Saffir-Simpson Scale

Classification system of hurricane intensity, which categorizes them from 1 (weak) to 5 (devastating), based on maximum sustained winds and the consequent damage.


Ensemble of particles of sand lifted by a strong and turbulent wind. Sand particles usually stay in the lowest 10 feet (3.5m) and rarely rise more than 50 feet (15m) above the surface. They tend to form during the day and cease at night.

Santa Ana wind

Dry and hot, Foehn-like desert wind which affects southern California, around the valley of Santa Ana and other passes, that carries southwestward desert air, proceeding from the northeast or east.


Artificial platform placed into orbit around the planet that carries devices to gather environmental data.


Condition in which vapor pressure is equal to the equilibrium vapor pressure over a plane surface of pure liquid water, or occasionally ice, at the same temperature and pressure. Saturated air is that moist air whose vapor pressure is the saturation vapor pressure, having a relative humidity of 100%.

Saturation mixing ratio

Mixing ratio of an air parcel, when it is saturated. Value of the mixing ratio of saturated air at a given temperature and pressure.


Process by which electromagnetic radiation is deflected in all directions by small atmospheric particles. By definition, this term subsumes reflection, refraction and even diffraction of electromagnetic waves. Usually, the term is applied to the radiation that is observed in all directions other than that of the source.

Sea breeze

Local wind in coastal regions, that blows from sea to land during the day, as a result of the temperature difference when the body of water is colder than the adjacent land.

Sea level

Generally used as a synonym of mean sea level. It is the arithmetic mean of hourly sea heights observed over a specified period.

Sea level pressure

Atmospheric pressure at mean sea level. It can be directly measured, but commonly, it is empirically determined from the observed station pressure.


Division of the year in four periods, winter, spring, summer and autumn, according to astronomical and climatic regularly recurrent events and phenomena.

Secondary rainbow

Less brilliant rainbow, which distinguishes from other rainbows by its angular radius (about 50° and 54° from the antisolar point), color order (red in the inside and violet to the outside) and brightness. It is supplementary to a primary rainbow.

Severe weather

Any potentially destructive or dangerous atmospheric condition, which results hazardous to human beings. Usually, it comes associated with extreme convective phenomena, such as tropical cyclones, severe thunderstorms, etc., and with freezing precipitation storms or blizzards.

Shadow bands / Crepuscular rays

Bands of shadows of clouds, with a dark blue coloration, situated on or below the horizon, that radiate from the Sun and cross the purple light during sunset. It is possible that these bands cross the sky completely, and then they are seen close to the antisolar point.

Short-range forecast

Forecast that has a period of validity shorter than three days. Sometimes, it refers to a weather forecast made for a 48 hours interval.


Precipitation, falling from a convective cloud, characterized by its sudden beginning and ending, and its rapid changes of intensity.

SI – International System of Units

(Abbreviated SI from French: Système international d’unités) It is the world’s most widely used system of measurement (established in 1960) based on the metric-kilogram-second system, which is used both in everyday commerce and in science. The system has been nearly globally adopted with the exception of UK and Canada who use it partially and the United States which is the only industrialized nation that does not use it in its commercial and standards activities.


Process by which sedimentary material, finer than sand and coarser than clay, consisting of very small rock fragments or mineral particles, existing in running or still water, is added to the composition of soil by virtue of flooding. Process whereby rivers and streams become choked by mud and soil that has washed off land through erosion.

Single cell thunderstorm

Thunderstorm that consists of one single cell, which is a group of convective clouds moving as a single unit, which appears when the atmosphere is unstable, but with little or no wind shear. They typically last 20 to 30 minutes.

Sky cover

Term that refers to: The amount of sky covered by clouds or by obscuring phenomena aloft. The amount of sky concealed by obscuring phenomena at ground level. A combination of the above mentioned.


Precipitation in the form of a mixture of rain and hail, or rain and ice pellets, or melting snow, or sudden short-term rainfall, with wind and hail.


Highly polluted fog. The term was coined as a combination of fog and smoke.


Particulate matter suspended in the atmosphere, produced by combustion processes.


Precipitation of white or translucent hexagonal ice crystals, that fall either isolated or agglomerated from a cloud.

Snow flurry

Light and short-lived snow shower.

Snow gauge

Instrument for measuring water equivalent of a snowfall; it could be by weighting it or by melting it.

Snow pellets

Solid precipitation falling from a cloud, consisting of conical or rounded, white and opaque ice particles, with diameters of up to 0.2 inches (5 mm).

Snow shower

Fairly brief period of snowfall, precipitating from a cumuliform cloud, which is characterized by its rapid start and end and by possible rapid changes of intensity.

Snowfall / Snow falling

Depth of fresh snow accumulated on the ground during a given period of time.


Aggregation or assemblage of a few too many ice crystals.

Solar altitude

The angle of the sun 90 degrees or less above the horizon.

Solar constant / Total Solar Irradiance (TSI)

Amount of solar radiational energy incident, per unit area and time, falling on a surface that is positioned at the top of the atmosphere and oriented perpendicular to the solar radiation, being the Earth at its mean distance from the sun.

Solar core / Core of the Sun

Inner region of the Sun, where its energy is produced through fusion reactions, caused by very high density and temperature. It is made up of an extremely hot and dense gas, in the plasma state, which is prevented from reaching the solid state because of such density and temperature.

Solar flares

A solar flare is a sudden release of magnetic energy that has built up in the solar atmosphere. It is a bright eruption from the Sun’s chromosphere that mostly occurs in active regions around sunspots. See (Coronal Mass Ejections (CME). Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square meter, W/m2) of 100 to 800 picometer X-rays near Earth, as measured in the GOES spacecraft.

Solar maximum / Solar max

It is the period of greatest solar activity in the solar cycle of the Sun, when its magnetic field lines are the most distorted due to the magnetic field on the solar equator rotating at a slightly faster pace than at the solar poles. The period of greatest solar activity is when large numbers of sunspots appear and also when large solar flares occur. The solar cycle takes an average of about 11 years to go from one solar maximum to the next (with observed variations from 9 to 14 years).

Solar storm

Term that can refer to: Geomagnetic storm: a temporary disturbance of the Earth’s magnetosphere, which is caused by a solar wind shock wave, occurred in the last 24 to 36 hours. Coronal mass ejections: an ejection of material from the solar corona. Solar flares: violent explosions in the Sun’s atmosphere.

Solar wind

Stream of ionized gas –basically hydrogen- in speedy and continuous flow from the Sun, with variable intensity, into the solar system. In its interaction with the Earth’s magnetic field, it produces different phenomena, such as the aurora.


Instrument that takes multispectral measurements, from which it can be derived vertical profiles of atmospheric temperature and humidity.

Sounding balloon

Free, unmanned balloon which carries either instruments for measuring meteorological elements high in the atmosphere, or an electronic device or a radar reflector, intending to obtain a sounding of the atmosphere, e.g.: determination of the upper winds.

Source region

Southern Oscillation

Cyclic change of pressure regimes on la large scale, backwards and forwards between the Pacific and the Indian Oceans (Walker Circulation). The pressure “seesaw” translates into a global pattern of anomalies in circulation, wind, temperature and precipitation.

Southern Oscillation Index (SOI)

Measure of the state of the Southern Oscillation. Its figure derives from this operation: sea level pressure at Tahiti minus the sea level pressure at Darwin, Australia, divided by the standard deviation of that quantity.

Space-geodetic positioning techniques

Spaghetti plots

Method of visual display of data from an ensemble forecast, by which each run of the model is drawn on a chart. The results are compared and they can be used to gauge the degree of uncertainty in the forecast. If the contours follow a certain pattern, it can be said that the confidence in the forecast can be high; if it has a chaotic appearance (like a plate of spaghetti) there should be low confidence in the accuracy of the forecast.


Sun Protection Factor. The SPF of a sunscreen is a laboratory measure of its effectiveness against UV-B (the ultraviolet radiation that causes sunburn) and UV-A (more associated with longer-term skin damage).


Ensemble of water droplets torn by the wind from the surface of a extensive body of water, generally wave crests and carried up a short distance into the air.


Atmospheric phenomenon characterized by a brief but sudden and large increase of wind speed, which ceases rather abruptly too. It often comes together with showers and thunderstorms.

Squall line

Line or narrow band of active thunderstorms, of a non-frontal nature, with or without squalls.

Stationary front

Front that is stationary or nearly so, moving at a speed of five knots or less. In synoptic chart analysis, a quasi-stationary front is one that has not changed position appreciably during the last three or six hours, that is since the last previous synoptic chart was completed.


Any disturbance of the atmosphere, involving perturbations of the prevailing pressure and wind field, ranging from tornadoes to extra-tropical cyclones, especially as affecting the Earth’s surface, involving inclement and probably destructive weather. Beaufort scale wind Force 10 (between 48 and 55 knots)

Storm surge / Storm tide, storm wave, hurricane tide

Rise and onshore surge of seawater, as a consequence of the winds of a storm and also of the surface pressure drop, close to the storm center. Actually, it is the difference between the actual water level (under the influence of a storm) and the level that it would have attended in the absence of such disturbance. Storm surges are responsible for most deaths in tropical cyclones worldwide.

Stratocumulus (Sc)

Abbreviated Sc. Principal cloud genus, basically stratiform, that appear as a gray or whitish sheet or layer of clouds, nearly always with dark parts, composed of tessellations, and non-fibrous rounded masses (except for virga), which may or may not be merged. Usually, they are arranged in groups, lines or undulations, generally flat in the top, smooth and large. The individual Sc element has an apparent width of more than 5°.


Top of the stratosphere.


Region of the atmosphere, situating between the tropopause and the stratopause, at a height of roughly 30 miles (50 km). Here, temperature generally increases with height.

Stratus (St)

Abbreviated St. Principal cloud genus, in the form of a generally gray cloud layer with a rather uniform base. Usually, this kind of clouds does not produce precipitation, but they might generate drizzle, ice prisms or snow grains. Often, they occur in the form of ragged patches o cloud fragments, and usually they do not produce halo phenomena.


Sinuous channel of very high ion density, propagating itself through a gas creating an electron avalanche just ahead of its advancing tip. There are special kinds of streamer, as the stepper leader, that appear in lightning discharges.

Storm track

Relatively narrow zones in the Atlantic and Pacific Oceans along which most of extra-tropical cyclones travel.

Subtropical high

Also called subtropical anticyclone, oceanic anticyclone or oceanic high, it is one of the semi-permanent highs of the subtropical high pressure belt, which generally appear between 20° and 40° latitude, lying over oceans. They are best developed in summer season.

Subtropical high pressure belt / Subtropical ridge

Series of subtropical highs aligned along two axes on each hemisphere, located approximately along the 35° parallel. These axes present a slight, annual, meridional fluctuation.

Suction vortex

Whirlwind within a tornado, containing the most intense winds and pressure changes; they hang down from the tornado, generating cloud and revolve around and in the same direction of the winds in the main funnel.


Sunlight, as rays or beams.


Burn to the skin produced by overexposure to ultraviolet radiation, usually from the sun’s rays.


Regions on the solar surface that appear visibly as dark spots due their lower temperature compared to the surrounding regions in the Sun’s photosphere. They are temporary phenomenon which develop and persist for periods ranging from hours to months that expand and contract as they are carried around the surface of the Sun by its rotation. They are caused by intense magnetic activity, which inhibits convection forming areas of reduced surface temperature. A typical sunspot consists of a dark central region called the umbra and a somewhat lighter surrounding region called penumbra. They can be large enough to be seen with the naked eye (average size is about the same size of the Earth).

Supercell thunderstorm

Persistent and severe updraught and downdraught coexisting in a thunderstorm in a quasi-steady state (they can last for hours) rather than in a more usual assemblage of cloud cells, each of which has a relatively short time. This type of thunderstorm has the capability to deviate from the mean wind. Usually, supercells produce copious amounts of hail, torrential rainfall, strong winds and downbursts.


World leading computer in terms of processing capacity, particularly speed of calculation.

Supercooled water

Liquid water at a temperature below the freezing point.

Supernumerary rainbows

Narrow, weakly colored arcs, eventually seen just inside the primary rainbow and rarely outside of the secondary rainbow, produced by diffraction interference.


Condition existing in a sample of moist air whose mixing ratio is greater that the saturation mixing ratio at the same temperature and pressure. That is, when a given portion of the atmosphere contains more water vapor than is needed to produce saturation, with respect to a plane surface of pure water or pure ice.

Surface albedo

Ratio, expressed as a percentage, of the amount of sunlight (electromagnetic radiation) reflected by the Earth’s surface, to the amount of incident upon it.

Surface drag

Frictional impedance offered by air to the motion of bodies passing through it.

Surface friction

Resistance mechanical offered by one medium or body to the relative motion of another medium or body in contact with the first. In the atmosphere, surface friction is related to turbulent drag.


Any system of water waves that has left its generating area.