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Meteorological Terms

Meteorology- This is the study of the Earth's atmosphere. A meteorologist is someone who studies the atmosphere. Physical meteorology deals with the physical aspects of the atmosphere, such as the formations of clouds, rain, thunderstorms, and lightning. Synoptic meteorology is the study and anlysis of large weather systems that exist for more than one day. Weather forecasting is one part of Synoptic meteorology.

Acid Precipitation- Rain or snow with a pH value of less than 5.6.

Adibiatic Temperature Change- A cooling or heating of the air caused by expansion of contraction of air molecules, as opposed to the gain or loss of heat. For example, adiabatic cooling takes place as air rises.

Advection- Horizontal movement of air, moisture, or heat.

Advection Fog- Fog formed by warm, humid air flowing over colder ground or water.

Air Mass- A large body of air with nearly uniform temperature and moisture content.

Airstream- A significant body of air flowing in the same general circulation.

Alberta Clipper- A small, fast-moving low-pressure system that forms in western Canada and travels southeastward into the United States. These storms, which generally bring little precipitation, generally precede an Arctic air mass.

Altitude- Height expressed as the distance above a reference point, which is normally sea level or ground level.

Anemometer- An instrument that measures wind speed.

Aneroid Barometer- An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.

Anti-Cyclone- A high-pressure system featuring clock-wise winds.

Arctic Air- A mass of very dry, very cold air that develops over the snow-and-ice-covered regions of the Far North.

Atmosphere- The mass of air surrounding the Earth.

Atmospheric Pressure (also called air pressure or barometric pressure)- The pressure asserted by the mass of the column of air directly above any specific point.

Back-Door Cold Front- A cold front that moves in from the northeast, rather than from the normal north or northwest direction.

Backing Wind- Shifting of the wind in a counterclockwise direction, usually resulting from the approach of a low-pressure system.

Barograph- An instrument that provides a continuous record of atmospheric pressure.

Barometer- An instrument for measuring atmospheric pressure.

Barometric Tendency- The amount and direction of change in barometer readings over a three-hour period.

Beaufort Wind Scale- A system used to classify wind speed, developed in 1805 by British Admiral Francis Beaufort.

Blizzard- A severe snow storm featuring sustained winds of more than 32 miles per hour, temperatures 10 degrees Fahrenheit or lower, and visibility of 500 feet or less.

Ceiling- The height above the ground of the base of the lowest layer of clouds, when at least 60 percent of the sky is covered by clouds.

Celsius Temperature Scale (also known as Centigrade Temperature Scale)- A temperature scale in which 0 degrees is the melting point of ice and 100 degrees is the boiling point of water.

Circulation- The pattern of the movement of air. General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.

Climate- A statistical portrait of weather conditions in a specific place over a long period.

Climatology- The study of climate.

Cloudburst- A sudden, intense rainfall that is normally of short duration.

Cold Front- The leading edge of a colder mass of air that displaces a warmer mass of air.

Condensation- The change of vapor to liquid.

Condensation Nuclei- Small particles in the air around which water vapor condenses.

Conduction- The transfer of heat by molecular action within a substance or when two substances are in direct contact.

Continental Air Mass- An air mass that forms over land. It is usually dry, but may be cold or warm.

Contrail- A cloud-like stream formed in cold, clear air behind the engines of an airplane.

Convection- The transfer of by movement of the heated material. In meteorology, it is often used to indicate the vertical movement of warm air (as opposed to advection, the horizontal movement).

Coriolis Effect- The curving motion of anything, such as air, caused by the rotation of the Earth.

Cyclone- A low-pressure system in which winds spin inward in a counterclockwise direction in the northern hemisphere.

Depression- An area of low atmospheric temperature.

Dew- Water that condenses onto grass and other objects near the ground.

Dew Point- The temperature to which a certain volume of air must be cooled to bring the relative humidity to 100 percent.

Disturbance- An area of low-pressure in which storm conditions occur.

Doppler Radar- Sophisticated radar that can measure the speed and direction of moving objects, such as wind.

Downburst- A sudden, strong, downward blast of air, usually from a thundercloud.

Drizzle- Precipitation featuring tiny water droplets, no more than .02 inches in diameter.

Drought- Abnormally dry weather in a region over an extended period.

Dust Devil- A small, whirling column of wind that picks up dirt and other loose material as it travels.

El Niņo- Linked ocean and atmospheric events, which have worldwide effects, characterized by warming of the water in the tropical Pacific from around the International Date Line to the coast of Peru.

Equinox- The twice-yearly occurrence (about March 21 and September 21) when the sun at its highest point crosses the equator.

Extratropical Cyclone- A cyclone that forms outside of the tropics.

Evaporation- The change in a substance from a liquid or solid state to a gaseous state.

Eye- The roughly circular area of relatively calm weather at the center of a hurricane.

Fahrenheit Temperature Scale- A temperature scale that uses 32 degrees as the melting point of ice and 212 degrees as the boiling point of water.

Flash Flood- Flooding caused by a rapid rise in the water level of rivers, streams, or lakes, usually as a result of heavy rains.

Fog- A cloud of water droplets suspended in the air that touches the ground.

Freezing- The change in a substance from a liquid to a solid state.

Freezing Nuclei- Particles suspended in the air around which ice crystals form.

Freezing Rain- Supercooled drops of water that turn to ice when they hit a cold surface.

Front- The boundary between two different air masses.

Frost- Ice crystals that form on grass and other objects when the temperature and dew point fall below freezing.

Fujita Scale- A scale for estimating damage caused by the winds of a tornado, developed by Theodore Fujita.

Glaze- A smooth coating of ice formed when supercooled water droplets spread out on a surface before freezing.

Graupel- Precipitation formed when water droplets freeze in layers around a falling ice crystal.

Greenhouse Effect- The warming that takes place when molecules in the atmosphere trap heat radiating away from the surface and redirect it back toward Earth.

Gulf Stream- A warm ocean current that flows from the Gulf of Mexico across the Atlantic to the coast of western Europe.

Gust- A sudden, brief increase in wind speed.

Hail- Chunks of ice that form in layers in the updrafts of thunderstorms.

Halo- A ring or arc of light around the sun or moon that is caused by ice crystals in the atmosphere.

Haze- Particles or fine dust suspended in the air that produce limited visibility.

Heat Index- This index is a measure of the contribution that high humidity makes with abnormally high temperatures in reducing the body's ability to cool itself.

Heat Lightning- Lightning that can be seen, but is too far away to be heard.

High- An area of higher atmospheric pressure.

Hurricane- A tropical cyclone in the western hemisphere that has sustained wind speeds of 74 miles per hour or greater.

Hydrosphere- The Earth's water.

Hygrometer- An instrument that measures the water vapor content of the air.

Ice Crystals- Frozen water vapor suspended in the air.

Instability- A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Inversion- A condition in which air near the ground is cooler than air above it....a condition opposite the normal decrease in temperature with height.

Isobar- A line on a weather map that surrounds an area with the same atmospheric pressure.

Isotherm- A line on a weather map that surrounds an area with the same temperature.

Jet Stream- A narrow band of winds blowing high in the troposphere at speeds in excess of 57 miles per hour or greater.

Kelvin Temperature Scale- A temperature scale in which 0 degrees is the point at which all molecular motion ceases (absolute zero).

Kilopascal- A metric unit of air pressure. It is simply millibars divided by 10.

Latent Heat- The energy that is stored when water evaporates. This energy is released when water condenses as a liquid or ice.

Lifting- The forcing of air in a vertical direction by an upslope in terrain or by the movement of a denser air mass.

Lightning- An electrical discharge produced by a thunderstorm.

Low- An area of low atmospheric pressure.

Maritime Air Mass- An air mass that forms over water. It is usually humid, and may be cold or warm.

Mean Temperature- The average of a series of temperatures taken over a period of time, such as a day or a month.

Mercury Barometer- An instrument that measures barometric pressure by measuring the level of mercury in a column.

Mesocyclone- A large, rotating column of air that forms in a violent thunderstorm and may spawn tornadoes.

Microburst- A downburst from a thunderstorm that is confined to a small area.

Mid-Latitudes- The areas in the northern and southern hemispheres between the tropics and the Arctic and Antarctic circles.

Millibar- A metric unit of air pressure measurement.

Monsoon- A wind that in summer blows from the sea to a continental interior, bringing copious rains.

NEXRAD- "Next Generation Weather Radar," a system now being installed across the country by the U.S. Weather Service, the Federal Aviation Administration, and the Department of Defense.

Nor'easter- A low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. Also called a Northeaster or Coastal Storm.

Normal- A numerical figure representing the average of conditions at a location over a period of years.

Numerical Forecasting- Forecasting the weather through digital computations carried out by supercomputers.

Occluded Front- A boundary between cold and warm air masses that acts like a cold front in some areas and a warm front in others.

Orographic Lifting- The upward flowing of air caused by rising terrain, such as a mountain range.

Overrunning- The flow of warm air over cold air in advance of a warm front.

Ozone- An unstable oxygen compound that is a pollutant at ground level, but that absorbs deadly ultraviolet rays in the stratosphere.

Ozone Hole- A thinning of the ozone layer over Antarctica, which occurs each spring.

Polar Air- A mass of very cold, very dry air that forms in polar regions.

Precipitation- Any liquid or solid form of water that falls from the atmosphere and reaches the surface of the Earth.

Pressure Gradient Force- Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure.

Prevailing Wind- The direction from which the wind blows most frequently in any location.

Psychrometer- An instrument that measures relative humidity of the air.

Radar- Stands for "radio detection and ranging." An instrument that detects and ranges distant objects by measuring the scattering and reflection of radio beams.

Radiation- The transferring of energy through electromagnetic waves.

Rain- Liquid precipitation with drops larger than .02 inches in diameter.

Rainbow- An arc or circle of colored light caused by the refraction of light by water droplets in the air.

Refraction- The bending of light as it passes through areas of different density, such as from air through ice crystals.

Relative Humidity- A measure of the amount of water vapor actually held by a specific volume of air in comparison to the maximum water vapor that air could hold at a constant temperature.

Ridge- An elongated area of high pressure that normally runs north and south.

Rime- Tiny balls of ice that form when tiny drops of water freeze on contact with the surface.

Saffir-Simpson Hurricane Damage Potential Scale- A scale that measures hurricane intensity, developed by Herbert Saffir and Robert Simpson.

Saturation- A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Secondary Cold Front- A front that follows a primary cold front and ushers in even colder air.

Shower- Snowfall or rainfall or brief duration that can either be heavy or light.

Sleet- Precipitation consisting of ice particles formed when raindrops freeze.

Smog- Air pollution caused by a mixture of smoke and fog.

Snow- Precipitation consisting of clumps of ice crystals.

Solar Energy- The energy produced by the sun.

Solstice- The time of year when the sun is the farthest north or the farthest south (about June 21 and December 21).

Squall Line- A line of thunderstorms that forms along a front.

Stable Air- Air in which temperature and humidity at various levels discourage the formation of convection currents.

Stationary Front- The border between cold and warm air masses that are not moving.

Storm Track- The path that storms generally follow in a given area.

Sublimation- The change of water vapor directly into ice crystals or ice crystals directly into water vapor.

Supercell Thunderstorm- An unusually violent thunderstorm that is capable of generating tornadoes.

Subsidence- The descent of a body of air, usually in a high pressure area, that warms the lower levels of air.

Supercooled Water- Water cooled to a temperature of less than 32 degrees Fahrenheit without freezing.

Thermometer- An instrument for measuring temperature.

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

Thunder- The sound produced by lightning discharges.

Thunderstorms- Storms that produce lightning and thunder.

Tornado- A violently rotating column of air that reaches from the base of a cloud to the ground.

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

Tropical Air- Warm, humid air masses that form in tropical regions.

Tropical Cyclone- A low-pressure system that forms in the tropics.

Tropical Depression- A tropical cyclone with winds that do not exceed 38 miles per hour.

Tropical Storm- A tropical cyclone with winds of 39-73 miles per hour.

Trough- An elongated low-pressure system that generally stretches north and south.

Typhoon- A tropical cyclone with winds of 74 miles per hour or greater that occurs west of the International Date Line.

Unstable Air- Air with temperature differences that encourage the formation of convection currents that can produce clouds and precipitation.

Updraft- An upward current of air, usually within a thundercloud.

Veering Winds- Winds that shift in a clockwise direction, a shift caused by a high-pressure system.

Visibility- The greatest distance at which one can see and identify objects.

Virga- Water droplets or ice crystals that fall from high clouds but that evaporate before hitting the ground.

Warm Front- The leading edge of a mass of warmer air that displaces a mass of colder air.

Warning- A threatening weather condition is occuring as indicated by a radar or spotter, or is expected to occur or strike an area very soon.

Watch- Conditions are favorable for a certain threatening weather condition to occur. It doesn't mean the threatening weather condition is occuring.

Waterspout- A tornado-like formation over water, usually much smaller and less vigorous than a true tornado. Waterspouts tend to occur most frequently in tropical waters, although they have been observed on the Great Lakes and on other northern lakes in Canada and the United States.

Water Vapor- The invisible gaseous form of water.

Wave- A small cyclonic circulation in the early stages of development that moves along a cold front.

Weather- The conditions in the atmosphere at any given time.

Wind- Air in motion that moves relatively horizontally in relationship to the surface of the Earth.

Windchill Factor- A measure of the effect of wind in increasing the heat loss from exposed flesh.

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

Wind Shear- A sudden shift in wind direction.