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CA

[cloud-to-air lightning flash] Used when describing lightning which branches from a cumulonimbus cloud and terminates in clear air. This is an uncommon type of lightning.(See also CC, CG & GC)

Calibration (radar)

As well as de-cluttering for permanent echoes, and other adjustments, rainfall radar returns are calibrated (in real-time) against a network of telemetering rain gauges. This means of course that if the return is over areas without rain gauges (e.g. the sea), over-reading can occur, and caution is needed in blindly following radar imagery to assess rainfall rates/accumulations for this reason.

CAPE

[ Convectively Available Potential Energy ] A measure of the energy released once convection is initiated, often from the surface (for high values), but mid-level convective initiation is also very important. Assessed on a thermodynamic diagram (e.g. a tephigram) by noting the area enclosed by the environment curve (i.e. actual temperature found by a radio-sonde), and the parcel-path curve where it intersects the environment curve at height. Used extensively in severe convective storm studies, although worth noting that just because high values of CAPE are observed, other factors need to be right for a severe storm to develop. (see also HERE)

CAT

[Clear Air Turbulence] Bumpy conditions in the upper atmosphere when no clouds are present to betray the possibility of such. Caused by sharp vertical and horizontal shear of wind, often (but not exclusively) in association with upper-level jet streams (see "What are jetstreams?"). Can occur in, or be enhanced by mountain wave activity.

CAV

[ Conservation of Absolute Vorticity ] -- The principle first outlined by Carl-Gustav Rossby in the 1930's which accounts for the tendency for upper atmospheric flow to take up a wave-like pattern. The theory can be used to predict the wavelength and speed of translation of the long-waves found in the atmosphere, which in turn govern the broad 'weather type' at any one point.

CAVOK

"CeilingAnd Visibility OK": No CB, no cloud with base < 1500m/5000ft or below the highest minimum sector altitude, whichever is greater & visibility 10km or more & no weather of 'significance' i.e. DZ, RA, SN, SG, PL, IC, GR, GS, FG, BR, SA, DU, HZ, FU, VA, PO, SQ, FC, DS, SS ... and variants (see this glossary for decodes).

CB (strictly Cb)

Abbreviation for cumulonimbus, the cloud type associated with a thunderstorm, when the upper portion of the cloud exhibits glaciation (supercooled water droplets converting to ice crystals). Broadly, there are two species: Cumulonimbus calvus (Cb cal) and Cumulonimbus capillatus (Cb cap). The former is used when glaciation has only just begun, and is often taken to be the start of the most active phase of development - transition from Cumulus congestus ('Towering CU'); the latter type exhibits the traditional 'anvil' shape, when major activity may be (but not necessarily) beginning to wane. (See also TCU)

CC

(abbr)[cloud-to-cloud lightning flash] Used when describing lightning that originates in cloud and terminates in cloud. Thus it describes lightning with passes from one cumulonimbus cloud to another and lightning which is contained within a single cumulonimbus cloud. This includes the diffuse 'lit from within' (sheet) type lightning, as well as that whose channel is directly visible as it loops out of the cloud before returning back into it. (see also CA, CG & GC)

CC

(abbr) Cirrocumulus (CC in aviation reports etc., Cc otherwise); a high level, layer cloud type, with elements of instability, but rarely of significance for aviation or general meteorology.

CCL

(abbr)[convective condensation level ] Provided the dew-point of an air parcel is high enough, then during convective ascent the resultant cooling will lead to condensation at some altitude as the air temperature=dew point. The precise height this is achieved will depend upon the difference between the initial air temperature and dew-point of the parcel, and also the amount of mixing with the environment of the air through which the parcel is rising. The level where condensation is achieved (and thus the theoretical cloud base for cumulus clouds), is known as the Convective Condensation Level (CCL).
[ to a rough approximation, the CCL is given by (T-D)*400, where T=air temperature at the surface, D=dew point temperature (as the surface): the answer will be in feet. DON'T use slavishly - regard as 'good' to the nearest couple-of-hundred of feet at best. ]

CET

(abbr)[Central England Temperature] -- A series used to track temporal changes in the average temperatures over a large area of 'central England'...see "What is the Central England Temperature Series?". CLICK HERE FOR LATEST DATA FROM THE HADLEY CENTRE.

CF

Abbreviation for cold front.

CG

(abbr)[cloud-to-ground lightning flash*] Used when describing lightning which branches from the cumulonimbus cloud to the ground. It is sometimes referred to as 'fork' lightning from its appearance. (see also CA, CC & GC)[ *The lightning discharge process is complex; it involves two discharges per stroke and there may be several strokes in one flash (which result in the flickering which is often observed). The initial and very weakly luminous discharge establishes a conductive ('ionised') and usually highly-branched path through the air. The second and intensely luminous discharge moves in the opposite direction and drains the charge from the ground/cloud/air to the cloud/ground. For instance, a CG refers to a stroke/flash where the initial discharge is from cloud to ground, although the intensely luminous discharge that we see is from ground to cloud. ]

Channel Rat

(Dutch=Kanaalrat)An intense (small scale) developing depression that 'scuttles' along the English Channel. It comes (and goes) in a matter of a couple of hours to half a day. Because of its speed of movement, coupled with its often rapid development (increase in wind speed), it can cause severe problems for areas adjacent to the Channel and southern North Sea. The term probably in use (in the Netherlands) since at least the early 1980's. Examples 12th May, 1983 & 28th May, 2000.

Climate

Average weather over fairly long intervals of time, usually greater than one year and often 30 years. Care should be taken to state (or ascertain) the period for which particular climate 'normals' are applicable.

Cloud head (strictly Baroclinic cloud head)

During the early stages of 'explosive cyclogenesis' (q.v.), a very marked area of dense layered cloud - convex away from the developing depression - can be observed in IR, VIS and WV imagery, detached from the cloud area associated with the development. This feature is the result of air rapidly ascending as the intense development gets underway. Studies have shown that all mid-latitude cyclogenetic events over oceanic areas giving rise to winds of hurricane force were preceded by such features. However, care is needed to correctly identify such and true detection is only possible with animated imagery. (See also Baroclinic leaf; Dry intrusion.)

Cluster

In ensemble forecasting (q.v.), individual members often show strong grouping around a few results. Each grouping is referred to as a cluster. The more members making up a particular cluster, the higher is the confidence in that particular solution. (See also Ensemble; Ensemble mean)

Cold advection

The replacement (usually quasi-horizontally) of a warm air mass by a colder one. The process can be gradual or abrupt: the latter often occurring at well-marked cold-frontal boundaries.

Cold anticyclone

Area of high pressure with a cold core (relative to surrounding air), with the cold / dense air dominant in the lower part of the troposphere contributing to the surface high pressure. Has a shallow circulation (i.e. 'high' characteristics confined to lower layers), with a low / warm tropopause etc. It often forms in the 'polar' air behind a depression, moving with the synoptic-scale features with which it is associated. However, if a major change of type (from 'mobile' to 'blocked') is underway, then the High may transform to a warm type. Persistent cooling of continental areas in winter at high latitudes (e.g. over Scandinavia, Russia, Siberia) produces semi-permanent cold anticyclones, with mean 'sea-level' pressure often above 1050 hPa. (See Anticyclones; Warm anticyclone)

Cold Occlusion

If air (at the surface) behind an occluded front is colder than the air it is displacing (the usual case in the NE Atlantic/maritime NW Europe), then the front is known as a cold occlusion. The occlusion may be shown on synoptic charts as a linear extension of the cold front. (see also Warm occlusion).

Cold pool

An area where the atmosphere (in depth) is colder than its surroundings. The temperature is not measured by means of surface-based sensors (such as screen temperatures), but often (though not necessarily) by using thickness values (q.v.), with the 500-1000 hPa measure (roughly sampling the lower half of the troposphere) the most commonly used for this purpose. Closed centres with low thickness value (relative to adjacent regions) will define a cold pool. [ The opposite term (for a closed centre of high relative values), is "warm dome", although this term will not be heard much nowadays. ]

Cold-Front wave

A secondary low pressure system forming on an extended cold front, where the thermal contrast across the front (in the troposphere) is large, and the upper pattern is conducive to falling pressure at the surface. The wave can move quite rapidly (in the direction of the general upper driving flow), and will lead to a hesitation in the clearance of the main cold front at the surface, or its return to areas that previously experienced a clearance. Not all such waves develop closed-low characteristics; some will just 'run' quickly along the length of the trailing cold front with little development, other than enhancing rainfall. Because of the small-scale of the initial development, NWP models don't always place and forecast these correctly. (See also warm-front wave.)

Cold undercut

A term often used in situations where advection of relatively milder air as traced by 850hPa variables (e.g. actual 850 temperature, ThetaE, ThetaW, partial thickness etc.) does not fully reflect events in the lowest 50 to 100hPa (i.e. within the planetary boundary layer). In winter & spring especially, warm-air advection may be indicated by levels at / above 850hPa, but the surface wind is well backed & coming from a colder direction - the cold (relatively denser) air 'undercuts' the overlying milder airstream: a 'decoupling' occurs, depressing temperatures and creating a marked inversion / potential cloudy mixing layer. The cold air may be either an airmass feed, or from a local source, e.g. cold air off the North Sea at the end of winter or early spring.

Colour states

METAR reports from military airfields operated by the RAF, some USAF and others may have a 'colour-code' appended (usually only when ATC is open), which describes the airfield 'fitness': these run from BLU best, through WHT GRN YLO (1 and 2), AMB and RED. The colour is based on the lowest cloud base (usually 3 oktas or more cover, but some use 5 oktas) and the horizontal 'MET' visibility. BLACK is also used, for airfield closed for non-weather reasons.

Condensation

The transformation of water vapour to liquid water; in the process, latent heat (of vaporisation) is either released or absorbed. [ also see Evaporation ]

Condensation nuclei

Microscopic particles in the atmosphere that act as a focus or stimulant for cloud-drop growth.

Conduction

Heat transfer through a substance from point-to-point by means of the movement (or 'excitement') of adjacent molecular motions.

Confluent

When streamlines (q.v.) approach one another, the pattern is a confluent one. However, note that because streamlines only define the wind direction, and not the wind speed, a confluent pattern is not necessarily a convergent pattern. [ The opposite of confluent is diffluent (often spelt difluent in North American texts.). This case denotes the spreading apart of streamlines. Again, such diffluent patterns are not necessarily divergent. ] (see also convergence, divergence.)

Conservative property

Meteorologists are always keen to 'label' an air mass using a value that can be calculated from variables measured within that air mass (at various levels), but which remain constant, or nearly so in vertical (adiabatic [q.v.]) motion. Many properties are defined, such as potential temperature (Theta), equivalent potential temperature (Theta-e) and wet-bulb potential temperature (Theta-w). This latter measure is often used in operational meteorology in NW Europe: At the 850hPa level, it is used as a 'tracer' for air masses, and is much used for defining frontal boundaries, and for defining the axes of warm 'plumes' of air. (See also the entry under Wet Bulb Potential Temperature)

Contours

Lines on an upper air (constant pressure) chart (actual or forecast) joining places of equal height, 700 mbar; 500 mbar etc., or of equal thickness.

Contrail

CONdensation TRAIL. Also abbreviated (from old coding practice) to COTRA. See "Why do some high flying aircraft leave white trails in their wake?" and MINTRA (this Glossary)

Control run

The principle of ensemble forecasting is to slightly perturb an analysis by small amounts and see what the outcome is for each change. Rather than use a centre's operational model analysis and forecast output (which needs much computing time and maximum data ingress), an 'early look' analysis and forecast run, using the same physics as the operational (OP/OPER) run, but performed at lower resolution (typically half-scale) is employed. This is known as the 'control' run. Given the increase in computing power in recent years, the 'control' can have higher specifications than some operational models of less than a decade ago!

Convection

The transfer of heat by the actual movement of the heated substance, such as air or water. In meteorology, convection also means vertical transport through density imbalance, transporting mass, water vapour, and aerosols as well as heat.

Convective condensation level

See CCL

Convective precipitation

For precipitation production (rain, snow etc.), other conditions being satisfied, there must be a supply of upward motion through the cloud producing the rain, snow, hail or whatever. In convective precipitation, upward motion is provided by the release of convection in an unstable environment. (See "Stable and unstable air masses"). [ Computer models in operational use cope with instability features via parametrisation schemes (q.v.), which model 'ideal' convective towers within each model grid square, taking into account entrainment of dry air, moist convective vigour & depth, temperature structure etc. Algorithms will assign model rainfall to either 'dynamic' or 'convective': the type giving the greatest rate of rainfall is (usually) that which appears on the output chart.](See also Dynamic and Orographic precipitation)

Convergence

When air flows in such a way that the area occupied by a particular 'group' of air particles lessens ('drawing together'), the pattern is said to be convergent. Convergence in the atmosphere is associated with vertical motion, and hence development (or weakening) of weather systems. For example, convergent flow near the surface is coupled to, and may be the primary cause of, upward motion, leading to cloud formation/shower initiation etc. (See also divergence, confluent.)

Convergence zone

Usually referring to a low-level feature, this is a narrow elongated area where two different airstreams are converging such that air within the zone must ascend, leading to enhanced cloud / precipitation formation, particularly if the airmass is potentially unstable. The zone may propagate downstream with time, and it's activity, location and extent will be governed by the synoptic patterns giving rise to the zone in the first place. [Marked on a synoptic chart by a solid line along the axis of the zone, with angled branches indicating the convergence.]

Conveyor

In synoptic systems (e.g. a developing depression) airflow is not uniformly horizontal, and the system velocity (i.e. the speed of translation of the Low) must also be allowed for. High-velocity air aloft overtakes the synoptic feature, whilst lower down, the system often moves faster in a given direction than the low level airflow. To cope with all this, the concept of 'conveyor belts' was adapted for use in synoptic and mesoscale meteorology as a means of explaining the movement of heat, moisture and momentum around such systems. For example, in a developing/mobile depression, a warm conveyor belt (WCB) is assumed to rise from low levels in the warm sector just ahead of the surface cold front, to middle and upper altitudes over and well forward of the surface warm front. A compensating cold conveyor belt (CCB), descends from medium/upper levels well ahead of the surface warm front underneath the WCB then tucks around the backside of the low merging with the boundary layer flow.

Coriolis effect

As a consequence of earth's rotation, air moving across it's surface appears to be deflected relative to an observer standing on the surface. The 'deflection' is to the right of movement in the northern hemisphere, to the left in the southern hemisphere. (also known as the Coriolis acceleration, or deflection)

Coriolis parameter

A important quantity in theoretical meteorology because it plays a major part in describing (mathematically) how air moves on our spinning planet under the influence of a pressure gradient. It is usually denoted as 'f', and defined as twice the product of the angular velocity of the earth and the sine of a particular latitude. The angular velocity of our spinning earth is (for practical purposes) constant, therefore the important variable is latitude: from the definition, f varies from a maximum at the poles (sine 90deg=1) to zero at the equator (sine 0deg=0). [ see also Absolute Vorticity ]

Cross-contour flow

See "Why does the wind blow?"

CS

(abbr) Cirrostratus (CS in aviation reports etc., Cs otherwise); a high level, layer cloud type, due to wide-scale ascent in the upper troposphere: of no significance for aviation but is a pre-cursor to frontal activity to come.

CU

(abbr) Cumulus (CU in METAR/SIGWX reports etc., Cu otherwise); a convective cloud type, with a base in the lower part of the troposphere, and varying from weak to vigorous vertical penetration, possibly into medium or upper levels. (see also TCU)

Cut-off time

NWP models that are used in operational meteorology must have a nominal time at which the 'gates are closed' to new data, and the forecast computation cycle is started. For models used for primary forecast guidance at short lead times, only a couple of hours at most is allowed after the nominal data time. So for example, the cut-off for 12UTC data might be around 1345UTC. For global models, i.e. those used for international aviation, a slightly longer time is allowed, but usually no more than 3.5hrs after data time. However, some centres (e.g. ECMWF) with less demand for immediate products allow over 9 hours or more of data to be assimilated.

Cyclogenesis

The formation of a major low pressure system along a baroclinic zone (q.v.) (or frontal boundary), with primary forcing due to imbalances along the upper jet.

Cyclones

(depressions) Weather systems characterised by low pressure and rising air flows. Wind circulation is anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

Cyclonic trough disruption

The southern (northern in the southern hemisphere) portion of a trough advances, perhaps developing a cut-off circulation, and slowly warming out, whilst the opposite (residual) portion of the trough becomes quasi-stationary, maintaining a cyclonic pattern at the surface. (also see: anticyclonic trough disruption)