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 3.13-Win32
* Origin: The Thunderbolt BBS - tbolt.synchro.net (618:250/33)

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 3.14-Win32
* Origin: The Thunderbolt BBS - Little Rock, Arkansas (618:250/33)