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Weather Glossary - C

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CA

Cloud-to-Air lightning. Lightning which travels from a cloud to clear air surrounding the storm.

Calm

Atmospheric conditions devoid of wind. In Oceanic terms it is the absence of wind and swell.

Cap

Also called a capping inversion.

A layer of relatively warm air aloft (usually a few thousand metres 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 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. This can lead to the ‘loaded gun’ type sounding. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability through excess convection or mixing - 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.

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 sounding.

However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.

CAPPI

Constant Altitude Plan Position Indicator. See also PPI.

CAPPI is a display format developed for visualising radar data. Data from several radar scans at multiple angles are combined to form a composite image that approximates a cross section through the atmosphere at a constant altitude. All publicly available Bureau of Meteorology radar data is transmitted in CAPPI format. CAPPI scans can be created a multiple elevations. The Bureau of Meteorology radar data has a CAPPI altitude of approximately 3000 metres.

Carbon dioxide

A gas (CO2) present in the atmosphere which plays an important role in the greenhouse effect.

Category

See Tropical Cyclone categories

Cb

See Cumulonimbus.

CC

Cloud-to-Cloud lightning. Lightning which travels either within one cumulonimbus cloud (intracloud lightning) or between two cumulonimbus clouds.

Ceiling

The lowest cloud layer that is reported as broken or overcast

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 multicell thunderstorm).

The term "cell" can also be used to describe the radar echo returned by an individual shower or thunderstorm, however this is a slang term.

Celsius

The temperature scale where zero (0C) is the temperature at which water freezes and 100C is where water boils (at sea level). Celsius can be obtained from value in degrees Fahrenheit by the following formula C=(F-32)x5/9.

Central Pressure

The atmospheric pressure at the centre of a high or low pressure system.

Centrifugal Force

The apparent force in a rotating system that deflects masses radially outward from the axis of rotation. This force increases towards the equator and decreases towards the poles.

Centripital force

The radial force acting to maintain an object in circular motion. An object (or parcel of air) in circular motion is constantly accelerating (changing its velocity) and the centripital force is the force which causes this acceleration. It acts towards the centre of rotation of the motion.

CG

Cloud-to-Ground lightning. A lightning strike which hits the ground.

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 sounding.

Cirrocumulus

(Cc) High-level, white cloud which does not produce shadows. It consists of very small granular or rippled elements. This cloud is often described as "mackerel sky" cloud.

Cirrocumulus

Cirrostratus

(Cs) High, thin, sheet like clouds that often cover the entire sky. Composed of ice crystals, they are often the cause of halos around the sun or moon.

Cirrostratus

Cirrus

(Ci) Latin - curl of hair
High-level clouds (5000 metres 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 are often semi-transparent. Cirrus clouds are also largely responsible for atmospheric optical phenomena such as halos and sundogs. Thunderstorm anvils are often a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

Cirrus

Cj

Colloquial abbreviation for Cumulus Congestus, properly abbreviated to TCu (Towering Cumulus).

Classic Supercell

See Supercell.

Clear

The state of the sky when no clouds or obscurations are observed.

Clear air turbulence

Name given to turbulence that may occur in clear air without any visual warning in the form of clouds.

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.

Climate

The atmospheric conditions over a long period of time. Generally refers to the normal or mean course of the weather. Climate includes the future expectation of long-term weather in the order of weeks, months or years in advance.

Climatology

The study of climate. It includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems.

Closed low

A low pressure area with a distinct centre of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term is usually 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 cut-off low).

Cloud

Mass of water droplets or ice crystals caused by water vapour in the atmosphere condensing or freezing. There are ten main cloud types, which are further divided into 27 sub-types according to their height, shape, colour and associated weather. They are given Latin names which describe their characteristics, e.g. cirrus (a hair), cumulus (a heap), stratus (a layer) and nimbus (rain-bearing).

Cloud cover forecasting terms

  • Clear - skies free from cloud, fog, mist, dust or haze.
  • Sunny - little chance of the sun being obscured by cloud. High-level cirrus clouds are often thin and wispy, allowing a considerable amount of sunlight to penetrate them, sufficient to produce shadows. In this case the day could be termed "sunny" even though more than half the sky may be covered by cirrus cloud.
  • Cloudy - Predominantly more cloud than clear sky for example, during the day the sun would be obscured by cloud for substantial periods.
  • Overcast - Sky completely covered by cloud.


Forecasts of cloud usually give an average, if no significant variations are expected. A clear day, for example, may at times see a few cloud patches. Forecasters expecting variations in cloud cover may use such terms as sunny breaks, sunny periods, cloudy periods, cloudy at times, mostly sunny, mostly cloudy. If expecting a major change in cloud cover, they usually indicate a distinct trend, eg becoming sunny, cloud increasing.

Cloud streets

Rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.

Cloud tags

Ragged, detached cloud fragments, also called fractus or scud.

CMC

The global implementation of the Global Environmental Multiscale (GEM) model run by Environment Canada, run twice daily at high resolution out to +144 hours.

Cold advection

Transport of cold air into a region by horizontal winds. See advection.

Cold air funnel

A funnel cloud or 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 core thunderstorms

Thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlays warmer surface air.

Cold front

A moving boundary that separates cooler air from warmer air. A cold front is named as such because the cold air is advancing on the warm air.On satellite imagery, cold fronts are generally clearly marked cloud bands up to several hundred kilometres wide. Preceding the front, winds normally tend north or northwesterly, often leading to warmer temperatures. As the front passes, the winds will normally swing west or southwesterly - and temperatures will in most cases drop significantly.If there is sufficient instability in the air preceding the front, showers or thunderstorms may develop. Often there is a band of rain accompanying the front, either directly ahead of or behind the front. Showers and thunderstorms are also possible behind the front, again depending on the instability of the colder air mass.Some cold fronts can develop along the southern coast of Australia (more specifically in Victoria and NSW). These fronts are often only a squally wind change, occasionally not even marked by significant cloudiness. See Southerly Buster.

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 in the upper atmosphere represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

Comma cloud

A synoptic-scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large, 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.). Often the early stages of tornado formation.

Condense

The phase change of a gas to a liquid.

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 diffluence. 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

Same as cumulus congestus or towering cumulus.

Contrail

(Condensation trail) A cloudlike streamer often seen behind jet aircraft flying in clear, cold, humid air. It is caused by the hot exhaust (largely composed of water) from aircraft engines mixing with the very cold air.

Convection

The 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 and ACCAS 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 Condensation Level (CCL)

The height at which a parcel of air, if heated sufficiently from below will rise adiabatically until it is saturated.

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.

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 favouale). Compare with Confluence.

Core punch

[Slang] A penetration into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting or chasing.

Coriolis force

The apparent force observed on a free-moving body in a rotating system. On the Earth, this deflective force results from the Earth’s rotation and causes moving particles to deflect to the left in the Southern hemisphere and to the right in the Northern hemisphere.

Corona

A pastel halo around the moon or sun created by the diffraction of water droplets.

Crystallisation

The phase change from a gas to a solid. The opposite of sublimation.

Cumuilform

Clouds which have vertical growth. Generally these clouds form from buoyant lifting rather than forcing (orographic effects, see stratiform). Cumuliform clouds produce showers rather than rain.

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.

Cumulonimbus

(Cb) Latin cumulus - to heap, nimbus - violent rain
Commonly known as thunderstorms. These clouds can produce heavy rain, hail, strong winds and lightning. Several variations of the cumulonimbus are the incus (anvil), which has a well-formed anvil of cirriform cloud. Calvus (bald), which lacks the anvil. Often this is when the storm is still in the developing stage. Cappilatus (hairlike, having hair) in which the anvil takes on a fibrous or striated structure.

Cumulus

(Cu) Latin - to heap
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. Cumulus humilus are cumulus clouds with little vertcal development. Cumulus mediocris show slightly more vertical development, but indicate fair weather, giving them the name fair weather cumulus. See also Cumulonimbus, Cumulus congestus, Towering cumulus.

Cumulus congestus

(CuCg, or Cu2) Latin congerere - to pile up
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. Same as towering cumulus. Another name used in Australia is Cj, but this is a slang term.

Cut-off low

A closed low which has become completely displaced (cut off) from the basic westerly current which flows across Australia's southern oceans, and moves independently of that current (not including Tropical cyclones). Cut-off lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression). An East Coast Low is an example of a cut-off low. A cut-off low is often accompanied by a blocking high. "Cut-off low" and "closed low" often are used interchangeably to describe low pressure centres 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 "cut-off low" only to those closed lows which are clearly 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 thunderstorm that undergoes only one cycle (pulse), and then dissipates, is known as a pulse thunderstorm or air-mass thunderstorm.

Cyclogenesis

Development or intensification of a low-pressure centre (cyclone).

Cyclone

Large scale atmospheric circulations in which the winds rotate clockwise in the Southern Hemisphere. Cyclones are areas of low atmospheric pressure and are often associated with strong winds, cloud and rainfall. Cyclone is a general term covering all cyclonic systems in the atmosphere and should not be confused with Tropical Cyclone. Interchangeable with low pressure system.

Cyclonic circulation

(or cyclonic rotation) Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., clockwise (in the Southern Hemisphere) as seen from above. Winds around synoptic-scale low pressure systems circulate cyclonically. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes or dust devils occasionally rotate anticyclonically (anticlockwise). Compare with anticyclonic rotation.

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