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: - http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm



(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: http://www.cimms.ou.edu/~doswell/ ... 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:- http://www.nhc.noaa.gov/


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:- http://www.metoffice.gov.uk/


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 uk.sci.weather 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.