Glossary and Acronyms: C

CAPE :

Convective Available Potential Energy.

CDO :

Central Dense Overcast. One of the six Dvorak Technique pattern types. On a visible satellite imagery, the CDO consists of a dense solid-looking mass of clouds covering the cloud tropical system centre. Depending on the width of the CDO, the tropical system will be more or less intense.

CCC :

Central Cold Cover. One of the six Dvorak Technique pattern types. The CCC pattern consists of an approximately circular, cold or dense overcast covering the cyclone centre, obscuring the expected signs of pattern evolution.

Celsius (degree, °C):

unit of temperature. The thermodynamic scale of temperature has been defined by the Swedish stargazer Anders Celsius in 1742. The freezing point of water at standard atmospheric pressure (1013,25hPa) is 0°C and the corresponding boiling point is 100°C.
Relationship Celsius vs Kelvin and Fahrenheit: t°C = TK-273.16 = (5/9)(T°F-32)
Inversely: T°F = 1,8(T°C+32) and TK = T°C + 273,15.

Centre of action:

extensive and almost stationary depression or anticylcone which controls the movement of atmospheric disturbances over a large area.

CF:

Central Feature. Acronym used for the analysis of a tropical system with the Dvorak Technique.

Ci:

Current Intensity number. The Ci number is used with the Dvorak Technique, and relates directly to the intensity of the tropical system.

CIMSS :

Cooperative Institute for Meteorological Satellite Studies (University of Wisconsin-Madison, USA).

Conditional Instability of the Second Kind (CISK):

a process whereby low-level convergence in the wind field produces convection and cumulus formation, thereby releasing latent heat.
This enhances the convergence and further increases convection. The atmospheric environment that favors CISK is found over warm, tropical oceans where there is an abundant supply of moisture, the Coriolis force is small, and air convergence is strong.

Climatology:

study of the mean physical state of the atmosphere together with its statistical variations in both space and time reflected in the weather behaviourover a period of many years.

Climatological standard normals :

Averages of climatological data calculated for the following consecutive 30-year periods : 1 January 1901 to 31 December 1930; 1 January 1931 to 31 December 1960, etc. ...

Cloud band:

dense aggregate of cloud extending along a straight or curved line.

Cloud cluster:

convective cloud system often associated with low latitude convergence zones and covering an area of 500 to 1500 km^2.

Cloud etage:

the atmosphere is divided vertically into three layers whose limitsoverlap slightly and vary depending on latitude; a distinction is made between polar and tropical regions, as in the following table.

Etage	Polar regions	Temperate regions	Tropical regions
High	3-8 km	5-13 km	6-18 km
Middle	2-4 km	2-7 km	2-8 km
Low	Below 2 km	Below 2 km	Below 2 km

CMS:

Centre de Météorologie Spatiale (Lannion, France). French Meteorological Space Centre.

Col:

saddle-backed region with a weak pressure gradient which appears between two depressions and two anticyclones arranged aletnately in a cross.

Confluence :

progressive drawing together of the streamlines in the direction of flow.

Convective activity:

see "convection"

Convection:

organized motions within a layer of air, leading to the vertical transport of heat, momentum, etc.
A convective cloud is a cumuliform (bulging appearance) cloud which forms in an atmospheric layer made unstable by heating at the base or cooling at the top.
In the tropical regions, the convective activity represents all the areas of convection where unstable clouds with considerable vertical extent - such as Cumulonimbus developed.
In tropical meteorology, an area of convection is a cluster of cumulonimbus more or less dense and oragnized.

Convergence:

contraction of a vector field. Divergence of negative sign.

CONW:

consensus (or mean) of Numerical Weather Prediction models. CONW is an average of 8 models maximum (depending on the availability of these models), computed by the american Navy. Since cyclone season 2004-2005, RSMC La Réunion receives the results of this consensus as messages with coordinates of the centre of the tropical system together with the value of the minimum sea level pressure for the available ranges.

Coriolis force:

(or deviating force). apparent force, due to the rotation of the Earth, which acts normal to, and to the right of the velocity of a moving article in the Northern Hemisphere, and to the left in the Southern Hemisphere. This force is equal to zero on the equator and increases with the latitude to be maximum at the pole.
Coriolis parameter (f) is defined by the relation f=2 Ω Sin Φ, where Ω is the angular velocity of the Earth's rotation and Φ, the latitude of the point considered.

CRC:

Cellule de Recherche Cyclone de La Réunion. Tropical Cyclone Reasearch Unit, created in 1998 at the Interregional Direction of Météo-France at La Réunion.

CSC:

Cloud System Center. Acronym used for the tropical system analysis with the Dvorak Technique.

Cumulonimbus (Cb):

cloud with a considerable vertical extent causing heavy rainfalls, and sometimes associated to thunderstorms. It is a heavy and dense cloud, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spread out in the shape of an anvil or a vast plume.
In the tropical areas, the top of a Cumulonimbus can reach altitudes above 10 km, often close to 15 to 18 km (level of the tropopause in the tropics). If very powerful, the Cb can even overshoot the tropopause. Temperature at the top of the Cb is usually below -40°C, even reaching -90°C for the more mature clouds.

Curved band (CB):

dense aggregate of cloud extending along a curved line.
Curved Band Pattern: one of the six Dvorak Technique pattern types. The intensity is derived by measuring the arc lenght of the curved band fitted to a 10° logarithmic spiral overlay.

Cyclone season:

In the SouthWest Indian Ocean, period from the 1st of July to the 31th of june of the next year (before 2002, it was from the 1st of August to the 31st of July). 9 disturbances out of 10 develop between the 15th of November and the 30th of April, actually this period corresponds to the "official cyclone season".

Cyclogenesis:

generally speaking, process of initiation or intensification of a low (or depression), tropical or not.
In tropical meteorology, process of initiation or intensification of a tropical system, which can evolve as a tropical storm and then as a tropical cyclone ("tropical cyclone": expression used in the South-West Indian Ocean to name a mature tropical system (see TC)).

Cyclolysis:

generally speaking, process for weakening or terminating a low (or depression), tropical or not.
In tropical meteorology, process for weakening or terminating a tropical system.

Cyclonic circulation:

atmospheric circulation asssociated with a depression. It is couterclockwise in the Norther hemisphere and clockwise in the Southern Hemisphere.

Cyclonic weather:

At La Réunion, this expression is used for the public, to describe the weather associated to the approach of a tropical system which generates or is forecast to generate maximum average winds above 100 km/h with gusts above 150 km/h.