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NOAA - Meteorology glossary Category: Sciences > Meteorology Date & country: 14/10/2013, USA Words: 668

Snow
Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow Advisory
Older terminology replaced by winter weather advisory. An advisory issued when 4, 5, or 6 inches of snow or sleet is expected in 24 hours. It is expected to create hazardous or restricted travel conditions, but not as severe as expected with a winter storm.

Snow Depth
The vertical height of frozen precipitation on the ground. For this purpose, frozen precipitation includes ice pellets, glaze, hail, any combination of these, and sheet ice formed directly or indirectly from precipitation.

Snow Flurries
Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow Grains
Precipitation of very small, white, opaque grains of ice.

Snow Pellets
Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).

Snow Shower
Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow Squalls
Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning.

Snowburst
Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

Snowfall
The depth of new snow that has accumulated since the previous day or since the previous observation.

Snowflake
White ice crystals that have combined in a complex branched hexagonal form.

Solar Energy
The energy produced by the sun.

Sounder
A special kind of radiometer that measures changes in atmospheric temperature with height, as well as the content of various chemical species in the atmosphere at various levels. The High Resolution Infrared Radiation Sounder (HIRS), found on NOAA polar-orbiting satellites, is a passive instrument. See passive system.

Sounding
A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location ( example). Soundings are used extensively in weather forecasting, e.g., to determine instability, locate temperature inversions etc.

Southern Oscillation
A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events.

Special Marine Warning
Issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more. This is usually associated with severe thunderstorms or waterspouts.

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
A small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

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.

Squall
A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute.

Squall Line
Any non-frontal line or narrow band of active thunderstorms. The term is usually used to describe solid or broken lines of strong or severe thunderstorms.

St. Elmo's Fire
A luminous, and often audible, electric discharge that is intermediate in nature between a spark discharge and a point discharge (with its diffuse, quiescent, and non-luminous character). It occurs from objects, especially pointed ones, when the electric field strength near their surfaces attains a value near 100,000 volts per m. Aircraft flying through active electrical storms often develop corona discharge streamers from antennas and propellers, and even from the entire fuselage and wing structure. It is seen also, during stormy weather, emanating from the yards and masts of ships at sea.

Stability
An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.

Stable Air
Air with little or no tendency to rise, that is usually accompanied by clear dry weather.

Standard Atmosphere
A hypothetical vertical distribution of the atmospheric temperature, pressure, and density, which by international agreement is considered to be representative of the atmosphere for pressure-altimeter calibrations and other purposes (29.92INS or 1013hPa).

Standing Lenticular Cloud
A, more or less, isolated cloud with sharp outlines that is generally in the form of a smooth lens or almond. These clouds often form on the lee side of and generally parallel to mountain ranges. Depending on their height above the surface, they may be reported as stratocumulus standing lenticular cloud (SCSL); altocumulus standing lenticular cloud (ACSL); or cirrocumulus standing lenticular cloud (CCSL).

Statement
Provides the public with information concerning the status of existing warnings.

Station Identifier
A group of four alphabetic characters used to identify a location that makes weather observations.

Station Pressure
The pressure that is read from a barometer but is not adjusted to sea level.

Stationary Front
The boundary between cool and warm air masses in that are not moving.

Stationary wave
Wave (flow pattern with periodicity in time and/or space) that is fixed relative to Earth.

Steam fog
Fog that is formed when water vapor is added to air which is much colder than the vapor's source. This is most common when very cold air drifts across relatively warm water.

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

Storm
In marine usage, winds 48 knots (55 mph) or greater.

Storm Surge
A rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.

Storm Track
the path that a low pressure area follows.

Storm Warning
A marine wind warning for sustained winds greater of 48 knots (55 mph) or more from a non-tropical system.

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.

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.

Stratocumulus
Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves.

Stratosphere-
The layer of atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km) generally characterized by an increase in

Stratus
A flat, 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.

Striations
Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud.

Sublimation
The change from ice directly to water vapor or from water vapor to ice with out going through the liquid water phase.

Subsidence
Downward moving (sinking) air over a broad area that is associated with warming air and little cloud formation.

Subtropical Jet
The branch of the jet stream that is found in the lower latitudes.

Subtropical storm
A low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation).

Supercell Thunderstorm
A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.

Supercooled Water
Water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point.

Supersaturation
The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.

Surface Hoar
The deposition (sublimation) of ice crystals on a surface which occurs when the temperature of the surface is colder than the air above and colder than the frost point of that air.

Surface Pressure
The pressure that is read from a barometer but is not adjusted to sea level.

Sustained Winds
The wind speed obtained by averaging the observed values over a one minute period.

SWEAT Index
Severe Weather ThrEAT index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds.

Synoptic Chart
Chart showing meteorological conditions over a region at a given time; weather map.

Synoptic Scale (Large Scale)
Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale.

TAF
A weather forecast for aircraft operations at an airport.

Tail Cloud
A low tail-shaped cloud extending outward from the northern quadrant of a wall cloud. Motions in the tail cloud are toward the wall cloud with rapid updraft at the junction of tail and wall cloud. This horizontal cloud is not a funnel or tornado.

Tail-end Charlie
The thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms.

Teleconnection
A strong statistical relationship between weather in different parts of the globe. For example, there appears to be a teleconnection between the tropics and North America during El Ni�o.

Temperate Zone
The area of the globe between the tropics and the polar regions.

Temperature
a measure of the warmth or coldness of an object or substance with reference to a standard value.

Terrestrial Radiation
The total infrared radiation emitted by the Earth.

Thermal
A small rising parcel of warm air produced when the earth's surface is unevenly heated.

Thermodynamics
In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thermometer
An instrument for measuring temperature.

Theta-e Ridge
An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Thunder
The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.

Thunderstorm
A storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail.

Tilted Storm or Tilted Updraft
A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Topography
Generally, the lay-out of the major natural and man-made physical features of the earth's surface. Bridges, highways, trees, rivers and fields are all components that make up this topography.

Tornadic Activity
The occurrence or disappearance of tornadoes, funnel clouds, or waterspouts.

Tornado
A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.

Tornado Alley
The area of the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River Valleys. Although no state is entirely free of tornadoes, they are most frequent in the Plains area between the Rocky Mountains and Appalachians

Tornado Family
A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Tornado Warning
Issued when there is likelihood of a tornado within the given area based on radar or actual sighting. It is usually accompanied by conditions indicated for Severe Thunderstorm Warning.

Total-Totals Index
A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb.

Trade Winds
Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low. They blow northeasterly in the Northern Hemisphere.

Transverse Bands
Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence.

Transverse Rolls
Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow.

Triple Point
The intersection point between two boundaries (dry line, outflow boundary, cold front, warm front etc.), often a focus for thunderstorm development.

Tropical Air
An air mass that has warm temperatures and high humidities and develops over tropical or sub-tropical areas.

Tropical Depression
Tropical mass of thunderstorms with a cyclonic wind circulation and winds near the surface between 23 mph and 39 mph.

Tropical Disturbance
An organized mass of thunderstorms in the tropics than lasts for more than 24 hours, has a slight cyclonic circulation, and winds less than 23 mph.

Tropical Storm
An organized low pressure system in the tropics with wind speeds between 38 and 74 mph.

Tropical Storm Warning
A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours.

Tropical wave
A kink or bend in the normally straight flow of surface air in the tropics which forms a low pressure trough, or pressure boundary, and showers and thunderstorms. Can develop into a tropical cyclone.

Tropics
The area of the globe from latitudes 23.5 degrees north to 23.5 degrees south.

Tropopause
The boundary between troposphere and the stratosphere. It is usually characterized by an abrupt change in temperature with height from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).

Troposphere
The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height. It's the layer of the atmosphere where most of the weather occurs.

Trough
An elongated area of relatively low atmospheric pressure surface or aloft. Usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Turbulence
Disrupted flow in the atmosphere that produces gusts and eddies. At times this can be violent and can cause the up and down movement of a plane.

Turkey Tower
A narrow, individual cloud tower that develops and falls apart rapidly.

TVS
Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation

Twister
A colloquial term for a tornado.

Typhoon
A hurricane that forms in the Western Pacific Ocean.

Ultraviolet radiation
The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.

Unstable Air
Air that rises easily and can form clouds and rain.

Updraft
A small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds.