaufort Scale

One 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.

Force Wind (Knots) Description Effects On the Water Effects On Land
0 Less than 1 Calm Sea surface smooth and mirror-like Calm, smoke rises vertically
1 1-3 Light Air Scaly ripples, no foam crests Smoke drift indicates wind direction, still wind vanes
2 4-6 Light Breeze Small wavelets, crests glassy, no breaking Wind felt on face, leaves rustle, vanes begin to move
3 7-10 Gentle Breeze Large wavelets, crests begin to break, scattered whitecaps Leaves and small twigs constantly moving, light flags extended
4 11-16 Moderate Breeze Small waves 1-4 ft. becoming longer, numerous whitecaps Dust, leaves, and loose paper lifted, small tree branches move
5 17-21 Fresh Breeze Moderate waves 4-8 ft taking longer form, many whitecaps, some spray Small trees in leaf begin to sway
6 22-27 Strong Breeze Larger waves 8-13 ft, whitecaps common, more spray Larger tree branches moving, whistling in wires
7 28-33 Near Gale Sea heaps up, waves 13-20 ft, white foam streaks off breakers Whole trees moving, resistance felt walking against wind
8 34-40 Gale Moderately high (13-20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks Whole trees in motion, resistance felt walking against wind
9 41-47 Strong Gale High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility Slight structural damage occurs, slate blows off roofs
10 48-55 Storm Very high waves (20-30 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility Seldom experienced on land, trees broken or uprooted, "considerable structural damage"
11 56-63 Violent Storm Exceptionally high (30-45 ft) waves, foam patches cover sea, visibility more reduced
12 64+ Hurricane Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced

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.

One should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.

An inflow band attached to a supercell thunderstorm. Note that the inflow band feeds into the thunderstorm at a level above the base of the storm.

Black Frost

Black 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.


Falling and/or blowing snow with winds exceeding 30 knots, temperatures below -7C and visibility less than 400m for more than 3 hours.

Blocking high

An 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.


Abbreviation for Bureau of Meteorology, the Australian government department responsible for the issuing of weather forecasts, warnings and taking observations across Australia.


An 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.

Boundary layer

In general, a layer of air adjacent to a bounding surface.

The term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the Earth, this layer is considered roughly the lowest kilometre of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and night-time radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.

There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 metres thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.

Bow Echo

A 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.


Describes cloud cover when between 5/8ths to 7/8ths of the sky is obscured by cloud.

Bubble High

A 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.

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.


In meteorology the upward force exerted upon a parcel of air by virtue of the density difference between itself and the surrounding air.


[Slang] An inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.

Buys-Ballot's Law

The 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).


Bounded 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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