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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • S band - See radar frequency band.

  • SAGE - Abbreviation for Stratospheric Aerosol Gas Experiment.

  • SAM - Abbreviation for Stratospheric Aerosol Measurement.

  • SAR - Abbreviation for synthetic aperture radar.

  • satellite - 1. An artificial platform placed into orbit around the earth, often carrying instruments to gather environmental data. 2. Any natural or man-made object that orbits about an astronomical body.

  • saturation - The condition in which vapor pressure is equal to the equilibrium vapor pressure over a plane surface of pure liquid water, or sometimes ice.

  • scale - 1. Regular markings on an instrument used to allow the reading of the measured quantity or setting. 2. A factor that relates the indication of the measuring instrument to the value of the quantity. 3. An order of magnitude aid in estimating meteorological parameters (e.g., mesoscale).

  • SCAPE - Abbreviation for slantwise convective available potential energy.

  • scatter - 1. Same as scattering; or, sometimes used in referring to the scattered radiation. 2. The relative dispersion of points on a graph, especially with respect to a mean value, or any curve used to represent the points. See scatter diagram, spread.

  • scattered - 1. In radiation, see scattering. 2. A sky coverage of 1/8 through 4/8. In U.S. weather observing procedures, this is reported with the contraction "SCT."

  • scope - The general abbreviation for an instrument of viewing, such as telescope, microscope, or oscilloscope. In radar installations, the cathode-ray oscilloscope indicators are commonly referred to as scopes or radarscopes.

  • SCR - Abbreviation for Selective Chopper Radiometer.

  • sea - 1. Same as ocean. 2. A subdivision of an ocean. All seas except "inland seas" are physically interconnected parts of the earth's total saltwater system. Two types are distinguished, mediterranean and adjacent. Mediterraneans are groups of seas, collectively separated from the major water body as an individual sea. Adjacent seas are those connected individually to the larger body. 3. Same as state of the sea. 4. Sea surface waves within their fetch; opposed to swell. See fully developed sea.

  • sea breeze - A coastal local wind that blows from sea to land, caused by the temperature difference when the sea surface is colder than the adjacent land. Therefore, it usually blows on relatively calm, sunny, summer days, and alternates with the oppositely directed, usually weaker, nighttime land breeze. As the sea breeze regime progresses, the wind develops a component parallel to the coast, owing to the Coriolis deflection. See lake breeze, brisa, doctor, virazon, sea-breeze front.

  • sea level - The level of the sea after averaging out the short-term variations due to wind waves. It is used loosely as a synonym for mean sea level.

  • sea surface temperature - The temperature of the ocean surface. The term sea surface temperature is generally meant to be representative of the upper few meters of the ocean as opposed to the skin temperature, which is the temperature of the upper few centimeters.

  • season - A division of the year according to some regularly recurrent phenomena, usually astronomical or climatic. See winter, spring, summer, autumn.

  • SEC - Abbreviation for South Equatorial Current.

  • secondary low - Same as secondary cyclone.

  • seeing - (Or astronomical seeing.) A term long used by astronomers for the degradation of images by the atmosphere.

  • SEM - Abbreviation for Space Environmental Monitor.

  • sequence - With respect to aviation weather observations, same as collective.

  • severe weather - Generally, same as severe storm.

  • shear - The variation (usually the directional derivative) of a vector field along a given direction in space. The most frequent context for this concept is wind shear.

  • shear line - In meteorology, a line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line; a line of maximum horizontal wind shear.

  • shield - See rain-gauge shield, radiation shield.

  • short wave - 1. (Or minor wave.) With regard to atmospheric circulation, a progressive wave in the horizontal pattern of air motion with dimensions of cyclonic scale, as distinguished from a long wave. A short wave moves in the same direction as that of the prevailing basic current through the troposphere. The angular wavenumber of short waves ranges between eight and twenty. See cyclone wave. 2. A wave with a relatively short wavelength and period. For ocean wind waves, this usually means waves with periods shorter than about 60 s.

  • shower - Precipitation from a convective cloud. Showers are characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky. In weather observing practice, showers are always reported in terms of the basic type of precipitation that is falling, that is, rain showers, snow showers, sleet showers. In aviation weather observations, these are encoded RW, SW, and EW, respectively. See airmass shower.

  • SI - Acronym for the Systeme Internationale, or the International System of Units.

  • SID - Abbreviation for sudden ionospheric disturbance.

  • sink - A route or reservoir by which a measurable quantity may exit a system, such as by accumulation (in a reservoir) or chemical conversion. Compare source.

  • sinking - A mirage in which the image of distant objects is displaced downward. Because the displacement increases with distance, a horizontal surface, such as that of a body of water, desert, or road, appears to bend downward and one's perception is that of being on top of an inverted bowl or possibly on a planet with a very much smaller radius. Indeed, the downward bending surface results in an (optical) horizon that can be very much closer to the observer than in the absence of a mirage. Sinking is an example of an inferior mirage. The opposite of sinking is looming. Sinking occurs when the concave side of light rays from a distant object is up, and this in turn occurs when the refractive index of the atmosphere increases with height. This only happens near a surface when the heat flux is upward and so the temperature gradient decreases with height. This is common over a warm surface (such as might occur over sun-warmed ground or a lake at night). Sinking is often accompanied by a two-image inferior mirage. Compare stooping.

  • site - See site of station.

  • skill - A statistical evaluation of the accuracy of forecasts or the effectiveness of detection techniques. Several simple formulations are commonly used in meteorology. The skill score (SS) is useful for evaluating predictions of temperatures, pressures, or the numerical values of other parameters. It compares a forecaster's root-mean-squared or mean-absolute prediction errors, Ef, over a period of time, with those of a reference technique, Erefr, such as forecasts based entirely on climatology or persistence, which involve no analysis of synoptic weather conditions: SS = 1 - (Ef/Erefr). If SS > 0, the forecaster or technique is deemed to possess some skill compared to the reference technique. For binary, yes/no kinds of forecasts or detection techniques, the probability of detection (POD), false alarm rate (FAR), and critical success index (CSI) may be useful evaluators. For example, if A is the number of forecasts that rain would occur when it subsequently did occur (forecast = yes, observation = yes), B is the number of forecasts of no rain when rain occurred (no, yes), and C is the number of forecasts of rain when rain did not occur (yes, no), then POD = A/(A + B) FAR = C/(A + C) CSI = A/(A + B +C). For perfect forecasting or detection, POD = CSI =1.0 and FAR = 0.0. POD and FAR scores should be presented as a pair.

  • sky - 1. The apparent surface against which all aerial objects are seen from the earth. 2. Same as sky condition, sky cover, state of the sky.

  • sky cover - In surface weather observations, a term used to denote one or more of the following: 1) the amount of sky covered but not necessarily concealed by clouds or by obscuring phenomena aloft; 2) the amount of sky concealed by obscuring phenomena that reach the ground; or 3) the amount of sky covered or concealed by a combination of 1) and 2). Opaque sky cover is the amount of sky completely hidden by clouds or obscuring phenomena, while total sky cover includes this plus the amount of sky covered but not concealed (transparent). Sky cover, for any level aloft, is described as thin if the ratio of transparent to total sky cover at and below that level is one-half or more. Sky cover is reported in tenths, so that 0.0 indicates a clear sky and 1.0 (or 10/10) indicates a completely covered sky. Amount of sky cover for any given level is determined according to the summation principle. The following classifications of sky cover are used in aviation weather observations: clear, scattered, broken, overcast, partial obscuration, obscuration.

  • sleet - 1. See ice pellets. 2. In British terminology, and colloquially in some parts of the United States, precipitation in the form of a mixture of rain and snow.

  • slope flow - Along-slope flows generated by heating or cooling of the slope. A warm slope surface, as produced by daytime heating, generates anabatic or upslope winds, whereas a cool surface, as from nocturnal cooling, generates katabatic or downslope winds. Slope flows are a component of mountain-valley and mountain-plains wind systems.

  • small hail - Hail with a diameter less than 0.64 cm (0.25 in.). See ice pellets.

  • sno - Same as elvegust.

  • snow - Precipitation composed of white or translucent ice crystals, chiefly in complex branch hexagonal form and often agglomerated into snowflakes. For weather-observing purposes, the intensity of snow is characterized as 1) light when the visibility is 1 km (5/8 statute mile) or more; 2) moderate when the visibility is less than 1 km (5/8 statute mile) but not more than 1/2 km (5/16 statute mile); and 3) heavy when the visibility is less than 1/2 km (5/16 statute mile).

  • snow cover - 1. The areal extent of snow-covered ground, usually expressed as percent of total area in a given region. 2. In general, a layer of snow on the ground surface. Compare snowfield, snowpack. 3. The depth of snow on the ground, usually expressed in inches or centimeters.

  • snow flurry - Common term for a light snow shower, lasting for only a short period of time.

  • snowfall - 1. In surface weather observations, usually expressed as centimeters or inches of snow depth per six-hourly period. 2. The accumulation of snow during a specified period of time.

  • socked in - In the early days of aviation, commonly used to describe weather at an airport when ceiling and/or visibility were of such low values that the airport was effectively closed to aircraft operations. The term probably originated from reference to the wind sock. When the sock was in the clouds or when the visibility was so badly impaired that the sock was not visible, the airport was "socked in." The expression is still widely used.

  • sound - A pressure fluctuation, usually in the range of audible frequencies, resulting from a displacement of a gas, liquid, or solid, that can be detected by a mechanical or electromechanical transducer (e.g., a barometer, microphone, or the human ear).

  • sounding - 1. In geophysics, any penetration of the natural environment for scientific observation. 2. In meteorology, same as upper-air observation. However, a common connotation is that of a single complete set of radiosonde observations. 3. The measurement of the depth of water beneath a vessel.

  • soundings - See atmospheric sounding.

  • source - In hydrodynamics, a point, line, or area at which mass or energy is added to a system, either instantaneously or continuously. Conversely, a sink is a point where mass or energy is removed from the system. An incompressible fluid will possess sources or sinks of mass only at points where the divergence of its velocity vector is nonzero; a source is associated with positive divergence and a sink with negative divergence (convergence). The fluid is usually assumed to pass outward from a source or inward to a sink equally in all directions along radial lines. The strength of a source, for example, the rate of mass flow of fluid of unit density across a curve enclosing the source, is given by Q = 2p[&pgr;] rvn, where r is the distance from the source and vn the radial speed.

  • southeaster - (Often contracted sou'easter.) A southeasterly wind, particularly a strong wind or gale, for example, the winter southeast storms of the Bay of San Francisco. A specific application to a local wind is the southeaster of Table Bay, South Africa (Table Mountain southeaster). When there are dark clouds over Table Mountain, it is a black southeaster; when there are no clouds, it is a blind southeaster. See Cape doctor.

  • souther - A south wind, especially a strong wind or gale.

  • southwester - (Often contracted sou'wester.) A southwest wind, particularly a strong wind or gale.

  • SPECI - In the METAR observation program, a surface observation issued on a nonroutine basis as dictated by changing meteorological conditions.

  • specific - Applied to a physical quantity, especially a thermodynamic variable, usually means per unit mass. For example, specific volume is volume divided by mass, specific enthalpy is enthalpy divided by mass, and so on. Specific quantities are common in meteorology, and hence specific is often omitted as a qualifier, it being understood.

  • spit - Colloquially, a fall of rain or snow in very light amount; a sprinkle, or snow flurry.

  • SPOT - Abbreviation for Satellite pour l'Observation de la Terre.

  • spread - 1. (Also called variability.) The general departure of individual values from central tendency. Spread is reflected geometrically in the probability curve as the width of the region over which the probability density is appreciable. See scatter, variance. 2. Popular contraction for dewpoint spread.

  • spring - The season of the year composing the transition period from winter to summer; the vernal season, during which the sun is approaching the summer solstice. In popular usage and for most meteorological purposes, spring is customarily taken to include the months of March, April, and May in the Northern Hemisphere, and September, October, and November in the Southern Hemisphere. Except in the Tropics, spring is a season of rising temperatures and decreasing cyclonic activity over continents. In much of the Tropics, neither spring nor fall is recognizable, and in polar regions, both are very short-lived.

  • Sq - The solar daily magnetic variations on quiet days. It is determined from the records at geomagnetic stations for days that are believed free of noncyclic disturbances.

  • squall - 1. A strong wind characterized by a sudden onset, a duration of the order of minutes, and then a rather sudden decrease in speed. In U.S. observational practice, a squall is reported only if a wind speed of 16 knots or higher is sustained for at least two minutes (thereby distinguishing it from a gust). See line squall, white squall, williwaw. 2. In nautical use, a severe local storm considered as a whole, that is, winds and cloud mass and (if any) precipitation, thunder and lightning. See squall line, arched squall, black squall, sumatra, Abrolhos squalls.

  • squall line - A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. The squall line is a type of mesoscale convective system distinguished from other types by a larger length-to-width ratio.

  • SSU - Abbreviation for Stratospheric Sounding Unit.

  • stability - 1. The characteristic of a system if sufficiently small disturbances have only small effects, either decreasing in amplitude or oscillating periodically; it is asymptotically stable if the effect of small disturbances vanishes for long time periods. A system that is not stable is referred to as unstable, for which small disturbances may lead to large effects. Some authors also distinguish a neutral or marginally stable case, in which disturbances do not vanish, but also do not grow without bound. Classically, stability was defined only with respect to systems in equilibrium. More recently it has been extended to apply to evolving systems, for which an unstable disturbance leads to an evolution that becomes uncorrelated with the undisturbed evolution. From this standpoint stability and predictability can be equated. 2. Same as static stability. 3. The property that each computed solution (in exact arithmetic) of a finite difference approximation remains bounded for all possible choices of the time step. See Lax equivalence theorem. 4. The ability of laminar flow to become turbulent in a fluid.

  • stable air - Air in which static stability prevails, a condition that depends on the vertical gradients of air temperature and humidity.

  • stable air mass - Air mass having a stable stratification in its lower layer, and consequently free from convection, having a low degree of turbulence, and containing either stratiform clouds, fog, or no clouds at all.

  • stage - The level of the water surface in a stream, river, or reservoir, measured with reference to some datum.

  • stand - The condition at high water or low water when there is no change in the height of the tide. Compare slack water.

  • standard - An established unit of measure, reference instrument, or method appropriate for the calibration of other instruments.

  • station - 1. In science generally, a permanent or temporary location where scientific observations and measurements are made. In meteorology, several types of stations are officially defined, including first-order station; first-order climatological station; second-order station; second-order climatological station; third-order climatological station; climatological substation; ocean weather station. 2. In oceanography, the geographic location at which any set of oceanographic observations was taken; also, the observations recorded at the location. The appropriate verbal phrase is "occupy a station." See also serial station; compare ocean station, ocean weather station.

  • stationary front - Same as quasi-stationary front.

  • statistic - Generally, a number describing some characteristic of a population or samples therefrom. Specifically, an estimate of a statistical parameter computed from a sample.

  • statistical - Pertaining to or characterizing random phenomena, or referring to statistics.

  • steady flow - A system in which the flow characteristics, such as depth of flow or mean velocity, at any given point, do not change with time.

  • steady state - (Also called steady motion, stationary motion.) A fluid motion in which the velocities at every point of the field are independent of time; streamlines and trajectories are identical. Sometimes it is further assumed that all other properties of the fluid (pressure, density, etc.) are also independent of time. All local derivatives in the fundamental equations then vanish. A steady-state solution to a theoretical problem suggests two further questions: how the steady state came to exist (the initial-value problem), and whether it will persist (the instability problem).

  • steam - 1. A popular term for mixing cloud. 2. Water vapor at a temperature greater than the boiling point.

  • steering - In meteorology, loosely used for any influence upon the direction of movement of an atmospheric disturbance exerted by another aspect of the state of the atmosphere. Thus, it might be said that a surface pressure system tends to be steered by isotherms, contour lines, streamlines aloft, warm-sector isobars, the orientation of a warm front, etc. Nearly always this principle is applied to the relationship between the velocity of a cyclone and the velocity of the basic flow in which it is embedded.

  • steering flow - (Or steering current.) A basic flow that exerts a strong influence upon the direction of movement of disturbances embedded in it. This is the concept of steering in meteorology.

  • still well - A device, used in evaporation pan measurements, that provides an undisturbed water surface and support for the hook gauge. The National Weather Service model consists of a brass cylinder, 8 in. high and 3.5 in. in diameter, mounted over a hole in a triangular galvanized iron base that is provided with leveling screws.

  • storm - 1. Any disturbed state of the atmosphere, especially as affecting the earth's surface, implying inclement and possibly destructive weather. There are at least three somewhat different viewpoints of storms. 1) In synoptic meteorology, a storm is a complete individual disturbance identified on synoptic charts as a complex of pressure, wind, clouds, precipitation, etc., or identified by such mesometeorological means as radar or sferics. Thus, storms range in scale from tornadoes and thunderstorms, through tropical cyclones, to widespread extratropical cyclones. 2) From a local and special interest viewpoint, a storm is a transient occurrence identified by its most destructive or spectacular aspect(s). In this manner we speak of rainstorms, windstorms, hailstorms, snowstorms, etc. Notable special cases are blizzards, ice storms, sandstorms, and duststorms. 3) To a hydrologist, "storm" alludes primarily to the space- and time-distribution of rainfall over a given region. See local storm, severe storm. 2. See magnetic storm 3. (Also called storm wind, violent storm.) In the Beaufort wind scale, a wind with a speed from 56 to 63 knots (64 to 72 mph) or Beaufort Number 11 (Force 11).

  • STP - Abbreviation for standard temperature and pressure, that is, 0°C and 1 atmosphere (1013.3 mb).

  • stratiform - Descriptive of clouds of extensive horizontal development, as contrasted to the vertically developed cumuliform types. See stratus, altostratus, cirrostratus, nimbostratus, stratocumulus.

  • stratocumulus - (Abbreviated Sc.) A principal cloud type (cloud genus), predominantly stratiform, in the form of a gray and/or whitish layer or patch, which nearly always has dark parts and is nonfibrous (except for virga). Its elements are tesselated, rounded, roll-shaped, etc.; they may or may not be merged, and usually are arranged in orderly groups, lines, or undulations, giving the appearance of a simple (or occasionally a cross-pattern) wave system. These elements are generally flat-topped, smooth, and large; observed at an angle of more than 30° above the horizon, the individual stratocumulus element subtends an angle of greater than 5°. When a layer is continuous, the elemental structure is revealed in true relief on its under surface. Stratocumulus is composed of small water droplets, sometimes accompanied by larger droplets, soft hail, and (rarely) by snowflakes. When the cloud is not very thick, the diffraction phenomena corona and irisation appear. Under ordinary conditions, ice crystals are too sparse even to give the cloud a fibrous aspect; however, in extremely cold weather, ice crystals may be numerous enough to produce abundant virga, and sometimes even halo phenomena. Mamma may be a supplementary feature of stratocumulus, in which case the mammiform protuberances may develop to the point where they seem about to detach themselves from the main cloud. Virga may form under the cloud, particularly at very low temperatures. Precipitation rarely occurs with stratocumulus. Stratocumulus frequently forms in clear air. It may also form from the rising of stratus, and by the convective or undulatory transformation of stratus, or nimbostratus, with or without change of height (Sc stratomutatus or Sc nimbostratomutatus). Stratocumulus is analogous to altocumulus and forms directly from the latter when the elements grow to a sufficient size (Sc altocumulomutatus). The further humidification, accompanied by turbulence and/or convection, of an already humid layer of air near the base of nimbostratus or even altostratus can form stratocumulus (Sc nimbostratogenitus or Sc altostratogenitus). If the ascending currents that produce cumulus or cumulonimbus approach an upper layer of stable air, they slow down, and all or a portion of the mother-cloud tends to diverge gradually and spread horizontally, often producing stratocumulus (Sc cumulogenitus or Sc cumulonimbogenitus). A particular form of Sc cumulogenitus, previously called stratocumulus vesperalis, often occurs in the evening when convection decreases, resulting in the gradual dissipation of both bases and tops of cumuliform clouds. Since stratocumulus may be transformed directly from or into altocumulus, stratus, and nimbostratus, all transitional stages may be observed. By convention, altocumulus is composed of apparently smaller elements (often simply because of its higher altitude); stratus and nimbostratus do not show regular subdivisions or wave form, and they have a more fibrous aspect. When the base of stratocumulus is rendered diffuse by precipitation, the cloud becomes nimbostratus. See cloud classification.

  • stratus - (Abbreviated St.) A principal cloud type (cloud genus) in the form of a gray layer with a rather uniform base. Stratus does not usually produce precipitation, but when it does occur it is in the form of minute particles, such as drizzle, ice crystals, or snow grains. Stratus often occurs in the form of ragged patches, or cloud fragments (stratus fractus), in which case rapid transformation is a common characteristic. Stratus clouds have characteristically low vertical velocities, usually less than 1 m s-1. When the sun is seen through the cloud, its outline is clearly discernible, and it may be accompanied by corona phenomena. In the immediate area of the solar disk, stratus may appear very white. Away from the sun, and at times when the cloud is sufficiently thick to obscure it, stratus gives off a weak, uniform luminance. The particulate composition of stratus is quite uniform, usually of fairly widely dispersed water droplets and, at lower temperatures, of ice crystals (although this is much less common). Halo phenomena may occur with this latter composition. Dense stratus often contains particles of precipitation. The prior existence of any other cloud in the low or middle levels is seldom required for the formation of stratus. A common mode of stratus development is the transformation of fog, the lower part of which evaporates while the upper part may rise (St nebulomutatus). As can be expected by its close relationship to fog, stratus follows a diurnal cycle with a maximum (over land) in the night and early morning. Insolation tends to dissipate this cloud rapidly, and often brings about the transformation of stratus fragments to cumulus clouds. Fog arriving from the sea frequently becomes stratus over the adjacent land. Coastal regions also provide the low-level moisture and frequently the lapse-rate stability conducive to its formation, and therefore these areas have the greatest stratus status. Stratus also develops from stratocumulus when the undersurface of the latter descends or for any reason loses its relief or apparent subdivisions (St stratocumulomutatus). Nimbostratus and cumulonimbus often produce stratus fractus, as precipitation from these clouds causes low-level condensation (St fractusnimbostratogenitus or St fractus cumulonimbogenitus). Stratus fractus in this form constitutes the accessory feature pannus of these mother-clouds. Stratocumulus and nimbostratus are the clouds most difficult to distinguish from stratus. Stratus is lower and lacks the uniform undulations or relief of stratocumulus. More difficulty is encountered when differentiating it from nimbostratus. Their modes of formation are different, nimbostratus usually having been formed from a preexisting mid- or low-level cloud; nimbostratus is more dense and has a wetter aspect, and its precipitation is of the ordinary varieties. As a final distinction, the wind accompanying nimbostratus is usually stronger than that with stratus. See cloud classification.

  • stream - Body of water moving under the influence of gravity, to lower levels, in a well-defined natural channel.

  • sub-tropical cyclone - A cyclone in tropical or subtropical latitudes (from the equator to about 50°N) that has characteristics of both tropical cyclones and midlatitude (or extratropical) cyclones. They occur in regions of weak to moderate horizontal temperature gradient and extract the associated available potential energy, as do baroclinic cyclones, but they also receive some or most of their energy from convective redistribution of heat acquired from the sea, as do tropical cyclones. These storms usually have a radius of maximum winds that is larger than what is observed in purely tropical systems, and their maximum sustained winds have not been observed to exceed about 32 m s-1 (64 knots). Subtropical cyclones sometimes become true tropical cyclones, and likewise, tropical cyclones occasionally become subtropical storms. Subtropical cyclones in the Atlantic basin are classified by their maximum sustained surface winds: Subtropical depressions have surface winds less than 18 m s-1 (35 knots), while subtropical storms have surface winds greater than or equal to 18 m s-1. While these storms are not given names, forecasters do issue warnings for them.

  • sub-tropical jet - A band of relatively strong winds concentrated between 20° and 40° latitude in the middle and upper troposphere. It can be present at any longitude but is generally strongest off the Asian coast. See jet stream.

  • sublimation - The process of phase transition from solid directly to vapor in the absence of melting. Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid. Colloquially the terms are used interchangeably for the solid-vapor transition (evaporation). For growth, the term sublimation has been replaced by deposition since the 1970s. There is evidence that deposition nucleation does occur, although there may be an adsorbed layer prior to nucleation. It appears that most nuclei in the atmosphere require near-water saturation before they initiate ice.

  • subsidence - A descending motion of air in the atmosphere, usually with the implication that the condition extends over a rather broad area.

  • subsidence inversion - An increase in temperature with height produced by the adiabatic warming of a layer of subsiding air. This inversion is enhanced by vertical mixing in the air layer below the inversion.

  • summer - Astronomically, between the summer solstice and autumnal equinox in the Northern Hemisphere, and beteen the winter solstice and vernal equinox in the Southern Hemisphere; the warmest season of the year everywhere except in some tropical regions. Popularly and for most meteorological purposes, summer is taken to include June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere, the opposite of winter. See Indian summer, Old Wives' summer, St. Luke's summer, St. Martin's summer.

  • sunrise - 1. The phenomenon of the sun's daily appearance on the eastern horizon as a result of the earth's rotation. Under normal conditions the upper limb of the sun appears to be on the horizon of an observer at sea level. The parallax of the sun is ignored. The observed time may differ from the tabular time because of a variation of the atmospheric refraction from the adopted value (34') and because of a difference in height of the observer and the actual horizon. 2. Contraction for "time of sunrise," which is defined by the U.S. Naval Observatory. See also twilight; compare sunset.

  • sunset - 1. The phenomenon of the sun's daily disappearance below the western horizon as a result of the earth's rotation. 2. Contraction for "time of sunset," which is defined by the U.S. Naval Observatory. Compare sunrise.

  • sur - A cold wind in Brazil.

  • surf - The wave activity occurring on a beach inshore of the point at which incoming waves break. Generally, surf consists of waves that have broken and therefore have air to some degree mixed in with the water.

  • surface front - See front.

  • surface pressure - In meteorology, the atmospheric pressure at a given location on the earth's surface. This expression is applied loosely and about equally to the more specific terms: station pressure and sea level pressure.

  • surface temperature - 1. In meteorology, the temperature of the air near the surface of the earth; almost invariably determined by a thermometer in an instrument shelter. 2. In oceanography, the temperature of the layer of seawater nearest the atmosphere. See sea surface temperature.

  • surface trough - An elongated area with relatively low pressure values when reduced to sea level.

  • surface wind - The wind measured at a surface observing station. This wind is customarily measured at a standard distance above the ground to minimize the distorting effects of local obstacles and terrain. See anemometer, wind vane, instrument exposure, winds aloft.

  • surge - 1. See storm surge. 2. See surge current. 3. See surge line. 4. In hydrology, a sudden change in discharge resulting from the opening or closing of a gate that controls the flow in a channel, or by the sudden introduction of additional water into the channel. 5. The fore and aft movement of the center of gravity of a ship. See heave, sway, ship motion. 6. Water transported up a beach by breaking waves.

  • sweep - 1. A single traversal of the electron beam along any coordinate axis on the face of a cathode-ray oscilloscope. 2. A single rotation of an antenna at fixed azimuth or elevation.

  • SWI - Abbreviation for severe weather threat index. See stability index.

  • synoptic - 1. In general, pertaining to or affording an overall view. In meteorology, this term has become somewhat specialized in referring to the use of meteorological data obtained simultaneously over a wide area for the purpose of presenting a comprehensive and nearly instantaneous picture of the state of the atmosphere. Thus, to a meteorologist, "synoptic" takes on the additional connotation of simultaneity. 2. A specific scale of atmospheric motion with a typical range of many hundreds of kilometers, including such phenomena as cyclones and tropical cyclones. Compare mesoscale.

  • synoptic model - Any model specifying a space distribution of some meteorological elements. The distribution of clouds, precipitation, wind, temperature, and pressure in the vicinity of a front is an example of a synoptic model.

  • synoptic scale - Used with respect to weather systems ranging in size from several hundred kilometers to several thousand kilometers, the scale of migratory high and low pressure systems (frontal cyclones) of the lower troposphere. See cyclonic scale.

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