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 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.
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).
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.
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.
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.
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.
Doldrums- the regions on either side of the equator where airpressure 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.
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.
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.
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.
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.
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.
Hail- 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.
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.
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.
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.
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.
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.
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.
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 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.
