Weather Glossary - S

Sago

Precipitation consisting of small (generally less than 5mm in diameter) transparent ice pellets. Often a precursor to snow.

SALR

Saturated Adiabatic Lapse Rate. The rate at which the temperature of saturated air (air at 100% relative humidity) will vary as it is raised or lowered through the atmosphere. The SALR varies with temperature and pressure, however it is always less than the DALR. The SALR can be as low as -3K/km (temperature reducing as the air is raised).

The SALR is less than the DALR due to latent heat release as water vapour in the air is condensed. This latent heat release acts to raise the temperature of the surrounding air.

Satellite image

Images of the Earth taken from a satellite. The most common is the infrared image which indicates the temperature of the cloud tops (or the land or sea in cloud free areas). Infrared satellite images are most commonly used, as they can be used day and night.Visible satellite images are useful for identifying low cloud features (which do not appear bright on infrared images), however visible images can only be taken during daylight hours.Water vapour images are used to identify moisture in the atmosphere that may not be in the form of clouds. These images show water vapour from around 800 hPa (2000m) and upwards.

Scattered

Describes the cloud cover when between 3/8ths and 4/8ths of the sky is obscured by cloud

Scud

(or fractus) Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gustfronts. Such clouds are generally associated with cool moist air, such as thunderstorm outflow.

Scud cloud forming ahead of a supercellular thunderstorm near Muswellbrook, NSW.

Sea waves

Waves generated by the wind blowing at the time, and in the recent past, in the area of observation. They are generally smaller and choppier than swell waves.

Sea breeze

A local onshore wind. Cooler, more humid air from over the sea flows onto the coast to replace the warm air rising over the land. On sunny days the land heats up more quickly and to a greater extent than the sea. The air in contact with the land warms and expands and the resulting changes in the pressure and temperature differences and distributions generate the sea breeze circulation. At night, when the land cools more quickly and to a greater extent than the sea, the reverse land breeze circulation can develop.

Sea fog

Fog which develops when the sea temperature is less than the dew point of the air above it. Sea fog is also called advection fog, because the formation of the fog nearly always requires the advection of warmer air over a cooler surface.

Seasons

In Australia, the seasons are defined by grouping the calendar months in the following way:

• Spring - the three transition months September, October and November.
• Summer - the three hottest months December, January and February.
• Autumn - the transition months March, April and May.
• Winter - the three coldest months June, July and August.

Severe thunderstorm

A thunderstorm which produces any or several of the following:

• Hail 2 cm in diameter or larger.
• Flash flooding
• Wind gusts in excess of 48 knots (96 km/h)
• Tornado

Shear

The variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e. the change in wind with height, but the term is also used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Shelf cloud

A low, horizontal wedge-shaped cloud, associated with a thunderstorm gustfront or a cold front, even in the absence of thunderstorms. Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Shortwave trough

(or shortwave) A disturbance in the mid or upper levels of the atmosphere which induces upward motion ahead of it. If other conditions are favourable, the upward motion can contribute to thunderstorm development ahead of a shortwave trough.

Showers

Intermittent precipitation (snow, sleet, liquid water) from cumuliform cloud, usually of short duration which starts and ends suddenly. Can be very heavy as in thunderstorms (cumulonimbus).

Significant wave height

The average of the highest one third of the waves. The likely maximum wave height can be up to twice the significant wave height, occuring around 1 in 2000 waves.

Sleet

A mixture of rain and snow or falling snow that is melting into rain.

Smog

Smog - contraction for 'smoke fog'. An unpleasant fog in which smoke or other atmospheric pollutants (sea spray, dust) have an important part in causing the fog to thicken.

Snow

Precipitation of ice crystals, most of which are branched (sometimes star shaped). In cold conditions, snowflakes may be tiny, individual crystals. At warmer temperatures, snowflakes may clump or freeze together to produce larger snowflakes.

Snow level

The elevation in mountainous terrain where precipitation changes from rain to snow.

Sounding

A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.

Southerly buster

An abrupt southerly wind change, often producing strong and squally winds and sometimes accompanied by thunderstorms. These occur along the NSW coastline, mainly during the summer months. The Southerly Buster is not associated with a low pressure system in the Tasman, rather it is a shallow cold front moving up the coast from Victoria and Tasmania.

Southern Oscillation Index

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.

Positive SOI values indicate increased probability of above average rainfall over northern and eastern Australia. Positive SOI,s are associated with lower air pressure over Darwin, stronger Pacific trade winds and warmer sea temperatures to the north of Australia. This is popularly known as a La Niña episode.

Negative SOI values suggest drier weather or a reduction in rainfall over eastern and northern Australia. Negative SOIs indicate higher air pressure over Darwin and a decrease in the strength of the Pacific Trade Winds. Sustained negative SOI values are linked to El Niño episodes.

Speed shear

The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 knots at 10,000 feet increasing to 50 knots at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Spin up

[Slang] A small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

Splitting storm

A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The right mover typically moves faster than the original storm, the left mover, slower.

Of the two, the right mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the left mover is the one most likely to become a supercell.

Spring

The transition months of September, October and November between winter and summer (in the Southern hemisphere). In Australia spring is characterized by the onset of the northern wet season and eastern thunderstorm season. Spring also brings an end to the southern wet season.

Squall

A sudden increase in the mean wind speed which lasts for several minutes at least before returning to near its previous value. A squall may include many gusts.

Squall line

A solid or nearly solid band of active thunderstorms. Generally, the distance between individual storms is less than the diameter of the individual storms. Accompanied by strong, squally winds. Generally occur in situations of little directional wind shear but strong speed shear.

SST

Sea Surface Temperature

Stability

Occurs when a rising air parcel is denser than the surrounding air.

Staccato lightning

A CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

Steering winds

A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Stevenson Screen

A standard white louvered box which contains standard meteorological instruments such as wet and dry-bulb thermometers. All BoM weather stations have their instruments inside a Stevenson screen.

Storm force winds

Winds with mean speed exceeding 48 knots or roughly 89 km/h. Storm force winds are the strongest winds used in midlatitudes. In tropical areas, Hurricane force is used to describe winds with a mean speed in excess of 64 knots.

Storm relative

Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm-scale

Referring to weather systems with sizes on the order of individual thunderstorms - generally around 10 km. See synoptic scale, mesoscale.

Straight line winds

Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds. Straight line winds occur from downbursts and can reach wind speeds similar to torndaic speeds.

Stratiform

Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

Stratocumulus

(Sc) Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of a low-level jet (if present).

Stratosphere

Layer of the atmosphere between about 10 and 50 kilometres above the ground.

Stratus

(St) Latin - layer
A low, generally grey cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog is usually a surface-based form of stratus.
Stratus

Striations

Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an LP storm.

Sub synoptic low

Essentially the same as mesolow.

Sub tropical jet

The boundary between subtropical air and tropical air, marked by a concentration of isotherms and vertical shear. Migrates north in the southern hemisphere winter.

Sublimation

The phase change from a solid to a gas. The opposite of crystallisation.

Subsidence

Sinking (downward) motion in the atmosphere, usually over a broad area. Generally associated with anticyclonic conditions, high pressure and clear skies.

Subtropical

The region between the tropical and temperate regions, between 23.5 and about 35 to 40 North and South. This is generally an area of semi permanent high pressure and low precipitation, explaining why much of Australia is arid or semi arid.

Suction vortex

(sometimes Suction spot) A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

Summer

The three hottest months December, January and February (for the southern hemisphere). Summer in Australia marks the middle of the northern wet season and southern dry season.

Sundog

A sun dog or sundog is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.

Sundogs may appear as a coloured patch of light to the left or right of the sun, 22° distant and at the same distance above the horizon as the sun, and in ice halos. They can be seen anywhere in the world during any season, but they are not always obvious or bright. Sundogs are best seen and are most conspicuous when the sun is low.

Sunshine

Direct radiation from the sun, as opposed to the shading of a location by other obstructions.

Supercell

A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the left of the main environmental winds (i.e., they are left movers).

Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm.

Storms exhibiting these characteristics are often called classic supercells; however HP storms and LP storms are also supercell varieties.

A supercell thunderstorm in inland Queensland.

Supercooled water

Water which has been cooled below 0 °C but remains in liquid form. Occurs when no seed crystal or nucleus is present around which a crystal structure can form.

Surface based convection

Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.

SWEAT Index

Severe Weather ThrEAT index. A stability index developed by the US Air Force which incorporates instability, wind shear, and wind speeds as follows:

SWEAT=(12 Td 850 ) + (20 (TT-49)) +( 2 f 850) + f 500 + (125 (s+0.2)) where

• Td 850 is the dew pint temperature at 850 mb,
• TT is the total-totals index,
• f 850 is the 850-mb wind speed (in knots),
• f 500 is the 500-mb wind speed (in knots), and
• s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).

Swell waves

Waves which have travelled into the area of observation after having been generated by previous winds in other areas. These waves may travel thousands of kilometres from their origin before dying away. There may be swell present even if the wind is calm and there are no 'sea' waves.

SYNOP

SYNOPtic report. Similar to a METAR, but gives additional details of the weather at a site. SYNOP's are only reported at most every three hours.

Synoptic chart

Any map or chart that depicts meteorological or atmospheric conditions.

Synoptic-scale

(or large scale) A size scale referring generally to weather systems with horizontal dimensions of several hundred kilometres or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.