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Saturation (of air) - the presence in air of the most water possible under existent pressure and temperature

Saturation Vapor Pressure - see equilibrium vapor pressure

Scattering - the process by which small particles are forced to change their direction of motion

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 gust fronts

Sea Breeze - a cooling breeze blowing generally in the daytime inland from the sea, caused by the temperature difference when the sea surface is cooler than the adjacent land

Sea Level Pressure - the atmospheric pressure computed from the station pressure for the given elevation of the station above mean sea level

Sensible Heat - the heat absorbed or transmitted when the temperature of a substance changes but the substance does not change state

Severe Thunderstorm - a thunderstorm with wind gusts of 50 knots (58 mph) or greater, hail at least three-quarters of an inch in diameter, and/or a tornado or funnel cloud

Shear - see wind shear

Shelf Cloud - a low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with 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 (or Shortwave Trough) - a disturbance in the middle or upper part of the atmosphere which induces upward motion ahead of it

Shortwave Radiation - in meteorology, radiation having a wavelength equal to or less than that of visible light

Shower - intermittent precipitation from a convective cloud, generally of short duration

Sky Cover - a term used to describe the amount of sky covered or concealed by clouds or obscuring phenomena. Classifications for sky cover include clear, broken, partly cloudy, and overcast.

Sleet - frozen or partly frozen falling rain; ice pellets

Smog - a natural fog made heavier and darker by smoke and chemical fumes

Snow - precipitation in the form or small tabular and columnar white ice crystals formed directly from the water vapor of the air at a temperature of less than 0 degrees Celsius

Snow Flurries - popular term for a light snow shower

Snow Pellets - precipitation in the form of white, opaque, approximately round ice particles, about 2 to 5 mm in diameter, with a snow-like structure

Snow Shower - intermittent snow falling from a convective cloud

Snowflake - a flake or crystal of snow

Solar Radiation - the radiation emitted by the sun

Solid - one of the three basic phases of matter; a substance that does not flow under moderate stress

Solstice - the times of the year when the sun appears to the farthest north or south of the equator, lying above either the Tropic of Cancer or the Tropic of Capricorn

Sounding - a plot of the vertical profile of temperature and dew point (and often winds) above a fixed location; used extensively in weather forecasting

Sounding-based Stability Index - an index calculated from balloon observations (e.g., CAPE, Lifted Index, K-Index, Bulk Richardson Number, Total-Totals Index) that provide guidance about the potential organization, type, and severity of thunderstorms (supercell, multi-cell, etc.)

Southern Oscillation - the reversal of typical surface air pressure patterns across the tropical Pacific that occurs during a major El Niño event

SPC (Storm Prediction Center) - a national forecast center in Norman, Oklahoma, which is part of NCEP; responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding), and severe winter weather over the contiguous United States

Specific Heat - the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius

Specific Volume - volume per unit mass; the reciprocal of density

Speed Shear - the component of wind shear resulting from a change in wind speed with height (e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet). Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Splitting Storm - a thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

Squall Line - any line or narrow band of active thunderstorms which is not directly along a frontal boundary

Standard Atmosphere - a hypothetical vertical distribution of atmospheric temperature, pressure, and density

Standard Atmospheric Pressure - the pressure exerted by a 760 millimeter column of mercury at sea level at a temperature of 0 degrees Celsius; equal to 1013.25 millibars (mb), 29.92 inches of mercury (in of Hg), or 14.7 pounds per square inch

Stationary Front - the boundary between two air masses neither of which is replacing the other

Station Model - the specific pattern for entering meteorological symbols on a weather map that describe the state of the weather at that geographical location

Station Pressure - the actual pressure measured at a given station location after being corrected for temperature, gravity, and instrument error

Stefan-Boltzmann Law - a mathematical relationship for electromagnetic radiation that states the irrandiance of a blackbody is proportional to the fourth power of the absolute temperature of the blackbody

Storm - a disturbance of the atmosphere marked by wind and usually by rain, snow, hail, sleet, or thunder and lightning

Storm Centroid - the location of the center of a given storm. The NEXRAD RPG runs a storm detection and tracking algorithm to determine storm centroids automatically for each volume scan

Storm-Relative - measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity

Storm-Relative Velocity - the wind velocity minus storm motion. The wind at a given location may be the combination of the environmental flow plus winds due to a thunderstorm. NEXRAD produces a storm-relative radial velocity product from the base velocity product by subtracting the average motion of all identified storms on the radar scope.

Storm-Scale - referring to weather systems with sizes on the order of individual thunderstorms

Storm Surge - an atypical rise of the sea along a shore primarily resulting from the winds of a storm, especially those of a hurricane

Straight-Line Winds - generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Stratiform - having extensive horizontal development, as opposed to the more vertical development characteristic of convection; stratiform precipitation, in general, is relatively continuous and uniform in intensity

Stratocumulus - a low-level cloud in the form of a gray and/or whitish flat layer or patch, which nearly always has dark parts and is non-fibrous

Stratus - a low-level cloud in the form of a gray layer with a rather uniform base

Sublimation - the process of changing from a solid directly to vapor or gas

Subsidence - sinking (downward) motion in the atmosphere, usually over a broad area

Subtropical High - a semi-permanent high pressure region near 30 degrees latitude

Suction Vortex (sometimes Suction Spot) - a small but very intense vortex within a tornado circulation; much of the extreme damage associated with violent tornadoes is attributed to suction vortices.

Summer - the period extending from the summer solstice, about 21 June, to the autumnal equinox, about 22 September

Summer Solstice - the solstice when the sun is highest in the sky; the first day of summer

Sundog - a colored luminous spot appearing 22 degrees (or somewhat more) on either side of the sun and at the same elevation as the sun

Sun Pillar - a luminous streak of white or slightly reddened light extending vertically above and below the sun, most frequently observed near sunrise or sunset

Supercell (or Supercell Storm) - a violent thunderstorm which can produce hail and large tornadoes and containing updrafts and downdrafts that are nearly in balance, allowing it to maintain itself for several hours

Synoptic - relating to or displaying conditions as they exist simultaneously over a broad area

Synoptic Scale - the scale of the migratory high and low pressure systems of the lower troposphere; generally considered 1000 to 2500 km in length


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