Weather Glossary - C

Calm

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

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.

Carbon dioxide

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

Cb

See Cumulonimbus.

Ceiling

The lowest cloud layer that is reported as broken or overcast

Celsius

The temperature scale where zero (0°C) is the temperature at which water freezes and 100°C 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.

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.

CG

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

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

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

Classic Supercell

See Supercell.

Clear air turbulence

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

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.

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 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.

Cloud tags

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

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 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.

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.

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.

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.

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.

Convective Condensation Level (CCL)

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

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.

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.

Crystallisation

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

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

(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.

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.

Cyclogenesis

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

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.