Absolutely Stable Air
An atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.
Absolutely Unstable Air
An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.
Adirondack Type Snow Sampling Set
In hydrologic terms, a snow sampler consisting of a 5-foot fiberglass tube, 3 inches in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch snow depth capacity.
Air
The mixture of gases comprising the earth's atmosphere.
Air Mass
A body of air covering a relatively wide area and exhibiting horizontally uniform properties.
Air Mass Thunderstorm
Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.

Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable.
Air Pollutant
Harmful substance or product introduced into the atmosphere.
Air Pollution Potential
The meteorological potential for air pollution problems, considered without regard to the presence or absence of actual pollution sources.
Air Quality Alert
This product is used by the National Weather Service to relay non-routine air quality alert messages issued by state and local agencies.
Air Quality Model
Mathematical or conceptual model used to estimate present or future air quality.
Air Stagnation
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
Air Stagnation Advisory
This National Weather Service product is issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time. This usually results from a stagnant high pressure system with weak winds being unable to bring in fresh air.
Air Toxin
Toxic air pollutant.
Air Transportable Mobile Unit
A modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.
Airborne Snow Survey Program
In hydrologic terms, Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.
AIRMET
Airman's Meteorological advisory (WA)
Atmospheric Circulation Model
A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs.
Backfire
A fire started to stop an advancing fire by creating a burned area in its path.
Circulation
The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.
Cirriform
High altitude ice clouds with a very thin wispy appearance.
Cirrocumulus
A cirriform cloud characterized by thin, white patches, each of which is composed of very small granules or ripples. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Cirrostratus
A cloud of a class characterized by a composition of ice crystals and often by the production of halo phenomena and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Cirrus
(abbrev. CI) High-level clouds (16,000 feet or higher), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
Clear Air Turbulence
(CAT) - In aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.
Cold Air Avalanche
Downslope flow pulsations that occur at more or less regular intervals as cold air builds up on a peak or plateau, reaches a critical mass, and then cascades down the slopes.
Cold Air Dam
A shallow cold air mass which is carried up the slope of a mountain barrier, but with insufficient strength to surmount the barrier. The cold air, trapped upwind of the barrier alters the effective terrain configuration of the barrier to larger-scale approaching flows.
Cold Air Damming (CAD)
The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain. Cold Air Damming often implies that the trapped cold air mass is influencing the dynamics of the overlying air mass, e.g. in an overrunning scenario. Effects on the weather may include cold temperatures, freezing precipitation, and extensive cloud cover
Cold Air Funnel
A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.
Conditionally Unstable Air
An atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.
Continental Air Mass
A dry air mass originating over a large land area. Contrast with tropical air mass.
Crown Fire
A fire where flames travel from tree to tree at the level of the tree's crown or top.
Cyclonic Circulation
Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, occasionally rotate anticyclonically (clockwise). Compare with anticyclonic rotation.
DIR
1. Direction

2. On a buoy report, the ten-minute average wind direction measurements in degrees clockwise from true North.
Direct Flood Damage
In hydrologic terms, the damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.
Direct Hit
A close approach of a tropical cyclone to a particular location. For locations on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind. Compare indirect hit, strike.
Direct Runoff
In hydrologic terms, the runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the hydrograph of a flood.
Direct Solar Radiation
The component of solar radiation received by the earth's surface only from the direction of the sun's disk (i.e. it has not been reflected, refracted or scattered).
Directional Shear
The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.
Dust Whirl
A rotating column of air rendered visible by dust.
Environment Canada
The Canadian federal government department responsible for issuing weather forecasts and weather warnings in Canada.
Environmental Lapse Rate
The rate of decrease of air temperature with height, usually measured with a radiosonde.
Environmental Temperature Sounding
An instantaneous or near-instantaneous sounding of temperature as a function of height. This sounding or vertical profile is usually obtained by a balloon-borne instrument, but can also be measured using remote sensing equipment.
Evapotranspiration
Combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere.
Fair
It is usually used at night to describe less than 3/8 opaque clouds, no precipitation, no extremes of visibility, temperature or winds. It describes generally pleasant weather conditions.
Fire Wind
A thermally driven wind blowing radially inward toward a fire, produced by horizontal temperature differences between the heated air above the fire and the surrounding cooler free atmosphere.
Firebrand
Any source of heat, natural or man made, capable of igniting wildland fuels; flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or gravity into unburned fuels.
Firn (Snow)
In hydrologic terms, old snow on top of glaciers, granular and compact and not yet converted into ice. It is a transitional stage between snow and ice. Also called Neve.
Firn Line
In hydrologic terms, the highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the accumulation area from the ablation area
First Law of Thermodynamics
The law of physics that states that the heat absorbed by a system either raises the internal energy of the system or does work on the environment.
General Circulation
The totality of large-scale organized motion for the entire global atmosphere.
General Circulation Models
(GCMs) - These computer simulations reproduce the Earth's weather patterns and can be used to predict change in the weather and climate.
Great Circle Track
A great-circle track is the shortest distance between two points on a sphere, and when viewed on a 2-dimensional map the track will appear curved. Swell waves travel along routes that mark out great circles.
Indirect Hit
Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane force winds (either sustained or gusts) or tides of at least 4 feet above normal.
Interception Storage Requirements
In hydrologic terms, water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.
IR
Infrared Satellite Imagery
Iridescence
Brilliant spots or borders of colors in clouds, usually red and green, caused by diffraction of light by small cloud particles. The phenomenon is usually observed in thin cirrus clouds within about 30° of the sun and is characterized by bands of color in the cloud that contour the cloud edges.
Iridescent Clouds
Clouds that exhibit brilliant bright spots, bands, or borders of colors, usually red and green, observed up to about 30 degrees from the sun. The coloration is due to the diffraction with small cloud particles producing the effect. It is usually seen in thin cirrostratus, cirrocumulus, and altocumulus clouds.
Irrigation
In hydrologic terms, the controlled application of water to arable lands to supply water requirements not satisfied by rainfall
Irrigation Requirement
In hydrologic terms, the quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes.
Jet Stream Cirrus
A loose term for filamentous cirrus that appears to radiate from a point in the sky, and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.
Maritime Air Mass
An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.
Maritime Polar Air Mass
An air mass characterized by cold, moist air. Abbreviated mP.
Maritime Tropical Air Mass
An air mass characterized by warm, moist air. Abbreviated mT.
Multipurpose Reservoir
In hydrologic terms, a reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control, power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc.
Nadir
The point on any given observer's celestial sphere diametrically opposite of one's zenith.
National Ambient Air Quality Standards
In the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.
National Fire Danger Rating System
A uniform fire danger rating system used in the United States that focuses on the environmental factors that impact the moisture content of fuels. Fire danger is rated daily over large administrative areas, such as national forests.
NOAA Weather Wire
Mass dissemination via satellite of National Weather Service products to the media and public.
PIREP
Pilot Report. A report of inflight weather by an aircraft pilot or crew member. A complete coded report includes the following information in this order: location and/or extent of reported weather phenomenon: type of aircraft (only with reports turbulence or icing).
Plume-dominated Fire
A fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the general wind.
Prescribed Fire
A management ignited or natural wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned fire treatment and resource management objectives.
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
Primary Swell Direction
Prevailing direction of swell propagation.
Pulse-Pair Processing
Nickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".
Reservoir
In hydrologic terms, a manmade facility for the storage, regulation and controlled release of water.
Saffir-Simpson Hurricane Wind Scale
The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. In general, damage rises by about a factor of four for every category increase. The maximum sustained surface wind speed (peak 1-minute wind at the standard meteorological observation height of 10 m [33 ft] over unobstructed exposure) associated with the cyclone is the determining factor in the scale. The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes.
Secondary Ambient Air Quality Standards
Air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.
Space Environment Center
(SEC) - This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.
Special Fire Weather
Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.
Spectral Wave Direction
On a buoy report, mean wave direction, in degrees from true North, for each frequency bin.
St. Elmo's Fire
The glow on a masthead produced by an extreme buildup of electrical charge. Unprotected mariners should immediately move to shelter when this phenomena occurs. Lightning may strike the mast within five minutes after it begins to glow.
Stair Stepping
In hydrologic terms, the process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared.
Swell Direction
The direction from which the swells are propagating.
Thermally Driven Circulation
A diurnally reversing closed cellular wind current resulting from horizontal temperature contrasts caused by different rates of heating or cooling over adjacent surfaces; includes along-slope, cross-valley, along-valley, mountain-plain and sea breeze circulations.
Transpiration
Water discharged into the atmosphere from plant surfaces.
Unstable Air
Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.
Upper-air Weather Chart
Weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.
Variable Wind Direction
A condition when
(1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Virga
Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.
Virtual Potential Temperature
The virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.
Virtual Temperature
The virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.
Visible Infra-Red Imaging Radiometer (VIIRS)
A medium-resolution sensor flown aboard the NOAA-20 and Suomi-NP satellites.
WDIR
On a buoy report, wind direction (the direction the wind is coming from in degrees clockwise from true N) during the same period used for WSPD.
Weir
In hydrologic terms,
(a) A low dam built across a stream to raise the upstream water level (fixed-crest weir when uncontrolled);
(b) A structure built across a stream or channel for the purpose of measuring flow (measuring or gaging weir).
Whirlwind
A small, rotating column of air; may be visible as a dust devil.
Wildfire
Any free burning uncontainable wildland fire not prescribed for the area which consumes the natural fuels and spreads in response to its environment.
Wind Direction
The true direction from which the wind is blowing at a given location (i.e., wind blowing from the north to the south is a north wind). It is normally measured in tens of degrees from 10 degrees clockwise through 360 degrees. North is 360 degrees. A wind direction of 0 degrees is only used when wind is calm.
Wire Weight Gage
In hydrologic terms, a river gage comprised of a weight which is lowered to the water level. The weight is attached to a cable; and as the weight is lowered, a counter indicates the length of cable released. The stage is determined from the length of cable required to reach the water level.

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