Weather Glossary


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S 1) South
2) Snow
S-Band Radar These were in use as network radars in the National Weather Service prior to the installation of the WSR 88-D radars. They were 10-centimeter wavelength radars.
S/W Shortwave - a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
S/WV Shortwave - a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
SafetyNET A satellite based part of the Global Maritime Distress and Safety System (GMDSS) for automatically disseminating safety information, including weather warnings and forecasts, to mariners almost anywhere on the world's oceans.
Saffir-Simpson Scale This scale was developed in an effort to estimate the possible damage a hurricane's sustained winds and storm surge could do to a coastal area. The scale of numbers are based on actual conditions at some time during the life of the storm. As the hurricane intensifies or weakens, the scale number is reassessed accordingly. The following table shows the scale broken down by central pressure, winds, and storm surge:

CategoryCentral Pressure (mb)Wind Speed (mph)Storm Surge (ft.)Damage
1980 or >74 - 954 - 5Minimal
2965 - 97996 - 1106 - 8Moderate
3945 - 964111 - 1309 - 12Extensive
4920 - 944131 - 15513 - 18Extreme
5< 920> 155> 18Catastrophic
Salinity (SAL) In oceanography, conductivity is measured and converted to salinity by a known functional relationship between the measured electrical conductivity of seawater temperature and pressure.
SAMEX Storm and Mesoscale Ensemble Experiment
Sampling Frequency The rate at which sensor data is read or sampled.
Sandstorm Particles of sand carried aloft by strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.
Santa Ana Wind In southern California, a weather condition in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions.
Sastrugi Ridges of snow formed on a snow field by the action of the wind.
SAT 1. Satellite (imagery)

2. Saturday
Satellite Hydrology Program A NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.
SATL Satellite
Saturation Vapor Pressure The vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase.
SAWRS Supplementary Aviation Reporting Station - the SAWRS program addresses the concerns of users who depend on weather observations for air operations. If the cooperator is collocated with a commissioned automated system, they ensure continuity during outage periods of the automated system. The requirement for a SAWRS arises from the FAA validated need for observations to satisfy FAR 121 or 135 operations or for the safe conduct of other aircraft.
SBCAPE Surface Based CAPE; CAPE calculated using a Surface based parcel.
SBND Southbound
SBSD Subside
SC Stratocumulus
SCA Small Craft Advisory
Scattered A layer whose summation amount of sky cover is 3/8ths through 4/8ths
Scattering The process in which a beam of light is diffused or deflected by collisions with particles suspended in the atmosphere.
SCT Scattered
Scud Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.
SE Southeast
Sea Breeze A thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.
Sea Breeze Convergence Zone The zone at the leading edge of a sea breeze where winds converge. The incoming air rises in this zone, often producing convective clouds.
Sea Breeze Front The leading edge of a sea breeze, whose passage is often accompanied by showers, a wind shift, or a sudden drop in temperature.
Sea Level Pressure The sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
Sea Surface Temperatures The term refers to the mean temperature of the ocean in the upper few meters.
Seas 1) This term is used in National Weather Service Marine Forecasts to describe the combination or interaction of wind waves and swells (combined seas) in which the spearate components are not distinguished. This includes the case when swells are negligible or are not considered in describing sea state.
2) Waves generated by the action of wind blowing at the time of observation or in the recent past. Seas become swell at some point
Second-Day Feet In hydrologic terms, the volume of water represented by a flow of one cubic foot per second for 24 hours; equal to 86,400 cubic feet. This is used extensively as a unit of runoff volume. Often abbreviated as SDF.
Secondary Ambient Air Quality Standards Air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.
Secondary Pollutant Pollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.
Sector Boundary In solar-terrestrial terms, in the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward or away from the sun.
Sector Visibility The visibility in a specific direction that represents at least a 45?Ǭ? arc of a horizontal circle.
Sectorized Hybrid Scan A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.
Sediment Storage Capacity In hydrologic terms, the volume of a reservoir planned for the deposition of sediment.
Seepage In hydrologic terms, the interstitial movement of water that may take place through a dam, its foundation, or abutments.
SEL A watch cancellation statement issued to terminate a watch before its original expiration time.
SELS Severe Local Storm
SELY Southeasterly
Sensible Heat Flux The flux of heat from the earth's surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.
Separation Eddy An eddy that forms near the ground on the windward or leeward side of a bluff object or steeply rising hillside; streamlines above this eddy go over the object.
Serial Derecho Type of derecho that consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows. A Serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the "progressive derecho."

It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low pressure system and strong short wave trough at 500 mb (strong dynamic forcing). Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (3-7 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential. The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots. Squall line movement is often less than 30 knots.
SERN Southeastern
Service Hydrologist The designated expert of the hydrology program at a WFO.
Servo Loop In radar meteorology, a generic description of hardware needed to remotely control the motion of the antenna dish.
Set The direction towards which a current is headed. For example, a current moving from west to east is said to be set to east.
Set-up The process whereby strong winds blowing down the length of a lake cause water to "pile up" at the downwind end, raising water levels there and lowering them at the upwind end of the lake.
Severe Icing The rate of ice accumulation on an aircraft is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
Severe Local Storm A convective storm that usually covers a relatively small geographic area, or moves in a narrow path, and is sufficiently intense to threaten life and/or property. Examples include severe thunderstorms with large hail, damaging wind, or tornadoes. Although cloud-to-ground lightning is not a criteria for severe local storms, it is acknowledged to be highly dangerous and a leading cause of deaths, injuries, and damage from thunderstorms. A thunderstorm need not be severe to generate frequent cloud-to-ground lightning. Additionally, excessive localized convective rains are not classified as severe storms but often are the product of severe local storms. Such rainfall may result in related phenomena (flash floods) that threaten life and property.
Severe Thunderstorm A thunderstorm that produces a tornado, winds of at least 58 mph (50 knots), and/or hail at least ?Ǭ" in diameter. Structural wind damage may imply the occurrence of a severe thunderstorm. A thunderstorm wind equal to or greater than 40 mph (35 knots) and/or hail of at least ?Ǭ?" is defined as 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 3/4 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 3/4 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 Analysis This WSR-88D radar product provides 3 base products (reflectivity (SWR), radial velocity (SWV), and spectrum width (SWW)) at the highest resolution available along with radial shear (SWS). These products are mapped into a 27 nm by 27 nm region centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. It is most effective when employed as an alert paired product with the product centered on alert at height that caused the alert. It is used to examine 3 base products simultaneously in a 4 quadrant display; and analyze reflectivity and velocity products at various heights to gain a comprehensive vertical analysis of the thunderstorm.
Severe Weather Potential Statement This statement is designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance. It is issued by the local National Weather Service office.
Severe Weather Probability This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.
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.
SEWD Southeastward
SFC Surface
Sferic In solar-terrestrial terms, a transient electric or magnetic field generated by any feature of lightning discharge (entire flash).
SG Snow grains
SGFNT Significant
Shallow Fog Fog in which the visibility at 6 feet above ground level is 5/8ths statute mile or more and the apparent visibility in the fog layer is less than 5/8ths statute mile.
SHARS Subtle Heavy Rainfall Signature
Shear Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.
Sheet Flow In hydrologic terms, flow that occurs overland in places where there are no defined channels, the flood water spreads out over a large area at a uniform depth. This also referred to as overland flow.
Sheet ice Ice formed by the freezing of liquid precipitation or the freezing of melted solid precipitation (see snow depth)
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.
SHFT Shift
SHLW Shallow
Shore ice In hydrologic terms, an ice sheet in the form of a long border attached to the bank or shore.; border ice.
Short Term Forecast A product used to convey information regarding weather or hydrologic events in the next few hours.
Short Wave Fade (SWF) In solar-terrestrial terms, a particular ionospheric solar flare effect under the broad category of sudden ionospheric disturbances (SIDs) whereby short-wavelength radio transmissions, VLF, through HF, are absorbed for a period of minutes to hours.
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.
Shortwave Also known as Shortwave Trough; a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
Shortwave Radiation In solar-terrestrial terms, shortwave radiation is a term used to describe the radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths (between about 0.1 and 2 micrometers).
Shortwave Trough Also called Shortwave; A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave trough.
Showalter Index (Abbrev. SWI) - a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.
Shower(s) A descriptor, SH, used to qualify precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.
SHRA Rain showers
SHRAS showers
SHRT Short
SHRTWV Shortwave - a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
SHSN Snow showers
SHWR Shower
Sidelobe A secondary energy maximum located outside the main radar beam. Typically, it contains a small percentage of energy compared to the main lobe, but it may produce erroneous echoes.
SIGMET Significant Meteorological Advisory
Signal-to-Noise Ratio A ratio that measures the comprehensibility of data, usually expressed as the signal power divided by the noise power, usually expressed in decibels (dB).
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.
SIGWX Significant Weather
Single Cell Thunderstorm This type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft. The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe (3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a tornado), but only briefly. In this case, they are called Pulse Severe Thunderstorms.
Sky Condition Used in a forecast to describes the predominant/average sky condition based upon octants (eighths) of the sky covered by opaque (not transparent) clouds.

Sky ConditionCloud Coverage
Clear / Sunny0/8
Mostly Clear / Mostly Sunny1/8 to 2/8
Partly Cloudy / Partly Sunny3/8 to 4/8
Mostly Cloudy / Considerable Cloudiness5/8 to 7/8
Fair (mainly for night)Less than 4/10 opaque clouds, no precipitation, no extremes of visibility/temperature/wind
SKYWARN Highly trained volunteer storm spotters
SL Sea Level
SLD Solid
Sleet Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of ?Ǭ?" or more.
Sleet Warning Issued when accumulation of sleet in excess of 1/2" is expected; this is a relatively rare scenario. Usually issued as a winter storm warning for heavy sleet.
SLGT Slight
Slight Chance In probability of precipitation statements, usually equivalent to a 20 percent chance.
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.
Sling Psychrometer An instrument used to measure the water vapor content of the atmosphere in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.
SLO Slow
SLP Sea Level Pressure
SLT 1. Slight (as in "slight chance")

2. Sleet
SLY Southerly
SMA The Soil Moisture Accounting Model.
Small Craft Generally a vessel under 65 feet in length.
Small Craft Advisory This is issued by the National Weather Service to alert small boats to sustained (more than 2 hours) hazardous weather or sea conditions. These conditions may be either present or forecasted. The threshold conditions for it are usually sustained winds of 18 knots (21 mph) (less than 18 knots in some dangerous waters) to 33 knots (38 mph) inclusive or hazardous wave conditions (such as 4 feet or greater). In the Great Lakes, this advisory relates to conditions within 5 nautical miles of shore. As a result, these will be only issued in the Nearshore Forecast. Along the coastal regions of the East Coast, Gulf of Mexico, and West Coast, this advisory relates to conditions out to as much as 100 nautical miles of shore (coastal waters). As a result, these will be only issued in the Coastal Marine Forecast. Mariners learning of this advisory are urged to determine immediately the reason by turning their radios to the latest marine broadcast. Decisions as to the degree of the hazard will be left to the boater, based on experience and size and type of boat. There is no legal definition for a "small craft."
Small Craft Advisory for Seas/Swell Issued for combined seas of 7 feet or greater. (locally defined criteria)
Small Craft Should Exercise Caution Issued for winds of 15 to 20 knots or combined seas of 6 feet. (locally defined criteria)
Small Hail Technically used to refer to snow pellets or graupel.
Small Stream Flooding In hydrologic terms, flooding of small creeks, streams, or runs.
Smog Originally smog meant a mixture of smoke and fog. Now, it means air that has restricted visibility due to pollution or pollution formed in the presence of sunlight--photochemical smog.
Smoke (abbrev. K) Smoke in various concentrations can cause significant problems for people with respiratory ailments. It becomes a more universal hazard when visibilities are reduced to ?Ǭ mile or less.
Smoke Dispersal Describes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere, i.e., a combination of mixing heights and transport winds.
Smoke Management The use of meteorology, fuel moisture, fuel loading, fire suppression and burn techniques to keep smoke impacts from prescribed fires within acceptable limits.
Smoothed Sunspot Number An average of 13 monthly RI numbers, centered on the month of concern.
SMW Special Marine Warning
SN snow
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 gage, air temperature sensor and a small shelter for housing electronics.
Snow Precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing [deposition] of the water vapor in the air.
Snow Accumulation and Ablation Model In hydrologic terms, a model which simulates snow pack accumulation, heat exchange at the air-snow interface, areal extent of snow cover, heat storage within the snow pack, liquid water retention, and transmission and heat exchange at the ground-snow interface.
Snow Advisory This product is issued by the National Weather Service when a low pressure system produces snow that may cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations. The advisory criteria varies from area to area. If the forecaster feels that it is warranted, he or she can issued it for amounts less than the minimum criteria. For example, it may be issued for the first snow of the season or when snow has not fallen in long while.
Snow Core A sample of either freshly fallen snow, or the combined old and new snow on the ground. This is obtained by pushing a cylinder down through the snow layer and extracting it.
Snow Cornice A mass of snow or ice projecting over a mountain ridge.
Snow Density The mass of snow per unit volume which is equal to the water content of the snow divided by its depth.
Snow Depth The combined total depth of both the old and new snow on the ground.
Snow Flurries Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).
Snow Grains Precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.
Snow Pack Same as Snowcover; the combined layers of snow and ice on the ground at any one time.
Snow Pellets Precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.
Snow Pillow 1) A window of snow deposited in the immediate lee of a snow fence or ridge.
2) In hydrologic terms, 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. One great advantage of snow pillows over a snow survey is the frequency of observations, which can be as high as twice per day.
Snow Shower A snow shower is a short duration of moderate snowfall. Some accumulation is possible.
Snow Squall A snow squall is an intense, but limited duration, period of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning (generally moderate to heavy snow showers). Snow accumulation may be significant.
Snow Stake A 1-3/4 inch square, semi-permanent stake, marked in inch increments to measure snow depth.
Snow Stick A portable rod used to measure snow depth.
Snow Water Equivalent The water content obtained from melting accumulated snow.
Snowboard A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.
Snowcover Also known as Snow Pack; the combined layers of snow and ice on the ground at any one time.
Snowflake An agglomeration of snow crystals falling as a unit.
Snowmelt Flooding In hydrologic terms, flooding caused primarily by the melting of snow.
Snowpack The total snow and ice on the ground, including both the new snow and the previous snow and ice which has not melted.
SNR Signal-to-Noise Ratio
SNWFL Snowfall
SOI Southern Oscillation Index - a numerical index measuring the state of the Southern Oscillation. The SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Ni?ɬo3.
Soil Moisture Water contained in the upper part of the soil mantle. This moisture evaporates from the soil and is the used and transpired by vegetation.
Solar Coordinates In solar-terrestrial terms, Central Meridian Distance (CMD). The angular distance in solar longitude measured from the central meridian
Solar Cycle In solar-terrestrial terms, the approximately 11-year quasi-periodic variation in frequency or number of solar active events.
Solar Maximum In solar-terrestrial terms, the month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a maximum.
Solar Minimum In solar-terrestrial terms, the month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a minimum.
Solar Noon The time of day at which the sun is the highest in the sky. This time varies through the year due to the change in speed of the earth's orbit around the sun.
Solar Sector Boundary (SSB) In solar-terrestrial terms, the apparent solar origin, or base, of the interplanetary sector boundary marked by the larger-scale polarity inversion lines.
Solar Wind The outward flux of solar particles and magnetic fields from the sun. Typically, solar wind velocities are near 350 km/s.
SOLN Solution
Solstice Either of the two times per year when the sun is at its greatest angular distance from the celestial equator: about June 21 (the Northern Hemisphere summer solstice), when the sun reaches its northernmost point on the celestial sphere, or about December 22 (the Northern Hemisphere winter solstice), when it reaches its southernmost point.
SOO Science and Operations Officer
Sounding A set of data measuring the vertical structure of an atmospheric parameter (temperature, humidity, pressure, winds, etc.) at a given time.
Southern Oscillation (SO) - a "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation". The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Nino occurs. The cold phase of the SO, called "La Nina" by some, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Nino Southern Oscillation or ENSO.
Southern Oscillation Index A numerical index measuring the state of the Southern Oscillation. The SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Ni?ɬo3.
Space Environment Center (SEC) - This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.
SPC Storm Prediction Center
SPCLY Especially
SPD 1) Speed
2) On a buoy report, ten-minute average wind speed values in m/s.
Spearhead Echo A radar echo associated with a downburst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During it's mature stage, the appendage turns into a major echo and the parent echo loses its identity.
Special Avalanche Warning Issued by the National Weather Service when avalanches are imminent or occurring in the mountains. It is usually issued for a 24 hour period.
Special Fire Weather Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.
Special Marine Warning This is issued by the National Weather Service for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings. These are tone alerted on NOAA Weather Radio. Boaters will also be able to get this information by tuning into Coast Guard and commercial radio stations that transmit marine weather information.
A warning issued for 2 hours or less by the National Weather Service to warn boaters of any of the following that is not adequately covered by the existing Coastal Waters Forecast:
1) thunderstorm or non-thunderstorm winds of 34 knots or more (39 mph)
2) waterspouts, detected by radar or observed
3) tornadoes moving from land to water
Special Tropical Disturbance Statement This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).
Specific Gravity The ratio of the density of any substance to the density of water.
Specific Humidity In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.
Specific Yield In hydrologic terms, the ratio of the water which will drain freely from the material to the total volume of the aquifer formation. This value will always be less than the porosity.
Spectral Density A radar term for the distribution of power by frequency.
Spectral Wave Density On a buoy report, energy in (meter*meter)/Hz, for each frequency bin (typically from 0.03 Hz to 0.40 Hz).
Spectral Wave Direction On a buoy report, mean wave direction, in degrees from true North, for each frequency bin.
Spectrum Width This WSR-88D radar product depicts a full 360 degree sweep of spectrum width data indicating a measure of velocity dispersion within the radar sample volume. It is available for every elevation angle sampled, it provides a measure of the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the velocity samples. It is used to estimate turbulence associated with boundaries, thunderstorms, and mesocyclones; check the reliability of the velocity estimates; and locate boundaries (cold front, outflow, lake breeze, etc.).
Spectrum Width Cross Section This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data
2) Estimate vertical extent of turbulence (aviation use).
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.
SPENES NESDIS Satellite Precipitation Estimates
Sphere Calibration Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.
Spillway In hydrologic terms, a structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.
Spillway Crest In hydrologic terms, the elevation of the highest point of a spillway.
Spin-Up Slang for a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.
SPKL Sprinkle
Split Flow A flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.
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.
SPLNS Southern Plains
Sporadic E In solar-terrestrial terms, a phenomenon occurring in the E region of the ionosphere, which significantly affects HF radiowave propagation. Sporadic E can occur during daytime or nighttime and it varies markedly with latitude.
SPOTNIL In solar-terrestrial terms, a spotless disk.
Spotting Outbreak of secondary fires as firebrands or other burning materials are carried ahead of the main fire line by winds.
Spray An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.
SPRD Spread
Spring 1. The season of the year comprising the transition period from winter to summer occurring when the sun is approaching the summer solstice. In the Northern Hemisphere, spring customarily includes the months of March, April and May.

2. In hydrologic terms, an issue of water from the earth; a natural fountain; a source of a reservoir of water.
Spring Tide A tide higher than normal which occurs around the time of the new and full moon.
SPS Severe Weather Potential Statement
SQLN Squall Line
Squall A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained more than 22 knots or more for at least one minute.
Squall Line A solid or broken line of thunderstorms or squalls. The line may extend across several hundred miles.
SRH Storm-Relative Helicity
SRN Southern
SS Sandstorm
SSHS Saffir/Simpson Hurricane Scale
SSTs Sea Surface Temperatures
ST Stratus
Stability The degree of resistance of a layer of air to vertical motion.
Stability Index The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.
Stable An atmospheric state with warm air above cold air which inhibits the vertical movement of air.
Stable Boundary Layer The stably-stratified layer that forms at the surface and grows upward, usually at night or in winter, as heat is extracted from the atmosphere's base in response to longwave radiative heat loss from the ground. Stable boundary layers can also form when warm air is advected over a cold surface or over melting ice.
Stable Core Post-sunrise, elevated remnant of the temperature inversion that has built up overnight within a valley.
Staccato Lightning A Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.
Stair Stepping In hydrologic terms, the process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared.
Standard Atmosphere A hypothetical vertical distribution of temperature, pressure and density which, by international consent, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.
Standard Atmosphere A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.
State Forecast Discussion This National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product. The forecaster will try to focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The term non-convective refers to weather that is not caused by thunderstorms. Intermediate State Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur. Most states are going away from this product and more toward the Area Forecast Discussion (AFD).
State Forecast Product This National Weather Service product is intended to give a good general picture of what weather may be expected in the state during the next 5 days. The first 2 days of the forecast is much more specific than the last 3 days. In comparison with the Zone Forecast Product, this product will be much more general.
State Weather Roundup This is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service (NWWS). It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point in Fahrenheit, the relative humidity, wind speed and direction, and finally additional information (wind chill, heat index, a secondary weather condition). These reports are broken up regionally. When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet with no significant weather and/or obstructions to visibility.
Station ID Five-digit WMO Station Identifier used by the Buoy Data Center since 1976. ID's can be reassigned to future deployments within the same 1 degree square.
Station Model A specified pattern for plotting, on a weather map, the meteorological symbols that represent the state of the weather at a particular observing station.
Station Pressure The absolute air pressure at a given reporting station. The air pressure is directly proportional to the combined weight of all air in the atmosphere located in a column directly above the reporting site. Consequently, the station pressure may vary tremendously from one location to another in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.
Stationary Front A front between warm and cold air masses that is moving very slowly or not at all.
STBL Stable
Steam Fog Fog that forms as cold air moves over warm water. Water evaporates from the warm water surface and immediately condenses in the cold air above. Heat from the water warms the lower levels of the air creating a shallow layer of instability. It rises like smoke from the warm surface. The low level convection can become quite turbulent. Steam fog is most common in Arctic regions where it is called "Arctic Sea Smoke", but it can and does occur occasionally at all latitudes.
Steepness In marine terms, on a buoy report, wave steepness is the ratio of wave height to wave length and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio, the wave becomes unstable and begins to break.
Steering Currents Same as Steering Winds; a prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
Steering Winds Same as Steering Currents; A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
Stepped Leader A faint, negatively charged channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.
STFR Stratus Fractus
STG Strong
Stilling basin In hydrologic terms, a basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.
STJ Subtropical Jet - this jet stream is usually found between 20?Ǭ? and 30?Ǭ? latitude at altitudes between 12 and 14 km.
STLT Satellite
STM Stratiform
STNRY Stationary
Stoplogs In hydrologic terms, large logs, timbers or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit providing a temporary closure versus a permanent bulkhead gate.
Storm Any disturbed state of the atmosphere, especially affecting the Earth's surface, and strongly implying destructive and otherwise unpleasant weather. Storms range in scale from tornadoes and thunderstorms to tropical cyclones to synoptic-scale extratropical cyclones.
Storm Data This National Climatic Data Center (NCDC) monthly publication documents a chronological listing, by states, of occurrences of storms and unusual weather phenomena. Reports contain information on storm paths, deaths, injuries, and property damage. An "Outstanding storms of the month" section highlights severe weather events with photographs, illustrations, and narratives. The December issue includes annual tornado, lightning, flash flood, and tropical cyclone summaries.
Storm Motion The speed and direction at which a thunderstorm travels.
Storm Relative Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.
Storm Relative Mean Radial Velocity Map (SRM): This WSR-88D radar product depicts a full 360?Ǭ? sweep of radial velocity data with the average motion of all identified storms subtracted out. It is available for every elevation angle sampled. It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms.
Storm Relative Mean Radial Velocity Regi (SRR): This WSR-88D radar product depicts a 27 nm by 27 nm region of storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator. It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product
Storm Scale Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale and mesoscale.
Storm Surge A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles wide and sweeps across the coastline around where the hurricane makes landfall. The maximum rises in sea-level move from under the storm to the right of the storm's track, reaching a maximum amplitude of 10 to 30 feet at the coast. The storm surge may even double or more in height when the hurricane's track causes it to funnel water into a bay. The storm surge increases substantially as it approaches the land because the normal water depth decreases rapidly as it approaches the beaches. The moving water contains the same amount of energy; thus, resulting in an increase of storm surge. Typically, the stronger the hurricane, the greater the storm surge.
Storm Tide The actual sea level resulting from astronomical tide combined with the storm surge. This term is used interchangeably with "hurricane tide."
Storm Total Precipitation This WSR-88D radar product displays the total precipitation (in inches) as a graphical image. It displays hourly precipitation total (in inches) as a graphical image. Currently , this product is done in a polar format with resolution 1.1 nm by 1 degree. It will reset after one hour of no precipitation. It is used to monitor total precipitation accumulation; observe short term trends of precipitation tracks with time lapse of this product; and estimate total basin runoff and ground saturation.
Storm Tracking Information This WSR-88D radar product displays the previous, current, and projected locations of storm centroids (forecast and past positions are limited to one hour or less). Forecast tracks are based upon linear extrapolation of past storm centroid positions, and they are intended for application to individual thunderstorms not lines or clusters. It is used to provide storm movement: low track variance and/or 2 or more plotted past positions signify reliable thunderstorm movement.
Storm Warning A warning of 1-minute sustained surface winds of 48 kt (55 mph or 88 kph) or greater, either predicted or occurring, not directly associated with tropical cyclones.
Stormwater Discharge In hydrologic terms, precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces but instead flows onto adjacent land or water areas and is routed into drain/sewer systems.
Straight-Line Hodograph The name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.
Straight-line Winds Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.
Stratiform Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds 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).
Stratiform Rings and Bands These occur between the active convective bands of a hurricane outside of the eye wall. Inner stratiform bands often exhibit the bright band aloft, a VIP Level 2, and in the lower layers typically show a VIP Level 1.
Stratocumulus Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. 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 between the stratosphere and mesosphere.
Stratosphere The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.
Stratospheric Ozone In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.
Stratus A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.
Streamflow In hydrologic terms, water flowing in the stream channel. It is often used interchangeably with discharge.
Streamline The path of an air parcel that flows steadily over or around an obstacle.
STRFM Stratiform
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 a Low Precipitation (LP) storm.
Strike For any particular location, a hurricane strike occurs if that location passes within the hurricane's strike circle, a circle of 125 n mi diameter, centered 12.5 n mi to the right of the hurricane center (looking in the direction of motion). This circle is meant to depict the typical extent of hurricane force winds, which are approximately 75 n mi to the right of the center and 50 n mi to the left.
Sub-synoptic Low Essentially the same as mesolow.
Subrefraction The bending of the radar beam in the vertical which is less than under standard refractive conditions. This causes the beam to be higher than indicated, and lead to the underestimation of cloud heights.
Subsidence 1. A descending motion of air in the atmosphere occurring over a rather broad area.
2. In hydrologic terms, sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater.
Subsidence Inversion A temperature inversion that develops aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high pressure areas.
Substation A location where observations are taken or other services are furnished by people not located at NWS offices who do not need to be certified to take observations.
Subsurface Storm Flow In hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called interflow.
Subtle Heavy Rainfall Signature This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68?Ǭ?F with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.
Subtropical Cyclone A low pressure system that develops over subtropical waters that initially has a non-tropical circulation, but in which some elements of tropical cyclone cloud structure are present. Subtropical cyclones can evolve into tropical cyclones.
Subtropical Depression A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 kph) or less.
Subtropical Jet (Abbrev. STJ) - this jet stream is usually found between 20?Ǭ? and 30?Ǭ? latitude at altitudes between 12 and 14 km.
Subtropical Storm A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 kph) or more.
Suction Vortex 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.
Sudden Commencement (SC) In solar-terrestrial terms, an abrupt increase or decrease in the northward component of the geomagnetic field, which marks the beginning of a geomagnetic storm.
Sudden Impulse (SI+ or SI-) In solar-terrestrial terms, a sudden perturbation of several gammas in the northward component of the low-latitude geomagnetic field, not associated with a following geomagnetic storm. (An SI becomes an SC if a storm follows.)
Sudden Ionospheric Disturbance (SID) In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.
SUF Sufficient
Summation Principle This principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover provided at all successively higher layers up to and including the layer in question.
Summer Typically the warmest season of the year during which the sun is most nearly overhead. In the Northern Hemisphere, summer customarily includes the months of June, July, and August.
Summer Solstice The time at which the sun is farthest north in the Northern Hemisphere, on or around June 21.
Sun Dog see Parhelion
Sun Pillar A bright column above or below the sun produced by the reflection of sunlight from ice crystals.
Sun Pointing Alignment of the radar antenna by locating the position of the sun in the sky, which has an exactly known position given the radar's location and the present time. This may be necessary to verify that when we think we're pointing "north", we actually are! The sun's signal is usually several dB above the background noise, and this technique is also sometimes used to examine the receiver sensitivity.
Sunny When there are no opaque (not transparent) clouds. Same as Clear.
Sunrise The phenomenon of the sun's daily appearance on the eastern horizon as a result of the earth's rotation. The word is often used to refer to the time at which the first part of the sun becomes visible in the morning at a given location.
Sunset The phenomenon of the sun's daily disappearance below the western horizon as a result of the earth's rotation. The word is often used to refer to the time at which the last part of the sun disappears below the horizon in the evening at a given location.
Sunspot In solar-terrestrial terms, an area seen as a dark spot on the photosphere of the sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.
Sunspot Group Classification
  • A: A small single unipolar sunspot or very small group of spots without penumbra.
  • B: Bipolar sunspot group with no penumbra.
  • C: An elongated bipolar sunspot group. One sunspot must have penumbra.
  • D: An elongated bipolar sunspot group with penumbra on both ends of the group.
  • E: An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10 deg. but not 15 deg.
  • F: An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15 deg.
  • H: A unipolar sunspot group with penumbra.
Sunspot Number In solar-terrestrial terms, a daily index of sunspot activity (R), defined as R = k (10 g + s) where S = number of individual spots, g = number of sunspot groups, and k is an observatory factor.
Supercell Short reference to Supercell Thunderstorm; potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours.
Supercell Thunderstorm Potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours. Supercells usually move to the right of the mean wind. These are called "Right Movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "Left Movers". These supercells typically don't last as long as their "Right Mover" cousins and they usually only produce large hail (greater than 3/4 inch in diameter) and severe wind gusts in the excess of 58 miles an hour. Left Movers are favored when you have backing winds.

Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 3/4 inch in diameter, wind gusts greater than 58 miles an hour, and possibly a tornado) weather.

Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development. Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.
Supercool To cool a liquid below its freezing point without solidification or crystallization.
Supercooled Liquid Water In the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.
Superrefraction Bending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.
Surcharge Capacity In hydrologic terms, the volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.
Surface Energy Budget The energy or heat budget at the earth's surface, considered in terms of the fluxes through a plane at the earth-atmosphere interface. The energy budget includes radiative, sensible, latent and ground heat fluxes.
Surface impoundment In hydrologic terms, an indented area in the land's surface, such as a pit, pond, or lagoon.
Surface Runoff In hydrologic terms, the runoff that travels overland to the stream channel. Rain that falls on the stream channel is often lumped with this quantity.
Surface Water Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.
Surface Weather Chart An analyzed synoptic chart of surface weather observations. A surface chart shows the distribution of sea-level pressure (therefore, the position of highs, lows, ridges and troughs) and the location and nature of fronts and air masses. Often added to this are symbols for occurring weather phenomena. Although the pressure is referred to mean sea level, all other elements on this chart are presented as they occur at the surface point of observation.
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.
Surge In solar-terrestrial terms, a jet of material from active regions that reaches coronal heights and then either fades or returns into the chromosphere along the trajectory of ascent.
Sustained Overdraft In hydrologic terms, long-term withdrawal from the aquifer of more water than is being recharged.
Sustained Wind Wind speed determined by averaging observed values over a two-minute period.
SVR 1. Severe

2. Abbreviation for Severe Thunderstorm Warning
SVRL Several
SW 1. Southwest

2. Snow Showers
SWD On a buoy report, Swell Direction is the compass direction from which the swell wave are coming from.
SWE Snow Water Equivalent (the amount of water content in a snowpack or snowfall).
SWEAT 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 upper air sounding analysis programs.
Swell Wind-generated waves that have travelled out of their generating area. Swells characteristically exhibit smoother, more regular and uniform crests and a longer period than wind waves.
SWH On a buoy report, swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.
SWLY Southwesterly
SWODY1 The Day-1 Convective Outlook, sometimes called the "AC" is a guidance product issued by the Operational Guidance Branch (OGB) unit of the Storm Prediction Center (SPC) in Norman, Oklahoma. The Day 1 outlook outlines areas in the continental United States where severe thunderstorms may develop during the next 6 to 30 hours.
SWODY2 The Day 2 Convective Outlook is very similar to the Day 1 Outlook. It is issued only twice a day, at 08Z and 18Z, and covers the period from 12Z the following day to 12Z the day after that. For example, if today is Monday then the Day 2 Outlook will cover the period 12Z Tuesday to 12Z Wednesday. The outlook issued at 08Z now qualifies the degree of risk like the Day 1 has (i.e. SLGT, MDT, and HIGH risk areas). The Day 2 Outlook has also includes a general thunderstorm outline.
SWP On a buoy report, Swell Period is the time (usually measured in seconds) that it takes successive swell wave crests or troughs pass a fixed point.
SWRN Southwestern
SWS Severe Weather Statement
SWWD Southwestward
SX Stability Index
SXN Section
Symmetric Double Eye A concentrated ring of convection that develops outside the eye wall in symmetric, mature hurricanes. The ring then propagates inward and leads to a double-eye. Eventually, the inner eye wall dissipates while the outer intensifies and moves inward.
Synchronous Detection Radar processing that retains the received signal amplitude and phase but that removes the intermediate frequency carrier.
SYNOP Synoptic - relating to the general weather pattern over a wide region, such as areas of high and low pressure or frontal boundaries, as opposed to mesoscale or smaller features such as a thunderstorm.
Synoptic Scale The spatial scale of the migratory high and low pressure systems of the lower troposphere, with wavelengths of 1000 to 2500 km.
Synoptic Track Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.
Synoptic Weather Weather occurring over a wide region on time scales exceeding 12 hours.
SYNS Synopsis
SYS System
Syzygy In solar-terrestrial terms, the instance (new moon or full moon) when the earth, sun, and moon are all in a straight line.