Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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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.
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.

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.

Chinook winds - They are warm, dry downsloping winds that race off the peaks to the west and down into the metro area and surrounding communities. The air warms as it descends at a rate of about 5.5 degrees for every thousand feet of descent. This warming is caused by compression of the air as it drops into a more dense atmosphere at lower elevations. An air mass that is at 10 degrees above zero at the Continental Divide, will warm by nearly 45 degrees by the time it falls into the Denver area. As the air warms, the relative humidity drops and the air becomes very dry. This warm, dry and gusty wind can quickly wipe out a snowpile in Denver or Boulder. The term Chinook comes from a Native American word meaning "Snoweater". Chinook winds are most common in mid-winter as the winds aloft are very strong, but surface heating from the sun is minimal. Once spring gets nearer, the higher sun angle warms the Earth more readily and this causes rising columns of air called "thermals". These rising air columns tend to break up the flow pattern of the Chinook winds and make them less of a factor by March and April. Chinook winds are not only warm and dry, they can be quite strong and sometimes cause gusts of over 100 mph in Jefferson County near Rocky Flats and around Boulder. These locations are close enough to the mouths of canyons or the ridges to the west, to get the really powerful gusts as the winds squeeze through the tight landforms to the west.

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.)

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.

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.

Cold Front - The leading edge of a cold air mass that is moving.

Collar Cloud - A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud.
This term sometimes is used (incorrectly) as a synonym for wall cloud.

Colorado Hooker - This low pressure forms east of the Rockies in Colorado, or in the panhandle of Texas. Bringing heavy snow to the eastern plains of our state, the storm pulls in moisture from the Gulf of Mexico dumping 6 to 12 inches of rather wet snow along its path. Easterly winds swirling against the foothills create upslope conditions that can greatly increase snowfall in some areas. If the low develops over northeast Colorado, most of the snow will fall near the Wyoming border and farther north. If the low forms over southeast Colorado, Denver and the Front Range get heavy snow. This snow system is called a "hooker" because of the curved or hook shaped path it takes toward the Great Lakes.

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 that has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo.

Condensation - A process that releases heat that occurs when a gas becomes a liquid. See evaporation.

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.

Condensation Nuclei - Tiny objects in the atmosphere such as dust where condensation of water vapor into water droplets take place.

Conduction - Transfer of heat by molecular activity. Heat always flows from hot to cold objects.

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 that offsets the (apparent) converging effect of the confluent flow.

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 without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Convective Outlook - 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 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 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.

Crepuscular Rays - One of the most glorious sights in our skies are the beams of light that stream out from clouds as if they were sent from Heaven. These light rays often take a subtle beautiful golden color in out late afternoon sky as they fan out from between the clouds. The light beams are called "crepuscular rays" and they are yet another optical effect that is very fascinating. The rays are visible because of dust in the air that reflects the light from the sun. The golden color is due to late afternoon or early morning filtering of the incoming sunlight as it travels through more of the atmosphere (due to the low angle of the sun). The more atmosphere the light has to traverse, the more the shorter wavelengths are filtered out by the dust, leaving only the longer yellow, orange and red light. The light rays seem to fan out from a central point, but it is really just an illusion. The light rays are actually parallel! This illusion is due to the fact that the source of the light (the sun) is so far away, that the light essential starts from an infinite point. The best way to think of this is to envision a pair of railroad tracks. If you look way down the tracks into infinity, the tracks seem to converge in the distance. Of course they do not, they remain parallel, but to our eye it would appear that the tracks are fanning out from a point way in the distance. The same illusion is what creates the fanning out of the crepuscular rays. Another phenomena that is seen on rare occasions are "anti-crepuscular rays". These take a little imagination to figure out. If the sun has already set from your vantage point (or has yet to rise), you may see beams of light seeming to come together along the opposite horizon (180 degrees from the sun). This is caused by the sunlight and shadows cast by clouds in front of the sun. At the cloud level (higher in the sky) the sun has already risen about the horizon. The light from the sun shines on to the clouds, casting parallel beams of light and shadow across the sky. In the distance behind you, those beams seem to converge just like the railroad tracks!

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, and 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 that undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.

Cyclone - An area of lower pressure dictated by stormy, unsettled weather. Wind flows counterclockwise around a "low" in the Northern Hemisphere and clockwise in the Southern Hemisphere. A red "L" on a weather map.

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.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society