This lab exercise was excerpted on 23 Jan 2007 from
NOAA Technical Memorandum NWS SR-145
A COMPREHENSIVE GLOSSARY OF
WEATHER TERMS FOR STORM SPOTTERS
Michael Branick
NOAA/NWS/WFO Norman
http://www.srh.noaa.gov/oun/severewx/glossary.php
-A-
AC - Convective outlook issued by the SPC. Abbreviation for Anticipated
Convection; the term originates from the header coding [ACUS1] of the
transmitted product. See SWODY1, SWODY2.
ACCAS (usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level
clouds (bases generally 8 to 15 thousand feet), of which at least a
fraction of their upper parts show cumulus-type development. These
clouds often are taller than they are wide, giving them a turret-shaped
appearance. ACCAS clouds are a sign of instability aloft, and may
precede the rapid development of thunderstorms.
Accessory Cloud - A cloud which is dependent on a larger cloud system
for development and continuance. Roll clouds, shelf clouds, and wall
clouds are examples of accessory clouds.
Advection - Transport of an atmospheric property by the wind. See cold advection, moisture advection, warm advection.
Air-mass Thunderstorm - Generally, a thunderstorm not associated with a
front or other type of synoptic-scale forcing mechanism. Air mass
thunderstorms typically are associated with warm, humid air in the
summer months; they develop during the afternoon in response to
insolation, and dissipate rather quickly after sunset. They generally
are less likely to be severe than other types of thunderstorms, but
they still are capable of producing downbursts, brief heavy rain, and
(in extreme cases) hail over 3/4 inch in diameter. See popcorn
convection.
Since all thunderstorms are associated with some type of forcing
mechanism, synoptic-scale or otherwise, the existence of true air-mass
thunderstorms is debatable. Therefore the term is somewhat
controversial and should be used with discretion.
Algorithm - A computer program (or set of programs) which is designed
to systematically solve a certain kind of problem. WSR-88D radars
(NEXRAD) employ algorithms to analyze radar data and automatically
determine storm motion, probability of hail, VIL, accumulated rainfall,
and several other parameters.
Anticyclonic Rotation - Rotation in the opposite sense as the Earth's
rotation, i.e., clockwise in the Northern Hemisphere as would be seen
from above. The opposite of cyclonic rotation.
Anvil - The flat, spreading top of a Cb (cumulonimbus), often shaped
like an anvil. Thunderstorm anvils may spread hundreds of miles
downwind from the thunderstorm itself, and sometimes may spread upwind
(see back-sheared anvil).
Anvil Crawler - [Slang], a lightning discharge occurring within the
anvil of a thunderstorm, characterized by one or more channels that
appear to crawl along the underside of the anvil. They typically appear
during the weakening or dissipating stage of the parent thunderstorm,
or during an active MCS.
Anvil Dome - A large overshooting top or penetrating top.
Anvil Rollover - [Slang], a circular or semicircular lip of clouds
along the underside of the upwind part of a back-sheared anvil,
indicating rapid expansion of the anvil. See cumuliform anvil,
knuckles, mushroom.
Anvil Zits - [Slang], frequent (often continuous or nearly continuous),
localized lightning discharges occurring from within a thunderstorm
anvil.
AP - Anomalous Propagation. Radar term for false (non-precipitation)
echoes resulting from nonstandard propagation of the radar beam under
certain atmospheric conditions.
Approaching (severe levels) - A thunderstorm which contains winds of 35
to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than
3/4 inch in diameter. See severe thunderstorm.
Arcus - A low, horizontal cloud formation associated with the leading
edge of thunderstorm outflow (i.e., the gust front). Roll clouds and
shelf clouds both are types of arcus clouds.
AVN - AViatioN model; one of the operational forecast models run at
NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800
GMT. As of fall 1996, forecast output was available operationally out
to 120 hours only from the 0000 and 1200 runs. At 0600 and 1800, the
model is run only out to 72 hours.
-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-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. (See Fig. 7,
supercell.)
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 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. 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.
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 pseudo-warm front.
Blue Watch (or Blue Box) - [Slang], a severe thunderstorm watch.
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 which is linear but bent outward in a bow shape
(Fig. 1). 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.
Box (or Watch Box) - [Slang], a severe thunderstorm or tornado watch. See blue box, red box.
BRN - See Bulk Richardson Number.
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 (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.
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.
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 extends upward into, and is
surrounded by, higher reflectivities aloft (Fig. 2). 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.
.