Weather Glossary: B's

Winds which shift in a counterclockwise direction with time at a given location (e.g. from south to southeast), or change direction in a counterclockwise sense with height (example: west wind 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.

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. See Baroclinic Zone.

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.

This WSR-88D radar product depicts a full 360° sweep of echo intensity data. It is available for every elevation angle that is sampled in a volume scan.

It is used to observe precipitation intensity and movement; determine storm structure; estimate hail potential; locate boundaries (cold front, outflow, lake breeze, etc.); identify cloud layers; and detect light snow, drizzle, birds, insects, and smoke plumes.

This WSR-88D radar product depicts a full 360° sweep of radial velocity data. It is available for every elevation angle that is sampled in a volume scan.

It is used to estimate wind speed and direction; determine regions of significant shear (convergence, etc.); locate boundaries (cold front, outflow, lake breeze, etc.); identify areas of circulation; and determine storm structure.

Slang for 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 southeast to northwest.

As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger.

Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically 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.

A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America 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 in the eastern part of the North Atlantic. Then it may be referred to as the Azores High.

• Sustained winds or frequent gusts to 35 miles per hour (56 km/h) or greater, and
• Considerable falling and/or blowing snow reducing visibility to less than 1/4 mile (0.4 kilometer).

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.

A radar feature that identifies where the strongest updraft is located in a supercell thunderstorm. This updraft is so strong that large particles do not have time to form in the lower- and mid-levels of the thunderstorm and they are prevented from falling back into the updraft core from updraft.

The weak echo region is bounded when, in a horizontal section, the weak echo region is completely surrounded or bounded by higher reflectivity values.

It is the ratio of the buoyancy (CAPE) of a lifted parcel to the vertical wind shear of the environment in which the parcel is lifted. It correlates well with observed storm type (single, multicell, supercell), especially for CAPEs between 1,500 and 3,000 J/kg.

BRN's less than 45 tend to support supercell structures, but multicellular convection is favored over 45. While the BRN has shown some value as a predictor of storm type, it is a poor predictor of storm rotation because BRN Shear is a "bulk" measure.

For example, it does not take in account the specific effects of directional and speed shear components. High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear.

Buttress dams are comprised of reinforced masonry or stonework built against concrete. They are usually in the form of flat decks or multiple arches. They require about 60% less concrete than gravity dams, but the increased form work and reinforcement steel required usually offset the savings in concrete.

Many were built in the 1930's when the ratio of labor cost to materials was comparatively low. However, this type of construction is not competitive with other types of dams when labor costs are high.