uk.sci.weather FAQ: GLOSSARY

(This issue dated.....08 JAN 2000)

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 by myself in association with Paul Bartlett, David Reynolds and 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. PLEASE NOTE HOWEVER THAT THIS WILL NOT INCLUDE EVERY ABBREVIATION TO DO WITH METEOROLOGY....THE LIST WOULD BE ENDLESS! 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 ng 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.

(Use these navigation bars to move to the alphabetical grouping required.)

A
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.)
Adiabatic: 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)].
Advection: 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.
Ana-front: 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.
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.)
Asynoptic: 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. (q.v.)

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

C
CA: [cloud-to-air lightning flash] Used when describing lightning which branches from a cumulonimbus cloud and terminates in clear air. This is an uncommon type of lightning.(See also CC, CG & GC)
Calibration (radar): as well as de-cluttering for permanent echoes, and other adjustments, rainfall radar returns are calibrated (in real-time) against a network of telemetering rain gauges. This means of course that if the return is over areas without rain gauges (e.g. the sea), over-reading can occur, and caution is needed in blindly following radar imagery to assess rainfall rates/accumulations for this reason.
CAPE: [ Convectively Available Potential Energy ] -- A measure of the energy released once convection is initiated, often from the surface (for high values), but mid-level convective initiation is also very important. Assessed on a thermodynamic diagram (e.g. a tephigram) by noting the area enclosed by the environment curve (i.e. actual temperature found by a radio-sonde), and the parcel-path curve where it intersects the environment curve at height. Used extensively in severe convective storm studies, although worth noting that just because high values of CAPE are observed, other factors need to be right for a severe storm to develop.
CAT: [Clear Air Turbulence] Bumpy conditions in the upper atmosphere when no clouds are present to betray the possibility of such. Caused by sharp vertical and horizontal shear of wind, often (but not exclusively) in association with upper-level jet streams (see the main FAQ, Q/A 2A.1). Can occur in, or be enhanced by mountain wave activity.
CAV: [ Conservation of Absolute Vorticity ] -- The principle first outlined by Carl-Gustav Rossby in the 1930's which accounts for the tendency for upper atmospheric flow to take up a wave-like pattern. The theory can be used to predict the wavelength and speed of translation of the long-waves found in the atmosphere, which in turn govern the broad 'weather type' at any one point.
CC: [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)
CET: [Central England Temperature] -- A series used to track temporal changes in the average temperatures over a large area of 'central England'...see the main FAQ, Q/A 2B.11.
CG: [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. ]
Cloud head (strictly Baroclinic cloud head):--During the early stages of 'explosive cyclogenesis' (q.v.), a very marked area of dense layered cloud - convex away from the developing depression - can be observed in IR, VIS and WV imagery, detached from the cloud area associated with the development. This feature is the result of air rapidly ascending as the intense development gets underway. Studies have shown that all mid-latitude cyclogenetic events over oceanic areas giving rise to winds of hurricane force were preceded by such features. However, care is needed to correctly identify such and true detection is only possible with animated imagery. (See also Baroclinic leaf; Dry intrusion.)
Cluster: in ensemble forecasting (q.v.), individual members often show strong grouping around a few results. Each grouping is referred to as a cluster. The more members making up a particular cluster, the higher is the confidence in that particular solution. (See also Ensemble; Ensemble mean)
Cold-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.)
Confluent: When streamlines (q.v.) approach one another, the pattern is a confluent one. However, note that because streamlines only define the wind direction, and not the wind speed, a confluent pattern is not necessarily a convergent pattern. [ The opposite of confluent is diffluent (often spelt difluent in North American texts.). This case denotes the spreading apart of streamlines. Again, such diffluent patterns are not necessarily divergent. ] (see also convergence, divergence.)
Conservative property: Meteorologists are always keen to 'label' an air mass using a value that can be calculated from variables measured within that air mass (at various levels), but which remain constant, or nearly so in vertical (adiabatic [q.v.]) motion. Many properties are defined, such as potential temperature (Theta), equivalent potential temperature (Theta-e) and wet-bulb potential temperature (Theta-w). This latter measure is often used in operational meteorology in NW Europe: At the 850hPa level, it is used as a 'tracer' for air masses, and is much used for defining frontal boundaries, and for defining the axes of warm 'plumes' of air. (See also the entry under Wet Bulb Potential Temperature)
Contours: Lines on an upper air (constant pressure) chart (actual or forecast) joining places of equal height, 700 mbar; 500 mbar etc., or of equal thickness.
Contrail: CONdensation TRAIL...also abbreviated (from old coding practice) to COTRA. See the main FAQ: Q/A 2A.10
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 the main FAQ Q/A 2A.4). ).[ Computer models in operational use cope with instability features via parametrisation schemes (q.v.), which model 'ideal' convective towers within each model grid square, taking into account entrainment of dry air, moist convective vigour & depth, temperature structure etc. Algorithms will assign model rainfall to either 'dynamic' or 'convective': the type giving the greatest rate of rainfall is (usually) that which appears on the output chart. ](See also Dynamic and Orographic precipitation)
Convergence: When air flows in such a way that the area occupied by a particular 'group' of air particles lessens ('drawing together'), the pattern is said to be convergent. Convergence in the atmosphere is associated with vertical motion, and hence development (or weakening) of weather systems. For example, convergent flow near the surface is coupled to, and may be the primary cause of, upward motion, leading to cloud formation/shower initiation etc. . ( see also divergence, confluent.)
Conveyor: In synoptic systems (e.g. a developing depression) airflow is not uniformly horizontal, and the system velocity (i.e. the speed of translation of the Low) must also be allowed for. High-velocity air aloft overtakes the synoptic feature, whilst lower down, the system often moves faster in a given direction than the low level airflow. To cope with all this, the concept of 'conveyor belts' was adapted for use in synoptic and mesoscale meteorology as a means of explaining the movement of heat, moisture and momentum around such systems. For example, in a developing/mobile depression, a warm conveyor belt (WCB) is assumed to rise from low levels in the warm sector just ahead of the surface cold front, to middle and upper altitudes over and well forward of the surface warm front. A compensating cold conveyor belt (CCB), descends from medium/upper levels well ahead of the surface warm front underneath the WCB then tucks around the backside of the low merging with the boundary layer flow.
Cut-off time: NWP models that are used in operational meteorology must have a nominal time at which the 'gates are closed' to new data, and the forecast computation cycle is started. For models used for primary forecast guidance at short lead times, only a couple of hours at most is allowed after the nominal data time. So for example, the cut-off for 12UTC data might be around 1345UTC. For global models, i.e. those used for international aviation, a slightly longer time is allowed, but usually no more than 3.5hrs after data time. However, some centres (e.g. ECMWF) with less demand for immediate products allow over 9 hours or more of data to be assimilated.
Cyclogenesis: the formation of a major low pressure system along a baroclinic zone (q.v.) (or frontal boundary), with primary forcing due to imbalances along the upper jet.
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.

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

E
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.
ECMWF: European Centre for Medium Range Weather Forecasts, located next to the (UK) Met Office College at Shinfield, on the southern edge of Reading, Berkshire, UK. Visit their web site at:- http://www.ecmwf.int/
Ensemble: a collection of NWP runs (typically in excess of 15) 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. 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 applying these in small amounts to the initial conditions of the new run. [ NB: these output are in addition to (and run 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 is close to the ensemble mean (q.v.)]
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.
EWP:(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).
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.)

F
Fog point: (strictly fog-point temperature): is 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 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).
Frontogenetic: 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.
Frontolysis: 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.
Frost point: (strictly frost-point temperature): is 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).

G
Gale: 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 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. ]
GC: [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)
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.
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.
Gust: 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 10 minutes. 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.

H
Helicity: (see the main FAQ Q/A 2A.24)
hPa: 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)]
Humidity: (see the main FAQ Q/A 2A.17)
Hurricane Force: This term (in UK Met.Office use) is only used in shipping bulletins and associated Gale/Storm warnings. It is strictly defined as a mean (10 minute) wind of 64 knots or more. (Gusts not defined) (See also comments at Severe Gale).

I
Initialisation: 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.
Insolation: 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.
Intervention: 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.
IPV: 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).
IR: Infra-red (used in connection with satellite imagery)...see the main FAQ Q/A 2A.8.

J
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.

K
Kata-front: 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.
Knot: (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. ]

L
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.
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').

M
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.
mb (or MB): often seen as the abbreviation for 'millibar' (see also mbar).
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/cm^2 (c.g.s. system)=10⁵ N/m² (Pa) ( SI system).
MCC: Mesoscale Convective Complex (q.v.)
MCS: Mesoscale Convective System (q.v.)
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.
Meridional: 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.
METAR: 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.
Mountain Waves: (also known as standing, or lee waves..often abbreviated to MTW) -- 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.
MRF: Medium Range Forecast (or Global Spectral) model (run by the NCEP). Has 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).
Multi-cell storm: (see the main FAQ Q/A 2A.23)

N
NCEP: 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: http://www.ncep.noaa.gov/
NGP: (see NOGAPS)
NOGAPS: 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: http://152.80.56.202/index3.html
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.
NWP: 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.

O
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 & 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.

P
Pa: Pascal - allocated in honour of Blaise Pascal, to a unit of one N/m², the basic unit of pressure in the SI system.
Parametrisation:
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.
Polar low: (or Polar depression) See the main FAQ.. Q/A 2B.19
Potential Instability: (also known as Convective Instability) is 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 (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.
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.
Precipitation: 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:-
http://www.booty.demon.co.uk/metinfo/bletters.htm#7)
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).
Progression: 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).
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 at Q/A 2A.23, 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.

Q

R
Relative humidity: (see the main FAQ Q/A 2A.17)
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: 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.)
Relaxation: 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.
Retrogression: When an upper trough (or ridge) moves against the normal west-to-east flow in mid-latitudes, the feature is retrogressing, or undergoing retrogression.
Ridge (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.

S
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.
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.)
SFUK: part of the WMO 'header' code used in bulletins that carry atmospheric reports, more commonly known as 'sferics, or 'SFLOCS'. (See the main FAQ Q/A 2B.13)
Shear (see wind shear):
SIGMET : 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 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.
Snow-line: (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 at:.. http://www.booty.demon.co.uk/tthkfaq.htm ]
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.
SPECI: (see METAR)
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'. Comparision 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 Bracknell charts when such a 'split' is well-marked. (See the main FAQ Q/A 2B.17)
SST: 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.
Storm: 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 & 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.)
Streamlines: 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.
Supercell storm: (see the main FAQ Q/A 2A.23)
SYNOP: 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 address at: - http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm

T
TAF: 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.
Tephigram: 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. )
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 at ... http://www.booty.demon.co.uk/tthkfaq.htm
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 at:- http://www.booty.demon.co.uk/metinfo/thdyndia.htm
Theta-W: 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.
Thickness: 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 Q/A 2A.5 of the FAQ)
TPC: 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:- http://www.nhc.noaa.gov/
Tropopause: 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.)
Trough: 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 - this glossary, and the main FAQ Q/A 2A.20)
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.
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 and Cyclonic trough disruption.)

U
UKMO(or UKMetO): United Kingdom Meteorological Office. HQ, (and central computing/NWP facility) located at Bracknell, Berkshire, UK. Visit the web site at:- http://www.met-office.gov.uk/
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.

V
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).
VIS: visible imagery ... see the main FAQ. Q/A 2A.8.
Vorticity: 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 meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the general atmospheric flow. However, the vertical component of vorticity is important in the study of tornadoes for example. (See also: Absolute vorticity; Relative vorticity; Potential vorticity.)

W
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).
WBPT: (see Wet Bulb Potential Temperature)
Wet Bulb Freezing Level: Because evaporative cooling is so important in rain versus snow forecasting (see the main FAQ, Q/A's 2A.25 and 2A.26), 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.
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.
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.
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.)
WV: water vapour imagery...see the main FAQ. Q/A 2A.8.

X

Y

Z
ZDL: Zero-degree (celsius) Level. A somewhat better description of this variable than 'freezing' level. (q.v.)
Zonal: 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 along two latitude circles through the sector.

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METAR (and TAF) weather decode:

[ Used in the order: Qualifier + Descriptor + Phenomenon ... thus for heavy thunderstorm with rain=+TSRA; light freezing drizzle=-FZDZ ]

>> Weather type:
Precipitation

* although PE was the original two-letter abbreviation recommended for the METAR/TAF weather code, since the change whereby two (or more) weather groups can be used came into force, some users objected to the possible combination of rain (RA) and pellets (PE).

Reduction of visibility

Other events

>> Qualifiers:
Intensity

( NB: all phenomena are considered to be 'at the station' unless prefixed by VC=vicinity; thus VCFG is fog in the area, but not affecting the airfield. 'Vicinity is defined as within 8 km of the airfield. )

Descriptor

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