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
direction with time at a given location (e.g. from southerly to
south-easterly), or change direction in a counter clockwise 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
south-easterly direction. Backing of the surface wind can increase
the potential for tornado development by increasing the directional
shear at low levels.
Back-sheared Anvil - [Slang], a thunderstorm anvil which
spreads upwind, against the flow aloft. A back-sheared anvil often
implies a very strong updraft and a high severe weather potential.
Barber Pole - [Slang], a thunderstorm updraft with a visual
appearance including cloud striations that are curved in a manner
similar to the stripes of a barber pole. The structure typically is
most pronounced on the leading edge of the updraft, while drier air
from the rear flank downdraft often erodes the clouds on the
trailing side of the updraft.
Baroclinic Zone - A region in which a temperature gradient
exists on a constant pressure surface. Baroclinic zones are favoured
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 unfavourable areas for severe thunderstorm development. See
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
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.
Beaver('s) 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 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
Blue Watch (or Blue Box) - [Slang],
a severe thunderstorm
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 kilometres 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 meters thick, but
again the exact depth is indeterminate. Like friction, the effects
of insolation and radiational cooling are strongest within this
Bow Echo - A radar echo which is linear but bent outward in a
bow shape (Fig. 1). Damaging straight-line winds often occur near
the "crest" or centre 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.