Glossary of Meteorological Terms

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

- A -

Acid Rain-
rain that is especially acidic as the result of water vapor condensing onto particles of sulfate or nitrogen oxide.

Adiabatic- changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere.

Advection- the horizontal transport of air, moisture, vorticity or other atmospheric properties; commonly used in describing the transport of moisture and temperature.

Advection Fog- a type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions.

AFOS- the Automation of Field Operations and Services; AFOS is the computer system that links National Weather Service offices and other computer networks, such as the NOAA weather wire, to transmit weather information.

AGL- above ground level.

Air- the mixture of gases that make-up the earth's atmosphere.

Air Mass- a large body of air that has similar temperature and moisture characteristics.

Albedo- the reflectiveness of a surface given as either a percentage or a decimal.

Anabatic- wind flowing up an incline, such as up a hillside; upslope wind.

Anemometer- an instrument used to measure wind speed.

Angular Momentum- the energy of motion of a spinning body or mass of air or water.

Angular Velocity- the rate at which a spinning body rotates.

Anticyclone- a body of air in which the atmospheric pressure is higher than the pressure in the surrounding air; a high or high pressure area.

Anticyclonic- describes the movement of air around a high pressure; and rotation about the local vertical oppostie the earth's rotation.

Arctic Air- a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Arctic High- a very cold high pressure that originates over the Arctic Ocean.

ASOS- the Automated Surface Observation System; this system observes sky condition, weather, temperature, dew point, wind direction and speed, pressure and precipitation.

Atmosphere- the mass of air surrounding the earth and bound to it more or less permanently by the earth's gravitational attraction.

Avalanche- a large mass of rapidly moving snow down a steep mountain slope.

AWIPS- the Advanced Weather Information Processing System; this new computer system integrates computer graphics, satellite and radar imagery. (This system is scheduled to be put in all National Weather Service offices by the end of the 1990s).

- B -

Backing- relates to time trend and
wind direction; backing winds trend counter-clockwise and are indicative of cold air advection.

Bar- an obstacle formed at the shallow entrance at the mouth of a river or bay that empties into the ocean; when a swift ebb current and heavy seas rolling in from the Pacific meet, large breakers develop over the bar.

Barometer- an instrument used to measure atmospheric pressure.

Beaufort Scale- a scale that indicates the wind speed using the effect wind has on certain familiar objects.

Black Ice- thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears "black" because of its transparency.

Blizzard- includes winter storm conditions of sustained winds greater than thirty-five mph that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile.

Blowing Dust- dust that is raised by the wind to moderate heights above the ground to a degree that horizontal visibility decreases to less than seven miles.

Blowing Snow- wind blown snow that reduces horizontal visibility.

Breaker- a large sea surface wave that has become too steep and unstable; as the wave breaks a whitecap will form.

Broken Clouds- opaque clouds that cover 6/10ths to 9/10ths of the sky.

- C -

Calm- the absence of apparent motion in the

Ceiling- the height of the lowest layer of broken and overcast clouds.

Celsius- a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Chinook- a strong downslope wind that causes the air to warm rapidly as a result of compressive heating; called a Foehn wind in Europe.

Clear- the appearance of the sky when it is without clouds, or less than 1/10th of the sky is covered by clouds.

Climatology- the scientific study of climate.

Cloud Condensation Nuclei- small particles in the air on which water vapor condenses and forms cloud droplets.

Cloudy- the state of the sky when 7/10ths or more of the sky is covered by clouds.

Coastal Flooding- the submersion of land areas along the ocean coast and other inland waters caused by sea water over and above normal tide action.

Coastal Waters- include the area from a line approximating the mean high water along the mainland or island as far out as sixty nautical miles including the bays, harbors and sounds.

Cold Front- the leading edge of a cold air mass as it moves toward warmer air; its movement is characterized by a drop in temperature and humidity after the front passes.

Cold Low- a low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

Combined Seas- the interaction of wind waves and swell; the combined seas height is equal to the square root of the sum of the squares of the wind waves and swell.

Condensation- the occurrence of vapor transforming into a liquid.

Conduction- the transfer of heat between bodies that are in contact.

Convection- the transfer of heat within a gas or liquid by their movement.

Corona- a disk of light surrounding the sun or moon; this is a result of the diffraction of light by small
water droplets.

Cyclone- a body of air in which the pressure is lower than that of the surrounding air; a low or low pressure area.

- D -

Degree Day- a measure of the departure of the daily mean temperature from the normal daily
temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty-five degrees Fahrenheit.

Dense Fog- a fog in which the visibility is less than one-quarter mile.

Depression- a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Dew- water drops on an object at or near the ground formed from condensation of water vapor from the surrounding air.

Dew Point- the temperature to which a certain amount of air must be cooled in order for saturation to occur (a cloud or fog to form); the drier the air the lower the dew point and the more air is needed to be cooled for a cloud or fog to form.

Disturbance- a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Divergence- refers to the spreading out of winds.

Doldrums- the regions on either side of the equator where air pressure is low and winds are light.

Doppler Radar- radar that determines the intensity of rainfall and velocities of water and air particles; the National Weather Service uses a type of Doppler Radar called the WSR-88D, or NEXRAD; velocities of detected targets are determined by Doppler shift in frequency.

Downburst- a vertical or nearly vertical downward burst of strong winds at ground level, usually associated with a thunderstorm.

Drizzle- a form of precipitation that consists of water droplets less than .5mm.

Dust Devil- a rapidly rotating column of air carrying dust, leaves and other light-weight material; dust devils usually develop during hot, sunny days over dry and dusty or sandy areas.

Dust Storm- an area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile.

- E -

Ebb Current- the movement of a tidal current away form the coast or down an estuary.

El Nino- a great warming of the equatorial waters in the eastern Pacific Ocean; El Nino events occur every three to seven years and are related to shifts in global weather patterns.

Evaporation- the process of a liquid changing into a vapor or gas.

Extended Outlook- a basic forecast of general weather conditions three to five days in the future.

- F -

FAA- the Federal Aviation Administration.

Fahrenheit- the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty-two degrees and the boiling point is two hundred and twelve degrees.

Fair- describes weather in which there is less than 4/10ths of opaque cloud cover and no precipitation; and there is no extreme visibility, wind or temperature conditions.

Fall Wind- a strong, cold, downslope wind.

Fathom- the common unit used to measure depth in the ocean; it is equivalent ot six feet.

Fetch- the area in which waves are formed by a wind with an approximately constant speed and direction.

Foehn Wind- European term for Chinook Wind; or warm, downslope wind.

Flood- a condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Fog- water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet).

Fogbow- a rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

Freezing Level- the altitude in the atmosphere at which the temperature drops to thirty-two degrees Fahrenheit.

Freezing Rain or Drizzle- is precipitation that falls in liquid form but freezes upon contact with cold objects.

Freshet- the annual spring rise of streams in cold climates as a result of snow melt; freshet also refers to a flood caused by rain or melting snow.

Frog Storm- the first bad weather in spring after a warm period.

Front- the boundary or transition zone of two air masses; if cold air advances and replaces warmer air the front is a cold front, and if warm air advances and replaces cooler air the front is a warm front.

Frost- the formation of thin ice crystals on the ground or other surfaces; frost develops under conditions similar to those of dew, except in temperature of the earth's surface and earthbound objects fall below thirty-two degrees Fahrenheit.

Funnel Cloud- a funnel shaped cloud that extends from a cumulus or cumulonimbus cloud; this cloud is related to a rotating column of air that is not in contact with the ground.

- G -

Gale Warning- a marine weather warning for
winds of thirty-four to forty-seven knots (thirty-nine to fifty-four mph).

Geostationary Satellite- a satellite positioned over the equator that revolves around the earth once every twenty-four hours, thus remaining in the same position relative to the earth's surface.

Glaze- a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops.

Gradient- the time rate or spatial rate of change of an atmospheric property.

Greenhouse Effect- the heating of the earth's atmosphere caused by imbalances in the atmosphere's radiation cycle, characterized by so called greenhouse gases such as water vapor and carbon dioxide, that trap the heat from the sun in the atmosphere and inhibit re-radiation of that heat into space from the earth at night. Theoretically, this results in a gradual warming of the atmosphere.

Ground Fog- fog with very little vertical extent (less than twenty feet deep).

Growing Season- the period of time between the last killing frost of spring and the first killing frost of autumn.

Gust Front- the leading edge of a thunderstorm's downdraft air that is most prominent beneath the rain-free base and the leading edge of a thunderstorm; this gust front may precede the thunderstorm by several minutes and have winds that can easily exceed eighty miles an hour.

- H -

precipitation in the form of balls of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms.

Halos- rings or arcs that seem to encircle the sun or moon and are the result of the refraction of light through the ice ctystals that make up cirrus clouds.

Hard Freeze- freeze where vegetation is killed and the ground surface is frozen solid.

Harmattan- a hot, dry, and dusty northeasterly or easterly
wind that occurs in West Africa north of the equator and is caused by the outflow of air from subtropical high pressure areas.

Haze- fine dry or wet dust particles suspended in the that reduce visibility.

Heavy Snow- usually means snowfall of four air inches or more in twelve hours or less, or six inches or more in twenty-four hours or less in lower elevation; in higher elevations it means twelve inches or more in twelve hours or less, or eighteen inches or more in twenty-four hours or less.

Heavy Surf- the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

High or High Pressure Area- an area of high pressure that is usually characterized by clear skies and fair weather carried by sinking air; winds rotate clockwise around and outward from the center of the high.

High Wind- the sustained wind of forty mph or greater and/or gust greater than fifty-eight mph.

Horse Latitudes- subtropical regions where anticyclones produce settled weather.

Humidity- the amount of water vapor present in the air.

Hurricane- a tropical cyclone with sustained winds over seventy-four mph.

Hydrologic Cycle- the composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport.

Hygrometer- an instrument used to measure humidity.

- I -

Ice Jam- an accumulation of broken river ice caught in a narrow channel that frequently produces local
floods during a spring break-up.

Ice Storm- liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Inversion- an increase in temperature with height, a reverse of the normal cooling with height.

Iridescence- brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

Isobar- the line of equal pressure denoted on surface weather maps.

Isotherm- the line of equal temperature denoted on surface weather maps.

- J -

January Thaw- a period of mild weather popularly supposed to recur each year in late January.

Jet Stream- a zone of strong
winds concentrated in a narrow band in the upper atmosphere; these winds are often referred to as the storm track since the jet stream often "steers" atmospheric storms.

- K -

wind blowing down an incline, such as down a hillside; downslope wind.

Knot- the unit used to measure wind speed, equal to 1.15 statute miles per hour.

- L -

Land Breeze- a coastal breeze that blows from land out to sea, and is the result of
temperature differences when the sea is warmer then the adjacent land.

Latent Heat- the heat energy that must be absorbed when a substance changes from solid to liquid and liquid to gas, and which is released when a gas condenses and a liquid solidifies.

Lightning- any and all forms of visible electrical discharges produced by thunderstorms.

Low or Low Pressure Area- an area of low pressure usually characterized by clouds of precipitation caused by rising air; often related to inclement weather such as winds flowing counter clockwise and into the center of the low.

- M -

Macroburst- a large
downburst within a 2.5 mile or larger outflow diameter and damaging winds lasting five to twenty-five minutes.

Meteorology- the study of phenomena of the atmosphere and all the processes that take place in the atmosphere and their relationships with processes at the surface of the earth.

Microclimate- a local climate that differs from the main climate around it.

Microwave Radiation- electromagnetic radiation which comprises the highest frequency radio energy.

Millibar- the scientific unit used in measuring and reporting atmospheric

Mist- very fine water droplets at ground level that occur in air with a halo.

Monsoon- a seasonal change in wind direction bringing dry air or heavy rain.

MSL- Mean sea level.

- N -

NOAA- Natural Oceanic and Atmospheric Administration; parent agency of the National Weather Service in the Department of Commerce.

NOAA Weather Wire- a computer network that transmits weather information.

- O -

Overcast- when more than 9/10ths of the sky is covered with clouds.

Ozone- a form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is a primary component of photochemical smog.

- P -

Permafrost- a soil layer below the surface of tundra regions that remains frozen permanently.

Partly Cloudy- the appearance of the sky when 3/10ths to 6/10ths of the sky is covered with clouds.

Precipitation- all forms of water that fall from the sky and reach the ground.

Pressure- a measure of the weight of the air, that is usually measured with a barometer in meteorology.

- Q -

Quality Of Snow- the amount of ice in a snow sample expressed as a percent of the weight of the sample.

- R -

Radar- a method of detecting the distance, size, and movement of objects by their reflection of radio waves.

Radiosonde- a balloon carrying instruments for measuring conditions in the upper

Rain- a type of precipitation that is in the form of water droplets larger than 0.5mm.

Rawinsonde- a balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere.

Relative Humidity- the amount of water vapor in the air as compared to the percent of the amount of water vapor it would take to saturate the air.

Ridge- an elongated high pressure area in the atmosphere that lies between two low pressure areas.

Rossby Wabes- long waves that form in air or water that flows almost parallel to the equator, which results form the effect of the earth's rotation.

- S -

Sea Breeze- a coastal breeze that blows from sea to land during daytime in summer, that is caused by warm
air rising over land and being replaced by cooler air from over the sea.

Shower- a type of precipitation that has a stopping-and-starting nature or rapid changes in intensity; and occurs when the atmosphere is unstable.

Sleet- a mixture of rain and snow that fall when the temperature is around the freezing level.

Small Craft Advisory- a marine advisory issued for sustained winds and/or frequent gusts greater than twenty knots.

Smog- a mixture of smoke and fog; a natural fog contaminated by industrial pollutants.

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

Statement- provides the public with information concerning the status of existing severe weather conditions; and is used to cancel a watch or warning or delete part of a watch or warning that refers to areas that are no longer being threathened by a certain weather hazard.

Stationary Front- the boundary between cool and warm air in which neither air mass is advancing.

Storm Surge- a rise above the usual water level along the shore that is the result of strong onshore winds and/or reduced atmospheric pressure; the actual surge height is the difference of the observed water level minus the predicted tide.

Storm Track- the path that a low pressure area follows.

Storm Warning- a marine wind warning for winds greater than forty-eight knots (fifty-five mph).

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

- T -

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

Tidal Piling- happens when unusually high
water levels occur as the result of an accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.

Tornado- a violent, rotating column of air extending from the ground to a thunderstorm.

Trade Winds- easterly-blowing winds that are found on either side of the equator and blow northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.

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

Trough- an elongated relatively low pressure area between two high pressure areas.

Typhoon- a hurricane that occurs in eastern Asia.

- U -

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

- V -

rain falling from a cloud that evaporates before reaching the ground.

Vorticity- the tendency of a moving gas or liquid to rotate around an axis, forming a vortex.

- W -

Warning- is used when severe weather conditions are occurring, imminent or are expected within the following twelve to twenty-four hours.

Watch- informs the public that conditions exist that are favorable for the occurence of severe weather conditions within twelve to thirty-six hours of the given announcement.

Water- a transparent, odorless, tasteless liquid; composed of hydrogen and oxygen.

Waterspout- a column of rapidly spiraling
air that develops over warm, usually shallow, water in seas or large lakes.

Water Vapor- water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere.

Weather Synopsis- a description of weather patterns affecting a large area.

Wind- air in motion relative to the surface of the earth.

Wind Aloft- the wind speeds and wind directions at various levels in the atmosphere above the area of surface weather observations, usually above about two thousand feet AGL.

Wind Chill- the additional cooling effect resulting from wind blowing on bare skin.

Wind Direction- the direction from which the wind is blowing.

Wind Shear- the change in wind direction; vertical wind shear is the change in wind speed with height.

Wind Wave- a wave that is caused by the action of wind on the surface of water.

Wiresonde- an atmospheric sounding instrument that is used to obtain temperature and humidity information between ground level and height of a few thousand feet; this instrument is supported by a captive balloon while traveling from the ground level.

- X -

X-Band- a frequency band of
microwave radiation in which radars operate.

- Y-

Yellow Wind- a strong, cold, dry west
wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.

Youg- a hot wind during unsettled summer weather in the Mediterranean.

Young Ice- newly formed flat, sea or lake ice generally between two and eight inches thick.

- Z -

Zigzag Lightning- ordinary
lightning of a cloud-to-ground discharge that appears to have a single lightning channel.

Zodiac- the position of the sun throughout a year as it appears to move through successive star groups or constellations.

Zonal Wind- the wind or wind componant along the local parallel of latitude.

Zone Of Maximum Precipitation- the belt of elevation at which the annual