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. 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.)
*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
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.
Cloud-to-Ground lightning flash.
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.)
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
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
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
Cold Advection -
Transport of cold air into a region by
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
This term sometimes is used (incorrectly) as a synonym for wall
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 .
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.
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
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.
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 favourable).
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.
Cooling Degree Day (CDD)- useful in determining cooling
energy requirements. Defined as (MT - 65)= CDD, where MT is Mean
Temperature (F). If MT is less than 65 degrees F, CDD=0. MT= Mean (average) Temperature
of the day.
MT=(High Temperature+Low Temperature)/2
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,
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
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
A storm which undergoes only one cycle (pulse), and then dissipates,
is known as a pulse storm.
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.