Glossary
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Acid Rain
Deposition of acidic substances, resulting from long-range atmospheric transport of pollutants such as sulfur and nitrogen, which produce enhanced environmental acidification when they reach the Earth's surface.

Air
Mixture of gases which composes the Earth's atmosphere.

Air Density
Mass of air per unit volume.

Air Mass
An extensive body of the atmosphere whose physical properties, particularly temperature and humidity, exhibit only small and continuous differences in the horizontal. It may extend over an area of several million square kilometers and over a depth of several kilometers.

Air Temperature
The temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.

Altocumulus
White or gray, or both white and gray, patch, sheet, or layer of cloud.

Altostratus
Grayish or bluish cloud sheet or layer of stratified, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the Sun at least vaguely, as through ground glass.

Amount of Precipitation
Depth to which the precipitation would cover, in liquid form, a horizontal projection from the Earth's surface, in the absence of infiltration, runoff or evaporation, and if all solid precipitation was melted.

Antarctic Air
Mass of air from the Antarctic Continent which is very cold in the lower levels.

Anvil Cloud
A Cirriform cloud, with an anvil shape, which forms the upper part of a well-developed Cumulonimbus. Its glaciated top spreads out horizontally upon reaching the tropopause or by the action of the winds aloft.

Arctic Air
Mass of air in the Arctic Regions, very cold in the lower levels, which invades lower latitudes at irregular intervals.

Arctic Front
Front which separates cold Arctic air originating at high northern latitudes from relatively warm polar air.

Atmosphere
Gaseous envelope which surrounds the Earth.

Atmospheric Pressure
(pressure) Pressure (force per unit area) exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the atmosphere.

Atmospheric Tide
(Solar Atmospheric Tide) Periodical oscillations of the atmosphere, caused by the gravitational action of the Moon and Sun.

Aurora
(Polar Aurora) Luminous phenomena, in the form of arcs, bands, drapes, or curtains in the high atmosphere (caused by charged particles from space), mainly at high latitudes.

Aurora Australis
(Southern Lights) Aurora of the Southern Hemisphere.

Aurora Borealis
(Northern Lights) Aurora of the Northern Hemisphere.

Avalanche
Mass of snow and ice falling suddenly down a mountain slope and often taking with it earth, rocks, and rubble of every description.

Banner Cloud
Stationary orographic cloud which forms in the neighborhood of a mountain crest or peak and takes the shape of a banner streaming downward from the mountain peak. This type of cloud must not be confused with snow which is blown off a mountain summit and carried downward.

Barometer
An instrument for measuring atmospheric pressure.

Blizzard
Violent winter storm, lasting at least 3 hours, which combines below freezing temperatures and very strong wind laden with blowing snow that reduces visibility to less than 1 km.

Blood Rain
Rain of a reddish color caused by dust particles containing iron oxide picked up by the raindrops.

Blowing Snow
Snow raised by the wind to moderate heights above the ground reducing the horizontal visibility at eye level.

Blue Moon
(Green Moon, Blue Sun, Green Sun) Phenomenon caused by the presence of large quantities of suspended particles in the atmosphere which selectively remove the longer lunar to solar visible wavelengths more than the blue or green wavelengths.

Boiling Point
Temperature of equilibrium between the liquid and vapor phases of a substance at a given pressure.

Breeze
(1) In general, a light moderate wind (2) On the Beaufort Scale, a wind speed ranging from 4 knots (lower limit of "light breeze") to 27 knots (upper limit of "strong breeze").

Bush
A large debris collar at the touchdown point of a tornado funnel.

Calm
Absence of air motion or wind with a speed of less than 1 knot.

Carbon Dioxide
A gas present in the atmosphere to the extent of more than 0.03% by volume and playing an important role in the greenhouse effect.

Celsius Temperature Scale
Thermodynamic scale of temperature defined as a function of the Kelvin temperature scale by the relationship C=K-273.16. The freezing point of water at a standard atmospheric pressure is 0 degrees Celsius and the corresponding boiling point is 100 degrees Celsius.

Chinook
Wind of foehn type blowing on the eastern side of the Rocky Mountains.

Cirrocumulus
Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separated, and more or less regularly arranged.

Cirrostratus
Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance totally or partially covering the sky, and generally producing halo phenomena.

Cirrus
Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both.

Clearing
(Clearance) (1) Decrease of total cloud amount from an initial cloudy state. (2) Time in which this decrease takes place. (3) Gap in cloud layer covering the entire sky.

Clear Sky
Sky with a total cloud cover of less than one okta.

Cloud
A hydrometeor consisting of minute particles of liquid water or ice, or both, suspended in the free air and usually not touching the ground. It may also include large particles of liquid water or ice and non-aqueous liquid or solid particles such as those present in fumes, smoke, and dust.

Cloud Bank
A fairly well-defined mass of cloud observed at a distance; it covers an appreciable portion of the horizon sky but does not extend overhead.

Closest Point of Approach
Point where hurricane eye makes closest contact to shore without making landfall.

Cloud Discharge
Lightning discharge which occurs within a thundercloud and produces a diffuse illumination usually without a distinct channel being fired.

Coastal Flood Warning
A warning that significant wind-forced flooding is to be expected along low-lying coastal areas if weather patterns develop as forecast.

Coastal Flood Watch
An alert that significant wind-forced flooding might be expected along low-lying coastal areas if weather patterns develop as forecast.

Comfort Index
An index which expresses the combined effects of temperature and humidity on bodily comfort and which can be used to indicate, by reference to a table, graph, or monogram, whether the conditions specified are (broadly speaking) comfortable or not.

Condensation
The physical process by which water vapor is transformed into dew, fog, or cloud droplets.

Continental Air
Mass of air which remains over a continent for several days and which, therefore, has a fairly low moisture content.

Cumuliform Cloud
Cloud with the bulging appearance of a Cumulus.

Cumulus
Detached cloud, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part is often like a cauliflower. The sunlit parts are mostly brilliant white and the base is relatively dark and nearly horizontal. Sometimes Cumulus clouds are ragged.

Cyanometer
Instrument for measuring the blueness of the sky.

Daily Maximum Temperature
Maximum temperature during a continuous time interval of 24 hours.

Daily Minimum Temperature
Minimum temperature during a continuous time interval of 24 hours.

Depth of Snow
(Snow Depth) Vertical distance between the top surface of a snow layer and the ground beneath; the layer is assumed to be evenly spread over the ground which it covers.

Desert Climate
(Arid Climate) A climatic type which is characterized by insufficient moisture to support appreciable plant life.

Dew
Deposit of water droplets on objects the surface of which is sufficiently cooled, generally by nocturnal radiation, to bring about the direct condensation of the water vapor from the surrounding air.

Dew-Point
(Dewpoint Temperature) Temperature to which a volume of air must be cooled at constant pressure and constant moisture in order to reach saturation; any further cooling causes condensation.

Dew-Point Depression
Difference between the air temperature and the dew-point.

Doldrums
(Equatorial Calms) Zone of calms or light variable winds, in the lower atmospheric layers.

Drifting Snow
An ensemble of snow particles raised by the wind to a small height above the ground. The visibility is not sensibly diminished at eye level.

Drizzle
Fairly uniform precipitation in very fine drops of water (diameter less than 0.5mm) very close to one another, falling from a cloud.

Dry Air
Air of low relative humidity.

Dry Snow
Snow from which a snowball cannot readily be made and which has a temperature less than zero degrees Celsius.

El Niño
An anomalous warming of ocean water off the west coast of South America, usually accompanied by heavy rainfall in the coastal region of Peru and Chile. This warming of Pacific Ocean waters near the Equator typically occurs every 3 to 7 years and dictates a shift in "normal" weather patterns.

Emergency Broadcast System
A system designed to permit government officials to issue up-to-date and continous emergency information and instructions to the public in a threatened or actual emergency.

Emergency Operations Center (EOC)
The county facility that serves as a central location for the coordinaton and control of all emergency preparedness and response.

Emergency Public Information
Information disseminated primarily, but not unconditionally, at the time of an emergency frequently including actions, instructions and direct orders.

Emergency Public Shelter
Generally a public school or other such structure designated by county officials as a place of refuge.

Equatorial Air
Mass of air which has been over regions close to the equator for several days and which, therefore, has become fairly warm.

Evacuation Time
The lead time that a populated coastal area must have to safely relocate all residents of vulnerable areas from an approaching hurricane. This time can also be perceived as the necessary amount of time between the local official evacuation order and the arrival of sustained gale forece winds and/or flooding.

Evaporation
(Evaporation of Water) Emission of water vapor by the free surface of liquid water at a temperature below the boiling point.

Exceptional Visibility
Visibility such that objects are readily visible at a great distance.

Eye of a Tropical Cyclone
The relatively clear and calm area inside the circular wall of convective clouds, the geometric center of which is the center of tropical cyclone.

Extent of Evacuation
The identification of vulnerable people to evacuate based on estimated damage and/or homes susceptible force winds.

Extratropical
A term used in advisories and tropical summaries to indicate that a cyclone has lost its tropical characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) proceses.

Fahrenheit Temperature Scale
A temperature scale with the freezing point of water at 32 degrees and its boiling point at 212 degrees. Conversion to the Celsius temperature scale is accomplished by the formula C=(F-32)5/9.

Firn
Old snow which has become granular and dense under the action of various processes of melting and refreezing, including sublimation.

Flash Flood
A flood that rises quite rapidly with little or no advance warning, usually as a result of intense rainfall over a small area, or possibly, an ice jam, a dam failure, etc.

Flood
(Flooding) The overflowing by water of the normal confines of a stream or other body of water, or the accumulation of water by drainage over areas which are not normally submerged.

Flood Forecast
Prediction of a stage, discharge, beginning and duration of a flood, especially of the peak discharge at a specific point on a stream resulting from precipitation and/or snow melt.

Flood Plain
Any part of a valley floor subject to occasional floods that threaten life and property.

Flood Stage
That stage, on a fixed river gauge, at which the overflow of the natural banks of a stream begins to cause damage in any portion of the reach for which the gauge is used as an index.

Flood Warning
The expected severity of flooding (minor, moderate or major) as well as where and when the flooding will begin.

Foehn
Wind warmed and dried by descent, in general on the lee side of a mountain.

Fog
Suspension of very small, usually microscopic water droplets in the air; generally reducing the horizontal visibility at the Earth's surface to less than 1 km.

Forecast
(Meteorological Forecast, Weather Forecast) Statement of expected meteorological conditions for a specific period and for a specific area or portion of air space.

Forward Speed
The rate of movement (propagation) of the hurricane eye in mph or knots.

Freezing Point
The temperature at which a liquid solidifies under the influence of a particular set of conditions.

Freezing Precipitation
(Freezing Drizzle, Freezing Rain) Precipitation drops freezing on impact to form a coating of clear ice (glaze) on the ground and on exposed objects.

Frost Damage
Damage to vegetation occurring when the water that is part of the cell structure of the plant solidifies, bursting cells' walls and deteriorating the plant materials.

Funnel
(Funnel Column, Trunk) Column of cloudy cone under the base of a cloud Cumulonimbus.

Funnel Cloud
Cloud formed at the core of a waterspout or tornado vortex, sometimes extended right down to the ground, caused by the reduction of pressure at the center of the vortex.

Gale
Wind with a speed between 34 and 40 knots.

Gale Warning
Sustained wins 39-54 miles an hour (34-47 knots) either predicted or occuring. Note: Gale warnings are not normally issued during tropical cyclones.

General Forecast
Forecast of the main features of the weather expected to occur over a large area.

Gentle Breeze
Wind with a speed between 7 and 10 knots.

Glaze
(Glazed Frost) A smooth compact deposit of ice, generally transparent, formed by the freezing of supercooled drizzle droplets or raindrops on objects the surface temperature of which is below or slightly above zero degrees Celsius.

Greenhouse Effect
Warming of the lower layers of the atmosphere due to its different absorption properties for long and short-wave radiation.

Ground Inversion
(Surface Inversion) Temperature inversion in the atmospheric layer next to the Earth's surface (often caused by nocturnal radiation).

Growing Season
(Vegetation Season) Season during which meteorological conditions are favorable to the growth of plants.

Gust
Sudden brief increase of the wind speed over its mean value.

Hail
Precipitation of either transparent, or partly or completely opaque particles or ice (hailstones) usually spheroidal, conical or irregular in form and of diameter very generally between 5 and 50 millimeters, which falls from a cloud either separately or agglomerated into irregular lumps.

Hailstone
Globule or piece of ice, with a diameter varying between 5 and 50 mm or even more, the fall of which constitutes hail. Hailstones consist almost entirely of transparent ice, or of a series of layers of transparent ice with a thickness of at least 1 mm, alternating with translucent layers.

Halo
(Lunar Halo, Solar Halo) Group of optical phenomena, in the form of rings, arcs, pillars or bright spots, produced by the refraction of reflection of light by ice crystals suspended in the atmosphere (cirriform clouds, diamond dust, etc).

Haze
Suspension in the atmosphere of extremely small, dry particles which are invisible to the naked eye but near enough to give the sky an opalescent appearance.

Heat Island
Dome of warm polluted air which covers an urban area and in which the temperature is higher than in the surroundings. It appears as an "island" in the pattern of isotherms on a surface map.

Heat Stress Index
A four-step index, based on the Temperature-Humidity Index or "Humuture" Index expressing the likelihood of heat stroke, sunstroke, or other acute symptoms of bodily stress.

Heatwave
Marked warming of the air, or the invasion of very warm air, over a large area; it usually last from a few days to a few weeks.

Heavy Rain
Rain with a rate of accumulation exceeding a specific value, e.g., 7.6 mm/h.

High-Level Cloud
(High Cloud) Cloud of the high cloud state. Cirrus, Cirrocumulus, and Cirrostratus are high-level clouds.

Horse Latitudes
Belts of light, variable winds and fine weather associated with the subtropical anticyclones at latitudes between about 30 and 35 degrees.

Humidity
Water vapor content of the air.

Hurricane
Name given to a warm core tropical cyclone with maximum surface wind of 118 km/h (64 knots, 74 mph or greater hurricane force wind) in the North Atlantic, the Caribbean, and the Gulf of Mexico, and in the Eastern North Pacific Ocean; maximum wind speed of 64 knots or more. Winds blow in a large spiral around a relatively calm center of extrememly low pressure known as the eye. Around the rim of the eye, winds may gust to more than 200 mph.

Hurricane Advisories
Notices numbered consecutively for each storm, describing the present and forecasted position and intensity. Advisories are issued as six-hour intervals at midnight, 6 a.m., noon and 6 p.m., Eastern daylight time. Bulletins provide additional information. Each message gives the name, eye position, instensity and forecast movement of the storm.

Hurricane Eye
The relatively calm area of extremely low pressure near the center of the storm. In this area, winds are light and the sky is often covered by clouds.

Hurricane Eye Landfall
When the eye, or physical center of the hurricane, reaches the coastline from the hurricane's approach over water.

Hurricane Path or Track
Line of movement (propagation) of the eye through an area.

Hurricane Season
The portion of the year having relatively high incidence of hurricanes. In the Atlantic, Caribbean and Gulf of Mexico, it is usually June 1 through Nov. 30.

Hurricane/Storm Probablities
The National Weather Service issues hurricane/tropical storm probablities in public advisories to realistically assess the threat of a hurricane or tropical storm hitting your community. The probablities are defined as the chance in percent that the center of the storm will pass within approximately 65 miles of 44 selected locations from Brownsville, Texas to Eastport, Maine.

Hurricane Warning
An alert added to an advisory when hurricane conditions are expected within 24 hours. Hurricane warnings identify coastal areas where winds of at least 74 mph are expected. A warning may also describe coastal areas where dangerously high water or exceptionally high waves are forecast, even though winds may be less than hurricane force.

Hurricane Watch
An alert added to an hurricane advisory covering a specificed area and duration. A watch means that hurricane conditions are a real possibility; it does not mean they are imminent. When a watch is issued, everyone in the area covered by the watch should listen for further advisories and be prepared to act quickly if hurricane warnings are issued.

Ice Storm
(Glaze Storm) Intense formation of ice on objects by the freezing, on impact, of rain or drizzle.

Icing
Any deposit or coating of ice on an object caused by the impact of liquid hydrometers, usually supercooled.

Inversion Layer
Atmospheric layer which the temperature increases or remains constant with height.

Jet Stream
Flat tubular, quasi-horizontal, current of air generally near the tropopause, whose axis is along a line of maximum speed and which is characterized by great speeds and strong vertical and horizontal wind shears.

Kelvin
(Unit of Thermodynamic Temperature) Unit of absolute temperature equal to 1/273.16 of the absolute triple point of water. Symbol: K.

Knot
Unit of speed equal to one nautical mile per hour (1.852 km/h).

Lake Effect Snowstorm
Snowstorm occurring on the shore of a lake or downwind from a lake, arising as a result of the modification of the air during its passage over the water.

Laser Radar
(Lidar-light detection and ranging) Method for investigating atmospheric behavior using pulsed light beams (lasers).

Light Air
Wind with a speed between 1 and 3 knots.

Light Breeze
Wind with a speed between 4 and 6 knots.

Lightning
Luminous manifestation accompanying a sudden electrical discharge which takes place from or inside a cloud, or, less often, high structures on the ground or from mountains.

Lightning Damage
Direct damage to aircraft, electrical transmission systems, appliances or other property, and indirect damage (due to a fire or accidents) caused by lightning.

Long-Range Forecast
Forecast whose period of validity is generally beyond ten days.

Low Cloud
(Low-Level Cloud) Cloud of the low cloud state. Stratus and Stratocumulus are almost invariably low-level clouds.

Lunar Atmospheric Tide
Atmospheric tide due to the gravitational attraction of the Moon, the main component of which is a 12-lunar hour component, as in the oceanic tides.

Maximum Temperature
Highest temperature attained during a specific time interval e.g., a day.

Mean Sea Level
(MSL) (1) The average sea surface level for all stages of the tide over a 19-year period, usually determined from the hourly heights observed above a fixed reference level. (2) In aerology, the reference surface for all altitudes. (3) In aviation, the level above which altitude is measured by a pressure altimeter.

Medium-Range Forecast
Forecast for a period extending generally from 3 to 10 days from the day of issue.

Melting Level
Level at which ice crystals and snowflakes melt during their descent through the atmosphere.

Meteorologist
Person who is professionally employed in the study or practice of meteorology.

Meteor Trail
Phenomenon which accompanies a body from space (meteoroid) during its passage through the atmosphere, such as the flash and streak of light, and the ionized trail.

Minimum Temperature
Lowest temperature attained during a specific time interval.

Mirage
Optical refraction phenomenon in the atmosphere of distant objects which are displaced from their true position.

Mist
Suspension in the air of microscopic water droplets or wet hydroscopic particles, which reduce the visibility at the Earth's surface.

Moderate Breeze
Wind with a speed between 11 and 16 knots.

Moist Air
(1) Air containing water vapor. (2) Air with a high relative humidity.

Monsoon
(Monsoon Circulation) Wind in the general atmospheric circulation, typified by a seasonally persistent wind direction and by a pronounced change in the direction from one season to another.

Monsoon Season
The period when the summer monsoon blows onto the continent.

Nimbostratus
Gray cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the Sun. Low, ragged clouds frequently occur below the layer, with which they may or may not emerge.

NOAA Weather Radio
A 24-hour continuous broadcast of existing and forecasted weather conditions.

Nocturnal Cooling
Lowering of air temperature caused by nocturnal radiation.

Norther
A strong, cold northerly wind in various parts of the Northern Hemisphere.

Ozone
Triatomic form of oxygen.

Ozone Shield
Protection given to the Earth's surface by the stratospheric ozone layer due to intense absorption of solar ultraviolet radiation by the gas.

Polar Air
Mass of air which has laid over high latitudes for several days and which, therefore, has become fairly cold, at least in the lower levels.

Pre-Eye Landfall Time
The time before actual hurricane eye landfall within which evacuation cannot be carried out because of earlier effects, such as the inundation of evacuation routes from the storm surge or rainfall and the arrival of sustained gale force wins. It is composed of the time of arrival of sustained gale-force winds or the time roadway inundation from storm surge/rainfall beings, whichever come first.

Public Information Officer
A person appointed by a County Emergency Operations Center to be responsible for the formulating and coordinating of the dissemination of emergency public information with both the electronic and written media, ensuring that accurate information is being released to the general public.

Radar
Radio method of determining at a single station by the direction and distance of an object. The distance is determined by the time taken by signals emitted by the station to reach a distant object and return. The term "radar" is derived from "Radio Detection and Ranging."

Radiation
Emission or transfer of energy in the form of electromagnetic waves or particles.

Radon
A gas emitted by radioactive material in the Earth's crust and forming a minute constituency of the air near the ground.

Rain
Precipitation of liquid water particles, either in the form of drops of more than 0.5 mm in diameter, or of smaller widely scattered drops.

Rainbow
Group of concentric arcs with colors ranging from violet to red, produced on a "screen" of water drops (raindrops, droplets of drizzle or fog) in the atmosphere by refraction and diffraction of light from the Sun or Moon.

Raindrop
Drop of water having a diameter of 0.5 mm or more.

Ridge
(Ridge of High Pressure) Region of the atmosphere in which the pressure is high relative to the surrounding region at the same level.

Saffir-Simpson Scale
The scale system used by the National Weather Service to give public safety officials an assessment of the potential wind and storm surge damage from a hurricane. Scale numbers are available to public safety officials when a hurricane is within 72 hours of landfall. Scale assessments are revised regularly as new observastions are made. Public safety organizations are kept informed of new estimates of the hurricane's disaster potential. Scale numbers range from 1 to 5.

Saint Elmo's Fire
(Corona Discharge) A more or less continuous luminous electrical discharge of weak or moderate intensity in the atmosphere, emanating from elevated objects at the Earth's surface (lightning conductors, wind vanes, masts of ships) or from aircraft in flight (wing tips, propellers, etc.)

Saturated Air
Moist air in a state of equilibrium with a plane surface of pure water or ice at the same temperature and pressure.

Severe Thunderstorm Warning
Indicates that severe thunderstorms have been sighted or indicated on radar.

Severe Thunderstorm Watch
Indicates that confitions are favorable for lightning, damaging winds greater than 58 miles an hour and hail and/or heavy rainfall.

Severe Tropical Storm
Maximum wind speed of 48 to 63 knots.

Shelter Period
The period in which people are forced to evacuate their homes. This time may vary from several hours to a couple of days depending of the severity of the hurricane.

Sleet
Depending on the region, precipitation of rain or snow mixed, or rain and hail, or rain and ice pellets, or melting snow, or sudden and brief rainfall with wind and hail.

SLOSH (Sea, lake and overland surges from hurricanes)
A computerized model that is able to estimate the overland tidal surge heights and winds that result from hypothetical hurricanes with selected characteristics in pressure, size, forward speed, track and winds. The resultant tidal surge is then applied to a specific locale's shoreline, incorporating the unique bay and river configurations, water depths, bridges, roads and other physical features. The model estimates open coastline heights as well as surge heights over land, thus predicting the degree of propagation or run-up of the surge into inland areas.

Small Craft Advisories
A warning of winds from 20 to 33 knots or for sea conditions either forecasted or occurring that are considered potentially hazardous to small boats in coastal waters.

Smog
Fog having a high pollution content.

Snow
Precipitation of ice crystals, isolated or agglomerated, falling from a cloud.

Snowstorm
Meteorological disturbance giving rise to a heavy fall of snow, often accompanied by strong winds.

Solar Flare
Bright eruption from the Sun's chromosphere.

Special Marine Warning
A warning for hazardous weather conditions, usually short and not adequately covered by existing marine warnings. Such conditions include sustained winds or gusts of 35 knots or more for 2 hours or less.

Spiral Band
The characteristic arrangement of radar echoes in the intense echo region surrounding the "eye" of a hurricane, a typhoon or a tropical storm, which allows the storm center to be located.

Spout
A phenomenon consisting of often a violent whirlwind, revealed by the presence of a cloud column or inverted cloud cone (funnel cloud), protruding from the base of a Cumulonimbus, and of a "bush" composed of water droplets raised from the surface of the seal or of dust, sand or litter, raised from the ground.

Squall
Atmospheric phenomenon characterized by an abrupt and large increase of wind speed by at least 18 mph and rising to 25 mph with a duration of the order of minutes, which diminishes rather suddenly. It is often accompanied by showers or thunderstorms.

Storm
(1) An atmospheric disturbance involving perturbations of the prevailing pressure and wind fields, on scales ranging from tornadoes (1km across) to extratropical cyclones (2000-3000km across). (2) Wind with a speed between 48 and 55 knots.

Storm Surge
The high and forceful dome of wind-driven waters sweeping along the coastline near where the eye makes landfall or passes close to the coast.

Stratiformis
(Stratiform Cloud [other than stratus]) Cloud spread out in an extensive horizontal sheet or layer thus presenting the appearance of Stratus. This term applies to Altocumulus, Stratocumulus, and, occasionally, to Cirrocumulus.

Stratus
Generally gray cloud layer with a fairly uniform base, which may give drizzle, ice prisms, or snow grains. When the Sun is visible through the cloud, its outline is clearly discernible; Stratus does not produce halo phenomena except, possibly, at very low temperatures; sometimes it appears in the form of ragged patches.

Streak Lightning
Lightning in the form of a white flash of light.

Strong Breeze
Wind with a speed between 22 and 27 knots.

Strong Gale
Wind with a speed between 41 and 47 knots.

Subjective Forecast
Forecast produced largely by methods involving the judgment and skill of a forecaster.

Temperature
A physical quantity characterizing the mean random motion of molecules in a physical body.

Temperature Inversion
Vertical temperature distribution such that temperature increases with height.

Thaw
Melting snow or ice at the Earth's surface due to a temperature rise above 0° C.

Thermometer
Instrument which measures the temperature.

Thunder
A sharp or rumbling sound which accompanies lightning. It is emitted by rapidly expanding gases along the channel of a lightning discharge.

Tornado
(Twister) A violently rotating storm of small diameter; the most violent weather phenomenon. It is produced in a very severe thunderstorm and appears as a funnel cloud extending from the base of a Cumulonimbus to the ground.

Tropical Air
Mass of air which has been over tropical or subtropical regions for several days and which, therefore, has become fairly warm and moist.

Tropical Depression
Wind speed up to 33 knots (38 mph).

Tropical Storm
Maximum wind speed of 34 to 47 knots (39 to 73 mph)

Tropical Storm Watch
An announcement that a tropical storm or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A watch normally should not be issued if the system is forecast to attain hurricane strength.

Tropical Wave
A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.

Trough
(Thalwet) An elongated area of relatively low atmospheric pressure.

Turbulence
(Turbulent Flow) Random and continuously changing air motion which are superposed on the mean motion of the air.

Twilight
An intermediate period of illumination of the sky before sunrise and after sunset; there are three definitions of twilight: civil, nautical, and astronomical.

Typhoon
Maximum wind speed of 64 knots or more.

Unstable Air Mass
Air mass having static instability in its lower layers; convective clouds and precipitation occur if its moisture content is sufficiently high.

Upper Air
(Aloft) Term used to signify the region from the base of a free atmosphere to the upper limit of the troposphere.

Violent Storm
Wind with a speed between 56 and 63 knots.

Warm Front
Any non-occluded front which moves in such a way that warm air replaces cold air.

Water Spout
A spout occurring over water; this behavior is characterized by a tendency to dissipate upon reaching shore.

Water Table
The surface separating the upper layer of non-saturated soil and the lower layer of saturated soil.

Wet Snow
Snow which contains a great deal of liquid water. If the water entirely fills the air spaces, which originally existed in the snow, it is classified as very wet snow.

Wind
Air motion relative to the Earth's surface. Unless otherwise specified, only the horizontal component is considered.

Wind Chill Index
(Wind Chill Factor, Chill Factor) Index used to determine the relative discomfort resulting from a specific combination of wind speed and air temperature, expressed by the loss of body heat in watts per square meter (of skin).

Wind Speed
Ratio of the distance covered by the air to the time taken to cover it.