C

Calm – the absence of apparent motion in the air.

Celsius – a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Chinook – a Chinook is a warm, dry, gusty wind that occasionally occurs to the leeward side of a mountain range, particularly the Rocky Mountains.

Cirrus cloud – high cloud, delicate, hair-like and feathery looking.

Clear – sky condition of less than 1/10 cloud coverage. In the United Kingdom, clear is defined as “No cloud”, and in Australia, it is defined as “Virtually cloud-free”.

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.

Climate – the prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind velocity and phenomena such as fog, frost, and hail storms. Climate cannot be considered a satisfactory indicator of actual conditions since it is based upon a vast number of elements taken as an average.

Cloudburst – a sudden, intense rainfall that is normally of short duration.

Cloudy – cloudy means that clouds cover more than 60 percent of the sky.

Coastal flood warning – issued when there is widespread coastal flooding expected within 12 hours, more than just a typical over wash.

Cold – in the winter, cold refers to four to seven degrees Celsius below normal.

Cold Advection (CAA) – transport of cold air into a region by horizontal winds.

Cold front – a narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing.

Cold low – a low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

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.

Collar cloud – frequently used as a synonym for wall cloud, although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.

Combined seas – the combined height of swell and wind waves.

Condensation – the process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor. In Australia, it is described as a change from 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 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) – a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).

Continental air mass – a dry air mass originating over a large land area.

Convection – the transfer of heat within the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

Convergence – an atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.

Cumulonimbus Cloud – a vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds. In Australia, it is described as a heavy, puffy, heaped, dark cloud of great vertical depth, often bringing rain. Some have a distinctive anvil shaped head.

Cumulus cloud – a cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. In Australia, it is described as a cloud with a woolly, heaped appearance that often produces rain.

Cumulus congestus – a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).

Cyclone – an area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. In Australia, it is described as atmospheric circulations that rotate clockwise in the southern hemisphere, and anti-clockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.