S

Sandstorm – particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Saturation – a condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Scattered – a cloud layer that covers between 3/8ths and 1/2 of the sky.

Scud clouds – small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Sea breeze – a wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Sea-level pressure – the pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Secondary cold front – a front that follows a primary cold front and ushers in even colder air.

Severe thunderstorm – a strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4 inch or more.

Shallow fog – fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.

Shelf-cloud – a low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front.

Short-fuse warning – a warning issued by the National Weather Service for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shower – precipitation that is intermittent, both in time, space or intensity.

Sky condition – the state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.

Sleet – rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes. In Australia, sleet refers to a mixture of rain and snow or falling snow that is melting into rain.

Slight chance – in probability of precipitation statements, usually equivalent to a 20 percent chance.

Slight risk (of severe thunderstorms) – severe thunderstorms are expected to affect between two and five percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.

Smog – pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.

Smoke – a suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 statute miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.

Snow – frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow burst – very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

Snow flurries – light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow grains – light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. In Canada, Snow grains are described as minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.

Snow pellets – precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). In Canada, snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers and are often accompanied by snowflakes or rain drops, when the surface temperature is around zero degrees Celsius.

Snow shower – snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow squalls – intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning. In Canada, it is described that a snow squall brings strong winds, flurries and poor visibility.

Snowfall – the depth of new snow that has accumulated since the previous day or since the previous observation.

Snowflake – white ice crystals that have combined in a complex branched hexagonal form.

Special marine warning – issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more. This is usually associated with severe thunderstorms or waterspouts.

Spray – an ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.

Squall – a strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. In Canada, a squall is defined as a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. In Australia, it is described as a sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value.

Stability – an indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.

Stable air – air with little or no tendency to rise, usually accompanied by clear dry weather.

Steam fog – fog that is formed when water vapor is added to air which is much colder than the vapor's source. This is most common when very cold air drifts across relatively warm water.

Steering winds (steering currents) – a prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Storm – in marine usage, winds 48 knots (55 miles per hour) or greater.

Storm surge – a rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.

Storm warning – a marine wind warning for sustained winds greater of 48 knots (55 miles per hour) or more from a non-tropical system.

Straight line winds – thunderstorm winds most often found with the gust front.

Subtropical storm – a low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation).

Sunny – sunny or a few clouds means that less than half the sky has clouds.

Supercell – a severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.

Supersaturation – the condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.