• Weather Glossary (A)

    From Daryl Stout@57:57/10 to All on Wed Oct 7 00:05:23 2020
    This weather glossary contains information on more than 2000 terms,
    phrases and abbreviations used by the National Weather Service (NWS)...
    the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
    territories.

    Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
    before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.

    ***

    A
    1. Abbrevation for hail in weather observations.

    2. Symbol used on long-term climate outlooks issued by CPC to indicate
    areas that are likely to be above normal for the specified parameter (temperature, precipitation, etc.).

    A AMS
    Arctic Air Mass

    A Index
    A daily index of geomagnetic activity derived as the average of the eight 3-hourly a indices.

    AAAS
    American Association for the Advancement of Science

    AAWU
    Alaskan Aviation Weather Unit

    Ablation
    Depletion of snow and ice by melting and evaporation.

    ABNDT
    Abundant

    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.

    Absorption
    The process in which incident radiant energy is retained by a substance
    by conversion to some other form of energy.

    ABT
    About

    Abutment
    The part of a valley or canyon wall against which a dam is constructed.
    Right and left abutments are those on respective sides of an observer
    looking downstream.

    Abutment Seeping
    Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.

    ABV
    Above

    AC
    1. Abbreviation for Altocumulus - a cloud of a class characterized by
    globular masses or rolls in layers or patches, the individual elements
    being larger and darker than those of cirrocumulus and smaller than those
    of stratocumulus. These clouds are of medium altitude, about 8000-20,000
    ft (2400-6100 m).

    2. Convective outlook issued by the Storm Prediction Center. Abbreviation
    for Anticipated Convection; the term originates from the header coding
    [ACUS1] of the transmitted product.

    ACCAS
    (usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds
    (bases generally 8 to 15 thousand feet), of which at least a fraction of
    their upper parts show cumulus-type development. These clouds often are
    taller than they are wide, giving them a turret-shaped appearance. ACCAS
    clouds are a sign of instability aloft, and may precede the rapid
    development of thunderstorms.

    Accessory Cloud
    A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.

    Accretion
    The growth of a precipitation particle by the collision of a frozen
    particle with a supercooled liquid water droplet which freezes upon
    impact.

    ACCUMS
    accumulation

    Accuracy
    Degree of conformity of a measure to a standard or true value; in other
    words, how close a predicted or measured value is to the true value.

    Acid Precipitation
    Precipitation, such as rain, snow or sleet, containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.

    Acid Rain
    Rain containing relatively high concentrations of acid-forming chemicals
    that have been released into the atmosphere and combined with water vapor; harmful to the environment.

    ACLD
    Above Cloud Level

    ACPY
    Accompany

    Acre-foot
    The amount of water required to cover one acre to a depth of one foot.
    An acre-foot equals 326,851 gallons, or 43,560 cubic feet.

    ACRS
    Across

    Action Stage
    The stage which, when reached by a rising stream, represents the level
    where the NWS or a partner/user needs to take some type of mitigation
    action in preparation for possible significant hydrologic activity. The appropriate action is usually defined in a weather forecast office (WFO) hydrologic services manual. Action stage can be the same as forecast
    issuance stage.

    Active
    (abbrev. ACTV). In solar-terrestrial terms, solar activity levels with at
    least one geophysical event or several larger radio events (10cm) per day (Class M Flares).

    Active Conservation Storage
    In hydrologic terms, the portion of water stored in a reservoir that can
    be released for all useful purposes such as municipal water supply, power, irrigation, recreation, fish, wildlife, etc. Conservation storage is the
    volume of water stored between the inactive pool elevation and flood
    control stage.

    Active Dark Filament (ADF)
    In solar-terrestrial terms, an Active Prominence seen on the Disk.

    Active Longitude
    In solar-terrestrial terms, the approximate center of a range of
    heliographic longitudes in which Active Regions are more numerous and
    more flare-active than the average.

    Active Prominence
    In solar-terrestrial terms, a prominence displaying material motion and
    changes in appearance over a few minutes of time.

    Active Prominence Region (APR)
    In solar-terrestrial terms, a portion of the solar limb displaying active prominences.

    Active Region (AR)
    In solar-terrestrial terms, a localized, transient volume of the solar atmosphere in which plages, sunspots, faculae, flares, etc. may be
    observed.

    Active Storage Capacity
    In hydrologic terms, the total amount of reservoir capacity normally
    available for release from a reservoir below the maximum storage level.
    It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.

    Active Surge Region (ASR)
    In solar-terrestrial terms, an Active Region that exhibits a group or
    series of spike-like surges that rise above the limb.

    ACTV
    Active. In solar-terrestrial terms, solar activity levels with at least
    one geophysical event or several larger radio events (10cm) per day
    (Class M Flares)

    ACYC
    Anticyclone - A large-scale circulation of winds around a central region
    of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

    Additive Data
    A group of coded remarks that includes pressure tendency, amount of precipitation, and maximum/minimum temperature during specified periods
    of time.

    ADDS
    Aviation Digital Data Service

    Adiabat
    A line on a thermodynamic chart relating the pressure and temperature of
    a substance (such as air) that is undergoing a transformation in which
    no heat is exchanged with its environment.

    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,
    with no exchange of heat with the surrounding air.

    Adiabatic Lapse Rate
    The rate of decrease of temperature experienced by a parcel of air when
    it is lifted in the atmosphere under the restriction that it cannot
    exchange heat with its environment. For parcels that remain unsaturated
    during lifting, the (dry adiabatic) lapse rate is 9.8 C per
    kilometer.

    Adiabatic Process
    A process which occurs with no exchange of heat between a system and its environment.

    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.

    ADJ
    Adjacent

    ADPC
    Acoustic Doppler Current Profiler

    ADVCTN
    Advection- Transport of an atmospheric property by the wind.

    Advection
    (Abbrev. ADVCTN)- Transport of an atmospheric property by the wind.

    Advection Fog
    A fog that forms when warm air flows over a cold surface and cools from
    below until saturation is reached.

    ADVIS
    In hydrologic terms, a program which combines the Antecedent Precipitation Index (API) method of estimating runoff with unit hydrograph theory to
    estimate streamflow for a headwater basin.

    Advisory
    (Abbrev. ADVY)- Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant
    inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

    ADVN
    Advance

    ADVY
    Advisory - Highlights special weather conditions that are less serious
    than a warning. They are for events that may cause significant
    inconvenience, and if caution is not exercised, it could lead to
    situations that may threaten life and/or property.

    Aeration Zone
    A portion of the lithosphere in which the functional interstices of
    permeable rock or earth are not filled with water under hydrostatic
    pressure. The interstices either are not filled with water or are filled
    with water that is no held by capillarity.

    Aeroallergens
    Any of a variety of allergens such as pollens, grasses, or dust carried
    by winds.

    Aerosol
    A system of colloidal particles dispersed in a gas, such as smoke or fog.

    AFCT
    Affect

    AFD
    Area Forecast Discussion - This National Weather Service product is
    intended to provide a well-reasoned discussion of the meteorological
    thinking which went into the preparation of the Zone Forecast Product.
    The forecaster will try to focus on the most particular challenges of
    the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The
    term non-convective refers to weather that is not caused by
    thunderstorms. An intermediate Area Forecast Discussion will be issued
    when either significant forecast updates are being made or if
    interesting weather is expected to occur.

    AFOS
    Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data. This system was installed
    in the early to mid 1980s and it is being replaced by Advanced Weather Interactive Processing System (AWIPS).

    AFRED
    Abbreviation for the A Index for Fredericksburg.

    AFSS
    Automated Flight Service Station

    AFT
    After

    Afterbay
    In hydrologic terms, the tail race of a hydroelectric power plant at the
    outlet of the turbines. The term may be applied to a short stretch of
    stream or conduit, or to a pond or reservoir.

    AFTN
    Afternoon

    AFTR
    after

    AFWA
    Air Force Weather Agency

    AGDISP
    A particular atmospheric disperison model used for treating the transport
    and diffusion of aerially sprayed pest control agents in agricultural applications.

    AGFS
    Aviation Gridded Forecast System

    Agglomerate
    An ice cover of floe formed by the freezing together of various forms of
    ice.

    AGL
    Above Ground Level

    AGN
    Again

    AHD
    Ahead

    AHOS
    Automatic Hydrologic Observing System

    AHOS-S
    Automatic Hydrologic Observing System - Satellite

    AHOS-T
    Automatic Hydrologic Observing System - Telephone

    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 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)

    AIV
    Aviation Impact Variables

    Alaska Current
    A North Pacific Ocean current flowing counterclockwise in the Gulf of
    Alaska. It is the northward flowing (warm) division of the Aleutian
    Current.

    Albedo
    Reflectivity; the fraction of radiation striking a surface that is
    reflected by that surface.

    Alberta Clipper
    A fast moving low pressure system that moves southeast out of Canadian
    Province of Alberta (southwest Canada) through the Plains, Midwest, and
    Great Lakes region usually during the winter. This low pressure area is
    usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".

    ALERT
    Automated Local Event Reporting in Real Time. Network of automatic
    raingauges that transmit via VHF radio link when precipitation occurs.
    Some sites are also equipped with other sensors such as temperature,
    wind, pressure, river stage or tide level.

    Alert Stage
    The stage which, when reached by a rising stream, represents the level
    where appropriate officials (e.g., county sheriff, civil defense
    officials, or bypass gate operators) are notified of the threat of
    possible flooding. (Used if different from action stage, and at the
    discretion of the WFO or river forecast center [RFC].) The term
    "alert stage" is to be used instead of warning stage. Monitor stage or
    caution stage may be used instead of alert stage in some parts of the
    country.

    Aleutian Current
    An eastward flowing North Pacific Ocean current which lies north of the
    North Pacific Current.

    Aleutian Low
    A semi-permanent, subpolar area of low pressure located in the Gulf of
    Alaska near the Aleutian Islands. It is a generating area for storms
    and migratory lows often reach maximum instensity in this area. It is
    most active during the late fall to late spring. During the summer, it
    is weaker, retreating towards the North Pole and becoming almost
    nonexistent. During this time, the North Pacific High pressure system dominates.

    ALF
    Aloft

    ALG
    Along

    Algorithm
    A computer program (or set of programs) which is designed to
    systematically solve a certain kind of problem. WSR-88D radars (NEXRAD)
    employ algorithms to analyze radar data and automatically determine
    storm motion, probability of hail, VIL, accumulated rainfall, and
    several other parameters.

    Alluvium
    Sediments deposited by erosional processes, usually by streams.

    Along-slope Wind System
    A closed, thermally driven diurnal mountain wind circulation whose lower
    branch blows up or down the sloping sidewalls of a valley or mountain.
    The upper branch blows in the opposite direction, thereby closing the circulation.

    ALQDS
    All Quadrants

    ALTHO
    although

    Altimeter
    An instrument that indicates the altitude of an object above a fixed
    level. Pressure altimeters use an aneroid barometer with a scale graduated
    in altitude instead of pressure.

    Altimeter Setting
    A correction of the station pressure to sea level used by aviation. This correction takes into account the standard variation of pressure with
    height and the influence of temperature variation with height on the
    pressure. The temperatures used correspond to the standard atmosphere temperatures between the surface and sea level.

    Altocumulus
    A cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).

    Altostratus
    A cloud of a class characterized by a generally uniform gray sheet or
    layer, lighter in color than nimbostratus and darker than cirrostratus.
    These clouds are of medium altitude, about 8000 to 20,000 ft
    (2400-6100 m).

    Ambient
    Of the surrounding area or environment.

    AMD
    Amend

    AMOS
    Automatic Meteorological Observing System

    Amplifier
    A device used to increase the strength of an analog signal

    Amplitude
    The maximum magnitude of a quantity. Often used to refer to the maximum
    height of a wave.

    AMS
    1. Air Mass - a body of air covering a relatively wide area and
    exhibiting horizontally uniform properties.

    2. American Meteorological Society

    AMT
    Amount

    AMVER
    Automated Mutual Assistance Vessel Rescue System. A system operated by
    the U.S. Coast Guard which computes the nearest available rescue vessels
    for vessels in distress using vessel track and position reports supplied
    by participating vessels.

    AMVER/SEAS
    A software program created by the National Weather Service intended to efficiently generate AMVER and VOS reports as part of a cooperative
    effort.

    Anabranch
    A diverging branch of a river which re-enters the main stream.

    Analog
    1. Class of measuring devices in which the output varies continuously as
    a function of the input (non-digital).

    2. A historical instance of a given meteorological scenario or feature
    that is used for comparison with another scenario or feature. For example,
    a long-range forecaster predicting conditions for the upcoming winter may
    make comparisons to analog seasons in which meteorological factors were
    similar to those of the upcoming season.

    Analog Signal
    A signal, such as voice, that varies in a continuous manner.

    ANBURS
    Alphanumeric Backup Replacement System

    Anchor Ice
    In hydrologic terms, submerged frazil ice attached or anchored to the
    river bottom, irrespective of its formation.

    Anchor Ice Dam
    An accumulation of anchor ice which acts as a dam and raises the water
    level.

    Anemometer
    An instrument used for measuring the speed of the wind.

    Aneroid Barometer
    An instrument for measuring atmospheric pressure in which a needle,
    attached to the top of an evacuated box, is deflected as changes in
    atmospheric pressure cause the top of the box to bend in or out.

    Angels
    Radar echoes caused by birds, insects, and localized refractive index discontinuities.

    Angle of Reflection
    The angle at which a reflected ray of energy leaves a reflecting surface.
    It is measured between the outgoing ray and a perpendicular to the
    surface at the point of incidence (i.e., where the ray strikes).

    Angstrom
    A unit of length equal to 10-8 cm.

    Annual Flood
    In hydrologic terms, the maximum discharge peak during a given water
    year (October 1 - September 30).

    Anomaly
    The deviation of a measurable unit (e.g., temperature or precipitation)
    over a period in a given region from the long-term average, often the thirty-year mean, for that region.

    Antedecent Precipitation Index
    (Abbrev. API) - an index of moisture stored within a drainage basin
    before a storm.

    Anthelion
    A luminous white spot that appears on the parhelic circle at the same
    altitude as the sun and 180 from it in azimuth.

    Anthropogenic Source
    A pollutant source caused or produced by humans.

    Anti-wind
    The upper or return branch of an along-valley wind system, as confined
    within a valley, and blowing in a direction opposite to the winds in the
    lower altitudes of the valley.

    Anticyclogenesis
    The formation or intensification of an anticyclone or high pressure
    center.

    Anticyclone
    A large-scale circulation of winds around a central region of high
    atmospheric pressure, clockwise in the Northern Hemisphere,
    counterclockwise in the Southern Hemisphere

    Anticyclonic Rotation
    Rotation in the opposite sense as the Earth's rotation, i.e., clockwise
    in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.

    Antilles Current
    A current which originates in the vicinity of the Leeward Islands as
    part of the Atlantic North Equatorial Current.

    Anvil
    The flat, spreading top of a cumulonimbus cloud, often shaped like an
    anvil. Thunderstorm anvils may spread hundreds of miles downwind from
    the thunderstorm itself, and sometimes may spread upwind.

    Anvil Crawler
    [Slang], a lightning discharge occurring within the anvil of a
    thunderstorm, characterized by one or more channels that appear to
    crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an
    active MCS.

    Anvil Dome
    A large overshooting top or penetrating top.

    Anvil Rollover
    Slang for a circular or semicircular lip of clouds along the underside
    of the upwind part of a back-sheared anvil, indicating rapid expansion
    of the anvil.

    Anvil Zits
    Slang for frequent (often continuous or nearly continuous), localized
    lightning discharges occurring from within a thunderstorm anvil.

    AO
    Arctic Oscillation - the Arctic Oscillation is a pattern in which
    atmospheric pressure at polar and middle latitudes fluctuates between
    negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about
    45 north latitude. The negative phase allows cold air to plunge
    into the Midwestern United States and western Europe, and storms bring
    rain to the Mediterranean. The positive phase brings the opposite
    conditions, steering ocean storms farther north and bringing wetter
    weather to Alaska, Scotland and Scandinavia and drier conditions to
    areas such as California, Spain and the Middle East. In recent years
    research has shown, the Arctic Oscillation has been mostly in its
    positive phase. Some researchers argue that the North Atlantic
    Oscillation is in fact part of the AO.

    AOA
    At or above

    AOB
    At or below

    AOPA
    Aircraft Owners and Pilots Association

    AP
    Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions. Sometimes referred to as "ground clutter".

    AP Index
    In solar-terrestrial terms, an averaged planetary A Index based on data
    from a set of specific stations.

    APD
    On a buoy report, the average wave period (seconds) of all waves during
    the 20-minute period.

    Aphelion
    The point on the annual orbit of a body (about the sun) that is farthest
    from the sun; at present, the earth reaches this point (152 million
    kilometer from the sun) on about 5 July. Opposite of perihelion.

    API Method
    In hydrologic terms, a statistical method to estimate the amount of
    surface runoff which will occur from a basin from a given rainstorm
    based on the antecedent precipitation index, physical characteristics of
    the basin, time of year, storm duration, rainfall amount, and rainfall intensity.

    Apogee
    The farthest distance between the moon and earth or the earth and sun.

    Apparent Temperature
    A measure of human discomfort due to combined heat and humidity (e.g.,
    heat index).

    Apparent Wind
    The speed and true direction from which the wind appears to blow with
    reference to a moving point. Sometimes called RELATIVE WIND.

    APRCH
    Approach

    APRCHG
    approaching

    APRNT
    apparent

    APST
    Aviation Products and Services Team

    Aquiclude
    In hydrologic terms, a formation which contains water but cannot transmit
    it rapidly enough to furnish a significant supply to a well or spring.

    Aquifer
    In hydrologic terms, permeable layers of underground rock, or sand that
    hold or transmit groundwater below the water table that will yield water
    to a well in sufficient quantities to produce water for beneficial use.

    Aquifuge
    In hydrologic terms, a geologic formation which has no interconnected
    openings and cannot hold or transmit water.

    ARAM
    Aviation, Range, and Aerospace Meteorology

    Arch Dam
    A concrete arch dam is used in sites where the ratio of width between
    abutments to height is not great and where the foundation at the
    abutments is solid rock capable of resisting great forces. The arch
    provides resistance to movement. When combined with the weight of
    concrete (arch-gravity dam), both the weight and shape of the structure
    provide great resistance to the pressure of water.

    Arch Filament System (AFS)
    In solar-terrestrial terms, a bright, compact plage crossed by a system
    of small, arched filaments, which is often a sign of rapid or continued
    growth in an Active Region.

    Arctic
    The region within the Arctic Circle, or, loosely, northern regions in
    general, characterized by very low temperatures.

    Arctic front
    The boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air.

    Arctic Oscillation
    (abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric
    pressure at polar and middle latitudes fluctuates between negative and
    positive phases. The negative phase brings higher-than-normal pressure
    over the polar region and lower-than-normal pressure at about 45
    north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to
    the Mediterranean. The positive phase brings the opposite conditions,
    steering ocean storms farther north and bringing wetter weather to
    Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has
    shown, the Arctic Oscillation has been mostly in its positive phase.
    Some researchers argue that the North Atlantic Oscillation is in fact
    part of the AO.

    Arctic Sea Smoke
    Steam fog, but often specifically applied to steam fog rising from small
    open water within sea ice.

    Arcus
    A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds
    both are types of arcus clouds.

    Area Forecast Discussion
    This National Weather Service product is intended to provide a
    well-reasoned discussion of the meteorological thinking which went into
    the preparation of the Zone Forecast Product. The forecaster will try to
    focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective
    watches, and non-convective warnings. The term non-convective refers to
    weather that is not caused by thunderstorms. An intermediate Area
    Forecast Discussion will be issued when either significant forecast
    updates are being made or if interesting weather is expected to occur.

    Area of Influence
    In hydrologic terms, the area covered by the drawdown curves of a given
    pumping well or combination of wells at a particular time.

    Area Source
    An array of pollutant sources, so widely dispersed and uniform in
    strength that they can be treated in a dispersion model as an aggregate pollutant release from a defined area at a uniform rate. Compare line
    source and point source.

    Area Wide Hydrologic Prediction System
    (Abbrev. AWHPS) - A computer system which automatically ingests areal
    flash flood guidance values and WSR-88D products and displays this data
    and other hydrologic information on a map background.

    Area-Capacity Curve
    In hydrologic terms, a graph showing the relation between the surface
    area of the water in a reservoir, the corresponding volume, and
    elevation.

    Arid
    An adjunctive applied to regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable
    without irrigation.

    ARINC
    Aeronautical Radio, Incorporated

    ARND
    Around

    ARR
    Arrive/Arrival

    Arroyo
    In hydrologic terms, a water-carved channel or gully in arid country,
    usually rather small with steep banks, dry most of the time, due to
    infrequent rainfall and the shallowness of the cut which does not
    penetrate below the level of permanent ground water.

    ARSI
    Atmospheric Research System, Inc.

    ART
    The Automatic Radiotheodolite. A ground-based radio direction finder
    that automatically tracks a ballon-borne radiosonde.

    ARTCC
    Air Route Traffic Control Center

    Artesian Well
    In hydrologic terms, a well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping.
    Also called a flowing well.

    Artificial Control
    In hydrologic terms, a weir or other man-made structure which serves as
    the control for a stream-gaging station.

    AS
    (NOTE: if this appears in an Area Forecast Discussion or other text
    product in context as the word "as," disregard the technical definition
    below).

    Abbreviation for Altostratus, a cloud of a class characterized by a
    generally uniform gray sheet or layer, lighter in color than nimbostratus
    and darker than cirrostratus. These clouds are of medium altitude, about
    8000 to 20,000 ft (2400-6100 m).

    ASAP
    1. AHOS SHEF Automatic Processing System

    2. As soon as possible (may be used in Area Forecast Discussions)

    ASAPTRAN
    The software component of ASAP.

    ASB
    Aviation Support Branch

    ASL
    Above Sea Level

    ASOS
    Automated Surface Observing System

    Astronomical Dawn
    The time at which the sun is 18 below the horizon in the morning.
    Astronomical dawn is that point in time at which the sun starts l
    ightening the sky. Prior to this time during the morning, the sky is
    completely dark.

    Astronomical Dusk
    This is the time at which the sun is 18 below the horizon in the
    evening. At this time the sun no longer illuminates the sky.

    Astronomical Unit
    (abbrev. AU)- The mean earth-sun distance, equal to 1.496x1013 cm, or
    214.94 solar radii.

    ATC
    Air Traffic Control

    ATDTDCS
    Automated Tone Dial Telephone Data Collection System - Data collection
    system where cooperative observers collect precipitation, stage, and temperature data then transmit the data to the NWS ATDTDCS computer
    through the telephone lines. The ATDTDCS computer transmits the data to
    AFOS.

    Atmosphere
    The air surrounding and bound to the Earth.

    Atmospheric Boundary Layer
    Same as Boundary Layer - in general, a layer of air adjacent to a
    bounding surface. Specifically, the term most often refers to the
    planetary boundary layer, which is the layer within which the effects
    of friction are significant. For the earth, this layer is considered to
    be roughly the lowest one or two kilometers of the atmosphere. It is
    within this layer that temperatures are most strongly affected by
    daytime insolation and nighttime radiational cooling, and winds are
    affected by friction with the earth's surface. The effects of friction
    die out gradually with height, so the "top" of this layer cannot be
    defined exactly.

    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.

    Atmospheric Pressure
    The pressure exerted by the earth's atmosphere at any given point,
    determined by taking the product of the gravitational acceleration at
    the point and the mass of the unit area column of air above the point.

    Atmospheric Radiation
    Infrared radiation (energy in the wavelength interval of 3 -80
    micrometer) emitted by or being propagated through the atmosphere. It
    consists of both upwelling and downwelling components. Compare with
    terrestrial radiation.

    ATMP
    On a buoy report, the air temperature (Celsius).

    Attenuation
    It refers to the reduction of the radar beam power due to the reflection
    or absorption of energy when it strikes a target. The greatest
    attenuation occurs when the radar beam goes through very heavy rain.

    ATTM
    At this time

    Augmented report
    A meteorological report prepared by an automated surface weather
    observing system for transmission with certified observers signed on to
    the system to add information to the report.

    Aurora
    A faint visual phenomenon associated with geomagnetic activity, which
    occurs mainly in the high-latitude night sky; typical auroras are 100
    to 250 km above the ground.

    Aurora Australis
    Same as Aurora Borealis, but in the Southern Hemisphere. Also known as
    the southern lights; the luminous, radiant emission from the upper
    atmosphere over middle and high latitudes, and centred around the
    earth's magnetic poles. These silent fireworks are often seen on clear
    winter nights in a variety of shapes and colors.

    Aurora Borealis
    Also known as the northern lights; the luminous, radiant emission from
    the upper atmosphere over middle and high latitudes, and centred around
    the earth's magnetic poles. These silent fireworks are often seen on
    clear winter nights in a variety of shapes and colors.

    Auroral Oval
    In solar-terrestrial terms, an oval band around each geomagnetic pole
    which is the locus of structured aurorae.

    Automated Event Reporting Gage
    (also see Tipping Bucket Rain Gage); for river stage gages, IFLOWS
    pressure transducer type gages can be programmed to report if water
    surface rises or falls by a predetermined amount.

    Automated Report
    A meteorological report prepared by an automated surface weather
    observing system for transmission, and with no certified weather
    observers signed on to the system.

    Automated Surface Observing System
    The ASOS program is a joint effort of the National Weather Service
    (NWS), the Federal Aviation Administration (FAA), and the Department
    of Defense (DOD). Completed in the mid-1990s, the ASOS systems serve
    as the nation's primary surface weather observing network. ASOS is
    designed to support weather forecast activities and aviation operations
    and, at the same time, support the needs of the meteorological,
    hydrological, and climatological research communities.

    Autumn
    The season of the year that is the transition period from summer to
    winter, occurring as the sun approaches the winter solstice.
    Meteorological autumn (different from standard/astronomical autumn)
    begins September 1 and ends November 30.

    Autumnal Equinox
    The equinox at which the sun approaches the Southern Hemisphere,
    marking the start of astronomical autumn in the Northern Hemisphere.
    The time of this occurrence is approximately September 22. On that day, daylight is everywhere 12 hours. Compare with vernal equinox, offset
    by six months.

    Avalanche
    A mass of snow, rock, and/or ice falling down a mountain or incline.
    In practice, it usually refers to the snow avalanche. In the United
    States, the term snow slide is commonly used to mean a snow avalanche.

    Avalanche Advisory
    A preliminary notification that conditions may be favorable for the
    development of avalanches in mountain regions.

    AVBL
    Available

    AVG
    Average

    AVHRR
    Advanced Very High Resolution Radiometer. Main sensor on U.S. polar
    orbiting satellites.

    AVN
    The Aviation model (120-hour numerical model of the atmosphere). The
    output from this model is now part of what is known as the GFS model.

    AVP
    On a buoy report, Average Wave Period is the average period (seconds)
    of the highest one-third of the wave observed during a 20 minute
    sampling period.

    AWC
    Aviation Weather Center

    AWHPS
    Area Wide Hydrologic Prediction System

    AWIPS
    Advanced Weather Interactive Processing System. This system replaced
    the Automation of Field Operations and Services (AFOS). This system
    allows the operator to overlay meteorological data from a variety of
    sources.

    AWOS
    Automated Weather Observation System

    Azimuth
    A direction in terms of a 360 compass. North is at 0,
    east is at 90, south is at 180, and west is at 270.

    Azimuth Angle
    The direction or bearing toward which a sloping surface faces (e.g.,
    a north-facing slope has an azimuth angle of 360; a
    northeast-facing slope, an azimuth angle of 45).

    Azores Current
    One of the currents of the North Atlantic subtropical gyre.

    Azores High
    Alternate term for Bermuda High - a semi-permanent, subtropical area
    of high pressure in the North Atlantic Ocean off the East Coast of
    North America that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced
    westward, during the Northern Hemispheric summer and fall, the center
    is located in the western North Atlantic, near Bermuda. In the winter
    and early spring, it is primarily centered near the Azores in the
    eastern part of the North Atlantic. Also known as Azores High.
    --- SBBSecho 3.11-Win32
    * Origin: The Thunderbolt BBS - tbolt.synchro.net (57:57/10)