Glossary terms A-F


Absolute drought

A period of at least 15 consecutive days, to none of which is credited 0.01 inches / 0.2 mm or more of precipitation. (See also drought; partial drought; dry spell)

Absolute vorticity

(or total vorticity) of air particles at any particular point is comprised of two elements: (i) on the rotating earth, air adopts the local vorticity due to the earth's solid-body rotation about its pole-to-pole axis, which is latitude dependent, and is known as the Coriolis parameter. This increases to a maximum over the poles and decreases to zero at the equator. The Coriolis rotation sense is always positive (or zero). (ii) the other element is known as the relative vorticity, the 'spin' tendency of air particles due to their motion relative to the earth - driven by atmospheric forces. Relative vorticity can be either positive (cyclonic sense) or negative (anticyclonic). (See also: Vorticity; Relative vorticity.)


(abbr) Altocumulus (AC for METAR/SIGWX charts etc., Ac otherwise); a medium level, layer cloud type, but with varying levels of instability associated - the extreme event being ACCAST.


(" Alto-Q-Cast ")(Altocumulus castellanus or castellatus; the proper abbreviation for the Daily Register is 'Ac cas') These are medium level clouds (between circa 8000 and 18000 ft; 2.5 to 5.5 km) which exhibit, at least in their upper parts, marked cumuliform / turreted appearance - the convective towers are often taller than the width of the individual base; the bases are connected and often appear to be arranged in lines or streets. These clouds are a good indicator of medium level instability and high moisture content - and often the precursor for widespread thundery activity within the following 24 to 48 hours. Observers used to SYNOP coding may also refer to these (along with Altocumulus floccus/Ac flo), as CM8 clouds.


A process where temperature changes occur in a 'system', without heat being supplied to, or lost from that system. In meteorology, used in connection with changes involving air parcels moving vertically in the atmosphere. [ If heat exchange is involved, the process is non-adiabatic (or diabatic)].

Adiabatic lapse rate

The change of temperature of an air parcel rising (cooling) or descending (warming) adiabatically. For dry (i.e. unsaturated) ascent or descent, the rate is ~10°C / km; for moist or saturated processes, it varies but a useful average is 5 or 6°C / km.


The transfer by horizontal air movement of heat, moisture (or humidity), momentum etc. The atmosphere at all levels is usually in some form of motion at most times, thus it is necessary to identify areas of significant advection. For the low/middle troposphere thickness products (e.g. 500-1000 hPa) are often used.


Solid particles suspended in the air. They include dust, salt particles, products of combustion etc. These are very important to the formation of water droplets or ice particles in the atmosphere, acting as nuclei for condensation / sublimation.

Ageostrophic effects

See "Why does the wind blow?"

Air mass

An extensive volume of air possessing uniform physical characteristics at similar heights in terms of humidity and temperature structure.


The proportion of solar incoming visible light (after passage through the atmosphere) reflected by a surface (e.g. sea, cloud-tops, ice etc.), expressed as a fraction or percentage of the incident light falling on that surface. Clouds have highly varying albedo, dependent upon thickness and composition. Old snow is about 55% (or 0.55), new snow around 80% (or 0.8). Water surfaces vary from very low (about 5% or so) at high sun elevation, to at least 70% (0.7) at low sun-angles: very smooth water surfaces with a low sun-angle can give rise to the phenomenon known as 'sun glint' sometimes seen in visible satellite imagery where the albedo value is very high.


(abbr) Above mean sea level.

Anabatic wind

A local-scale wind which blows up-slope, after strong heating of the hill/mountain-side by the sun. Such upslope winds can sometimes drag fog/stratus in the valley bottom (formed after a cold night) to upland areas that were previously clear of these phenomena. Airfields that were previously clear can suddenly 'fog-out' well after sunrise due to this effect.


When warm air ascends relative to the cold air at a frontal surface, the front is said to be an ana-front. Such fronts are normally 'active', in that thick/precipitation producing clouds (possibly with embedded instability), are usually located in the warm air associated with both a warm and cold front.


A pressure feature where a maximum of pressure is surrounded by relatively lower values. On a synoptic chart, a system of closed isobars will be found, enclosing the central 'High'. The circulation (of wind) is clockwise in the northern hemisphere (anticlockwise in the southern hemisphere). The structure of such features in the vertical is that high-level convergence coupled to gentle divergent-outflow at the surface leads to descent (or subsidence) of air within the anticyclone: this in turn leads to a decrease in humidity and an increase in the stability of the air, often producing an inversion close to the surface. Two types are defined: cold and warm; see Cold anticyclone; Warm anticyclone.

Anticyclonic trough disruption

If the northern (southern in the southern hemisphere) portion of an upper trough moves forward and warms out, leaving a quasi-stationary cut-off low in the base of the trough, the process is described as anticyclonic trough disruption - because the net result is a strong build of pressure/new high cell formation behind the retreating trough. (See also cyclonic trough disruption)

Arctic Oscillation

The Northern Hemispheric broad-scale oscillation of long-wave type, of which the North Atlantic Oscillation (NAO) is our regional-scale component. (q.v. here)


(abbr) Altostratus (AS for METAR/SIGWX charts etc., As otherwise); a medium level, layer cloud type, formed by wide-scale rising motion in the troposphere, varying from thin, non-precipitating type through which the sun/moon can be seen, to thick layer(s) associated with frontal development giving persistent, significant precipitation, & moderate in-flight turbulence & icing.


Over the many years that operational meteorology has developed, certain hours have been designated 'synoptic' hours, and observation times standardised around these points; the MAIN synoptic hours currently being 00, 06, 12 and 18 UTC(formerly GMT), with intermediate hours at 03, 09, 15 and 21 UTC. Increasingly however, observing systems (e.g. satellite, radar-networks, drifting buoys etc.) provide data at times other than these 'fixed hours' - these are designated non, or 'asynoptic' observations. NWP models can assimilate these observations during the initialisation process.


Abbreviation used to stand for 'aviation'.



A term applied when a forecast wind direction changes in an anti-clockwise sense, i.e. from south back to northeast, via east. The opposite term is veering, thus " southwest 4 veering north 5 or 6 ", would imply a wind originally force 4 from the southwest, becoming a northerly force 5 or 6 by the end of the forecast period, passing via west.


The temperature along a constant pressure surface (say 500 mbar) varies; a thickness gradient exists. The degree of baroclinicity is given by the product of the layer thermal wind (q.v.) and the Coriolis parameter. For practical purposes, the strength of the thermal wind alone is a good guide.

Baroclinic leaf

An elongated cloud pattern formed within the jet stream zone associated with marked baroclinicity (i.e. strong thermal contrast). The boundary (in satellite imagery) on the polar air-mass side of the development is well defined, and has the look of a 'stretched-out' "S" shape. The downstream/warm air-mass edge is less distinct. This feature represents the initial (or frontogenetic) stage of a system development, certainly in the mid-troposphere, and often (but not always) at the surface. Not all baroclinic leaves lead to marked cyclogenesis although they will be the first stage of such. (See also Dry Intrusion).

Baroclinic zone

Area where there is a marked contrast between cold and warm air masses. Can be determined on a thickness chart by a "packing together" of thickness (q.v.) contours. Usually associated on a msl chart with classical fronts, and therefore an area for potential cyclonic development.


A (theoretical) state in which surfaces of constant pressure and constant temperature coincide at all levels. The atmosphere cannot sustain development, and thickness (q.v.) gradients are zero. If thickness contours are widely spaced (the realistic state), the atmosphere is said to be quasi-barotropic.


(abbr) Patches (as in BCFG in METAR coding, meaning patches of fog).

Beaufort wind scale

This scale was originally devised by Francis Beaufort (later Admiral Sir Francis, Hydrographer of the Royal Navy), who lived from 1774 to 1857. He had a very active naval career and was also interested from an early stage in meteorological observations afloat. In 1805 & 1806, he devised a scale for his own use, which was based upon the amount of canvas a sailing vessel could carry in the given conditions. The scale underwent various modifications and was not introduced into general RN use until the middle part of the 19th century but thereafter, it quickly gained world-wide acceptance. However, various versions developed , and in 1906, the UK Meteorological Office attempted to co-ordinate the usage, and at the same time provided the first definitive wind speed equivalents for each level of 'force', and since 1920, the scale has been used to define forecast wind conditions in the Shipping Forecasts for waters in the NE Atlantic/NW Europe continental shelf. For a description of the current scale in use, see http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm


(abbr) "Becoming", used in aerodrome (TAF) issues and others. A permanent change in conditions.


(abbr) "Broken", 5 to 7 oktas (eighths) of cloud cover.


(abbr) "Blowing", used in connection with snow, sand etc.


[for the U.K. Met Office only - other services will have different criteria and the definition has changed over time; it has not always been so strictly defined] "The simultaneous occurrence of moderate or heavy snowfall with winds of at least force 7, causing drifting snow and reduction of visibility to 200 m or less". (Moderate snow is said to occur when the visibility is 'substantially' impaired, and the snow cover increases in depth at a rate up to about 4 cm per hour. Heavy snow should reduce the visibility to a 'low value' (in the low hundreds of metres), and the snow cover increases at a rate exceeding 4 cm per hour. )

Blocked pattern

Large scale obstruction of the normal west to east progression of surface cyclones in the mid-latitudes. The upper flow changes from predominantly zonal (q.v.) to meridional (q.v.). In a 'meridional' block, the upper flow divides upwind of the block and flows around quasi-stationary vortices - one anticyclonic and the other cyclonic. In the 'omega' block case the strongest flow is diverted to lower latitudes, leaving a slow-moving anticyclonic vortex on the poleward flank of the displaced zonal flow.

Blocking anticyclone

Acellular pattern of high pressure in the mid-latitudes, which diverts or prevents the normal west-to-east motion of depressions (areas of low pressure).


A name applied to mid-latitude depressions which deepen violently. The term was coined by Sanders & Gyakum (US Monthly Weather Review), in a 1980 paper dealing with such events, and requires a pressure fall in the depression centre of 24 hPa (or mbar) or more in 24 hours at latitude 60degN for the name to be applied. At latitude 45degN, the required value is 19hPa, and at 55degN, 23hPa. (see Explosive cyclogenesis.)

Boundary layer

In operational/synoptic meteorology this is usually taken to be the layer at the bottom of the atmosphere wherein surface friction is important. It can vary in depth from as little as 100 m or less on a still, cold night to upwards of 1 km or more in a windy, well-mixed situation. Also known variously as the 'mixed layer' or 'friction layer' and is a function of wind speed, vertical temperature profile (i.e. stability) and surface roughness. (N.B: micrometeorologists regard the boundary layer as the first few cm of the lower atmosphere and this can lead to confusion when reading some texts. )


Mist (abbr. from French): vis >=1000m and not more than 5000m, where obscuration is caused by water droplets in suspension. Used in aviation reports, forecasts etc.

Bright band effect

As snow descends through the melting (or freezing) level, the melting snowflakes 'look like' huge raindrops causing radar reflectance to increase sharply, implying heavier precipitation than is actually occurring. Corrections can be applied, provided the calibration system has some knowledge of the vertical temperature profile. The effect is usually confined to a layer about 1000 ft (300 m) thick.

Buys Ballot's Law

As originally formulated, " if you stand with your back to the wind (in the northern hemisphere), then low pressure lies on your left-hand side ". [ For the southern hemisphere, then the low pressure is on the right-hand side. ] (Personally, I feel we should face the wind to see what is coming, so I would reverse these, i.e. face the wind, low pressure on right etc.)

[ Prof. C.H.D. Buys Ballot (1817-1890) was a famous Dutch Meteorologist who founded the Royal Netherlands Meteorological Institute (KNMI) in 1854 and played an important role in establishing the first international organisation for meteorology - after the inaugural meeting of the International Meteorological Congress in Vienna in 1873. However, he is now best known for the 'Law' as above; it is possible that the credit for a rigorous treatment of the physics might be credited to Alexander Buchan, long-serving (1860-1907) Secretary of the Scottish Meteorological Society. ]



[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.


[ 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)


[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.


[ 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.


"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)


(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)


(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.


(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. ]


(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.


Abbreviation for cold front.


(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.


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.)


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.


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.


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


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)


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.


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!


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


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)


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.]


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?"


(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.


(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.


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.


(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)



(abbr) Dry Adiabatic Lapse Rate. The rate of cooling (for ascending air), or warming (for descending air) when air parcels are displaced by whatever mechanism. Usually taken to be 10degC/1km (or 3degC/1000ft).


Dekametres (i.e. 10's of metres) - often used on upper air charts: thus a 500 hPa height quoted as 540 dam is equivalent to 5400 metres. (NB: although 'DM' may still be seen on some model output, this is regarded as a non-standard abbreviation. 'dm' should definitely not be used, as it is the abbreviation for decimetres, i.e. tenths of a metre)

Daughter cell

As the precipitation downdraught associated with a marked Cumulonimbus event meets the ground, it will spread out in all directions. Where this cold outflow current meets the low level inflow (relative to the cloud motion) 'head-on', then this is a point of maximum convergence, leading to forced lifting of the air at that point, and provided the air is unstable enough, and convection is not otherwise inhibited (e.g. widescale descent), then a new convective cloud event will be initiated - a daughter cell.


Even given the strongest pressure gradients, surface-based friction will slow airflow in the lowest 800 m or so of the atmosphere, leading to the familiar cross-isobaric flow (from high to low pressure). With strong 'free-air' gradients (Vgr) (roughly above 25 knots or 12 m/s), surface winds will bear some relationship to Vgr; however, below these (approximate) levels, come nightfall under clear skies, surface cooling will lead to stabilisation of the lowest layers and the atmosphere finds it increasingly difficult to transfer momentum from the 'free-air' levels to the near-surface. The surface wind may drop away completely as the surface-based inversion develops (often within the course of half-an-hour), allowing mist, fog or surface frost to form (other factors being in place): this process has come to be known as 'decoupling' of the boundary-layer air from the flow inferred by the isobaric flow. Once the flow is decoupled, then the surface cools even more efficiently, thus reinforcing the nocturnal inversion. (See "Stable and unstable air masses" for discussion of stability etc., and "Why does the wind blow?" for matters concerning surface wind-flow.)


(used in METAR reports) - fog dispersal operations are in progress (probably obsolete now so included for historical purposes).

Deterministic forecast

A forecast that says rain will occur at such-and-such a place within a given time band, i.e. a 'yes/no' forecast, is an example of deterministic forecasting. (See also probability forecasting.)

Dew Point

(strictly dew-point temperature) The temperature (of an air sample that contains water vapour), to which that sample must be cooled (Pressure and humidity content being held constant) to achieve saturation with respect to a water surface. It can be measured indirectly using a wet & dry hygrometer (ordinary dry bulb thermometer, and another/adjacent thermometer with its bulb covered in a damp muslin - hygrometric tables or calculator then being used to calculate the dew point, relative humidity, vapour pressure); also by a 'dew-cell' type of instrument that measures relative humidity, from which the dew point can be calculated, or it can be measured directly by a dew-point hygrometer. The screen/surface dew-point temperature is used in air mass analysis, and also in the calculation of night-minimum and fog-point temperatures, as well as being used in the estimation of convective condensation levels, human-comfort indices, probability of snow at the surface etc. Dew point values above the surface (from radio-sonde ascents) are used to define cloudy or potentially cloudy layers etc., in the upper air (see also Frost point).

Dew-point depression (DPD)

The numerical difference between the temperature of a sample and its dew-point. The greater the difference, the lower the relative humidity. Values (deg. C) of less than 3 would be considered to indicate 'high' relative humidity; those of 7 or greater would indicate 'low' relative humidity. (See "What is the dew point depression?")


(see Confluent)


Where a steep gradient (i.e. sharp change over a small horizontal distance) occurs in a meteorological variable (i.e. temperature, humidity, wind direction etc.), there is said to exist a discontinuity in that variable.

Diurnal cycle

Changes which take place over the course of a 24hr period. The most obvious cycle is the rise and fall of surface temperature.


When air flows in such a way that the area occupied by a particular 'group' of air particles grows ('spreads apart'), the pattern is said to be divergent. Divergence in the atmosphere is also (along with convergence/q.v.) associated with vertical motion, and hence development (or weakening) of weather systems, depending upon the level where the divergence is dominant in a particular atmospheric column. For example, divergent flow aloft is coupled to, and may be the primary cause of, upward motion, leading to widespread cloud formation/cyclogenesis etc. ( see also diffluent.)


(obsolete abbreviation for dekametre - see entry for dam)

Downward penetration of Snow

Falling snow modifies the temperature structure of the atmospheric boundary layer as both melting & evaporation takes place. (See this question in the FAQ). Even if snow does not initially penetrate to the surface (after having fallen out of the parent cloud), if the Wet Bulb Freezing Level (q.v.) is low enough, the intensity of the precipitation is more than just 'light' and the mean wind strength in the melting layer is not too strong, then the snow level can descend considerably below initial conditions. The depth of this 'downward penetration of snow' as it is called, increases as the intensity of rain increases, and/or the wind speed decreases. It will be immediately apparent that the prospect for error in snow forecasting due to these variables in 'marginal' situations will be large!


(abbr) (see Dew-point depression)


Used in METAR reports - low drifting (snow, sand etc.), not appreciably affecting the visibility, e.g. DRSN.


Partial & absolute droughts are terms that are no longer used in official summaries; they were introduced in 1887 by G.J. Symons in 'British Rainfall', (with the term 'dry spell' added in 1919) but ceased to be used circa 1960. Drought hydrology is a complex field of study and as statistical & data-processing techniques have become sophisticated, there was no longer an official requirement for the use of these rather crude definitions. It is however useful to know what the definitions were, and even today, for 'hobby-use', defining periods of drought using these standards can be an interesting exercise. They are detailed elsewhere in this Glossary. (See Absolute drought; partial drought; dry spell).

Droughts - types of

Droughts as defined above are essentially meteorological; in other words, they are defined in terms of the amount of rain (or rather lack of rain) that occurs. Hydrological drought episodes by contrast take account of the wider 'cycle' of water use, with focus on the imbalance between precipitation input (rain, snow etc.) against water availability via aquifer storage, reservoir levels, land-surface run-off / riverflow etc. Finally, agricultural droughts are usually defined in terms of the soil moisture deficit (SMD) across a growing season - the degree of irrigation (natural or artificial) needed to bring a particular soil type back to 'optimal' production - balancing the outgoing moisture due to evaporation & transpiration.

Dry Intrusion

(or 'dry slot') -- A narrow region, virtually cloud-free which separates a baroclinic leaf (q.v.), and the adjacent frontal cloud. This region is the result of abruptly descending upper tropospheric/lower stratospheric air into a rapidly developing and potentially damaging low pressure system -- hence the low humidity content/absence of cloud. Water vapour imagery (see "What are the various types of Satellite imagery available?") in particular is used to diagnose this feature, and the rate of 'darkening' of the dry slot gives a clue to the rate of development of the whole storm complex.

Dry spell

A period of at least 15 consecutive days, to none of which is credited 0.04 inches / 1.0 mm of precipitation. (See also drought, absolute drought, partial drought).


Duststorm (used in METAR/TAF reports etc.); visibility generally < 1km due to dust raised by strong winds over a large area.


Dust (widespread, in suspension) (used in METAR/TAF reports etc.) Visibility is 5000 m or less.


Deutscher Wetterdienst (German Weather Service), based at Offenbach. Visit their web site at: http://www.dwd.de/

Dynamic precipitation

For precipitation production, other conditions being satisfied (i.e. enough humidity, required temperature structure, sufficient depth of cloud), there must be a supply of upward motion through the cloud producing the rain, snow or whatever. In the case of dynamic precipitation, the primary agent for providing upward motion is broad-scale ascent due to, for example, short-wave troughs in the prevailing upper flow, jetstream developmental areas, mass convergence or strong warm advection. [ Computer models in operational use deduce dynamic precipitation by testing for super-saturation of a layer taking into account the total water content (all phases) in a layer: the excess found is precipitated out. The type (dynamic or convective) giving the greatest amount is (usually) that seen on output charts. ](See also Convective and Orographic precipitation).


Drizzle (as in METAR/TAF reports).


Eclipse (of a geostationary satellite)

The earth's equator (and therefore a geostationary satellite's orbit) is inclined to the orbit of the earth around the sun. This inclination allows sunlight to power the satellite on-board systems for most of the year. However, there is a period of about 3 weeks either side of the vernal and autumnal equinoxes when a satellite will be in the earth's shadow for about 70 minutes each day (around local midnight). Because most of these platforms do not carry sufficient battery power to tide them over this gap, no imagery is generated and thus a local-midnight image is missing.


European Centre for Medium Range Weather Forecasts, located on the southern outskirts of Reading, Berkshire, UK. Visit their web site at http://www.ecmwf.int/

Eden Winter Snow Index

See Snow Index


See Environmental Lapse Rate


Embedded (in cloud, as in CB embedded in LYR cloud).


A collection of NWP runs (typically in excess of 15, many having 50 or more) from the same start time (t=0) and using the same model physics, but each run (or 'member') having a slightly perturbed (altered) set of initial conditions from the 'control' run (q.v). The alterations are constrained within limits which are calculated in various ways - one example being that of performing a separate short-range model run and identifying the errors that would grow most over a 48 hr period. These errors are then applied in varying amounts to the initial conditions before performing the operational ensemble run. Another technique is to use (known) errors from a previous run and apply these in small amounts to the initial conditions of the new run. [ NB: these output are in addition to (and run some time after) the 'operational' model output, i.e. the deterministic run which is the set of charts most often seen on web sites: it should not be assumed that the operational run (OP/OPER: q.v.) is close to the ensemble mean (q.v.) or mode - significant deviations can and do occur at longer lead times. Also note that a particular centre's operational model is often run at a higher spatial resolution than that used for the ensemble generation - the control.]

Ensemble mean

An average of the ensemble output from a particular computer run - this is usually more accurate than just following one of the individual forecasts that make up the average. Further, by comparing the individual members 'spread' about the mean, some estimate can be made of the reliability of the forecast: if there is strong agreement and therefore small divergence from the mean solution, then high confidence can be assigned to the average solution. Wide divergence, or clustering of groups of individual members well away from the mean will lead to considerable caution regarding using the output too slavishly and lower confidence in issued forecasts.

Environmental Lapse Rate

The actual plot of temperature against height (or equivalent) on a thermodynamic diagram.


The transformation of liquid water to water vapour - in the process 'absorbing' latent heat (of vapourisation).


(abbr)(also EWR) England and Wales Precipitation (or Rainfall). A data series combining the rainfall (and melted snowfall) amounts from a matrix of recording stations (well over 30) averaged to produce a single figure for an area taken to represent 'England and Wales'. The series runs from 1766 - maintained (separately) by the Hadley Centre (EWR) and the University of East Anglia (EWP), though I understand that a unified data-set is to be (has been?) produced. CLICK HERE FOR LATEST DATA FROM THE HADLEY CENTRE.

Explosive cyclogenesis

Sometimes, in an otherwise 'normal' cyclogenetic situation, factors are conducive to rapid falls of pressure leading to very tight isobaric gradients / extreme low pressure. These situations often give rise to 'damaging' or stormy / hurricane force winds: watch for 3-hourly pressure falls in excess of 10.0 mbar. (sometimes referred to as 'bombs', particularly in North American meteorological circles.)



(abbr.) Funnel Cloud - (in METAR/TAF, this includes tornado/waterspout, so differs from the classical distinction between a funnel cloud not touching down, and one that does . See entry for Funnel cloud).


1 or 2 oktas cloud amount, used in Aviation/METAR reports etc. (see also SCT)


(abbr) Fog (vis < 1000m, except when qualified by MI, BC, PR, VC); used in METAR/TAF reports etc.


(abbr) Flight level (e.g. FL240 ... 24000ft amsl/standard atmosphere); used in aviation reports, forecasts etc.

Foehn (or Föhn) effect

Mechanisms which give rise to a warm, dry wind on the leeward side of mountains or significant hills. Broadly, there are two: (i) the 'subsidence' type where air at & just above the hill/mountain crest descends by lee-wave action, becoming even drier & warmer than when it started out; (ii) all air in a moist airstream on the upstream side of the hill/mountain rises, leading to cloud/precipitation formation, thence lowering the humidity content, this air then descending/warming adiabatically on the leeside.


Reduction in visibility to under 1 km caused by suspension of minute water droplets (water fog) or ice crystals (Ice fog - q.v.). Water fogs are further sub-divided according to the process by which the fog forms, e.g. Radiation fog (caused mainly due to loss of surface heat from the ground at night in conditions of near-calm wind and high relative humidity); Advection fog (caused by movement of humid air over a relatively colder surface); Upslope fog (adiabatic cooling of air having high relative humidity as it climbs over high ground) and Evaporation fog (caused by evaporation into cold air which lies over a relatively warm water surface).
( If the visibility is below 200M but greater than ~50M then it is usually referred to as 'Thick Fog' (& colloquially in the UK as 'motoring fog') & if below 50M then 'Dense Fog'. However, there are different criteria for climatological stations, and other services will have different rules - treat this note as a guide only. )

Fog point

(strictly fog-point temperature) The air temperature (as measured in a standard thermometer screen) at which fog is expected/does form. Its calculation (before an event) is usually based on empirical work which employs either the surface air temperature/dew point at some time earlier in the day, or by construction on a thermodynamic diagram. The fog point is lower than the air-mass dew point, because as air cools through the evening and night, moisture is condensed out on contact with the chilled land surface, and this lowers the dew point from afternoon values.

Freezing fog

As for the definition of fog (above), but the droplets are super-cooled (i.e. temperature below zero), and strictly, the fog should be depositing rime-ice. However, in METAR/TAF coding, as long as the air temperature is below zero degC, then fog is coded as 'freezing' irrespective of whether rime is observed.

Freezing level

Taken as the altitude where the air temperature is 0 deg.C. However, it should be carefully noted that in the free atmosphere, liquid water does not necessarily freeze at this level, or indeed at altitudes some way above this value -- it should more correctly be called the melting level, or as in operational /aviation meteorology, the level (or altitude) of the zero degree isotherm. (see ZDL and Wet Bulb Freezing Level).


A boundary separating two air masses such as warm, moist air and cold, dry air. If the cold air pushes into a region of warm air, a cold front occurs and if the warm air advances relative to the cold, a warm front occurs.

Frontal fracture

During rapid cyclogenesis events, a weakness appears along the portion of the cold-front nearest to the depression centre, thought to be due to a combination of subsidence in this region, plus differential thermal advection, as, unlike in the 'Norwegian' model, cold air is not advected so quickly eastwards to maintain the baroclinicity in this region. (See "What is the Shapiro-Keyser cyclone model?")


Any atmospheric process which leads to frontal formation, or an existing weak frontal zone to become enhanced, is termed frontogenetic. On charts issued by some national meteorological services, such fronts are shown with the normally solid line defining the front broken by spaces and large dots.


When fronts weaken markedly due to, for example, marked anticyclonic subsidence across the front, then the feature is undergoing frontolysis. On charts issued by some national meteorological services, such fronts are shown with the line defining the front 'struck-through' by short inclined strokes.


See "What is the difference between a ground frost and an air frost?"

Frost point

(strictly frost-point temperature) The temperature (of an air sample that contains water vapour), to which that sample must be cooled (Pressure and humidity content being held constant) to achieve saturation with respect to an ice surface. (see also dew point).


(abbr) Frequent (hardly or not separated, as in FRQ CB in aviation forecasts).


(abbr) Smoke (as used in METAR/TAF reports etc.)

Funnel cloud (FC)

A visible rotating 'tube' of condensation particles, formed as the pressure falls in an intensifying vortex (extending below a cumulonimbus cloud) - perhaps reaching the ground/sea. It should be noted that the funnel cloud simply betrays the zone where the pressure is low enough and humidity high enough for cloud to form - the vortical circulation may well be in contact with the ground, but of such relatively weak intensity that it either causes little or no damage, or is detected only by surface dust / soil disturbance. There are well documented cases where tornadoes (as defined elsewhere) do not have cloudy funnels all the way to the surface. (based on Doswell, C.A. III, 2001)


(abbr) Freezing (used in connection with rain, drizzle, fog, all giving rise to ice deposition due to supercooled water droplets impacting upon surfaces with temperatures below 0.0degC, OR when temperature < 0.0degC anyway, whether or not ice deposits are observed.)