Glossary
Saffir-Simpson Scale
a scale developed to estimate the possible damage a hurricane’s sustained winds and storm surge could do to a coastal area
|
Category |
Central Pressure (mb) |
Wind Speed (mph) |
Storm Surge (ft) |
Damage |
|
1 |
980 or > |
74-95 |
4-5 |
Minimal |
|
2 |
965-979 |
96-110 |
6-8 |
Moderate |
|
3 |
945-964 |
111-130 |
9-12 |
Extensive |
|
4 |
920-944 |
131-155 |
13-18 |
Extreme |
|
5 |
< 920 |
> 155 |
> 18 |
Catastrophic |
Sandstorm (SS)
particles of sand carried aloft by strong winds. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground. One kind of sandstorm is a haboob.
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 elevated, inland desert regions.
Satellite (SAT, SATL, or STLT)
an object that orbits a larger object. In weather, we usually refer to a man-made object that orbits the Earth and provides weather data as a satellite.
Saturation (of air)
the presence of the most water possible in the air. Saturation depends on the pressure and temperature of the air. If more water is added to already saturated air, the excess will be condensed from the air (moving from gas to liquid form).
Scary Looking Cloud (SLC)
a dark, ominous-looking cloud. Many SLCs are part of the turbulent flow (which is often seen as small areas of rotation) under a shelf cloud. This rotation does not usually produce tornadoes; nevertheless, it has been known to lead to "tornado" reports. Most SLCs are harmless.
Science and Operations Officer (SOO)
a position in a National Weather Service Forecast Office. The primary focus of the Science and Operations Officer is to ensure the scientific integrity of the hydrometeorological products and services provided to the public by the WFO. He or she also leads or participates in joint research projects with the collocated university/research center. The SOO monitors and evaluates the accuracy and scientific basis of forecast and warning products and services provided by the forecast office.
Scud (or Fractus)
small, ragged, low cloud fragments that are unattached to a larger cloud base. They are often seen with and behind cold fronts and thunderstorm gust fronts. Scud can also rise up and eventually form part of a wall cloud on a supercell thunderstorm.
Sea Breeze
a cooling breeze blowing generally inland from the sea in the daytime, caused by the temperature difference when the sea surface is cooler than the adjacent land
Sea Level Pressure (SLP)
the atmospheric pressure computed from the station pressure for the elevation of the station above mean sea level
Sea Surface Temperature (SST)
this term refers to the mean/average temperature of the ocean in the upper few meters.
Second Law of Thermodynamics
entropy (a measure of chaos) tends to increase in energy systems. For example, solid crystals, the most organized form of matter, have low values of entropy. Gases, which are highly disorganized, have high values of entropy.
Sensible Heat
the heat absorbed or transmitted when the temperature of a substance changes but the substance does not change state
Severe Thunderstorm
a thunderstorm with wind gusts of 50 knots (58 mph) or greater, hail at least one inch (quarter-sized) in diameter, and/or a tornado or funnel cloud
Severe Thunderstorm Watch (WW)
conditions are favorable for the development of severe thunderstorms.
Severe Weather Threat Index (SWEAT)
incorporates instability, wind shear, and wind speeds. Values of 250-300 or greater indicate a greater potential for severe weather.
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. Many shelf clouds are SLCs, and tornadoes usually do not form in this region.
Shortwave (or Shortwave Trough) (S/W, S/WV, SHRTWV)
a disturbance in the middle or upper part of the atmosphere that induces upward motion ahead of it (it creates lift)
Shortwave Radiation
in meteorology, radiation having a wavelength equal to or less than that of visible light
Skew-T, Log P Diagram
skewed temperature, logarithmic pressure diagram. This is the chart used to plot radiosonde data (a sounding).
Sky Cover
a term used to describe the amount of sky covered or concealed by clouds or obscuring phenomena. Classifications for sky cover include clear, broken, partly cloudy, and overcast.
SKYWARN
a nationwide network of volunteer weather spotters who report to and are trained by the National Weather Service. These spotters report many forms of significant or severe weather such as severe thunderstorms, tornadoes, hail, heavy snow, or flooding.
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.
Snow (SN, SNW)
precipitation in the form of small hexagonal ice crystals formed directly from the water vapor of the air at a temperature of less than 0 degrees Celsius (below 32 degrees Fahrenheit)
Snow Grains (SG)
precipitation consisting of white, opaque ice particles usually less than 1mm in diameter.
Snow Pellets
precipitation in the form of white, opaque, approximately round ice particles, about 2 to 5 mm in diameter, with a snow-like structure
Snow Squall
an intense, but limited duration, period of moderate to heavy snowfall accompanied by strong, gusty surface winds and possibly lightning. Snow accumulation may be significant.
Snowflake
either a single hexagonal (6-sided) ice crystal that grows through deposition, or an accumulation of ice crystals that stick together and fall as a unit.
Solar Constant
the total power collected per unit area. The units are meters squared. Since the orbit of Earth is an ellipse (the Sun-Earth distance varies), the solar constant changes with time.
Solstice
the times of the year when the sun appears to the farthest north or south of the equator, lying above either the Tropic of Cancer or the Tropic of Capricorn
Sonic Detection and Ranging (SODAR)
a type of wind profiler that measures the scattering of sound waves by turbulence, giving information on wind speed at various heights above the ground. SODAR is similar to radar, except that sound waves instead of radio waves are used for detection.
Sounding
a plot of the vertical profile of temperature and dew point (and often winds) above a fixed location; used extensively in weather forecasting. See Skew-T, Log P Diagram.
Sounding-based Stability Index
an index calculated from balloon observations (e.g., CAPE, Lifted Index, K-Index, Bulk Richardson Number, Total-Totals Index) that provides guidance about the potential organization, type, and severity of thunderstorms (supercell, multi-cell, etc.)
Southern Oscillation
the reversal of typical surface air pressure patterns across the tropical Pacific that occurs during a major El Niño event
Southern Oscillation Index (SOI)
an index that monitors the Southern Oscillation using the difference between sea level pressures at Darwin, Australia and Tahiti. Large negative values of the SOI indicate a warm event (El Niño) and large positive values indicate a cold event (La Niña).
Space Weather Prediction Center (SWPC)
one of 9 NCEP centers; provides space weather alerts and warnings for disturbances that can affect people and equipment working in space and on Earth. It is located in Boulder, Colorado.
Spaghetti Plot
an ensemble (group) of forecasts or observed values that looks like spaghetti. For example, if we plot critical thickness lines on a map, most of the lines will not match up. Instead, they tend to look like spaghetti tossed on a plate.
Specific Area Message Encoding (SAME)
a tone alert system that allows NOAA Weather Radio receivers to sound an alert for only certain weather conditions or within a limited geographic area such as a county.
Specific Heat
the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius; the higher the specific heat, the more heat is required to warm a substance. For example, water has a high specific heat--it takes a long time for it to heat up or to cool down. The ground has a relatively low specific heat (compared to water)--it takes a short time for it to heat up or to cool down.
Speed Shear
the component of wind shear resulting from 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 that 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 moves slower. Of the two, the left mover is more likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonically-rotating storm), while the right mover is the one more likely to produce severe weather and retain a supercell structure.
Squall
a strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots (18 mph) and is sustained at 22 knots (25 mph) or more for at least 1 minute.
Stable (STBL)
if a parcel is displaced (or moved), it will go right back to its original position and will not continue to rise.
Stable Boundary Layer
the stable and 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.
Stalled Front/System
a front or weather system that is moving very slowly or not at all. Sometimes the stalling of a front occurs because the two air masses involved have very similar temperature and moisture characteristics. If this happens, or if weak winds are behind the original front, the front is not likely to move.
Standard Atmosphere
a hypothetical distribution with height of atmospheric temperature, pressure, and density
Standard Atmospheric Pressure
the pressure exerted by a 760 millimeter column of mercury at sea level at a temperature of 0 degrees Celsius; equal to 1013.25 millibars (mb), 29.92 inches of mercury (in of Hg), or 14.7 pounds per square inch
Station Model
the specific pattern for entering meteorological symbols on a weather map. It describes the state of the weather at that geographical location
Station Pressure
the actual pressure measured at a given station location after being corrected for temperature, gravity, and instrument error
Stationary Front
the boundary between two air masses, neither of which is replacing the other--they’re at a stand-off.
Stefan-Boltzmann Law
a mathematical relationship for electromagnetic radiation, which states that the irradiance of a blackbody is proportional to the fourth power of the absolute temperature of the blackbody
Stepped Leader
a faint, negatively charged channel that emerges from the base of a thunderstorm and moves toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.
Stomata
microscopic openings on the underside of leaves. Water is lost through the stomata during transpiration.
Storm
a disturbance of the atmosphere marked by wind and usually by rain, snow, hail, sleet, or thunder and lightning
Storm Centroid
the computer- derived location of the center of a given storm. The NEXRAD RPG runs a storm detection and tracking algorithm to determine storm centroids automatically for each volume scan
Storm Prediction Center (SPC)
a national forecast center in Norman, Oklahoma, which is part of NCEP (one of nine centers); responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding), and severe winter weather over the contiguous United States
Storm Surge
an atypical rise of the sea along a shore, primarily resulting from the winds of a storm, especially those of a hurricane
Storm Top
the height of the top of the clouds. The higher the cloud top, the more likely it is that the cloud will become a thunderstorm.
Storm-Relative
measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity
Storm-Relative Velocity
the wind velocity minus storm motion. The wind at a given location may be the combination of the environmental flow plus winds due to a thunderstorm. NEXRAD produces a storm-relative radial velocity product from the base velocity product by subtracting the average motion of all identified storms on the radar scope.
Straight-Line Winds
generally, any winds that are not associated with rotation, used mainly to differentiate them from tornadic winds.
Stratified
the atmosphere is divided into distinct layers, each with different densities or temperatures. Stably stratified means that density decreases with height (e.g., cold, dense air is on the bottom).
Stratiform (STM, STRFM)
having extensive horizontal development, as opposed to the more vertical development characteristic of convection; stratiform precipitation, in general, is relatively continuous and uniform in intensity
Stratocumulus (SC)
a low-level cloud in the form of a gray and/or whitish flat layer or patch, which nearly always has dark parts and is non-fibrous. Elements often are arranged in rows, bands, or waves.
Stratopause
the layer between the Stratosphere and the Mesosphere, located at an altitude approximately between 50 and 55 km (around 31 to 34 miles above the ground).
Stratosphere
a layer above the Troposphere. This layer is where most of the ozone resides, so temperature increases with height (ozone absorbs ultraviolet radiation, warming this layer).
Stratus (ST)
a low-level cloud in the form of a gray layer with a rather uniform base; one example is fog.
Stratus Fractus (STFR)
lower in height and darker in color than cumulus fractus, this cloud also has a ragged, shredded appearance.
Streamer
a positively charged channel that emerges from the ground or from an object (e.g., top of a tree) and moves toward the sky, initiating a lightning stroke if it meets with a stepped leader.
Subgeostrophic
around a trough or low-pressure system, the wind speed is weaker than it would have been if it were blowing in a straight line.
Subtropical Jet (STJ)
this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.
Suction Vortex (sometimes Suction Spot)
a small but very intense vortex within a tornado circulation; much of the extreme damage associated with violent tornadoes is attributed to suction vortices.
Summer
the period extending from the summer solstice, about 21 June, to the autumnal equinox, about 22 September
Sun Pillar
a luminous streak of white or slightly reddened light extending vertically above and below the sun, most frequently observed near sunrise or sunset
Sundog
a colored luminous spot appearing approximately 22 degrees on either side of the sun and at the same elevation as the sun. See Parhelion.
Supercell (or Supercell Storm)
a violent thunderstorm that can produce hail and large tornadoes. It contains tilted updrafts and downdrafts that are nearly in balance, allowing it to maintain itself for several hours. Radar characteristics often (but not always) include a hook echo, 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), 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.
Supercooled
a condition in which the temperature of water is below freezing (0ºC or 32ºF), but the state of matter is still liquid. The liquid water will not freeze until its molecules are aligned in a certain way--this alignment can be provided if the droplet comes into contact with ice nuclei. As soon as a supercooled liquid hits an object that is below freezing, it will freeze on contact with the object (e.g., freezing rain).
Supergeostrophic
around a ridge or high pressure system, the wind speed is stronger than it would have been if it were blowing in a straight line
Synoptic Scale
the scale of the high and low pressure systems of the lower troposphere; generally considered to be 1000 to 2500 km in length
