This lab exercise was excerpted on 25 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
-C-
CA - Cloud-to-Air lightning.
Cap (or Capping Inversion) - A layer of relatively warm air aloft
(usually several thousand feet above the ground) which suppresses or
delays the development of thunderstorms. Air parcels rising into this
layer become cooler than the surrounding air, which inhibits their
ability to rise further. As such, the cap often prevents or delays
thunderstorm development even in the presence of extreme instability.
However if the cap is removed or weakened, then explosive thunderstorm
development can occur. See CIN and Fig. 6, sounding.
The cap is an important ingredient in most severe thunderstorm
episodes, as it serves to separate warm, moist air below and cooler,
drier air above. With the cap in place, air below it can continue to
warm and/or moisten, thus increasing the amount of potential
instability. Or, air above it can cool, which also increases potential
instability. But without a cap, either process (warming/moistening at
low levels or cooling aloft) results in a faster release of available
instability - often before instability levels become large enough to
support severe weather development.
CAPE - Convective Available Potential Energy. A measure of the amount
of energy available for convection. CAPE is directly related to the
maximum potential vertical speed within an updraft; thus, higher values
indicate greater potential for severe weather. Observed values in
thunderstorm environments often may exceed 1,000 joules per kilogram
(j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with
other indices or indicators, there are no threshold values above which
severe weather becomes imminent. CAPE is represented on a sounding by
the area enclosed between the environmental temperature profile and the
path of a rising air parcel, over the layer within which the latter is
warmer than the former. (This area often is called positive area.) See
also CIN and Fig. 6, sounding.
*Cb - Cumulonimbus cloud, characterized by strong vertical development
in the form of mountains or huge towers topped at least partially by a
smooth, flat, often fibrous anvil. Also known colloquially as a
"thunderhead."
CC - Cloud-to-Cloud lightning.
Cell - Convection in the form of a single updraft, downdraft, or
updraft/downdraft couplet, typically seen as a vertical dome or tower
as in a cumulus or towering cumulus cloud. A typical thunderstorm
consists of several cells (see multi-cellular thunderstorm).
The term "cell" also is used to describe the radar echo returned
by an individual shower or thunderstorm. Such usage, although common,
is technically incorrect.
*CG - Cloud-to-Ground lightning flash.
Chaff - Small strips of metal foil, usually dropped in large quantities
from aircraft or balloons. Chaff typically produces a radar echo which
closely resembles precipitation. Chaff drops once were conducted by the
military in order to confuse enemy radar, but now are conducted mainly
for radar testing and calibration purposes.
CIN - Convective INhibition. A measure of the amount of energy needed
in order to initiate convection. Values of CIN typically reflect the
strength of the cap. They are obtained on a sounding by computing the
area enclosed between the environmental temperature profile and the
path of a rising air parcel, over the layer within which the latter is
cooler than the former. (This area sometimes is called negative area.)
See CAPE and Fig. 6, sounding.
Cirrus - High-level clouds (16,000 feet or more), composed of ice
crystals and appearing in the form of white, delicate filaments or
white or mostly white patches or narrow bands. Cirrus clouds typically
have a fibrous or hairlike appearance, and often are semi-transparent.
Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds
are not associated with thunderstorms.
Classic Supercell - See supercell.
Clear Slot - A local region of clearing skies or reduced cloud cover,
indicating an intrusion of drier air; often seen as a bright area with
higher cloud bases on the west or southwest side of a wall cloud. A
clear slot is believed to be a visual indication of a rear flank
downdraft.
Closed Low - A low pressure area with a distinct center of cyclonic
circulation which can be completely encircled by one or more isobars or
height contour lines. The term usually is used to distinguish a low
pressure area aloft from a low-pressure trough. Closed lows aloft
typically are partially or completely detached from the main westerly
current, and thus move relatively slowly (see cutoff low).
Cloud Streets - Rows of cumulus or cumulus-type clouds aligned parallel
to the low-level flow. Cloud streets sometimes can be seen from the
ground, but are seen best on satellite photographs.
Cloud Tags - Ragged, detached cloud fragments; fractus or scud.
Cold Advection - Transport of cold air into a region by horizontal winds.
Cold-air Funnel - A funnel cloud or (rarely) a small, relatively weak
tornado that can develop from a small shower or thunderstorm when the
air aloft is unusually cold (hence the name). They are much less
violent than other types of tornadoes.
Cold Pool - A region of relatively cold air, represented on a weather
map analysis as a relative minimum in temperature surrounded by closed
isotherms. Cold pools aloft represent regions of relatively low
stability, while surface-based cold pools are regions of relatively
stable air.
Collar Cloud - A generally circular ring of cloud that may be observed
on rare occasions surrounding the upper part of a wall cloud. See Fig.
7, supercell.
This term sometimes is used (incorrectly) as a synonym for wall cloud.
Comma Cloud - A synoptic scale cloud pattern with a characteristic
comma-like shape, often seen on satellite photographs associated with
large and intense low-pressure systems.
Comma Echo - A thunderstorm radar echo which has a comma-like shape. It
often appears during latter stages in the life cycle of a bow echo (see
Fig. 1).
Condensation Funnel - A funnel-shaped cloud associated with rotation
and consisting of condensed water droplets (as opposed to smoke, dust,
debris, etc.). Compare with debris cloud.
Confluence - A pattern of wind flow in which air flows inward toward an
axis oriented parallel to the general direction of flow. It is the
opposite of difluence. Confluence is not the same as convergence. Winds
often accelerate as they enter a confluent zone, resulting in speed
divergence which offsets the (apparent) converging effect of the
confluent flow.
Congestus (or Cumulus Congestus) - same as towering cumulus.
Convection - Generally, transport of heat and moisture by the movement
of a fluid. In meteorology, the term is used specifically to describe
vertical transport of heat and moisture, especially by updrafts and
downdrafts in an unstable atmosphere. The terms "convection" and
"thunderstorms" often are used interchangeably, although thunderstorms
are only one form of convection. Cbs, towering cumulus clouds, and
ACCAS clouds all are visible forms of convection. However, convection
is not always made visible by clouds. Convection which occurs without
cloud formation is called dry convection, while the visible convection
processes referred to above are forms of moist convection.
Convective Outlook (sometimes called AC) - A forecast containing the
area(s) of expected thunderstorm occurrence and expected severity over
the contiguous United States, issued several times daily by the SPC.
The terms approaching, slight risk, moderate risk, and high risk are
used to describe severe thunderstorm potential. Local versions
sometimes are prepared by local NWS offices.
Convective Temperature - The approximate temperature that the air near
the ground must warm to in order for surface-based convection to
develop, based on analysis of a sounding. See Fig. 6.
Calculation of the convective temperature involves many
assumptions, such that thunderstorms sometimes develop well before or
well after the convective temperature is reached (or may not develop at
all). However, in some cases the convective temperature is a useful
parameter for forecasting the onset of convection.
Convergence - A contraction of a vector field; the opposite of
divergence. Convergence in a horizontal wind field indicates that more
air is entering a given area than is leaving at that level. To
compensate for the resulting "excess," vertical motion may result:
upward forcing if convergence is at low levels, or downward forcing
(subsidence) if convergence is at high levels. Upward forcing from
low-level convergence increases the potential for thunderstorm
development (when other factors, such as instability, are favorable).
Compare with confluence.
Core Punch - [Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm.
Core punching is not a recommended procedure for storm spotting.
Cumuliform Anvil - A thunderstorm anvil with visual characteristics
resembling cumulus-type clouds (rather than the more typical fibrous
appearance associated with cirrus). A cumuliform anvil arises from
rapid spreading of a thunderstorm updraft, and thus implies a very
strong updraft. See anvil rollover, knuckles, mushroom.
Cumulus - Detached clouds, generally dense and with sharp outlines,
showing vertical development in the form of domes, mounds, or towers.
Tops normally are rounded while bases are more horizontal. See Cb,
towering cumulus.
Cumulus Congestus (or simply Congestus) - Same as towering cumulus.
Cutoff Low - A closed low which has become completely displaced (cut
off) from basic westerly current, and moves independently of that
current. Cutoff lows may remain nearly stationary for days, or on
occasion may move westward opposite to the prevailing flow aloft (i.e.,
retrogression).
"Cutoff low" and "closed low" often are used interchangeably to
describe low pressure centers aloft. However, not all closed lows are
completely removed from the influence of the basic westerlies.
Therefore, the recommended usage of the terms is to reserve the use of
"cutoff low" only to those closed lows which clearly are detached
completely from the westerlies.
Cyclic Storm - A thunderstorm that undergoes cycles of intensification
and weakening (pulses) while maintaining its individuality. Cyclic
supercells are capable of producing multiple tornadoes (i.e., a tornado
family) and/or several bursts of severe weather.
A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.
Cyclogenesis - Development or intensification of a low-pressure center (cyclone).
*Cyclonic Circulation (or Cyclonic Rotation) - Circulation (or
rotation) which is in the same sense as the Earth's rotation, i.e.,
counterclockwise (in the Northern Hemisphere) as would be seen from
above. Nearly all mesocyclones and strong or violent tornadoes exhibit
cyclonic rotation, but some smaller vortices, such as gustnadoes,
occasionally rotate anticyclonically (clockwise). Compare with
anticyclonic rotation.
-D-
dBZ - Nondimensional "unit" of radar reflectivity which represents a
logarithmic power ratio (in decibels, or dB) with respect to radar
reflectivity factor, Z.
The value of Z is a function of the amount of radar beam energy
that is backscattered by a target and detected as a signal (or echo).
Higher values of Z (and dBZ) thus indicate more energy being
backscattered by a target. The amount of backscattered energy generally
is related to precipitation intensity, such that higher values of dBZ
that are detected from precipitation areas generally indicate higher
precipitation rates. However, other factors can affect reflectivity,
such as width of the radar beam, precipitation type, drop size, or the
presence of ground clutter or AP. WSR-88D radars can detect
reflectivities as low as -32 dBZ near the radar site, but significant
(measurable) precipitation generally is indicated by reflectivities of
around 15 dBZ or more. Values of 50 dBZ or more normally are associated
with heavy thunderstorms, perhaps with hail, but as with most other
quantities, there are no reliable threshold values to confirm the
presence of hail or severe weather in a given situation. See VIP for
threshold dBZ values associated with each VIP level.
*Debris Cloud - A rotating "cloud" of dust or debris, near or on the
ground, often appearing beneath a condensation funnel and surrounding
the base of a tornado.
This term is similar to dust whirl, although the latter typically
refers to a circulation which contains dust but not necessarily any
debris. A dust plume, on the other hand, does not rotate. Note that a
debris cloud appearing beneath a thunderstorm will confirm the presence
of a tornado, even in the absence of a condensation funnel.
Delta T - A simple representation of the mean lapse rate within a layer
of the atmosphere, obtained by calculating the difference between
observed temperatures at the bottom and top of the layer. Delta Ts
often are computed operationally over the layer between pressure levels
of 700 mb and 500 mb, in order to evaluate the amount of instability in
mid-levels of the atmosphere. Generally, values greater than about 18
indicate sufficient instability for severe thunderstorm development.
Derecho - (Pronounced deh-REY-cho), a widespread and usually
fast-moving windstorm associated with convection. Derechos include any
family of downburst clusters produced by an extratropical MCS, and can
produce damaging straight-line winds over areas hundreds of miles long
and more than 100 miles across.
Dew Point (or Dew-point Temperature) - A measure of atmospheric
moisture. It is the temperature to which air must be cooled in order to
reach saturation (assuming air pressure and moisture content are
constant).
Differential Motion - Cloud motion that appears to differ relative to
other nearby cloud elements, e.g. clouds moving from left to right
relative to other clouds in the foreground or background. Cloud
rotation is one example of differential motion, but not all
differential motion indicates rotation. For example, horizontal wind
shear along a gust front may result in differential cloud motion
without the presence of rotation.
Difluence (or Diffluence) - A pattern of wind flow in which air moves
outward (in a "fan-out" pattern) away from a central axis that is
oriented parallel to the general direction of the flow. It is the
opposite of confluence.
Difluence in an upper level wind field is considered a favorable
condition for severe thunderstorm development (if other parameters are
also favorable). But difluence is not the same as divergence. In a
difluent flow, winds normally decelerate as they move through the
region of difluence, resulting in speed convergence which offsets the
apparent diverging effect of the difluent flow.
Directional Shear - The component of wind shear which is due to a
change in wind direction with height, e.g., southeasterly winds at the
surface and southwesterly winds aloft. A veering wind with height in
the lower part of the atmosphere is a type of directional shear often
considered important for tornado development.
Diurnal - Daily; related to actions which are completed in the course
of a calendar day, and which typically recur every calendar day (e.g.,
diurnal temperature rises during the day, and diurnal falls at night).
Divergence - The expansion or spreading out of a vector field; usually
said of horizontal winds. It is the opposite of convergence. Divergence
at upper levels of the atmosphere enhances upward motion, and hence the
potential for thunderstorm development (if other factors also are
favorable).
Doppler Radar - Radar that can measure radial velocity, the
instantaneous component of motion parallel to the radar beam (i.e.,
toward or away from the radar antenna).
*Downburst - A strong downdraft resulting in an outward burst of
damaging winds on or near the ground. Downburst winds can produce
damage similar to a strong tornado. Although usually associated with
thunderstorms, downbursts can occur with showers too weak to produce
thunder. See dry and wet microburst.
Downdraft - A small-scale column of air that rapidly sinks toward the
ground, usually accompanied by precipitation as in a shower or
thunderstorm. A downburst is the result of a strong downdraft.
Downstream - In the same direction as a stream or other flow, or toward the direction in which the flow is moving.
Dry Adiabat - A line of constant potential temperature on a thermodynamic chart. See Fig. 6, sounding.
Dry Line - A boundary separating moist and dry air masses, and an
important factor in severe weather frequency in the Great Plains. It
typically lies north-south across the central and southern high Plains
states during the spring and early summer, where it separates moist air
from the Gulf of Mexico (to the east) and dry desert air from the
southwestern states (to the west). The dry line typically advances
eastward during the afternoon and retreats westward at night. However,
a strong storm system can sweep the dry line eastward into the
Mississippi Valley, or even further east, regardless of the time of
day. A typical dry line passage results in a sharp drop in humidity
(hence the name), clearing skies, and a wind shift from south or
southeasterly to west or southwesterly. (Blowing dust and rising
temperatures also may follow, especially if the dry line passes during
the daytime; see dry punch). These changes occur in reverse order when
the dry line retreats westward. Severe and sometimes tornadic
thunderstorms often develop along a dry line or in the moist air just
to the east of it, especially when it begins moving eastward. See LP
storm.
Dry-line Bulge - A bulge in the dry line, representing the area where
dry air is advancing most strongly at lower levels (i.e., a surface dry
punch). Severe weather potential is increased near and ahead of a dry
line bulge.
Dry-line Storm - Generally, any thunderstorm that develops on or near a
dry line. The term often is used synonymously with LP storm, since the
latter almost always occurs near the dry line.
Dry Microburst - A microburst with little or no precipitation reaching
the ground; most common in semi-arid regions. They may or may not
produce lightning. Dry microbursts may develop in an otherwise
fair-weather pattern; visible signs may include a cumulus cloud or
small Cb with a high base and high-level virga, or perhaps only an
orphan anvil from a dying rain shower. At the ground, the only visible
sign might be a dust plume or a ring of blowing dust beneath a local
area of virga. Compare with wet microburst.
Dry Punch - [Slang], a surge of drier air; normally a synoptic-scale or
mesoscale process. A dry punch at the surface results in a dry line
bulge. A dry punch aloft above an area of moist air at low levels often
increases the potential for severe weather.
Dry Slot - A zone of dry (and relatively cloud-free) air which wraps
east- or northeastward into the southern and eastern parts of a
synoptic scale or mesoscale low pressure system. A dry slot generally
is seen best on satellite photographs.
Dry slot should not be confused with clear slot, which is a storm-scale phenomenon.
Dust Devil - A small atmospheric vortex not associated with a
thunderstorm, which is made visible by a rotating cloud of dust or
debris (dust whirl). Dust devils form in response to surface heating
during fair, hot weather; they are most frequent in arid or semi-arid
regions.
*Dust Plume - A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.
If rotation is observed, then the term dust whirl or debris cloud should be used.
*Dust Whirl - A rotating column of air rendered visible by dust.
Similar to debris cloud; see also dust devil, gustnado, tornado.
Dynamics - Generally, any forces that produce motion or affect change.
In operational meteorology, dynamics usually refer specifically to
those forces that produce vertical motion in the atmosphere.