Aeronautical Terms beginning with S


The change of (a) ice to water vapor or (b) water vapor to ice.


More than normal bending of the radar beam resulting from abnormal vertical gradients of temperature and/or water vapor. (3)


Less than normal bending of the radar beam resulting from abnormal vertical gradients of temperature and/or water vapor.

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.

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.


The condition of the atmosphere when actual water vapor present is the maximum possible at existing temperature.


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. Compare land breeze.

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.


Wind shear. The rate of change of wind velocity (direction and/or speed) per unit distance; conventionally expressed as vertical or horizontal wind shear.


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.

Slant visibility

For an airborne observer, the distance at which he can see and distinguish objects on the ground.


Ice pellets mixed with rain or snow.


A mixture of smoke and fog.


A restriction to visibility resulting from combustion.


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.


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.

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.


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).


A state of the atmosphere in which the vertical distribution of temperature is such that a parcel will resist displacement from its initial level.

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.

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.

Station pressure

The actual atmospheric pressure at the observing station.

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.


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.


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.


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.


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 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.


In meteorology, a line whose tangent is the wind direction at any point along the line. A flowline.


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 water

Liquid water at temperatures colder than freezing.

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.

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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