Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

B

Back-building Thunderstorm - A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.

Backing Winds - Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly 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 southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.

Back-Door Cold Front - A cold front that moves south-southwest out of Canada into the Northeastern United States along the Atlantic Seaboard.

Backing Wind - A wind that changes direction in a counter-clockwise sense with respect to time (north to northwest to west). A backing wind with respect to height usually signifies the arrival of colder air.

Barometer - An instrument that measures air pressure.The barometric pressure is Colorado is always going to be lower than the pressure at sea level, due to our higher elevation - there simply is less air up here. In general, Denver's pressure is about 15% lower than at Sea Level. In Leadville, it is about 25% lower! In meteorology, we "correct" the pressure readings to read as if we were at Sea Level. This is done by convention by request of the World Meteorological Organization is order for us to get an accurate depiction of the "relative surface pressure". In other words, for forecasting it is more important to pinpoint the areas of higher and lower pressure due to storm systems. The absolute pressure is useful for other applications (such as aviation or drag racing - for manifold pressure ), but in weather we need to pick out the storm systems. If we did not correct the pressure to Sea Level, Denver would always be a permanent low-pressure area compared to Los Angeles, New York, Miami, etc.

Baroclinic Zone - A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.

Barotropic System - A weather system in which 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 unfavorable areas for severe thunderstorm development.
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 cutoff 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.

Bear's Cage - [Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm.
The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.

Bora Wind - It is a down-sloping wind along the Front Range, but it is different from the Chinook. Bora winds are very cold winds as they typically form just after a cold front has passed through the region. The pressure gradient is very tight after the front passes by and the winds can reach speeds of 50-60 mph. Despite the warming caused by the air sinking down from the mountains, the Bora is still a cold wind because the air was so chilly to begin with. In addition, Bora winds come in from the northwest instead of the west, so the mountains are not as tall over southern Wyoming and Larimer county.

Boundary Layer - In general, a layer of air adjacent to a bounding surface. Specifically, 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 to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime 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 meters 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 that is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.

BRN - See Bulk Richardson Number.

Bubble High - A meso-scale 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 (or 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 favorable for supercell development.

BWER - Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extend upward into, and is 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.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society