Aeronautical Terms beginning with S
Santa Ana
A hot, dry, foehn wind, generally from the northeast or east, occurring west of the Sierra Nevada Mountains especially in the pass and river valley near Santa Ana, California.
Saturated adiabatic lapse rate
The rate of decrease of temperature with height as saturated air is lifted with no gain or loss of heat from outside sources; varies with temperature, being greatest at low temperatures.
Saturation
The condition of the atmosphere when actual water vapor present is the maximum possible at existing temperature.
Scud
Small detached masses of stratus fractus clouds below a layer of higher clouds, usually nimbostratus.
Sea breeze
A coastal breeze blowing from sea to land caused by the temperature difference when the land surface is warmer than the sea surface. The sea breeze usually occurs during the day and alternates with the land breeze that blows in the opposite direction at night.
Sea fog
A type of advection fog formed when air that has been lying over a warm surface is transported over a colder water surface.
Sea level pressure
The atmospheric pressure at mean sea level, either directly measured by stations at sea level or empirically determined from the station pressure and temperature by stations not at sea level; used as a common reference for analyses of surface pressure patterns.
Sea smoke
Same as steam fog—fog formed when cold air moves over relatively warm water or wet ground.
Sector visibility
Meteorological visibility within a specified sector of the horizon circle.
Sensitivity time control
A radar circuit designed to correct for range attenuation so that echo intensity on the scope is proportional to reflectivity of the target regardless of range.
Severe Icing
The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and accumulation occurs in areas not normally prone to icing, such as aft of protected surfaces and other areas identified by the manufacturer. A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour6 on the outer wing. Immediate exit is required by many Airworthiness Directives (AD), flight manuals, and operations under part 91, §§ 91.13(a) and 91.527; part 121, § 121.341; part 125, § 125.221; and part 135, § 135.227.7
Severe Weather Forecast Alerts
Preliminary messages issued in order to alert users that a Severe Weather Watch Bulletin (WW) is being issued. These messages define areas of possible severe thunderstorms or tornado activity. The messages are unscheduled and issued as required by the Storm Prediction Center (SPC) at Norman, Oklahoma.
Shear
Wind shear. The rate of change of wind velocity (direction and/or speed) per unit distance; conventionally expressed as vertical or horizontal wind shear.
Shower
Precipitation from a cumuliform cloud; characterized by the suddenness of beginning and ending, by the rapid change of intensity, and usually by rapid change in the appearance of the sky; showery precipitation may be in the form of rain, ice pellets, or snow.
Significant Meteorological Information
SIGMET.
Significant Meteorological Information (SIGMET)
Information about in-flight weather of operational significance to the safety of all aircraft. SIGMETs may include severe icing. (See CWA and AIRMET.)
Significant weather prognostic
Presents four panels showing forecast significant weather.
Skyspotter
A pilot who has received specialized training in observing and reporting inflight weather phenomena.
Slant visibility
For an airborne observer, the distance at which he can see and distinguish objects on the ground.
Sleet
Ice pellets mixed with rain or snow.
Smog
A mixture of smoke and fog.
Smoke
A restriction to visibility resulting from combustion.
Snow
Precipitation composed of white or translucent ice crystals, chiefly in complex branched hexagonal form.
Snow flurry
Popular term for snow shower, particularly of a very light and brief nature.
Snow grains
Precipitation of very small, white opaque grains of ice, similar in structure to snow crystals. The grains are fairly flat or elongated, with diameters generally less than 0.04 inch (1 mm.).
Snow pellets
Precipitation consisting of white, opaque approximately round (sometimes conical) ice particles having a snow-like structure, and about 0.08 to 0.2 inch in diameter; crisp and easily crushed, differing in this respect from snow grains; rebound from a hard surface and often break up.
Snow shower
Precipitation from a cumuliform cloud; characterized by the suddenness of beginning and ending, by the rapid change of intensity, and usually by rapid change in the appearance of the sky; showery precipitation in the form of ice pellets or snow.
Solar radiation
The total electromagnetic radiation emitted by the sun.
Sounding
In meteorology, an upper-air observation; a radiosonde observation.
Source region
An extensive area of the earth’s surface characterized by relatively uniform surface conditions where large masses of air remain long enough to take on characteristic temperature and moisture properties imparted by that surface.
SPC
Storm Prediction Center
SPECI
Non-Routine (Special) Aviation Weather Report
Specific humidity
The ratio by weight of water vapor in a sample of air to the combined weight of water vapor and dry air. Compare mixing ratio.
Squall
A sudden increase in wind speed by at least 15 knots to a peak of 20 knots or more and lasting for at least one minute. Essential difference between a gust and a squall is the duration of the peak speed.
Squall line
Any nonfrontal line or narrow band of active thunderstorms (with or without squalls).
St. Elmo’s Fire
A luminous brush discharge of electricity from protruding objects, such as masts and yardarms of ships, aircraft, lightning rods, steeples, etc., occurring in stormy weather. Also called corposant.
Stability
A state of the atmosphere in which the vertical distribution of temperature is such that a parcel will resist displacement from its initial level.
Stagnation Point
The point on a surface where the local air velocity is zero. The region of maximum icing collection efficiency is near this point.
Standard atmosphere
A hypothetical atmosphere based on climatological averages comprised of numerous physical constants of which the most important are: (1) A surface temperature of 59° F (15° C) and a surface pressure of 29.92 inches of mercury (1013.2 millibars) at sea level; (2) A lapse rate in the troposphere of 6.5° C per kilometer (approximately 2° C per 1,000 feet); (3) A tropopause of 11 kilometers (approximately 36,000 feet) with a temperature of -56.5° C; and (4) An isothermal lapse rate in the stratosphere to an altitude of 24 kilometers (approximately 80,000 feet).
Standing cloud
A lenticular cloud. (standing lenticular altocumulus) A species of cloud whose elements have the form of more or less isolated, generally smooth lenses or almonds. These clouds appear most often in formations of orographic origin, the result of lee waves, in which case they remain nearly stationary with respect to the terrain (standing cloud), but they also occur in regions without marked orography.
Standing wave
A wave that remains stationary in a moving fluid. In aviation operations it is used most commonly to refer to a lee wave or mountain wave.
Station pressure
The actual atmospheric pressure at the observing station.
Stationary front
Same as quasi-stationary front—a front which is stationary or nearly so; conventionally, a front which is moving at a speed of less than 5 knots is generally considered to be quasi-stationary.
Stationary Front
A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.
Steam fog
Fog formed when cold air moves over relatively warm water or wet ground.
Storm detection radar
A weather radar designed to detect hydrometeors of precipitation size; used primarily to detect storms with large drops or hailstones as opposed to clouds and light precipitation of small drop size.
Stratiform
Descriptive of clouds of extensive horizontal development, as contrasted to vertically developed cumuliform clouds; characteristic of stable air and, therefore, composed of small water droplets.
Stratocumulus
A low cloud, predominantly stratiform in gray and/or whitish patches or layers, mayor may not merge; elements are tessellated, rounded, or roll-shaped with relatively flat tops.
Stratoshere
A layer of the atmosphere above the tropopause extending to a height of approximately 160,000 feet.
Stratosphere
The atmospheric layer above the tropopause, average altitude of base and top, 7 and 22 miles respectively; characterized by a slight average increase of temperature from base to top and is very stable; also characterized by low moisture content and absence of clouds.
Stratus
A low, gray cloud layer or sheet with a fairly uniform base; sometimes appears in ragged patches; seldom produces precipitation but may produce drizzle or snow grains. A stratiform cloud.
Stratus Clouds.
Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.
Stratus fractus
A stratus layer (low, gray cloud layer or sheet) that has a base in the form of irregular shreds (fractus), appearing as if torn.
Streamline
In meteorology, a line whose tangent is the wind direction at any point along the line. A flowline.
Structural icing
The accumulation of ice on the exterior of the aircraft.
Sublimation
Process by which a solid is changed to a gas without going through the liquid state.
Sublimation
A process in which ice turns directly into water vapor without passing through a liquid state.
Subrefraction
Less than normal bending of the radar beam resulting from abnormal vertical gradients of temperature and/or water vapor.
Subsidence
A descending motion of air in the atmosphere over a rather broad area; usually associated with divergence.
Summation principle
The principle states that the cover assigned to a layer is equal to the summation of the sky cover of the lowest layer plus the additional coverage at all successively higher layers up to and including the layer in question. Thus, no layer can be assigned a sky cover less than a lower layer, and no sky cover can be greater than 1.0 (10/10).
Superadiabatic lapse rate
A lapse rate greater than the dry-adiabatic lapse rate.
Supercooled Large Drops (SLD)
Water drops with a diameter greater than 50 micrometers (0.05 mm) that exist in a liquid form at air temperatures below 0 °C. SLD conditions include freezing drizzle drops and freezing raindrops.
Supercooled water
Liquid water at temperatures colder than freezing.
Supercooled water droplets
Water droplets that have been cooled below the freezing point, but are still in a liquid state.
Superrefraction
More than normal bending of the radar beam resulting from abnormal vertical gradients of temperature and/or water vapor. (3)
Surface analysis chart
A report that depicts an analysis of the current surface weather. Shows the areas of high and low pressure, fronts, temperatures, dewpoints, wind directions and speeds, local weather, and visual obstructions.
Surface inversion
An inversion with its base at the surface, often caused by cooling of the air near the surface as a result of terrestrial radiation, especially at night.
Surface visibility
Visibility observed from eye-level above the ground.
SVRWX
Severe Weather
SWSL
Supplemental Weather Service Locations
Synoptic chart
A chart, such as the familiar weather map, which depicts the distribution of meteorological conditions over an area at a given time.
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