Weather Glossary - S
Search
SagoPrecipitation consisting of small (generally less than 5mm in diameter) transparent ice pellets. Often a precursor to snow. |
SALRSaturated 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). |
Satellite imageImages 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. |
ScatteredDescribes 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. |
Sea wavesWaves 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 breezeA 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 fogFog 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. |
SeasonsIn Australia, the seasons are defined by grouping the calendar months in the following way:
These definitions reflect the lag in heating and cooling as the sun appears to move southward and northward across the equator. They are also useful for compiling and presenting climate-based statistics on time scales such as months and seasons. These definitions do not apply in tropical regions. There are two more distinct seasons - wet and dry. |
Severe thunderstormA thunderstorm which produces any or several of the following:
|
ShearThe 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 cloudA 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. |
ShowersIntermittent 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 heightThe 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. |
SleetA mixture of rain and snow or falling snow that is melting into rain. |
SmogSmog - 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. |
SnowPrecipitation 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 levelThe elevation in mountainous terrain where precipitation changes from rain to snow. |
SoundingA 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 busterAn 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 IndexThe Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. |
Speed shearThe 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 stormA 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. |
SpringThe 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. |
SquallA 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 lineA 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. |
SSTSea Surface Temperature |
StabilityOccurs when a rising air parcel is denser than the surrounding air. |
Staccato lightningA CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching. |
Steering windsA prevailing synoptic scale flow which governs the movement of smaller features embedded within it. |
Stevenson ScreenA 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 windsWinds 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 relativeMeasured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity. |
Storm-scaleReferring to weather systems with sizes on the order of individual thunderstorms - generally around 10 km. See synoptic scale, mesoscale. |
Straight line windsGenerally, 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. |
StratiformHaving 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). |
StratosphereLayer of the atmosphere between about 10 and 50 kilometres above the ground. |
Stratus(St) Latin - layer |
StriationsGrooves 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 lowEssentially the same as mesolow. |
Sub tropical jetThe boundary between subtropical air and tropical air, marked by a concentration of isotherms and vertical shear. Migrates north in the southern hemisphere winter. |
SublimationThe phase change from a solid to a gas. The opposite of crystallisation. |
SubsidenceSinking (downward) motion in the atmosphere, usually over a broad area. Generally associated with anticyclonic conditions, high pressure and clear skies. |
SubtropicalThe 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. |
SummerThe 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. |
SundogA 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.
|
SunshineDirect radiation from the sun, as opposed to the shading of a location by other obstructions. |
SupercellA 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). |
Supercooled waterWater 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 convectionConvection 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 IndexSevere Weather ThrEAT index. A stability index developed by the US Air Force which incorporates instability, wind shear, and wind speeds as follows:
SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs. |
Swell wavesWaves 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. |
SYNOPSYNOPtic 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 chartAny 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. |
Weather News
Townsville airport closes, but Tropical Cyclone Kimi weakens, is unlikely to cross coast
01:15 EDT
The Bureau of Meteorology (BOM) says it is now unlikely weakening Tropical Cyclone Kimi will significantly impact the north Queensland coast.
South-east Queensland lashed by 'very dangerous' storms and intense rainfall
23:34 EDT
Dangerous thunderstorms that hit south-east Queensland on Monday night have brought substantial rain to the region, with one bayside suburb recording 118mm in one hour.