Weather Glossary - O thru S  

 

Weatherwx

Terminology Page

This Weather Glossary, features an alphabetical listing of over 1000 weather-related terms, phrases and abbreviations covering the environmental sciences. The terms with *asterisks before them are related to Storm Spotting and Chasing. The purpose of this glossary is to aid the general public in better understanding the numerous weather related terms and related NWS products.

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Shortcut...just click on the first letter of the word you're looking for

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P| Q | R | S | T | U | V | W | X| Y | Z

 

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-O-

Obscuration - Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. According to the National Weather Service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog) haze, mist, sand and blowing sand smoke, blowing spray, and volcanic ash. It is reported as "X" in an observation and on the METAR. See a partial obscuration.

Observation - In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.

Occluded Front - Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet. See cold front and warm front.

Occluded Mesocyclone - A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.

Ocean - The intercommunicating body of salt water occupying the depressions of the earth's surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.

Oceangraphy - The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.

OMEGA Block - A warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts. It is an example of a blocking high.

Opaque - A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.

Orographic - Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic Lift - Lifting of air caused by its passage up and over mountains or other sloping terrain.

Orographic Lifting - Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, and the dissipation of clouds. Also called an orographic uplift.

Orphan Anvil - [Slang], For an anvil from a dissipated thunderstorm, below which no other clouds remain.

Outflow - Also sometimes referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and temperature drop. See bubble high or meso high.

Outflow Boundary - A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc...; see triple point).

Outflow Winds - Winds that blow down fjords and inlets from the land to the sea. When cold arctic air flows from the interior of BC onto the coast, the windspeeds through mainland inlets can reach over 100 km/h.

Overcast - The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

Overhang - A Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).

Overruning - This occurs when a relatively warm air mass is forced above a cooler air mass of greater density that is at the surface. Weather generally associated with this event includes cloudiness, cool temperatures, and steady rain.

*Overshooting Top (or Penetrating Top) - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top, (anvil dome), often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm.

Oxygen (O₂) - A colorless, tasteless, ordorless gas that is the second most abundant consituent of dry air. It comprises 20.946% of the atmosphere.

Ozone (O₃) - A pungent-smelling, nearly colorless gas to slightly bluish gas which is a close chemical cousin to molecular oxygen. It is composed of an oxygen molecule made up of three oxygen atoms instead of two. About 90% of the earth's ozone is located in a natural layer far above the surface of the globe, in a frigid region of the atmosphere known as the stratosphere. Here in this outer region it protects the earth and its inhabitants from the harmful effects of ultraviolet radiation from the sun.

Ozone Layer - An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.

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Palmer Drought Index (PDI) - A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agriculture) Joint Agricultural Weather Facility. The index depicts prolonged times, as in months or years, of abnormal dryness or wetness. It responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration.

Palouser - A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley in northern Idaho and eastern Washington. Also call a Cow-Killer.

Parcel - A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

Parhelion - The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns.

Partial OBScuration - Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. It often reduces horizontal visibility but not the vertical. It is reported as "-X" in an observation and on the METAR. See an obscuration.

Partly Cloudy - The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. The National Weather Service does not have an amount of sky cover for this condition. Refer to clear, few, scattered, broken, and overcast.

Pascal - The unit of pressure produced when one newton acts on about one square meter.

Pascal's Law -When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. Formulated by Blaise Pascal (1623-1662), a French mathematician, theologian, and physicist.

PDS Watch - [Slang], For a tornado watch with enhanced wording (Particularly Dangerous Situation).

Peak Gust - The highest instantaneous wind speed observed or recorded.

Pendant Echo - A Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Penetrating Top - Same as overshooting top.

Perigee - The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of apogee.

Perihelion - The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000 year cycle, currently it occurs around January, when the earth is about 3 million miles closer to the sun than at aphelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of aphelion.

Photometer - Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not directly cause adverse weather. They include halos, coronas, rainbows, and fogbows.

Photosphere - The intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000 degrees Fahrenheit, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.

Pilot Ballon - A small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. Also known as a pibal.

Pilot Report - A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.

Plan Position Indicator - Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.

Plow/Plough Wind - The spreading downdraft and strong straight-line winds preceding a thunderstorm. So called in the American Midwest because of its ability to flatten tall grasses as it passes. See first gust.

Poles/Polar - The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 60 and 90 degrees latitude, both North and South.

Polar Air Mass - An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime polar air (mP), produced over warmer waters, is less stable with high moisture.

Polar Front - A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An integral part of an early meteorological theory known as the Polar Front Theory.

Polar Jet - Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.

Polar-Orbiting SATellite - A satellite whose orbit passes over both of the earth's between poles. Compare with a geostationary satellite.

Pollutant - Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on man or his surroundings. Something unfavorable to health and life that has been added to the environment.

Popcorn Convection - [Slang], For, cumulus clouds, showers thundershowers or thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to asair-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Positive CG - A CG lightning flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)

Positive-tilt Trough - An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Positive VORTicity ADVection - The rotation of the atmosphere advects higher values of vorticity into an area. This is created by cyclonic turning and is often associated with upward motion of air. When found in advance of a short wave aloft, it often enhances thunderstorm potential. Contrast with negative vorticity advection.

Potential Temperature - The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

Pounds Per Square Inch (PSI) - A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one square inch.

PPINE - Plan Position Indicates No Echoes, is a term referring to the fact that a radar detects no precipitation within its range.

Precipitation - Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

Probability Of PRECIPitation - Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period. Measurable means at least 0.01 Inch of rain or the water equivalent of snow. For example, a 40% probability of rain today means there are 4 chances in 10 that it will rain.

Profiler - An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Pre-Frontal SQUALL Line - A line of thunderstorms that precedes an advancing cold front. Refer to a squall line.

Pre-Frontal Trough - An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction. Refer to a trough.

Pressure -The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. Also known as atmospheric pressure or barometric pressure.

Pressure Altimeter - An aneroid barometer calibrated to indicate altitude in feet instead of unit of pressure. It read accurately only in a standard atmosphere and when the correct altimeter setting is used.

Pressure Altitude - The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.

Pressure Change -The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.

Pressure Characteristic - The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.

Pressure Gradient - The amount of pressure change that occurs over a fixed distance at a fixed altitude.

Pressure Jump - A sudden increase in the observed atmospheric pressure or station pressure.

Pressure Tendency - The pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation.

Prevailing Wind - A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

Prevailing Visibility - It is considered representative of visibility conditions at the observation station. It is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous.

Profiler - A type of Doppler radar that measures both wind speed and direction from 1,500 feet up through 55,000 feet in the atmosphere.

Prognostic Chart - A chart of forecast predictions that may include pressure, fronts, precipitation, temperature, and other meteorological elements. Also known as a prog.

Pseudo-Cold Front - A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. See also pseudo-warm front.

Pseudo-Warm Front - A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.

Psychrometer - An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

Pulse - A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic radiation emitted by the radar.

*Pulse Storm - A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

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Quantitative Precipitation Forecast (QPF) - A forecast of rainfall, snowfall or liquid equivalent of snowfall.

QUASI-Stationary Front - A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.

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RADAP II - RAdar DAta Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, accumulated rainfall, etc.

Radar - Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back. Doppler radar and NEXRAD are examples.

Radarsonde Observation - An upper air observation used to determine winds, and other meteorological data, by tracking the range, elevation, and azimuth of a radar target carried aloft . A type of rawinsonde.

Radial Velocity - A type of velocity that expresses the component of motion toward or away from a given location. In Doppler radar, it is the component of motion that is parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, cannot be detected.)

Radiation - The process by which energy is propagated through any medium by virue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

Radiational Cooling - >The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it is whenever the earth's surface suffers a net loss of heat due to the reradiation of surface heat. See terrestrial radiation.

Radiation Fog - Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning. It may also be called ground fog.

RadioSonde - An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends. Often call a RAOB, an acronym for RAdiosonde OBservation.

Rain - Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on the METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.

*Rain Foot - [Slang], For a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.

Rainbow - Rainbows occur when sunlight is refracted and then reflected by raindrops. The raindrops act like a prism and form a luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun. On occasion the light can be reflected from both the front and back of the raindrops and two rainbows are visible, with the colour bands in the second opposite to those in the primary rainbow.

Rainfall - The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge. Refer to rain for rates of intensity and the quantitative precipitation for forecasting.

Rain Forest - A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.

*Rain-free Base - A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.

Rain Gauge - An instrument used to measure the amount of rain that has fallen. Measurement is done in hundredths of inches (0.01").

Rain Shadow - Also referred to as a precipitation shadow, it is the region on the lee side of a mountain or similar barrier where the precipitation is less than on the windward side. For example, the relatively dry Washoe Valley of western Nevada is in the rain shadow of the Sierra Nevada.

Range Resolution - The ability of radar to distinguish between targets on the same azimuth but at different ranges.

RAWINSonde - An upper air observation that evaluates the winds, temperature, relative humidity, and pressure aloft by means of a balloon-attached radiosonde that is tracked by a radar or radio direction-finder. It is a radiosonde observation combined with a winds-aloft observation, called a rawin.

*Rear Flank Downdraft (or RFD) - A region of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant; thus the presence of a hook or pendant may indicate the presence of an RFD.

Reconnaissance (RECCO) Code - An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.

Red Tide - A phenomena that occurs near the surface of the sea and is caused by an abnormal growth of dinoflagellates, single-celled plankton. The toxin secreted by the dinoflagellates is capable of killing fish through a cyclic effect. As decomposition continues, it creates more nutrients, which feed the plankton bloom, and the greater quantities can color the water red.

Red Watch or Red Box - [Slang], For a tornado watch.

Reflectivity - A measure of the process by which a surface can turn back a portion of incident radiation back into the medium through which the radiation approached. Reflectivity of a given surface is know as albedo.

In meteorlogy it is a Radar term referring to the ability of a radar target to return energy; used to derive echo intensity, and to estimate precipitation intensity and rainfall rates. It is also the degree by which a precipitation type is able to reflect a radar beam, and to return energy; this is used to estimate precipitation intensity and rainfall rates See dBZ, VIP.

Refraction - The bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.

Relative Humidity - A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

Relative VORTicity - The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis. Refer to vorticity.

Report - A weather report is a statement of the actual weather conditions observed at a specific time at a specific site.

Resolution - In relation to radar, it is the ability to read two distinct targets separately. The clearer the resolution, the nearer the two objects can be to each other and still be distinguishable.

Retrogression - In meteorology, it is the movement of a weather system in a direction opposite to the direction of the basic flow in which it is embedded. Often used in reference to a long wave trough or other macroscale feature. For example, a long wave trough that may move slightly westward when the "normal" movement and flow is eastward.

Ridge - An elongated area of high atmospheric pressure that is associated with an area of maximum anticyclonic circulation. The opposite of a trough.

Right Entrance Region (or Right Rear Quadrant) - The area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.

*Right Mover - A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.) See left mover, splitting storm.

Rime - The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing. See glaze, hail, ice pellets, or snow pellets.

Rip Current - It is formed by a strong surface water movement, or current, of a short duration that flows seaward from the shore. The return flow is piled up onshore by the incoming waves and wind. It is localized, of narrow width, and its position relative to the beach can change as the wave condition changes. Therefore, the higher the waves, the stronger the current.

RocketSonde - A type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data during its parachute descent from a higher position in the atmosphere than a balloon could reach.

*Roll Cloud - A relatively rare, low-level, horizontal, tube-shaped arcus cloud. Although they are associated with a thunderstorm, gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base of the cumulonimbus cloud or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds.

*Rope (or Rope Funnel) - A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.

Rope Cloud - In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.

*Rope Stage - The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.

Rossby Waves - The movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling the earth. Named for Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee, who first theorized about the existence of the jet stream in 1939.

Rotation - The spinning of a body, such as the earth, about its axis.

Rotor Cloud - An altocumulus cloud formation that can be found in the lee of a mountain or similar barrier. The air rotates around a horizontal axis, creating turbulence. Altocumulus lenticularis is an example.

RUC - Rapid Update Cycle - A numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

Runway Visual Range (RVR) - It is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.

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Saffir-Simpson Scale Damage-Potential Scale - Developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Director of the National Hurricane Center, it is a measure of hurricane intensity on a scale of 1 to 5. The scale categorizes potential damage based on barometric pressure wind speeds, and storm surge.

Saffir-Simpson Scale:

     Cat. 1 Hurricane: Winds 74-95 mph (64-82 kt or 119-153 km/hr). - Moderate Damage.
     Cat. 2 Hurricane: Winds 96-110 mph (83-95 kt or 154-177 km/hr). -  Considerable damage. 
     Cat. 3 Hurricane: Winds 111-130 mph (96-113 kt or 178-209 km/hr). - Significant damage.
     Cat. 4 Hurricane: Winds 131-155 mph (114-135 kt or 210-249 km/hr). -  Devastating damage.
     Cat. 5 Hurricane: Winds greater than 155 mph (135 kt or 249 km/hr). - Castrophic damage.

For complete details on the Saffir-Simpson Scale click here.

Salinity - A measure of the quantity of dissolved salts in sea water. The total amount of dissolved solids in sea water in parts per thousand by weight.

Salt Water - The water of the ocean, distinguished from fresh water by its appreciable salinity.

Sand - Loose particles of hard, broken rock or minerals. In observing, sand is reported when particles of sand are raised to sufficient height that reduces visibility. It is reported as "SA" in an observation and on the METAR.

Sandstorm - A strong wind carrying sand particles through the air. They are low level occurances, usually only ten feet in height to not more than fifty feet above the surface. Due to the frequent winds created by surface heating, they are most predominate during the day and die out in the night. Visibility is reduced to betweeen 5/8ths and 6/16ths statute mile, and if less than 5/16ths, then the storm is considered a heavy sandstorm. It is reported as "SS" in an observation and on the METAR.

Santa Ana Winds - The hot, dry winds, generally from the east, that funnel through the Santa Ana river valley south of the San Gabriel and San Bernadino Mountains in southern California, including the Los Angeles basin. Classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind.

Sargasso Sea - An area of the North Atlantic Ocean between Bermuda and the Azores. It is in the middle of the North Atlantic oceanic gyre, with converging surface waters. Consequently, it has less biological features than any other region of the ocean because the lack of not mixing with more nutrient-rich waters.

SATellite - A manufactured object that orbits a celestial body, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, such as GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.

SATellite Pictures (Photos) - A photograph of the earth taken by weather satellite that reveal information, such as the flow of water vapor, the movement of frontal systems, and the development of a tropical system. Looping individual pictures aids meteorologists in forecasting. One way a picture can be taken is as a visible shot, that is best during times of visible light (daylight). Another way is as an IR (infrared) shot, that reveals cloud temperatures and can be used day or night.

Saturate - To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.

Saturation Point - The point when the water vapor in the atmosphere is at its maximum level for the existing temperature

Scattered - The amount of sky cover for a cloud layer between 3/8ths and 4/8ths, based on the summation layer amount for that layer.

Scattering - The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. When working with radars, this often refers to the more or less random changes in direction of radio energy.

*Scud (or Fractus) - Small, ragged, low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. Such clouds generally are associated with cool moist air. They are often along and behind cold fronts and thunderstorm gust fronts and thunderstorm outflows. When observed from a distance, they are sometimes mistaken for tornadoes.

Sea Breeze - A daytime diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

Sea Breeze Front - A coastal phenomena, it is restricted to large bodies of water and their immediate coast lines. This is usually the landward extent of the sea breeze. Due to the imbalance of heating between land and water, a region of maximum upward motion or convergence occurs by mid-afternoon in the summer some 10 to 15 miles inland. Several air mass thunderstorms will form, or a line of towering cumulus clouds with showers. At the beach, there are blue skies and a light breeze. This often occurs along the coast of the Gulf of Mexico and Flordia's east coast.

Sea Fog - A type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water. See Arctic Sea Smoke for an example.

Sea Ice - Ice that is formed by the freezing of sea water. It forms first as small crystals, thickens into sludge, and coagulates into sheet ice, pancake ice, or ice floes of various shapes and sizes.

Sea Level - The height or level of the sea surface at any time. It is used as a reference for elevations above and below. Refer to mean sea level.

Sea Level Pressure - The atmospheric pressure at mean sea level, usually determined from the observed station pressure.

Sea Mile - A unit of length distinguished from a nautical mile. One sea mile is equivalent to 1,000 fathoms (6,000 feet).

Season - A division of the year according to some regularly recurring phenomena, usually astronomical or climatic. For example, in the Northern Hemisphere, winter is said to begin on the winter solstice and end on the vernal equinox when spring begins, covering the months of December, January, and February. In the tropics, there is the dry and the rainy season, depending on the amount of precipitation.

Sea Spray - Sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave. See blowing spray as a factor in visibility at sea and as a possible condensation nuclei.

Sea Surface Temperature (SST) - The temperature of the water's surface. It is measured using buoy and ship data, infrared satellite imagery, and coastal observations.

SELS - The acronym for the SEvere Local Storms Unit, the former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).

SEMI-Permanent Pressure Systems - A relatively stable, stationary pressure-and-wind system where the pressure is predominately high or low with the changing season. They are not of a transitory nature, like migratory lows that develop from temperature and density differences. Examples include the Icelandic Low and the Bermuda High in the North Atlantic.

Severe Weather - Generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes.

Severe Thunderstorm - A thunderstorm with winds measuring 50 knots (58 mph) or greater, 1 inch hail or larger, or tornadoes. Severe thunderstorms may also produce torrential rain and frequent lightning. See a supercell for an example. Structural wind damage may be imply with the occurrence of a severe thunderstorm. See approaching (severe).

Severe Thunderstorm Warning - This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail 1" inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criteria for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.
Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning. If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning. If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.
After it has been issued, the affected NWFO will follow it up periodically with Severe Weather Statements. These statements will contain updated information on the severe thunderstorm and they will also let the public know when the warning is no longer in effect.

Severe Thunderstorm Watch - This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces 1" inch hail or larger in diameter and/or winds equal or exceed 58 miles an hour. The size of the watch can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.
A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement. During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.

Severe Weather Statement - A National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.

Shear - It is the rate of change over a short duration. In Meteorlogy it refers to wind shear. In wind shear, it can refer to the frequent change or variation in wind speed (speed shear) within a short distance. It can occur vertically, such as a change with height, or horizontally, but usually refers to the vertical wind shear, the change in wind with height. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.

Shear Line - A line of maximum horizontal wind shear. A narrow zone across which there is an abrupt change in the horizontal wind component parallel to it.

*Shelf Cloud - A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Short-Fuse Warning - A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Short Wave (or Shortwave Trough) - A progressive wave or disturbance of smaller amplitude, wave length, and duration than a long wave. It is a disturbance in the mid or upper part of the atmosphere. It moves in the same direction as the basic current in which it is embedded, and may induce upward motion ahead of it, possibly contributing to thunderstorm development. They are more numerous than long waves and often disappear with height in the atmosphere.

Showalter Stability Index - A measure of the local static stability of the atmosphere. It is determined by lifting an air parcel to 500 millibars and then comparing its temperature to that of the environment. If the parcel is colder than its new environment, then the atmosphere is more stable. If the parcel is warmer than its new environment, then the atmosphere is unstable and the potential for thunderstorm development and severe weather increases.

Shower - It is precipitation from a convective cloud. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE) ,and is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. It is reported as "SH" in an observation and on the METAR.

Siberian Express - A fierce, cold flow of air that originates in Siberia, then moves into Alaska and northern Canada before moving southward into the United States.

Siberian HIGH - The semi-permanent high pressure area that forms over Siberia during the winter. The average central pressure exceeds 1030 millibars from late November to early March. It is one the main contributors to a monsoon in southern Asia.

Sidereal Time The measure of time as defined by the diurnal motion of the vernal equinox. A sidereal day is equivalent to one complete rotation of the earth relative to the equinox, which is 23 hours, 56 minutes, and 4.091 seconds. A sidereal year is the interval required for the earth to make one absolute revolution around the sun, which is 365 days, 6 hours, 9 minutes, and 9.5 seconds. Compare with the solar day.

Significant Weather Outlook - A narrative statement produced by the National Weather Service, frequently issued on a routine basis, to provide information regarding the potential of significant weather expected during the next 1 to 5 days.

Skew T-LOG P Diagram - A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is used to evaluate and forecast air parcel properties. Some values that can be determined are the Convective Condensation Level (CCL), the Lifting Condensation Level (LCL), and the Level of Free Convection (LFC).

Sky - The vault-like apparent surface against which all aerial objects are seen from the earth.

Sky Cover - The amount of the celestial dome that is hidden by clouds and/or obscurations.

Sleet - Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface. It is reported as "PE" in an observation and on the METAR.

Slight Risk (of severe thunderstorms) - Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. See high risk, moderate risk, convective outlook.

Slush - Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.

Small Craft Advisory - An advisory issued for marine interests, espcially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).

Smoke - Small particles produced by combustion that are suspended in the air. A transition to haze may occur when the smoke particles have traveled great distance (25 to 100 miles or more), and when the larger particles have settled out. The remaining particles become widely scattered through the atmosphere. It is reported as "FU" in an observation and on the METAR.

Snow - Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes. It is reported as "SN" in an observation and on the METAR.

Snow Advisory - A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns of less severe weather conditions than a winter storm warning.

Snow Banner - A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.

Snow Blindness - Temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. The medical term is niphablepsia.

Snowburn - A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.

Snow Cover - The areal extent of ground covered by the snow. It is usually expressed as a percent of the total area of a given region.

Snow Creep - A continuous, extremely slow, downhill movement of a layer of snow.

Snow Crust - The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.

Snow Depth - The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms.

Snow Devil - A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a waterspout). Formed mechanically by the convergence of local air currents. Also called a snowspout.

Snow Eater - Any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow. Generally applies to the Chinook wind in the Rocky Mountains.

Snowfall - The rate at which snow falls, usually expressed in inches of snow depth over a six hour period.

SnowFlakes - An ice crystal or an aggregate of ice crystals which fall from clouds.

Snow Flurry/Flurries - Light showers of snow, generally very brief without any measurable accumulation. May be reported as "SHSN--" in an observation and on the METAR.

Snow Garland - Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by the surface tension of thin films of water bonding individual snow crystals.

Snow Grains - Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle. It is reported as "SG" in an observation and on the METAR.

Snow Level - The elevation in mountainous terrain where the precipitation changes from rain tosnow, depending on the temperature structure of the associated air mass.

Snow Line - The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.

Snowpack - The amount of annual accumulation of snow at higher elevations.

Snow Pellets - Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail. It is reported as "GS" in an observation and on the METAR.

Snow Roller - The product of moist, cohesive snow that when initiated by wind rolls down a hill, gathering snow until it can no longer move, either due to size or lack of slope. It is shaped like a rolled sleeping bag, some reaching four feet across and seven feet in diameter.

Snow Shower - Frozen precipitation in the form of snow, characterized by its sudden beginning and ending. It is reported as "SHSN" in an observation and on the METAR.

Snow SQUALL - A heavy snow shower accompained by sudden strong winds, or a squall.

Solar Day - The complete rotation of the earth in relation to the sun. Although it varies, an average has determined a mean solar day of 24 hours. It is universally used for civil purposes. Compare with the sidereal day.

Solar Eclipse - An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.

Solstice - The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21 and the summer solstice on or about June 21.

Sounding - A plot or vertical profile of the atmosphere, using data from upper air or radiosonde observations. Usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location. Soundings are used extensively in weather forecasting, e.g., to determine instability, and to , measure the strength of the cap, and to locate temperature inversions etc. They are also used to obtain the convective temperature profile, etc...
( example).

Southern Oscillation - A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. It is represents the distribution of temperature and pressure over an oceanic area.

SPC - The acronym for the - Storm Prediction Center. A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States.

SPECIFIC Humidity - The ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. It is expressed in grams per gram or in grams per kilograms. The specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel.

Speed Shear - The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

*Spin-up - [Slang], For a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

*Splitting Storm - A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

Spring - The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the North Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.

Spring Tide - A tide of increased range, which occurs about every two weeks when the moon is new or full. Compare with a neap tide.

SQUALL - A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust. It is reported as "SQ" in an observation and on the METAR.

*SQUALL Line - A narrow band or line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a mesohigh.

ST. Elmo's Fire - A luminous, and often audible, electric discharge that is intermediate in nature. It occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. It often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. Also known as corposant or corona discharge.

Stable/Stability - Occurs when a rising air parcel becomes denser than the surrounding air. It will then return to its original position. When the density of the air parcel remains the same as the surrounding air after being lifted, it is also considered stable, since it does not have the tendency to rise or sink further. Contrast with unstable air and instability.

Staccato Lightning - A CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

Stagnation Area - An area that has a combination of stable stratification, weak horizontal wind speed, and little, if any, significant precipitation. It is usually associated with a warm area of high pressure.

Standard Atmosphere - A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO). It assumes a mean sea level temperature of 15 degrees Celsius, a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury, and a temperature lapse rate of 0.65 degrees Celsius per 100 meters up to 11 kilometers in the atmosphere.

Standard Surface Pressure - The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter.

Standing Cloud - Any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain. An example is altocumulus lenticularis.

Standing Wave - An atmospheric wave that is stationary with respect to the medium in which it is embedded. A mountain wave is an example.

Stationary Front - A front which is nearly stationary or moves very little since the last synoptic position. Also known as a quasi-stationary front.

Station ELEVation - The vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station. At Pepper Ridge North Valley the station elevation is 1469ft above sea level.

Station Pressure - The atmospheric pressure with respect to the station elevation.

Steam Fog - A type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. Also called sea smoke when it occurs over the ocean. An example is Arctic Sea Smoke.

Steering Winds (or Steering Currents) - A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Storm - An individual low pressure disturbance, complete with winds, clouds,and precipitation. Examples include thunderstorms, tornadoes, or even tropical cyclones. The name is associated with destructive or unpleasant weather. Storm-scale refers to disturbances the size of individual thunderstorms.

Storm Prediction Center (SPC) - A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the United States. Formerly known as the Severe Local Storms (SELS) unit of the National Severe Storms Forecast Center. For further information, contact the SPC, located in Norman, Oklahoma.

Storm-relative - Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm-scale - Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale.

Storm Surge - The increase in sea water height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.

Storm Tracks - The path or tracks generally followed by a cyclonic disturbance.

Storm Winds - On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).

*Straight-Line Winds - Generally, any surface wind that is not associated with rotation. An example is the first gust from a thunderstorm, as opposed to tornadic winds.

Stratiform - Clouds composed of water droplets that exhibit no or have very little vertical development and has more extensive horizontal development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus. The opposite in type are the vertical development of cumuliform clouds. Stratiform clouds then to cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

StratoCumulus - A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

Stratopause - The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temp with increasing altitude.

Stratosphere - The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.

*Stratus - One of the three basic cloud forms (the others are cirrus and cumulus). It is also one of the two low cloud types. It is a sheetlike cloud with a fairly uniform base that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth's surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces heavy precipitation. Clouds producing heavy precipitation may exist above a layer of stratus.

Stratus Fractus - Stratus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds.

*Striations - Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an LP storm.

Sublimation - The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). The opposite of crystalization.

Subpolar - The region bordering the polar region, between 50 and 70 degrees North and South latitude. This is generally an area of semi-permanent low pressure that exists and where the Aleutian and Icelandic Lows may be found. However, a dome of high pressure may form over the cold continental surfaces during the winter, for example, the North American High and the Siberian High.

SubreFraction - Less than normal bending of light or a radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. Contrast with superrefraction.

Subsidence - A sinking or downward motion of air, often seen in anticyclones. It is most prevailent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.

Sub-synoptic Low - Essentially the same as mesolow

SubTropical - The region between the tropical and temperate regions, an area between 35 and 40 degrees North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.

Sub-Tropical Air - An air mass that forms over the subtropical region. The air is typically warm with a high moisture content due to the low evaporative process.

Sub-Tropical Jet - Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25 and 35 degrees North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.

*Suction Vortex (sometimes call Suction Spot) - A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

Summation Layer Amount - The amount of sky cover for each layer is given in eighths of sky cover attributable to clouds or obscurations. The summation amount for any given layer is equal to the sum of the sky cover for the layer being evaluated plus the sky cover for all lower layers, including partial obscuration. A summation amount for a layer can not exceed 8/8ths.

Summer - Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the Southern Hemisphere.

Summer Solstice - Astronomically, this is the date when the sun reaches it's northern most point or furtherest point from the the celestial equator, the Tropic of Cancer. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. the summer solstice, occurs in the Northern Hemisphere, on or about June 21.

Sun Dog - Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. The scientific name for sun dogs is parhelion and they are also known as mock suns.

Sun Pillar - Horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's surface, reflect sunlight into vertical sun pillars for a spectacular display.

Sunrise - The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison.

Sunset - The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.

*SuperCell - A severe thunderstorm characterized by a persistent rotating, long-lived, intense updraft. Although not very common (rare), they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS.

Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.

Supercooling - The reduction of the temperature of any liquid below the melting point of that substance's solid phase. Cooling a substance beyond its nominal freezing point. Supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.

Superrefractionn - Greater than normal bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. Contrast with subrefraction.

*Surface-based Convection - Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.

Surface Boundary Layer - The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it. May also be called the boundary layer or the friction layer.

SWEAT Index - The acronym for Severe Weather ThrEAT Index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows: SWEAT = (12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where Td 850 is the dew point temperature at 850 mb, TT is the total-totals index, f 850 is the 850-mb wind speed (in knots), f 500 is the 500-mb wind speed (in knots), and s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer). SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

Swell - Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch.

SWODY1, SWODY2, SWODY3 (sometimes pronounced swoe-dee) - The day-1, day-2 and day 3 convective outlooks issued by SPC.

SYNOPTIC Chart - Any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time.

SYNOPTIC Scale (or Large Scale) - The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more. Contrast with macroscale, mesoscale, and storms.. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.

Syzgy - The points in the moon's orbit about the earth at which the moon is new or full.

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