Glossary of Meteorological Terms (S)

S

Saffir–Simpson hurricane wind scale

Classifies hurricane intensity on a scale from 1 to 5 based on the maximum surface wind speed and the type and extent of damage done by the storm. Developed by engineer Herbert Saffir and meteorologist Robert Simpson in the early 1970s.

Salinometer

Any device or instrument for measuring salinity, especially one based on electrical conductivity methods.

SAM

Station for Atmospheric Measurements. A portable meteorological station used to provide weather data to the ALOHA air model that predicts how a cloud of pollutant gas might disperse in the atmosphere after an accidental release. Also see CAMEO.

Saturation

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

Savart polariscope

A polariscope consisting of a specially constructed double plate polarizer and a tourmaline plate analyzer. Polarized light passing through the instrument is indicated by the presence of parallel colored fringes, while unpolarized light results in a uniform field.

SAWRS

Supplemental Aviation Weather Reporting Station. A facility where weather observations are taken, prepared, and transmitted by a local operator under federal government supervision.

SCADA

Supervisory Control and Data Acquisition. A type of industrial process control system used for gathering data in real time from remote locations in order to control equipment and conditions.

Scale

The array of indicating marks and figure in relation to which the position of an index is observed, e.g. a scale plate on a recorder.

Scatter

The process by which small particles suspended in a medium of a different refractive index diffuse a portion of the incident radiation in all directions. In scattering no energy transformation results, only a change in the spatial distribution of the radiation. Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere.

Scattered radiation

Solar radiation scattered by particles in the atmosphere.

Scattering coefficient

A measure of the attenuation due to scattering of light as it traverses a medium containing scattering particles.

Scintillation

Generic term for rapid variations in apparent position, brightness, or color of a distant luminous object viewed through the atmosphere.

Scintillometer

A type of photoelectric photometer used to measure high-altitude winds on the assumption that stellar scintillation is caused by atmospheric inhomogeneities being carried along by wind near the tropopause level.

Scud

Ragged low clouds, usually stratus fractus. Most often applied when such clouds are moving rapidly beneath a layer of nimbostratus.

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 rainbow

Same as marine rainbow.

Sea water thermometer

A thermometer designed for use in measuring the temperature of sea water. One form consists of a mercury-in-glass thermometer protected by a perforated metal case. Another form consists of a mercury-in-glass thermometer surrounded by a metal case which forms a well around the bulb of the thermometer. When the thermometer is raised from the water, a sample is retained in the well. See bucket thermometer, reversing thermometer.

Secchi disk

A white disk 12" or more in diameter which is lowered into the sea to estimate transparency of the water. The depths are noted at which it first disappears when lowered and reappears when raised.

Secondary instrument

An instrument whose calibration is determined by comparison with an absolute instrument.

Secondary rainbow

A rainbow of angular radius of about 50° often seen outside the primary rainbow of 42° radius. The secondary rainbow is formed by two internal reflections (rather than one as in the primary rainbow), plus two refractions. Its spectral color sequence is from red inside to violet outside. Because each reflection introduces light losses, the secondary bow is much less bright than the primary bow.

Seisomograph

An instrument used to measure and record earthquake vibrations and other earth tremors.

Sensing element

The element directly responsive to the value of the measured variable.

Sensitivity

The ratio of the output of an instrument to the input value, e.g. a rain gauge with a sensitivity of 1 tip per 0.01".

Sensor

The part of a measuring instrument which responds directly to changes in the environment.

Serial data transmission (serial output)

A form of data transmission in which the bits of each character are sent one at a time along a single communication path. Compare to parallel data transmission.

Short-wave radiation

Radiation with wavelengths less than 4 microns.

Shower

Precipitation from a cumuliform cloud. Characterized by the suddenness of beginning and ending, by the rapid change in intensity, and usually by a rapid change in the condition of the sky. The solid or liquid water particles are usually bigger than the corresponding elements in other types of precipitation.

Sigma Theta (wind direction)

The standard deviation of wind direction. Provides an indication of the variability of the wind direction. Used in calculations of atmospheric stability.

SIGMET information

Meteorological information issued by a watch office concerning the occurrence or forecast of weather phenomena which may affect the safety of aircraft operations.

Signal conditioning

The processing of the form or mode of a signal so as to make it intelligible to, or compatible with, a given device.

Significant level

In a radiosonde observation, a level (other than a standard level) for which values of pressure, temperature, and humidity are reported because temperature and/or humidity data at that level is sufficiently important or unusual to warrant the attention of the forecaster.

Simplex

Operation mode of a communication circuit in which one end can only transmit and the other end can only receive.

Sine galvanometer

A magnetometer of the electromagnetic type which is used to measure the horizontal intensity of the earths magnetic field.

Siphon barograph

A recording siphon barometer.

Siphon barometer

A mercury barometer in which the tube is U-shaped and the upper and lower mercury surfaces have the same diameter.

Six's thermometer

A thermometer, invented by James Six in 1782, which simultaneously indicates the maximum and minimum temperatures attained during a given interval of time. A U-tube min/max thermometer.

Sky cover

The amount of sky covered or concealed by clouds or obscuring phenomena. It is reported in tenths, so that 0.0 indicates a clear sky and 1.0 (or 10/10) indicates a completely covered sky. The following classifications are used in aviation weather observations: clear, scattered, broken, overcast, partial obscuration, obscuration.

Skyhook balloon

A large plastic constant-level balloon for duration flying at very high altitudes.

Slant range

The line-of-sight distance between two objects.

Sleet

Frozen or partly frozen rain.

Sling psychrometer

Psychrometer to which a small chain or rotary handle is attached so that the observer can rotate the instrument rapidly to properly ventilate the thermometer bulbs.

SNOTEL

SNOw TELemetry. An automated network of snowpack data collection sites. The Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), has operated the Federal-State-Private Cooperative Snow Survey Program in the western United States since 1935. A standard SNOTEL site consists of a snow pillow, a storage type precipitation gauge, an air temperature sensor, and a small shelter for housing electronics.

Snow

Precipitation composed of white or translucent ice crystals, chiefly in complex branched hexagonal forms.

Snow board

A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground or snow surface to obtain more accurate measurements of snowfall and water content.

Snow bridging

An effect noted primarily in wet snow conditions when snow clings to the sides of a precipitation gauge and gradually accumulates until the gauge orifice is capped with accumulated snow. This effect can be minimized by using large collectors, and wind screens around gauges.

Snow core

A sample of either freshly fallen snow, or the combined old and new snow on the ground. obtained by pushing a cylinder down through the snow layer and extracting it.

Snow course

An established line or transect of measurements of snow water equivalent across a snow field in representative mountainous terrain, where appreciable snow accumulates, to monitor seasonal snowpack.

Snow cutter

A saw-toothed piece of metal which slips over the top of the overflow can of a standard rain gauge when it is used to cut snow samples for the determination of water content of snow on the ground. Not required except where ice or dense snow accumulation are persistent problems.

Snow flurry

Snow shower, particularly of a very light and brief nature.

Snow gauge

Apparatus designed to measure the amount of precipitation falling in the form of snow. The device determines the weight of the snow or the volume of water after the snow melts.

Snow grains

Precipitation of very small, white, opaque particles of ice, fairly flat or elongated, with diameters less than 1 mm. The solid equivalent of drizzle.

Snow pellets (soft hail)

Precipitation of white, opaque, spherical or conical ice particles that are crisp and easily crushed and that have diameters of 2 to 5 mm.

Snow pillow

An instrument used to measure snow water equivalents. Snow pillows typically have flat stainless steel surface areas. The pillow below this flat surface is filled with antifreeze solution and the pressure in the pillow is related to the water-equivalent depth of the snow on the platform.

Snow sampler

Snow gauge composed of a metal cylinder, closed at one end, used to obtain a sample of snow from which the water is measured after melting.

Snow stake

Graduated fixed stake used in regions of abundant snowfall to facilitate the measurement of snow depth.

Snow stick

A portable rod used to measure snow depth.

Snow survey

Determination of the total amount of snow covering a watershed or a given region. Both depth and water content of the snow may be measured, and the results may be used to predict the amount of water that will be available after melting.

Snow tube

Same as snow sampler.

Snow water equivalent (SWE)

The depth of water that would result from the melting of snow or ice, assuming measurement on a horizontal surface and no infiltration or evaporation.

Software

The programs and instructions which direct a computer.

Soil evaporimeter

Instrument used to measure the amount of water evaporated from the soil surface during a given time interval.

Soil moisture

Moisture contained in the soil above the water table, including water vapor which is present in the soil pores. In some cases this term refers strictly to the humidity contained in the root zone of plants.

Soil thermometer (geothermometer)

Thermometer for measuring the temperature in the soil at different depths.

Solar constant

Amount of solar radiation incident, per unit area and time, on a surface which is perpendicular to the radiation and is situated at the outer limit of the atmosphere, the earth being at its mean distance from the sun. It equals approximately 2.00 ly/min (1400 W/m²).

Solarimeter

Name sometimes used in place of pyranometer as a generic term.

Solar radiation

The total electromagnetic radiation emitted by the sun. About 99.9 percent of its energy output falls within the wavelength interval from 0.15 microns to 4.0 microns, with peak intensity near 0.47 microns. About one-half of the total energy in the solar beam falls in the visible spectrum from 0.4 to 0.7 microns, and most of the other half falls in the near infrared, a small additional portion falling in the ultraviolet.

Solar radiation shield

See radiation shield.

Solid-state device

An element that can control current without moving parts, heated filaments, or vacuum gaps.

Sonic anemometer

An anemometer which measures wind speed by means of the properties of wind-borne sound waves. It operates on the principle that the propagation velocity of a sound wave in a moving medium is equal to the velocity of sound with respect to the medium plus the velocity of the medium. The sonic anemometer is an absolute instrument and has the advantages of a very short time-constant and an absence of moving mechanical parts.

Sonic thermometer

A thermometer based upon the principle that the velocity of a sound wave is a function of the temperature of the medium through which it passes. Sonic thermometers possess very short time-constants and eliminate radiation error.

Sounding

Same as an upper air observation, but commonly used to refer to a single complete radiosonde observation.

Span

The algebraic difference between the upper and lower limits of the measuring range of an instrument, e.g. a thermometer with a range of -35 to 50°C has a span of 85°C.

Specific humidity

In a system of moist air, the dimensionless ratio of the mass of water vapor to the total mass of the system.

Spectral hygrometer

A hygrometer which determines the amount of precipitable moisture in a given region of the atmosphere by measuring attenuation of radiant energy caused by the absorption bands of water vapor.

Spectral solar radiation

Solar radiation of selected wavelengths.

Spectroheliograph

An instrument for taking photographs of an image of the sun in monochromatic light.

Spectrohelioscope

Similar to the spectroheliograph, but used for visual instead of photographic purposes.

Spectrophotometer

A photometer which measures the intensity of radiation as a function of the frequency (or wavelength) of the radiation.

Spectropyreheliometer

An instrument which measures the spectral distribution of the intensity of direct solar radiation.

Spirit thermometer

A liquid-in-glass thermometer which uses an organic substance such as alcohol as the thermometer liquid. This type of thermometer has a low freezing point and a high coefficient of expansion. It is less accurate, however, than a mercury thermometer.

Splayed tail

A type of wind vane having a split or V-shaped tail. The apex orients itself to the direction of the wind.

Squall

A strong wind characterized by a sudden onset, a duration on the order of minutes, and a rather sudden decrease in speed.

Staff gauge

A graduated scale placed in a position so that the stage of a stream may be read directly from it. Staff gauges may be placed on bridge piers or pilings, etc., or placed on specially constructed supports.

Stage

The elevation of the water surface in a stream as measured by a river gauge with reference to some arbitrarily selected zero datum.

Standard atmosphere

A standard unit of atmospheric pressure, defined as the pressure exerted by a 760 mm column of mercury at standard gravity (980.665 cm/sec²) at 0°C. 1 standard atmosphere is equal to 760 mm Hg, 29.9212 inches Hg, or 1013.250 mb. Also, a hypothetical vertical distribution of atmospheric temperature, pressure, and density which, by international agreement, is taken to be representative of the atmosphere.

Standard error

The standard deviation (positive square-root of the variation) of the errors associated with physical measurements of an unknown quantity, or statistical estimates of an unknown parameter or of a random variable.

Standard level (mandatory level)

One of several constant-pressure levels in the atmosphere for which a complete evaluation of data derived from upper air observations is required.

Static pressure vent

A vent used with pressure sensors to reduce the effect of wind on the pressure inlet. It is normally mounted remotely and connected to the sensor using airtight tubing.

Station model

A specified pattern for entering, on a weather map, the meteorological symbols that represent the state of the weather at a particular observing station. Also known as a station model plot.

Station pressure

The atmospheric pressure computed using station elevation as the reference datum level. Station pressure is usually the base value from which sea level pressure and altimeter setting are determined.

Statute mile

A unit of distance equal to 5280 feet. It is sometimes referred to as a land mile.

Stevenson screen

A type of instrument shelter. It is a wooden box painted white with double louvered sides and mounted on a stand four feet above the ground. Designed by Thomas Stevenson, civil engineer (father of Robert Louis Stevenson).

Still well (or stilling well)

A cylinder installed in a body of water or an evaporation pan to hold a sensor, such as a float to measure water level or a hook gage. The stillwell is constructed so that there is free movement of water in and out of it, and it therefore provides a representative sample of the water body. It functions to protect the sensor in some cases and to provide an undisturbed water surface in other cases.

Storm

Wind with a speed between 56 and 63 knots (64 and 72 mph); Beaufort scale number 11.

Stream gauge

Same as river gauge.

Strong breeze

Wind with a speed between 22 and 27 knots (25 and 31 mph); Beaufort scale number 6.

Strong gale

Wind with a speed between 41 and 47 knots (47 and 54 mph); Beaufort scale number 9.

Sun dog

Same as parhelion.

Sunshine recorder

An instrument designed to record the duration of sunshine at a given location without regard to intensity. See Campbell-Stokes recorder, Jordan sunshine recorder, Marvin sunshine recorder, Pers sunshine recorder.

Supercooled water

Liquid water at temperatures colder than freezing.

Supernumerary rainbows

A set of weakly colored rainbow arcs sometimes discernable inside a primary rainbow.

Super-pressure balloon

See constant-level balloon.

Switching power supply

A power supply which achieves its output regulation by means of one or more active power handling devices which are alternately placed in the "off" or "on" states. It is more efficient than linear supplies which vary the conduction of power devices to achieve output regulation.

Synchro

A motorlike device containing a rotor and a stator and capable of converting an angular position into an electrical signal, or an electrical signal into an angular position. When several synchros are correctly connected, all of the rotors will align themselves into the same angular position. This is useful, since one synchro whose angular motion is forced to change, can drive another synchro to indicate the angular change.

Synchronous

Having a specific relationship to a time base or clock. In synchronous communications, data characters are sent according to a timing signal which synchronizes the two communicating devices.

Synoptic

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 obtaining a comprehensive and nearly instantaneous picture of the state of the atmosphere. Thus, to a meteorologist, "synoptic" takes on the additional connotation of simultaneity.

Synoptic weather observation

A surface weather observation, made at periodic times, of sky cover, state of the sky, cloud height, atmospheric pressure reduced to sea level, temperature, dew point, wind speed and direction, amount of precipitation, hydrometeors and lithometeors, and special phenomena that prevail at the time of the observation or have been observed since the previous specified observation. Compare to aviation weather observation.

Systematic error

That part of the inaccuracy of a measuring instrument, or statistical estimate of a parameter, that is due to a single cause or small number of causes having the same sign, and hence, in principle, is correctable; a bias or constant offset from the true value. In the absence of random errors, the true value is equal to the instrumental reading or statistical mean estimate minus the systematic error.