There now follows an alphabetical list of some commonly used words/phrases/abbreviations that you might see in the newsgroup (or via other weather-related sites on the WWW), but which may not be readily understood. The list has been prepared in association with Paul Bartlett, David Reynolds and many others: I would welcome suggestions for inclusion, but I will also scan the posts in the newsgroup and if I see a word/phrase/abbreviation that is causing problems, I will include same.

Where a longer explanation is required, a Q/A in the FAQ will be worked up, and referred to hereunder. For the professional/academic community, these terms etc., will not be rigorous enough, but I ask for some understanding in this respect, as the list is intended to convey the 'idea' of a concept, process etc., so that casual readers in the newsgroup can keep up with discussions without having to delve too deeply into a meteorological textbook. However, if I've made a fundamental mistake, then by all means let me have a corrected entry for consideration.


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


(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:

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

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.)


Glossary terms G-L



The word 'gale' is used in everyday speech in a rather loose way to describe any strong wind, for example ..." it's blowing a gale outside", when it may be just a 'strong blow' in inland areas of the southern Britain. Meteorologists must work to a strict definition of a gale. For operational forecasting (UK Met Office practice) both for land and sea use, a gale [ Force 8 on the Beaufort scale ] is defined as a mean wind (over 10 minutes) of 34 knots (39mph, 63 km/hr, 17 m/s ) or more, or gusts of 43 knots (49 mph, 79 km/hr, 22 m/s) or more. This definition is also used for verifying Shipping Forecasts and Gale Warnings. Isolated gusts accompanying squalls or thunderstorms are not counted. However, for climatological purposes (i.e. post-event analysis), only the mean wind is considered, i.e. a mean wind of 34 knots or more, as specified in the Beaufort wind scale (q.v.). [ see also definitions for Severe Gale, Storm, Violent Storm and Hurricane Force. ]


(abbr)[ground-to-cloud lightning flash] Used when describing lightning which branches from the ground to the cloud. The upward branching often results in an appearance like a trident, etc. This is an uncommon type of lightning. (See also CA, CC & CG)


(abbr) Global Ensemble Forecast System (of NCEP / q.v.)


"Potential energy per unit mass of a body due to the earth's gravitational field, referred to an arbitrary zero" (The Meteorological Glossary/UK; in meteorology, mean sea level is the reference level). A geopotential metre (by this definition) is related to the dynamic metre (straightforward unit of length) by the expression: 1 gpm=0.98 dynamic m. Geopotential height differs from geometric height where the value of the gravitational 'constant' (g) departs from 9.8 m/s^2. Gravity does vary, both by altitude and latitude, but for practical purposes, when looking at NWP output on the web, you can ignore these slight differences. Geopotential heights are used in meteorology because flow along a geopotential surface involves no loss or gain of energy, whereas flow along a geometric surface may do so - so for strict physical / mathematical calculations within computer models, the distinction between the two must be maintained.

Geostrophic wind

Defined as the (theoretical) wind that would blow on a rotating planet which results from a balance between the pressure gradient causing the initial displacement of the air, and the apparent (to us on the earth) deflecting force due to the planetary rotation. Many corrections are needed to find the 'true' wind vector amongst which are the effects of friction and the several forces involved when the pressure pattern changes - which is the usual case. However, by this definition we get the general statement that the speed of the geostrophic wind is proportional to the pressure gradient, or inversely proportional to the distance between isobars/contours. Curvature of the flow must also be taken into account ... see Gradient wind.


(abbr) Global Forecast System

Global Forecast System

The primary forecast model (NWP) from the US NCEP service (q.v.). The model suite is run to T + 384 hr, in two 'bursts'; one to T + 120 (5 days) then a further run to the 16 days (384hr). The model is run four times daily, though not all WWW sites hold all runs (or full output for each run).


Hail (abbr. from French) dia: > = 0.5 cm; used in METAR / aviation reports etc.

Gradient wind

When the path that an air parcel takes is curved (relative to the earth's surface), as so often in meteorology, that airflow is subject to an additional force necessary to maintain a curved path. For cyclonic flow, the 'true' wind that blows will be less than the theoretical/geostrophic wind; for anticyclonic flow the true wind is greater, subject to a limiting maximum. This is why, for example, around what initially looks like a dramatically intense depression, the wind may not be quite so excessive: cyclonic curvature will account for substantial negative correction to the theoretical value. Around a surface ridge, the wind is often surprisingly stronger than might be implied by isobaric spacing.


Small hail dia: <0.5cm; used in METAR/aviation reports etc.


Given that the wind in the surface boundary level varies markedly about the mean wind (q.v.), it is often necessary to report the accompanying instantaneous maximum (or gust speed) in a defined period. For METAR reports, then the period over which this peak wind is reported is between 2 and 10 minutes (depending upon the country). For SYNOP reports, the period is either the last hour (most likely in NW Europe), or the period covered by the past weather group in the report - reference to the accompanying amplifying groups will usually sort this out.



See hPa


See "What is Helicity?"

Horizontal vorticity

Even the classical 'vertical' vorticity term (q.v.) has some upward / downward component, but this is usually ignored for practical synoptic-scale meteorology. However, when coming down a scale or two, to local / mesoscale development, ( e.g. severe convective storms ), then vorticity about a horizontal axis is most important. It is often assessed in the lowest 3 km of atmosphere, and is 'driven' by two terms: vertical speed shear (increase / decrease of wind with increasing altitude) and directional ('twisting') shear, the change of direction with increasing altitude. If, in the lowest 3 km of atmosphere (up to 700 hPa), there is both a sharp increase of wind speed and a directional veer of wind with height, then horizontal vorticity will be potentially significant, provided it is coupled to the vigour of a developing cumulonimbus complex. (See also Vorticity; Vertical vorticity and "What is Helicity?"


(abbr) Hectopascal - equivalent to a millibar (q.v.). An attempt to use SI units without doing away with the idea of millibars (from the c.g.s. system). [ 1 hPa=100 Pa (or N/m2)]


See "What is the difference between humidity and relative humidity?"

Hurricane Force

This term (in UK Met Office use) is only used in shipping bulletins and associated Gale/Storm warnings in the form "Hurricane Force 12", from the modified Beaufort scale. It is strictly defined as a mean (10 minute) wind of 64 knots or more. (Gusts not defined) (See also comments at Severe Gale).

[ Please note carefully that just because an area of low pressure produces winds to 'hurricane' force as defined here, it does NOT make that feature a Hurricane! For more on this, see this question on the October 1987 storm ]


Haze: used in METAR/TAF reports etc., when visibility is reduced in a 'dry' atmosphere. (visibility > = 1km, relative humidity roughly < 90%).



(abbr) Ice crystals (also known as diamond dust); used in METAR/aviation reports.


When used in aviation weather reports/forecasts, implies aircraft superstructure icing.

Ice day

A period of 24hr (conventionally beginning 0900UTC), during which the air temperature is less than 0 degC.

Ice fog

Visibility reduced to less than 1000 m by suspension of minute collection of ice crystals in high concentration. The crystals will glitter and may give rise to optical phenomena. (NB: this is NOT the same as freezing fog, which is composed of water droplets - see definition elsewhere.)


The process whereby a model 'analysis' is produced by utilising model fields from an earlier run, and integrating synoptic, and asynoptic observations to produce the 'initial state' at t=0. The model analysis may not be the same (in detail), as a hand-drawn analysis, and intervention (q.v.) is sometimes needed as a result to preserve some small scale features which can influence the forecast run.


Radiant energy received from the sun on any particular surface. Often used when discussing receipt of infra-red radiation on the surface of the earth.

Instant Occlusion

[ or pseudo-occlusion ] The name that has been coined to label the cloud mass associated with an active trough in the cold air, that comes close to, and interacts with a pre-existing baroclinic zone, forming a pattern that looks superficially as if it was part of a traditional occlusion process.

Intertropical Convergence Zone

(usually abbr. ITCZ) A zone (often rather broad, but sometimes quite narrow), which separates the 'air-masses' brought together by the low-level outflow from the sub-tropical high pressure belts north and south of the equator. Over the oceans, the zone can be well marked; over land, sensible heating usually leads to 'breaks' or other anomalies, and the regional-scale monsoon circulations also distort, or swamp the idealised structure of the ITCZ. Cloudiness (and hence precipitation activity) can vary sharply over a period of 24hr. Day-to-day change of position is often small, but the zone migrates north & south through the course of a year, roughly in sympathy with the changing position of the sun.


A process where forecasters force acceptance of a report rejected in the model initialisation routine ('supporting'), or use 'bogus' observations to input a conceptual model observed in imagery.


(abbr) Intensifying (as used in SIGMETs for a phenomenon becoming more intense or extensive).

Inversion (of temperature)

A layer in the atmosphere (usually very shallow < 0.4 km), where temperature rises with increasing height. Two of the best-known in operational meteorology are the nocturnal inversion (formed due to strong cooling of land surfaces after sunset), and the subsidence inversion (due to descent & adiabatic warming of air associated with anticyclones). Another near-surface type is that formed when warm air travels over a cold surface (e.g. cold seas or ice/snow).


(abbr) Isentropic Potential Vorticity - the product of the absolute vorticity of an air parcel, and its static stability, calculated along a constant surface of 'theta' (potential temperature), hence the 'isentropic'. Anomalies in IPV around the level of the tropopause (and hence in the region of the driving jet stream) can be related to developments through the troposphere, leading to cyclogenesis. Because IPV is a highly conservative property for any sample of air, it is found to be particularly useful for tracking the path that stratospheric air (high IPV values) will take as it enters the upper troposphere during rapid cyclogenesis events. NWP models can be programmed to output the height of a particular IPV value - defined such that it 'samples' air in the model stratosphere. These patterns are then overlaid on water vapour imagery, and any mis-match between model and reality are quickly seen and allowed for. (See also Potential Vorticity) and also this article on Water Vapour Imagery.


(abbr) Infra-red (used in connection with satellite imagery) See "What are various types of satellite imagery available?"


(abbr) International Standard Atmosphere. A standard reference for temperature, pressure, and relative density structure in the troposphere and lower stratosphere, used for the calibration of (pressure) altimeters.


A line on a synoptic chart joining points of equal atmospheric pressure.


Isolated (as in ISOL CB etc.)


A line connecting points of equal temperature.


Intertropical Convergence Zone (Sometimes seen as ICZ, or perhaps, erroneously, ITF [ intertropical front ]).


Jet entrance

Given a jet core, the area where the speed increases markedly upstream is referred to as the jet entrance region.

Jet exit

Given a jet core, the area where the speed decreases markedly downstream is referred to as the jet exit region.

Jet streak

A small-scale but intense core of strong winds, usually within (and moving quickly along) a well-defined jet stream (See "What are jetstreams?"). They generally do not have a length much more than two to three-hundred kilometres, and can be associated with explosive cyclogenesis events (q.v.). Difficult to pick up via conventional observations (e.g. radiosonde wind-finding), but can be detected in WV imagery. (See also "What are the various types of satellite imagery available?")

Jet Stream

A band of high winds usually found in the upper troposphere: wind speeds can exceed 90 m/s. Jet streams are also located in the stratosphere and, with lower speeds, in association with the atmospheric boundary layer. (See also "What are jetstreams?").


Katabatic wind

A wind that blows down a slope which is strongly cooled after sunset. Given the right topography and surface composition, surprisingly high speeds can be achieved in otherwise light-wind situations.


When the warm air associated with a frontal surface descends relative to the cold air, the front is a kata-front. Such fronts are usually weak/ill-defined, but often mask more complex mechanisms, including upper frontal structures, and mesoscale processes.


(abbr. kt or kn) One nautical mile per hour (for speed of wind, ship movement, depression movement etc.) [ 1 knot = 0.515 m/s = 1.85 km/hr = 1.151 mph. As a working approximation, to obtain m/s, halve the reported wind in knots. ]


Knots (nautical miles per hour, as used in METAR/TAF reports etc.)



Abbreviation used in aviation work to stand for "over-land".

Lapse rate (of temperature)

The decrease of temperature with height in the atmosphere. Confusingly, the opposite case, an increase in temperature with height, is known as a negative lapse rate.

Latent heat

The amount of energy needed to accomplish a phase change. Latent heat of fusion is the amount of energy required to melt ice, and at 0°C is 3.34 * 105J kg-1 (or about 80 cal/g). The latent heat of vaporisation is the amount of energy needed to evaporate liquid water. It is equivalent to 2.50 * 106 J kg-1 (or about 600 cal/g) at 0°C. The latent heat of sublimation is the energy needed to carry out a change from solid (ice) to gas (vapour). It is the sum of the latent heats of fusion and vaporisation, i.e. 2.83 * 106 J kg-1 (or about 680 cal/g) at 0°C. When water freezes, condenses or changes from a gas to a solid, 80 cal/g, 600 cal/g and 680 cal/g are released to the environment respectively. The processes are all reversible.

Left exit

Lies on the cold side of the jet axis, in the region of marked deceleration of flow. A preferred region for cyclonic development.

Lenticular clouds

These form within the crest(s) of orographic (or lee) wave-trains, over and downwind of hills / mountains / islands, provided of course that the air is humid enough. The clouds are formed because air cools as it is forced to rise and if condensation takes place, lens-shaped clouds are observed with clear space in between the elements. The cloud forms within the upwind leg of each wave-crest and dissipates (evaporates) on the downwind leg: the air is therefore flowing through the cloud, with the cloud itself staying quasi-stationary; change in the cloud requires an alteration in the windflow or temperature / humidity environment. Sometimes, under very special circumstances, a 'pile of plates' is observed, where lenticular clouds are stacked vertically. The most common form of wave-forced cloud is perhaps Altocumulus lenticularis (Ac len), but lenticular cloud forms are found at all levels. Standing wave motion can also lead to a previously uniform sheet of cloud developing a lenticular appearance, and on occasion, complete dispersal. (See also MTW).

Loaded gun scenario

On a day of instability through a great depth of the troposphere, and high values of CAPE (q.v.), rising surface temperatures will at some point ensure that convection parcels leave the surface, the condensation level will be reached, cloud will grow (given sufficient moisture), and a heavy shower, or even a thunderstorm will result. It sometimes happens though that although the atmosphere is markedly unstable above, say, 2 km a 'lid' opposing surface-based convection exists at or below this level, due often to a layer of warm/dry air that has become entrained in the airflow from some source. This means that surface temperatures must become very high to overcome this 'lid', often requiring additional triggers, such as low-level convergence or release of medium level potential instability by a mid-level trough, thus lifting the whole column and releasing the 'pent-up' energy in a sudden burst... and the 'loaded gun' will be 'fired', perhaps leading to a severe storm/supercell event. (see 'Spanish plume').


Layer(s) (as used in cloud forecasting in aviation products).


Glossary terms M-R


Man-machine mix

The concept whereby a forecast result is achieved as a combination of human interaction with (by intervention), and interpreting (using experience of the atmosphere and conceptual models) NWP output, rather than allowing the models to predict the weather with no interpretation.


Abbreviation used to stand for maritime environments, often seen in aviation forecasts.

mb (or MB)

Often seen as the abbreviation for 'millibar' (see also mbar).


An abbreviation for 'millibar', being one-thousandth part of a bar. The 'bar' is the basic unit of atmospheric pressure as defined in the c.g.s. system of measurement (now regarded as obsolete). 1 bar = 103 millibars = 106 dynes/cm2 (c.g.s. system) = 105 N/m2 (Pa) (SI system).


(abbr) Mesoscale Convective Complex


(abbr) Mesoscale Convective System

Mean wind

The atmosphere in motion near the surface of the earth (the 'wind') exhibits marked variations over very short periods of time, depending on such variables as low level thermal stability, ground-induced friction effects, vertical shear etc. Average or 'mean' winds are defined over periods such as 1, 2 or 10 minutes for reporting in meteorological bulletins. (60 minute periods are often used for climatological purposes). For SYNOP and METAR reports, the period is 10 minutes in most countries, though in tropical storm advisories and measurements associated with US-based organisations (e.g. NHC, JTWC), 1 minute averages are used, and often referred to as 'sustained' winds.


Both predominantly north-to-south and south-to-north airflows (over distances of 100's of km) are termed meridional.

Mesoscale Convective Complex (MCC)

This might be regarded as the 'super' version of an MCS. It is strictly defined in terms of areal extent of coldest tops of the cloud sensed by IR imagery (typically over 300 km), and must last at least 6 hr, with many active systems lingering for upwards of 12hr. Would be regarded as extremely rare for NW Europe - more a feature of central and eastern Europe with average annual frequency around 5.

Mesoscale Convective System (MCS)

(as defined in 'Images in weather forecasting' - other services may define differently)... " a mesoscale grouping of deep convective and stratiform cloud and precipitation, together with associated circulations. The system exists much longer (at least four hours) than the lifetime of an individual constituent cloud. The individual convective clouds contribute to a common upper-tropospheric outflow shield or anvil. The system's convective-scale downdraughts merge at some time to form a continuous zone of cold air in the lower troposphere". What is visible on satellite imagery is the amalgamation of many cirrus outflow heads from the many cells comprising the system. Frequent cloud-to-cloud lightning is a feature of these systems. Although mainly a feature of the central plains of the United States and over continental Europe, they can on occasions affect Britain, mainly affecting the 'English lowlands' with an average frequency 2 to 3 times per year - when they do occur, they are responsible for some extreme/violent convective weather, and behave as a weather system in their own right, often modifying the enviroment in which they form considerably.


METeorological Aviation Report -- a weather observation for a specific airfield at a given time, containing the minimum information necessary for air operators for safe usage: wind/visibility/significant weather type/cloud amount+base/temperatures/pressure settings being the 'core' elements. Usually only issued with all elements when the airfield is operational (i.e. Air Traffic Control is open), but increasingly automated observations are now appearing of varying quality. Major/civil airports issue at HH+20 and HH+50 (i.e. 20 and 50 minutes past each hour), with others hourly only. METAR reports may also have TREND forecasts appended giving a short-range (usually 2hr) forecast of significant changes. SPECI reports are issued when meteorological variables deteriorate/improve through defined levels. For a brief listing of the weather decode for a METAR/TAF/SPECI, click on this link, and there is now a full explanation of the METAR code on this site :- HERE


(abbr) Shallow (below eye level); used in aviation reports in connection with fog (FG).


To aid the forecasting of condensation trails emitted (or not) from high-flying aircraft, a line marking the critical temperatures (altitude dependent), above which trails are not possible, is marked on a tephigram . The values are approximately -24degC at 1000 hPa (i.e. roughly sea-level), -39degC at 250 hPa (34000ft / 10.4 km) and about -45degC at 130 hPa (50000feet/15km). Using the MINTRA line (as it has come to be called - based on experiments by JK Bannon during World War II with the piston-engined Spitfire), a forecaster will mark two further lines on a tephigram: MINTRA minus 11degC (A) and MINTRA minus 14degC (B). If the ambient temperature (from the tephigram air temperature plot) lies between (A) and (B), then short, non-persistent trails are possible. If colder than (B), then long, persistent trails should be expected. However, some note should be paid to the relative humidity - high values will tip the balance to trailing (or longer/persistent trails.), even with air temperatures warmer than (A); ultra-low rh% will reduce the risk of condensation trails - the design of engines will have an effect as well. In broad terms, warm Tropical Maritime airmasses with a high but cold tropopause will result in a good deal of trailing, whilst cold, polar air-masses with a low, relatively warm tropopause will seldom give rise to significant aircraft trails. (See also "Why do some high flying aircraft leave white trails in their wake?")


Visibility reduced due to the suspension of minute water droplets in the atmosphere: the visibility is >=1km (less than this, and fog is reported), and the relative humidity is ~>95%. However, slavish adherence to this latter figure is not advised!

Mountain Waves

(also known as standing, lee or orographic waves. Often abbreviated to MTW in aviation circles) -- Under certain conditions, which must include a stable layer around/just above a mountain/hill range, air flowing across the range will be found to oscillate vertically in a standing wave configuration with well defined crests/troughs in the wavetrain. The horizontal flow must be reasonably brisk and within roughly 30 degrees of the crest of the hill/mountain range, with speed increasing with height, and directional shear must be small. Airflow is often smooth, but the vertical velocities in the upward/downward legs of the standing waves can be strong, and may break down to give local severe turbulence. Surface winds downstream of the ridge may be notably gusty as well, but paradoxically, they may also be extremely light, or even reverse direction. The presence of MTW activity may be betrayed in visible satellite imagery, if the flow is moist enough, as a series of near-parallel bars of cloud downwind of the ridge causing the airflow displacement. (See entry under: "Lenticular clouds") [ Although standing-wave motion is often referred to as 'lee' wave action, upward motion will be found on the windward / upslope side and of course across the hill / mountain crest too; there are also well documented cases of wave motion being found some considerable distance upwind of a topographical obstacle - perhaps due to some element of 'backwards' propagation of the wave-train. Wave motion may also be seen in cloud patterns where the triggering is due to marked shear in the windflow, well away from a range of hills. ]


(abbr) [historical only - no longer used] Medium Range Forecast (or Global Spectral) model (run by the NCEP). Had two basic formulations: for short-range work, the "Aviation" (AVN) run to 72 hours, and for extended range work (also known as the MRF run) to beyond 144 hours - in one iteration to 240 hours (i.e. 10 days). Now (2002), subsumed within the main NCEP NWP suite as the Global Forecast System (GFS) (q.v.).


(abbr) Mean sea level pressure (also seen as PMSL).

Multi-cell storm

See the FAQ here.



(abbr) National Centers for Environmental Prediction (part of the US National Oceanic and Atmospheric Administration [NOAA], National Weather Service [NWS]). NCEP comprises 9 centres; one of those centres is responsible for running the atmospheric prediction models (see MRF). To find out more, visit:

Net radiation

The difference between absorbed and emitted radiation.




(abbr) see Noctilucent cloud.

Noctilucent cloud (NLC)

These are clouds in the high atmosphere (around 83km) which are thought to be composed of small ice-coated particles (possibly meteoritic debris), which can be observed in mid-summer (June/July in the Northern Hemisphere) against a twilit sky: the sun must be between 6 and 16 degrees below the local horizon. They appear as very tenuous, often interwoven filaments of cloud, mostly white or slightly blue in tone. (Also known as polar mesospheric clouds or PMC's.) [ For a link to more on this, and an aid to identification, see this entry in the main FAQ or this article.


(abbr) Navy Operational Global Atmospheric Prediction System (also abbreviated to NGP). This model is provided by the US Navy, Fleet Numerical Meteorology and Oceanography Center (FNMOC). To find out more, visit:

Normal (environmental) lapse rate

The average change (decrease or increase) of temperature with height. In the troposphere it is 6.5°C / km, or ~2°C / 1000ft.

North Atlantic Oscillation

See "What is the North Atlantic Oscillation?".

Norwegian model

The classical idea of a travelling wave depression on the polar front running forward and deepening, with the cold front moving faster than the warm front, thus 'occluding' the warm sector, with the parent low slowing/turning to the left (in northern hemisphere), and filling up.


(abbr) Nimbostratus (NS in METAR/SIGWX charts etc., Ns otherwise); a deep cloud extending from low through to higher mid-levels, formed by vigorous and widespread rising motion, giving rise to heavy precipitation & a high risk of severe icing.


(abbr) No significant cloud - (No CB, no cloud < 1500m/5000ft or below the highest minimum sector altitude, whichever is greater, and CAVOK is not appropriate); used in aviation forecasts (and latterly in actuals).


(abbr) No significant weather; used with aviation weather forecasts, e.g. TAFs.


(abbr) Numerical weather prediction. The processes that make up the atmospheric system can be represented by fundamental equations, which can be solved in discrete time steps to achieve a numerical forecast. The concept was well understood from early in the 20th century, but needed the arrival of electronic computers in the early 1950's to be of practical use.



(abbr) Outside air temperature: often used in connection with aviation weather reports.


Abbreviation for occluded front (or occlusion).


In general use, often used as an abbreviation for observations; in the specific case of the newsgroup, used as part of the 'Subject' line where the body text contains observations in some structured (perhaps semi-coded) way and in a time sequence. The (suggested) format for the Subject is:

[OBS] < location >< date >

(this entry may be amended with experience and validation of the current trial - see also WR)

Occluded front

The merging of two fronts, as when a cold front overtakes a warm front.


(abbr) Occasional (well separated, as in CB in aviation forecasts.)


Abbreviation for occluded front (or occlusion).


(abbr.) 'operational' model


(abbr.) 'operational' model

Operational model

A term used to differentiate the primary NWP output from a particular centre from any ensemble products from the same source. The operational model will almost always be run at a higher resolution than that used for ensemble output. It must not, however be assumed that the 'Op/OPER' is necessarily the best outcome, particularly beyond 3 days or so. (see Ensemble)

Orographic forcing

An airstream encountering a barrier to its passage is forced to go around or over the obstacle. The upward deflection of the airflow is sufficient to give rise to adiabatic cooling, and if the air is moist enough, the formation of clouds, precipitation etc. In addition, convergence of the flow on the windward side (due to a rapid decrease in velocity) when the air encounters a sharply graded barrier not only enhances the vertical motion, but also leads to a deformation of the flow which in turn alters the vorticity of the air particles. Thus, hill and mountain ranges are most important in a study of meteorology.

Orographic rainfall/snowfall

For precipitation to occur, other conditions being satisfied (i.e. enough humidity, required temperature structure, sufficient depth of cloud etc.), there must be a supply of upward motion through the cloud producing the rain, snow or whatever. In orographic precipitation, the forcing agent is provided by large ranges of hills/mountains blocking the flow of humid air in such a way that vertical (upward) currents of air are produced, leading to adiabatic cooling >> condensation >> cloud formation/enhancement >> precipitation element growth. Orographic forcing OF ITSELF usually only produces small amounts of precipitation, but can be the means of enhancing or triggering other mechanisms (e.g. convective activity), and is one of the important elements in the 'seeder-feeder' model (q.v.). Computer models in operational use do now have sufficiently realistic orography and vertical resolution to model such, but the output (usually) does not explicitly define orographic precipitation.

Orographic waves

See Mountain waves.


(in an NWP ensemble suite) When considering a collection of solutions at a particular lead time (from a single-centre, or as part of a 'Poor Man's ensemble), some 'clustering' is usually observed - i.e. a large number of members pointing to a similar outcome. However, as lead times get longer (especially beyond 72 hours), one or two members may depart significantly from the ensemble mean, (and/or mode of larger clusters) - these are termed 'outliers'; such indications carry small weight, but cannot be totally ignored. In particular, at extended range (beyond about 7 days), there may be no clear 'clustering' signal, and an 'outlier' is just as likely to be right as a solution nearer the mean/median of the output. (See also ensemble, clusters, ensemble mean etc.)


(abbr) Overcast 8 oktas (cloud amount, as used in aviation reports, forecasts etc.)


The promise of a fine, sunny day is sometimes spoiled because cumulus cloud builds and spread out into an almost unbroken sheet of stratocumulus by late morning - which then refuses to break up for the rest of the day*. For this to occur, there must be a marked inversion (see "What is an inversion?") within 100 to 300 hPa of the surface, which must be intense enough to stop convective currents 'breaking - through' the inversion even at maximum temperature; the convective condensation level (CCL) must be at least 60 hPa below the inversion level, and the layer between CCL and inversion must have a reasonably high relative humidity. For some rather obscure reason, this phenomenon has come to be called 'overconvection' (at least in the UK, probably originating within the gliding community) - possibly because convective cumulus 'spills-over' to cover the sky? (* though, other conditions being right, the cloud may disperse around or just after dusk.)

Over-running trough

An active, mid-latitude frontal system is associated with a marked short-wave trough. The 'active' weather associated with the front lies forward of the trough, driven by the dynamics associated with it. At some stage in its life though, the trough (or a portion of it) will 'relax' (and effectively weaken), allowing the trough to run well ahead of the lower-tropospheric portion of the frontal system - it 'over-runs' the (surface) location of the front, and the activity at that position will decay. Note however that the upper trough will still have 'weather' associated with it - and may be the means of driving an upper/split frontal structure well away from the classical surface front as drawn on conventional analyses. See also "Why fronts die".



Pascal - allocated in honour of Blaise Pascal, to a unit of one N/m2, the basic unit of pressure in the SI system.


Some atmospheric processes are below the grid-scale/wavelength of operational meteorological computer models and cannot be handled explicitly by such schemes - for example individual showers, which are not only important for local weather, but have a feedback effect within the atmosphere that needs to be included in the NWP routines to maintain a realistic model of the real atmosphere. Larger scale model parameters (e.g. wind vector, temperature, humidity) are used to diagnose and represent the effects of such sub-gridscale processes: this is know as parametrisation. [ See HERE ]

Partial drought

A period of at least 29 consecutive days, whose mean daily precipitation does not exceed 0.01 inches / 0.2 mm. (See also drought; absolute drought; dry spell).


(abbr) Pressure gradient force


Is the study of times of naturally occurring events, such as the first blossom appearance in a long established species, or the departure of migratory birds. From 1875 to 1948, a register of such events was maintained by the Royal Met. Society, but after a period when the science was in the doldrums, the Woodland Trust and Centre for Ecology & Hydrology combined in the late 1990's to 'kick-start' the observing network, recognising that such data can complement studies into long-term climate change. For more detail, see:-


(abbr) Ice Pellets (was PE); used in aviation weather reports.


(abbr) Polar mesospheric clouds (or Noctilucent clouds).


(abbr) Pressure at mean sea level: often seen in connection with NWP model products.


(abbr) Well developed sand/dust swirls; as used in aviation weather reports.

Polar front

A boundary that separates polar air masses from tropical air masses.

Polar low

(or Polar depression or Polar meso-cyclone) See "What is a polar low?".

Polar mesocyclones

A term now used to encompass the whole 'family' of disturbances resulting from arctic air flowing equatorward over progressively warmer seas; the term 'Polar Low' (q.v/above) is now often used only for 'extreme' systems where gale or near gale-force winds are observed.

Polar mesospheric clouds

(PMC) See entry under Noctilucent clouds.

"Poor Man's" Ensemble Technique

A true NWP ensemble (q.v.) is the product of multiple iterations of a single atmospheric model on a single centre's computer: the individual members of the ensemble run are obtained by perturbing the initial conditions very slightly to simulate the uncertainty that is always present at analysis time. However, long before these techniques were perfected, operational forecasters would (and still do) absorb the differing output from various international centres (e.g. EC, NCEP, DWD etc.) and / or different 'runs' from the same centre - treating all the various outputs as 'members' of what has been dubbed a 'Poor Man's Ensemble'. As with true ensembles, the more model runs that agree at a certain lead time, the higher is the confidence in that particular solution.

Potential Instability

(also known as Convective Instability) Said to exist when forced lifting (e.g. ascent over mountains or broad- scale/dynamic ascent) causes a layer, initially (just) stable to such forced ascent to become unstable. Decreasing humidity aloft is required within the layer, and heavy rain/thunder can be the result. Theta-W or Theta-E(q.v.) difference charts are often used to find such areas of potential instability: the usual levels used are at 850hPa and 500hPa. The value at 850hPa is subtracted from that found for 500hPa, and negative values so found indicate potential instability. Only slightly negative differences can lead to some significant convective activity .. all other factors being favourable of course. [ Such layers can also be inferred using a thermodynamic diagram (or tabular listing of Theta E or Theta W), noting where values decrease with increasing altitude within the low-to-middle troposphere (roughly up to 400hPa). ]

Potential Vorticity

The ratio of the absolute vorticity (q.v.) of an atmospheric column to the (defined) pressure difference across the column. This quantity is used to label air in much the same way as we use other conservative properties. As a column of air moves along, it 'shrinks' vertically (due to mass divergence) in just the right amount to decrease its absolute vorticity; as it expands vertically (due to mass convergence), its absolute vorticity increases. Therefore, Potential Vorticity tends to remain constant following the motion of the flow, for adiabatic motion.


Anything 'precipitated' by clouds (rain, snow, hail, drizzle etc.) is covered by this noun. Often abbreviated to 'ppn' or 'pptn'. (for definitions of various types of precipitation, see:- "Beaufort Letters")

Pressure Gradient

The difference in atmospheric pressure over a defined (usually horizontal) distance. (See "Why does the wind blow?")

Pressure Gradient Force

(abbr. = PGF)The force exerted on the air due to a pressure gradient, causing a tendency for movement (i.e. 'wind') from areas of high pressure to areas of low pressure.(See "Why does the wind blow?")

Prevailing visibility

Used in METAR (& TAF) coding - see Visibility [ reports ].

Prevailing wind

The most frequent wind direction for any particular location in a given period, e.g. a day, month, year or climatological period.


Partial fog (i.e. fog "banks"; substantial portion of airfield covered by fog - but not completely; visibility < 1000m.)


Probability (as used in aviation forecasts, e.g. TAFs; in the latter, under current [2002] rules, only PROB30 or PROB40 are allowed, e.g. 'moderate' probability of an event occurring.

Probability forecasting

Given that there is always a measure of uncertainty in forecasting the weather, the likelihood of an event happening can be expressed as a probability: thus a 70% chance of rain, 20% chance of thunderstorms etc. Often useful in finely balanced situations i.e. rain vs. snow; severe storms vs. no storm etc. (see also Deterministic forecasts).


When large scale features in the upper air, such as a 500 or 300 hPa trough/vortex drift west-to-east this is said to be a 'normal' progression of the pattern. (See also retrogression).


(abbr: Polar Stratospheric Clouds) During the polar 'night' (i.e. the period in the middle of the winter when insolation does not penetrate to ultra-high latitudes), the stratosphere cools significantly leading to 'closed-loop' circulations (both vertical and horizontal) which virtually isolate these polar stratospheric regions - the "Polar night vortex" is found, within which temperatures can be found well below (minus)75degC. In these extremely cold conditions, clouds are observed to form in the stratosphere, which appear to be composed of a combination of nitric acid and water. Stratospheric clouds can also form from ordinary water ice (i.e. as in the troposphere) but these are much less common at these high altitudes as the stratosphere is normally very dry and water-ice clouds only form at the lowest temperatures. The presence of PSC's and the part they play in the chemical interactions at these levels have been a subject of much debate in recent years. (See main FAQ here for the Stratospheric Night Jet and here for Stratosphere & various web-sites dealing with upper atmosphere ozone depletion.)

Pulse storms

(a term often used in North America) Random air-mass thunderstorms forming in an environment of little or no vertical wind shear, which appear as individual returns (without any obvious organisation) on radar/high-resolution satellite imagery systems. They usually last 20 to 30 minutes, perhaps up to 60 minutes, and give rise to small hail, sometimes heavy rain and perhaps weak tornadoes. They can be regarded as a more intense version of the single-cell convective type discussed in the main FAQ here, i.e. higher CAPE values are involved than for an 'ordinary' shower.

PVA region

An area where marked advection (movement) of positive, or cyclonic vorticity (q.v.) is occurring - hence Positive Vorticity Advection; often associated with a small upper trough running through the broadscale upper pattern. Cyclonic development will occur - other factors being favourable.



Pressure at airfield level; set on an aircraft (pressure) altimeter when height above local aerodrome level (strictly the official threshold elevation) is required.


Pressure at mean sea level (reduced according to actual/mean temperature).


Pressure at mean sea level (reduced according to ISA profile); set on an aircraft (pressure) altimeter when height above local mean sea level is required.



(abbr) Rain; as used in aviation (e.g. METAR/TAF) reports.


This is the transmission of energy by electromagnetic waves, which may be propagated through a substance or through a vacuum at the speed of light. Electromagnetic radiation is divided into various classes on the basis of wavelengths; these are, in order of increasing wavelength: gamma radiation, X-rays, ultra-violet (UV) radiation, visible (VIS) light, infra-red (IR) radiation and radio waves.


(sometimes abbreviated to 'R/S') An instrument that measures temperature, pressure and humidity of the atmosphere as it is carried aloft on a balloon. The "sonde" transmits its measurements to a ground-based radio receiver via radio signals, and by accurate tracking (radar or satellite) of the sonde unit, upper winds can be deduced.

Rain day

A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 0.2mm or more has been recorded. (See also Wet day).

Relative humidity (RH)

See the main FAQ here.

Relative Vorticity

The vorticity (or tendency for air particles to 'spin') relative to the earth. It can be considered for practical purposes (and crudely assessed on meteorological charts) as the combination of two factors: (i): the 'spin' imparted due to the curved path that air takes in its passage through the atmosphere (cyclonically curved contours=positive, anticyclonically curved contours=negative). (ii) the other factor is due to the shear developed along the flow due to the differing velocities of the moving particles. Swiftly moving air will generate a 'twist' element relative to the lower-velocity flow on either side [ shear vorticity ]: where the 'twisting' generated is in a cyclonic sense, that is counted as positive; where in the anticyclonic sense then it is negative. ( See also Vorticity; Absolute Vorticity.)


When the amplitude of a trough decreases with time, the trough is said to have undergone relaxation. The change is usually measured in terms of a latitude change of a chosen contour or thickness line.


When an upper trough (or ridge) moves against the normal west-to-east flow in mid-latitudes, the feature is retrogressing, or undergoing retrogression.


(abbr)Relative Humidity (expressed as a % value).


See Upper ridge

Ridge amplification

When contour heights along the axis of an upper ridge increase, the ridge is amplifying.

Right entrance

On the warm side of the jet core, in the region of maximum acceleration of flow. Often associated with marked cyclogenesis.


Glossary terms S-Z



(abbr) Sand, visibility 5000m or less. (as used in aviation reports, forecasts etc.)


(abbr) Saturated adiabatic lapse rate. The rate of cooling (variable) of a saturated air sample rising in the atmosphere. (see "Stable and unstable air masses")


The condition air reaches when it contains the most water in the vapour state that it is capable of holding at any particular temperature. If any more vapour is injected into the sample (or if the sample is cooled), then condensation will occur.


(abbr) Stratocumulus (SC in METAR/SIGWX charts etc., Sc otherwise); a low-level cloud type, varying from thin, well broken layers with little impact for aviation/general weather, to deep, sometimes unstable character giving rise to persistent PPN, and a risk of moderate turbulence & moderate (some situations severe) icing.


(abbr) Scattered (3 or 4 oktas); cloud amounts used in aviation reports, forecasts etc. (of historical note, SCT used to mean 1 to 4 oktas, until the introduction of 'FEW' on revamp of the METAR code in the 1990's).


In mid-latitudes, we are used to the idea of the four seasons: spring, summer, autumn and winter. For climatological 'accounting' purposes, these are defined using three calendar month blocks thus:- March, April & May = spring; June, July & August = summer; September, October & November = autumn and December, January & February = winter. (For more, see "How are the seasons defined?")

Seclusion process

During the process of rapid cyclogenesis(q.v.), the standard 'Norwegian' theory of development leading to an occluded front is not appropriate. What appears to happen is that the original cold front becomes weak/ill-defined (close to the low centre), and a new cold front appears further to the west. (This is effectively what has been drawn in the past as a 'back-bent' occlusion). So, what happens to the warm air associated with the warm frontal zone near the low centre? Around and immediately to the equatorward side of the low, it becomes trapped or 'secluded' from the rest of the development in a discrete region enclosed by relatively colder air encircling the development - a so-called 'seclusion'. (This is therefore a different process from that producing the classical 'occlusion' whereby warm-sector air is lifted by the advancing cold air.) (see "What is the Shapiro-Keyser clyclone model?")

Seeder/feeder mechanism

When very moist (e.g. tropical maritime) air flow is forced to rise over upland areas, thick layers of stratus or stratocumulus cloud form. As noted elsewhere, these 'orographic' clouds of themselves produce relatively little rainfall (in a thermally stable environment). If however rain is already occurring from medium layer cloud (thick altostratus, nimbostratus) [seeder clouds], it will have to fall through the low-level [feeder] cloud, with collision/collection processes markedly enhancing the net rainfall rate at the surface. This effect often produces prolonged heavy rainfall in the warm conveyor regime within a warm sector, particularly if the system is slow-moving.

Sensible heat transfer

The transfer of heat by conduction and convection (i.e. it can be 'sensed' or detected directly).


(abbr) Severe (as in SEV ICE, for severe icing).

Severe Gale

The definition of a 'Severe Gale/Force 9' is strict for operational (UK) forecasting for maritime purposes. Either the mean (10 minute) wind must be 41 knots or more, up to 47 knots; or the gusts must be 52 knots or more, up to 60 knots. The term will also be heard on broadcast weather forecasts, although it's arguable that the general population cannot be expected to know what this definition is, and the practice now is to explicitly forecast gust values rather than just relying on the adjective 'severe' to imply possible problems. (See also Gale, Storm and notes at the Beaufort wind scale.)


Part of the WMO 'header' code used in bulletins that carry atmospheric reports, more commonly known as 'sferics, or 'SFLOCS'. (See "What are sferics?'")


(abbr) Snow grains; as used in aviation weather reports.


(abbr) Showers; as used in aviation weather reports/forecasts.

Shapiro-Keyser cyclone model

See "What is the Shapiro-Keyser clyclone model?")


See wind shear.

Shear vorticity

See Relative vorticity


Short (!) for short-wavelength upper trough): A minor trough of small amplitude moving at speeds varying from 'steadily' to 'rapidly' through the long-wave upper pattern. Often best detected and monitored in water vapour imagery, and associated with development or de-stabilisation of the synoptic pattern. (See here)


Issued by meteorological offices responsible for aviation forecasting. When significant flight/weather events are observed or forecast, then a SIGMET is issued by the office responsible (Bracknell [ to end-August 2003]/ Exeter [from start September 2003] for London, Scottish and the Shanwick Oceanic FIR's (Flight Information Regions); Dublin for the Shannon FIR) for such as embedded (EMBD) or frequent (FRQ) cumulonimbus (CB) or thunderstorms (TS); severe icing (SEV ICE) in frontal cloud; severe clear air turbulence (CAT) etc. Issued when there is a high degree of confidence, and for a short (usually max. 4hr) period only.

Significant Wave height

The average of the highest one-third waves observed at a point and is approximately equal to the wave height an experienced observer would visually estimate for a given sea state.


(abbr) Significant weather, as in significant flight weather charts for route planning.


Climatologists have always been alive to the fact that similar weather patterns/types occur at certain times of the year with varying degrees of regularity - an annual 'singularity'. For a while, before dynamical methods of long-range forecasting were used, singularities were very popular, though controversial. The best known (in the British Isles) are Buchan's spells and Lamb's singularities.


(abbr) Sky clear (as used in aviation weather reports, though from 2005, it should NOT be used in METAR reports).


Strictly (by WMO regulation), this is defined as rain and snow falling in a mixed fashion, or snow that is melting as it lands. Unfortunately, in North America, it has a different definition where it is used to denote ice pellets - a totally different phenomenon.


(abbr) Snow (as used in aviation weather reports, forecasts etc.)


(abbr) Runway/airfield closed due to snow cover.

Snow Index

(Eden Winter Snow Index): Philip Eden has defined as follows: ' Add together the snow depth in centimetres for all days with snow-lying at 0900Z. Treat a slight cover as 1cm, but ignore less than 50% snow-cover. Thus one morning with a 10cm cover, and ten mornings with a 1cm cover, would each score 10. The units, for the sake of argument, might be called "cm days" '. This can be used as a 'running-total' through a particular winter to compare different areas of the country (alongside such as numbers of days of snow-lying at 0900, numbers of days air minimum temperature < 0.0C (or other threshold) etc., or to put winter 'snowiness' in historical context. [ to see the procedures for reporting falls of snow - see here ]


(in forecasting) Often taken to be the 528 dekametre thickness contour line in the U.K. It is useful, but rather a crude guide as to whether snow will fall at sea level. Meteorologists will prefer to use other parameters, such as the 850-1000 mbar partial thickness, or the wet bulb freezing level, but even these must be used with care. [ See the Thickness FAQ ]


(more correctly - runway state report) A group added to the end of a METAR (q.v.) which gives information relating to ice / snow (and other 'slippery') conditions ON THE RUNWAY. (See this section of the site)

Solar radiation

Short-wave electromagnetic energy from the sun.

Source region

Extensive areas where air remains in the same place long enough to acquire the characteristics of an air mass.

Spanish Plume

A catchy name applied to what is in reality quite a complex process producing the conditions necessary for severe local storms over maritime N.W. Europe. Strictly, the 'spanish plume' is the warm/dry ex-Saharan air, that has passed over the Iberian peninsula; been lifted by forced ascent (due to near-jet level forcing), cooling and moistening and producing outbreaks of thundery rain from medium level cloud. Initially providing a 'lid' (see 'loaded gun') which inhibits deep/vigorous convection, its breakdown allows the sudden release of potential instability, with the fuel for the subsequent severe storms being provided by air of a high theta-W value often running in from the SSE. Thunderstorms, often severe, are most likely within the tongue of highest theta-W air (> 18 degC or so), and where there are low-level forcing agents: e.g. isobaric troughing, sea breezes, coastal convergence etc.



Specific heat

The amount of heat energy required to raise the temperature of a specified mass of a substance by unit measurement of temperature. The specific heat of liquid water (at 0°C) is 4218 J deg-1 kg-1. For ice, it is approximately half this value.

Specific humidity

The mass of water vapour in a unit mass of moist air.

Split-frontal type

A situation where the vertically thick/cold-top upper cloud (usually producing the significant rain) has moved well forward, and moves notably faster than the classically analysed surface front (wind shift, dew point drop etc.). With dry air over-running the rearward feature, the precipitation on the surface cold front is often light & 'drizzly'. Comparison of IR imagery (showing the sharp rearward boundary of the upper front), with the VIS imagery (showing the break from stratiform to broken, or cumuliform cloud structures), will identify such features very well. Sometimes an upper cold front will be analysed on UK Met Office charts when such a 'split' is well-marked. (See the FAQ here)


(abbr) Squall(s) (as defined below - used in aviation weather reports).


(SQ) A squall is differentiated from a gust by its greater duration: generally lasting for several minutes before decaying again. Squalls are often associated with the passage of fronts, particularly cold fronts, or well defined troughs, or with the 'gust front' from a well defined/mature supercell Cb. To qualify as a line squall, other marked changes are often observed, e.g. change of wind direction, fall of temperature etc. The following definition is used when estimating wind speeds using the Beaufort scale of wind speed: ".... a sudden increase of wind speed by at least three levels of the Beaufort scale, the speed rising to F6 or more and lasting for at least one minute."


(abbr) Sandstorm (vis generally < 1km); as used in aviation weather reports, forecasts etc.


Sea surface temperatures. The subject of much discussion regarding reliability, methodology etc. For basic synoptic forecasting, accuracy to within 1 degC is fine; for climate change studies, tenths of a degree are of vital importance: such differences are easily introduced using differing methods of measurement e.g. bucket versus engine intake.


Abbreviation for Sea Surface Temperature Anomalies, i.e. the difference between a short-period analysis (e.g. 5-day, 7-day or monthly 'snapshot' of a particular area of ocean) and the long-period 'normal' for a specified period such as 30 years. SSTA's are (or should be) a key component in any reliable long-range forecasting scheme.


Stratus (ST in METAR, aviation charts etc., St otherwise); a low-level cloud type, varying from thin, well broken fragmentary pockets of cloud to deeper, overcast and extensive layers giving rise to hill fog and occasional drizzle.

Stable air

Air is stable when an air parcel sinks or rises to its original position, when the force that initially moved it is no longer operating. [ See "Stable and unstable air masses" ]

Sting jet

In a small fraction of mid-latitude cyclonic storms, highly damaging surface winds are observed that cannot be attributed to the 'normal' gusts produced within an intense gradient flow, nor to those due to suspected tornadic development. It is thought that a zone of strong winds, originating from within the mid-tropospheric cloud head of an explosively deepening depression, are enhanced further as the "jet" descends, drying out and evaporating a clear path through snow & ice particles, the evaporative-cooling leading to the air within the jet becoming denser - helping to accelerate the downward flow. The "jet" hits the surface as a relatively narrow zone of highly damaging winds (~ 80kt / ~150km/h, or higher) running around the southern flank (Northern Hemisphere) of the parent low: these high winds (at the surface) are found just ahead of the hook-like tip of the cloud stream which is being wrapped around the southern semi-circle of the low - the shape of which in satellite cloud imagery gives rise to the name "Sting Jet" (allusion to a scorpion's tail). [refs: Browning, Clark, Hewson; thanks to the latter for help with this entry.]


Stationary, as used in aviation forecasts, SIGMET's etc.


The definition in any good dictionary usually involves a mention of a 'strong wind' but also couples the term to such as thunderstorms, hail, heavy rain etc. When used within the UK Met Office Shipping Forecast, High Seas forecasts (and associated Gale and Storm Warnings), then 'Storm/Force 10' is strictly defined as either the (10 minute) mean wind 48 to 55 knots, or gusts 61 to 68 knots. (See also comments at Severe Gale).

Storm surge

When persistent, severe gales (usually stronger), markedly low atmospheric pressure* and geographic 'funnelling' of the wind-driven sea water are combined with astronomically high tides, then the resulting storm (or tidal) surge can cause coastal sea levels to rise several metres above the astronomically predicted level, with inundation of low-lying areas. Notable examples in regions bordering the North Sea occurred in 1099, 1236, 1287, 1421, 1697 and 1953. The North Sea is particularly prone to such events because it is shallow relative to the open Atlantic - often the source region for storm-driven waters - and its depth decreases still further towards its southern/narrow end. (* a decrease in pressure of roughly 10 mbar produces a sea-level rise of about 10cm.)


Stratocumulus (Sc) formed (generated) by the spreading out of Cumulus (Cu), hence Sc cugen. There are three main mechanisms by which this cloud form develops: (a): on a morning of cumulus formation, when there is a well-defined 'lid' (inversion) to convective development, and that inversion is moist, then cumulus development is arrested and 'spreading-out' occurs .. see 'Overconvection'; (b): at the end of a day of cumulus (or cumulonimbus, Cb) development, as surface-based thermals weaken, the convective towers lose vigour, with subsidence beginning to dominate and the Cu (Cb) 'flattens-out' towards or around dusk; (c): around vigorous Cu/Cb towers, there is always subsidence - leading to small-scale, subsidence-formed stable layers adjacent to the convective towers - some of the cloud taking part in the shower/thunder development 'leaks out' sideways underneath these small-scale inversions, again leading to Sc cugen (or Sc cbgen in the case of Cb)


(Latin,stratum=layer) A layer of high stability (in thermodynamic terms) such that air motion is primarily horizontal, although near the boundary with the troposphere (q.v.), marked vertical motion can occur (forced by jet-stream actions), which are important in driving developments in the troposphere. (See the main FAQ here)


Rather than drawing isobars (lines joining places of equivalent mslp), or contours (lines joining places of equivalent geopotential height), it is sometimes better to describe graphically the wind flow by drawing lines with arrows showing the direction of the wind at any level. Often used on / near the surface. Although long used in tropical / sub-tropical areas, ( where isobaric analysis is of dubious value ), streamlines are also very useful at mid-latitudes, for instance to determine likely areas of convective activity due to convergent triggering.


The process of a solid being transformed directly to a gas or vice-versa. (See Latent Heat)

Sunshine recording

The 'classic' method of recording sunshine duration has relied on the use of a glass globe to focus the rays of the sun onto a medium (usually stout card) that is scorched when the sunshine is strong enough. By adding up the length of the scorch marks (which are related to bright sunshine duration), the total sunshine for the day can be assessed. This method has been used for over a century: the instrument based on this principle which is in common use is the Campbell-Stokes recorder (CSR); one of the drawbacks of this unit is that is over-estimates, due to "burn-spread", the sunshine duration on days of strong but intermittent sunshine. There are other problems, not least that a human is needed to change the card daily and assess the burn pattern. (See the Observer's Handbook for more on this). In recent years, sensors have been developed which measure direct solar radiation above a defined threshold (currently set at 120 W/m^2), and these units can be left unattended and require no day-to-day intervention, with the results automatically fed into standard weather reports both hourly and over an aggregate of 24hr. (See here): the instrument chosen by the UK Met Office is the Kipp and Zonen sensor (KZ). Comparisons confirm the long-held belief that the CSR over-estimates 'true' sunshine duration primarily when solar elevation is large and cloud well broken. The difference between the methods (CSR > KZ) may be as much as 15 - 20% in the summer, falling to less than 10% in mid-winter. These differences must be acknowledged when comparing sunshine data, particularly when looking for 'records' or trends.

Supercell storm

See the main FAQ here.

Supercooled water

Liquid water at temperature below 0°C.

Sustained wind

The standard (WMO) averaging period for the surface wind for synoptic reports is 10 minutes. However, for use in tropical storm forecasting, and more generally in North America and the Caribbean region, a one-minute period is used: these winds are referred to as sustained winds in tropical bulletins. A rough conversion is given by: SUSTAINED (1 MIN MEAN)=1.14 * WMO-STANDARD (10 MIN MEAN).

Sutcliffe development theory

In 1947, R.C. Sutcliffe published his seminal paper relating surface developments (in terms of vorticity forcing) to the patterns to be found on thickness charts, and developing an equation that could quantify such in terms of the thermal wind in the layer (500-1000hPa) and the vorticity diagnosed objectively across the layer and at the surface. The thermal wind appears in each of the equation's three terms, confirming the subjective observation that the stronger the thermal (tighter thickness gradient), the more intense is the subsequent synoptic development. [ For more on all this, see HERE. ]


Wind waves that have travelled out of the area in which they were generated or can no longer be sustained by the wind in the generating area. (see also Wind waves)


(from 'synoptic'... see the main FAQ here) A fully coded version of a meteorological report from a weather reporting station - in groups of 5 figures. For a good site which deals with decoding SYNOP data, visit Dave Wheeler's web site at: -



(strictly Frontal T-Bone) See "What is the Shapiro-Keyser cyclone model?"


Terminal Aerodrome Forecast (also known formerly as a TAFOR) A meteorological forecast for a specific airport/airfield for a period covering 9 to 24 hours in 'self-briefing' code.


(abbr) Tropical cyclone; used in aviation reports, forecasts etc.


Abbreviation used in the METAR/TAF code (as defined by ICAO), for a cloud type that meteorologists would call cumulus congestus (Cu con) - cumulus clouds which are characterised by marked 'sprouting' limbs, and which often extend to a high altitude - certainly well above the zero degree level. The bulging upper part is sometimes said to resemble a cauliflower. The cloud type is important for aviation as they can generate significant icing and turbulence in flight, and heavy precipitation, reduced visibility on/near the surface. (see also Cb)


Temporarily; a temporary fluctuation in (aviation/airfield) conditions expected to last less than one hour in each instance and not occur in total during more than half of the period indicated.


Takes its name from Temperature and Entropy (formerly noted by the greek letter 'phi'). Used in the UK Met Office to plot upper air soundings, and assess such as instability, depth of moisture/cloud layers, fog points, temperature of free convection, condensation level etc. Designed by Napier Shaw early in the twentieth century. Other thermodynamic diagrams are used by other services (e.g. the pseudoadiabatic or Stüve chart [ common on web sites ]; skewT/log(p) diagram: ( see also the entry for Thermodynamic diagrams. )

Terrestrial radiation

Long-wave (infra-red) radiation emitted by the earth (and other components of the earth-atmosphere system).

Thermal steering

Mid-latitude, mobile surface features (frontal depressions, mobile highs etc.) are often found to 'follow' the pattern of the thickness field over them. In particular, classic warm-sector depressions can be forecast to move in the direction of the thermal wind over the wave tip, with a speed 4/5 of the average thermal wind within 300 nautical miles (5 deg. of latitude) of the centre. As development takes place, the thermal field becomes increasingly distorted and the rule is less applicable. [Strictly speaking, it is the surface centres of positive and negative vorticity forcing that are being steered, rather than the feature itself. Sutcliffe quantified this as one of the terms of his development equation. ]

Thermal wind

A theoretical (vector difference) wind that relates the magnitude of the horizontal temperature gradient in a defined layer to the real winds that blow at the top and base of that layer. The speed of the thermal wind is proportional to the temperature gradient. (see the thickness FAQ)

Thermodynamic diagram

(also known as an Aerological diagram)A graphical plot of the observations of temperature and humidity, against pressure, as obtained via a Radio-Sonde ascent, or derived by sensing returns of radiant energy using an artificial satellite. Many diagrams are in current use, the three most often found are described here.


From the greek letter 'theta' and subscript 'w', used to denote wet bulb potential temperature (q.v.) - one of a group of pseudo-conservative (q.v.) properties of air masses.


The difference in height between two layers in the upper air. The most commonly used being the thickness between 500 mbar and 1000 mbar, and normally expressed in dekametres. The larger the value of thickness, the warmer the column of air (warm air expands). (See also "Thickness: what is it?")


The World Meteorological Organisation defines as follows: " One or more sudden electrical discharges, manifested by a flash of light (i.e. lightning) and a sharp or rumbling sound (i.e. thunder)". Cumulonimbus (Cb) clouds are required, providing the necessary 'factory' for the vigorous updraughts / downdraughts, which in turn generate the enhanced charge-separation mechanisms within the cloud, and thus the marked positive and negative areas found within a Cb. Positive charge is found concentrated in the upper parts of the cloud, and negative in the lower, but just above the zero degC level, with small, but important positive charge areas in the base area, just below the zero degC level. (for the meteorological conditions necessary for a thunderstorm to occur [ in mid-latitudes ], see "Conditions for thunderstorms".

Thundery rain

A phrase used in weather forecasts when some quite sharp bursts of rain are expected from large areas of unstable medium cloud - with some 'rumbles' of thunder; the thunder not the most significant feature of the situation, and no well developed thunderstorms with associated hail, squally winds etc., are expected.

Thundery showers

A phrase akin to "wintry showers" which has crept into the language because many forecasts have severe word constraints. If well-developed and 'vigorous' thunderstorms are expected, then the forecast should say so, but when you only have 4 words to play with, some short-hand phraseology is necessary on a showery day with perhaps some scattered 'rumbles' of thunder.

Time Standard

The international meteorological community use UTC (or GMT) as the standard 'clock' for observations and forecasts. The 'day' is based on 00UTC and 12UTC, with multiples of 6 hours used for main observations. However, some countries (e.g. Australia), will use slightly differing conventions. When making an observation, it is important to state what 'clock' you are using, e.g. local time, GMT, etc.


A vortex extending upward from the surface (land or sea) at least as far as the cloud base (with that cloud base associated with deep moist convection), and is intense enough at the surface to do damage at one or more points along its path. (Doswell, C.A. III, 2001 / via: ... this will change in the near future)


Tropical Prediction Center (part of NCEP) also the National Hurricane Center (NHC) for the United States. For a wealth of information relating to tropical storm meteorology, visit the NHC site at:-


A two-hour 'significant weather' forecast appended to a routine (or special) weather report intended for aviation purposes. (see also METAR).


The point where a cold, warm and occluded front all meet. Given the right upper-air structure, a discrete area of low-pressure can form here.


The (usually) abrupt change from falling temperatures with height in the troposphere, to near-uniform, or rising temperatures in the stratosphere. For coding purposes, defined as the lowest level at which the lapse rate decreases to 2 degC/km or less. (with caveats to rule out lower level inversions.) (See the FAQ entry on the various levels of the atmosphere, here)


(Greek, tropos=turn ) lowest layer of the atmosphere, with an average depth of 16 to 18 km around the equator, 9 to 12 km temperate latitudes and well below 9 km much of the time in arctic regions. There is a general fall of temperature with height (i.e. a positive lapse rate), with an average value of some 6.5 degC/km (or 2 degC/1000ft). (See the FAQ entry on the various levels of the atmosphere, here)


A feature on a weather map where mean sea level pressure (or upper contour heights) are lower than surrounding areas of the atmosphere, with a 'V' shape to the isobars/contours evident in the pattern. Often associated with unsettled/cloudy weather, but not always. (see also upper trough, and "What is a trough?".

Trough disruption

The process whereby part of an upper trough moves 'forward' (usually west to east), leaving a portion of the old trough behind. (See anticyclonic trough disruption and cyclonic trough disruption.)

Trough extension

If an upper trough or thermal trough develops a markedly increased amplitude it is said to have undergone meridional extension. The change is usually defined in terms of a latitude change of a defined contour or thickness isopleth.


(abbr) Thunderstorm (as used in aviation weather reports, forecasts etc.)


(abbr) Turbulence (as used in aviation weather reports, forecasts etc: e.g. MOD TURB).


Atmospheric motion that shows irregular and random motion over very small distances and over short intervals of time.



(or UKMetO) United Kingdom Meteorological Office. The HQ, (and central computing / NWP facility) was formerly located at Bracknell, Berkshire, UK., but since late summer of 2003 is now in Exeter, Devon [ together with the Hadley Centre for Climate Prediction and Research]. (NB: they now like to be called 'the Met Office'). Visit the web site at:-


See Cold undercut

Unstable air

An air parcel will continue to move away (rise or fall) from its original location when the initial force exerted on it is removed. [ see the FAQ entry on stable and unstable air masses.

Upper Air

Normally taken to be the levels between 850 mbar (about 5000 ft), and 200 mbar (39000 ft). The most common heights used being 500 mbar (18000 ft) and 300 mbar (30000 ft).

Upper fronts

It has long been recognised that the simple 'Norwegian model' (q.v.) of a frontal zone extending with a defined slope from the surface to the top of the troposphere did not often fit the observed weather experienced on the ground or by aircraft in flight. Often, bands of precipitation which were not ideally tied to the classical front were observed, and with the coming of satellite imagery, structures could be detected which are now classified as upper fronts. On actual/prognostic charts issued by meteorological centres (strictly these are surface charts), such features are shown using the classical symbology, but the triangles (cold fronts) and 'bobbles' (warm fronts) are not filled in, and are termed upper cold and upper warm fronts respectively.

Upper Ridge

A ridge on an upper air chart - evidence of warm air in depth through the troposphere.

Upper Trough

A trough on an upper air chart - evidence of cold air in depth through the troposphere.



(abbr) Volcanic ash: as used in aviation reports, forecasts etc.


(abbr) In vicinity: used in aviation reports, forecasts etc., e.g. VCFG, fog in vicinity but not at airfield.


See Backing

Vertical vorticity

Synoptic-scale vorticity about a vertical axis, caused by horizontal differences in wind speed (the 'shear' term), and / or horizontal 'curving' of the air-flow (the 'curvature' term). It is often assessed at the 500 hPa level (around 5.5 km). This property is linked to divergence / convergence at upper levels, which in turn will lead to enhancement of vertical air movement - key to synoptic developments. (See also Vorticity; Horizontal vorticity.)

Violent Storm

This term is only used in bulletins for shipping, and associated Gale/Storm Warnings. A 'Violent Storm/Force 11' is defined as the (10 minute) mean wind of between 56 and 63 knots. (Gusts not defined) (See also comments at Severe Gale).


Visible imagery ... see the FAQ entry on satellite imagery.

Visibility [ horizontal ]

(from the Observer's Handbook) " the greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky, or, in the case of night observations, could be seen and recognized if the general illumination were raised to the normal daylight level. With the advent of automatic sensors to detect this parameter, the WMO re-defined the above in terms of the 'meteorological optical range' (MOR), which is given by the path-length over which a light source of known intensity/colour is reduced to a certain percentage of the original flux.

Visibility [ reports ]

As used in SYNOP, SHIP and METAR observations: the value reported as the visibility in surface reports has for many years been the lowest visibility (as defined above) that an observer can determine. So, for example, if the general value were 6 to 9 km, but in one particular direction (say towards a major town or over adjacent coast) it was as low as 3000 m, then it would be this latter value that was recorded, e.g. 3000 m. Until 2003-2004, this was also the scheme adopted for METAR reports world-wide. However, the concept of 'prevailing' visibility has been introduced for these latter, and can be taken to be the dominant visibility (applicable to at least 50% of the local horizon, continuously or otherwise), with separate groups for significantly lower values reported as required. For more on this, see the page relating to the METAR code.

Visibility [ vertical ]

Used in aviation reports to assess how far upwards a surface-based observer can 'see' through fog or snow. (See VV).

Volcanic effects

The effect on the atmosphere due to a major volcanic eruption depend critically upon the plume maximum-altitude and its sulphur content. Unless the plume penetrates into the stratosphere (and does so for a substantial length of time), material only affects the troposphere, and will be washed-out by precipitation (rain, snow etc.): in such cases, the effects will be regional and short-lived. In broad terms, sulphate aerosols penetrating to the stratosphere in quantity warm the stratosphere (due to absorption) and cool the troposphere (by radiation back-scatter), but the effect is highly size-dependent; only the smallest particles lead to long-term near-surface cooling.


Voice weather broadcast for in-flight briefing, often using automated interpretation of weather bulletins.


A measure of the 'spin' of a portion of a fluid - in our case, of atmospheric particles. Vorticity in a cyclonic sense is designated 'positive', and in an anticyclonic sense is designated 'negative'. In synoptic meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the atmospheric flow as depicted on classical 'weather maps': vertical vorticity. However, the vertical component of vorticity is important in the study of tornadoes for example: this is horizontal vorticity. (See also: Absolute vorticity; Horizontal vorticity; Relative vorticity; Potential vorticity; Vertical vorticity.)


(abbr) Variable wind direction, as used in aviation weather reports, forecasts etc.


(abbr) Vertical speed, as used in aviation weather reports, forecasts etc. Particularly used with respect to forecasts of mountain wave motion.


(abbr) Vertical visibility, as used in aviation weather reports, forecasts etc., e.g. VV002, vertical visibility 200ft. Where vertical visibility cannot be determined (or realistically forecast), then the group will be seen as VV///.



(abbr) World Area Forecast Centre - there are two such centres covering high-level / intercontinental air routes across the globe: "London" and "Washington" (actually located at Exeter, UK & Kansas, USA respectively). In addition, RAFC (Regional Area Forecast Centres) are provided by various major weather forecasting services around the world to cater for medium-level work on a continental/sub-continental scale, though increasingly even this work is being concentrated on just a few centres. Purely low-level work (roughly below 10000ft) is the prerogative of national weather services (NWS's), or equivalent military formations. All charts issued by WAFC's, RAFC's & NWS offices are produced to standards agreed between member states of the International Civil Aviation Organisation (ICAO). For more on SIGWX charts see here.

Warm advection

The replacement of cold air by warm (usually) due to horizontal movement of air-masses. The process can be gradual or abrupt: the latter often being marked by a 'warm front' on a synoptic chart.

Warm anticyclone

This feature has a warm core (relative to surrounding air), with cold / dense air dominant in the upper part of the troposphere / lower stratosphere, contributing to the net high pressure observed at the surface. Has a deep circulation (i.e. 'high' / ridge found on charts extending to tropopause levels), with that tropopause high/cold etc. Often associated with major long-wave blocking patterns. However, note that 'warm' anticyclones can have cold air trapped in the lowest layers under the surface inversion, and can of course be the means of advecting cold air (if it was there in the first place) around it's periphery. (See Anticyclones; Cold anticyclone)

Warm occlusion

If air (at the surface) behind an occluded front is warmer than the air it is displacing (a typical example being when a winter-time Polar Continental 'block' is being eroded by an incursion of Polar Maritime air), then the front is known as a warm occlusion. The occlusion is shown on synoptic charts as a linear extension of the warm front; the cold front leaving the occlusion/warm front at a fairly sharp angle. (see also Cold occlusion).

Warm-front wave

A secondary disturbance, often accompanied by a shallow closed-low circulation, that forms at some point on a marked warm frontal boundary a good way (at least 1000 km) from the parent (occluding) depression. Once formed, it moves quickly away from the parent depression (in the Northern Hemisphere east or southeastwards). Although not common, they are often responsible for considerable forecast errors, and are of particular importance in winter (snow-situation) forecasting as mild, maritime air attempts to displace a cold, continental blocking anticyclone. (See also cold-front wave).

Warm sector

The area between a warm and cold front in the classical Norwegian model frontal system.

Warnings - UK Public Service

The Met Office is responsible for providing advice and warning of forthcoming Severe Weather within the United Kingdom. There are various categories; see here.


See Wet Bulb Potential Temperature


(abbr) Widespread, as used in aviation weather reports, forecasts etc.

Wet Bulb Freezing Level

Because evaporative cooling is so important in rain versus snow forecasting (see the main FAQ entries, here and here), the wet-bulb 0degC level is a better guide to snow-risk level than the actual ZDL. The wet-bulb is the lowest temperature to which air can be cooled by evaporating water into a sample, which is a mechanism often important in determining whether it will rain or snow at or near the surface. However, note carefully that advection, both cold and warm must be taken into account - the former is particularly important in marginal situations in the vicinity of a well-marked cold front. (see also "Downward penetration of snow" in this Glossary).

Wet Bulb Potential Temperature

(often abbreviated to WBPT, or 'theta-W') A relatively conservative property within any one air mass that is derived from the temperature and humidity values of a particular air sample for a particular level: usually 850 or 500 hPa. Very warm/very humid southerlies for example (in NW Europe) would have typical 850hPa WBPT values well in excess of 16 degC, and perhaps as high as 20 degC or more; polar maritime air streams would have values typically 5 to 10 degC, but these values would be much lower in the depths of winter.

Wet day

A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 1.0mm or more has been recorded. (See also Rain day).


Abbreviation for warm front.

Wind shear

The change in wind direction, or speed, or both, either in a vertical or horizontal plane. Vertical shears are important in the study of convection, particularly for severe storm development; horizontal shear, particularly speed shear, contributes to relative vorticity terms in synoptic development.

Wind waves (sea)

Waves generated by the wind blowing over the surface of the ocean. (See also Swell).

Wintry precipitation

(often used as 'wintry showers') When the air temperature is close to zero deg.C (either side), it is sometimes easier to use this shorthand term for showers producing soft hail, sleet, snow, 'sleety-rain' etc. Frowned upon by proper meteorologists but a useful term nonetheless. (However, we try to avoid it when the showers are much heavier, and are expected to give significant snowfall.)


(abbr) Weakening, as used in aviation forecasts, particularly SIGMETs.


(abbr) Used in the newsgroup to report a 'weather event'; entered in the subject line thus: [WR] snow becoming heavy at Tilehurst 1540UTC. The use of this abbreviation allows those who do not wish to read such reports to ignore them during 'busy' weather periods. (See also OBS)


(abbr) Wind shear, as appended to METAR reports etc. This abbreviation should only be used to indicate actual (or forecast) conditions experienced by aircraft in the atmospheric boundary layer.


Water vapour imagery...see the FAQ entry on satellite imagery.



Zulu time (GMT/UTC), as used in aviation reports, forecasts and more generally by the meteorological fraternity used to military usage. It is gradually being ousted though by UTC (or sometimes just UT).


Zero-degree (celsius) Level. A somewhat better description of this variable than 'freezing' level. (q.v.)


A predominantly west-to-east airflow is termed zonal (and an east-to-west airflow is negative zonal).The strength of the flow in any sector may be expressed in terms of a zonal index given by the difference in average contour height between two latitude circles through the sector.