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(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: -