From Daryl Stout@57:57/10 to All on Thu Oct 22 00:05:02 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
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.
1) Fahrenheit- The standard scale used to measure temperature in the
United States. On this scale, the freezing point of water is 32øF and the boiling point is 212øF. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32: øF = (øC * 9/5) + 32
2) Fog- Water droplets suspended in the air at the Earth's surface. Fog
is often hazardous when the visibility is reduced to ¬ mile or less.
In solar-terrestrial terms, of the white-light corona (that is, the
corona seen by the eye at a total solar eclipse), that portion which is
caused by sunlight scattered or reflected by solid particles (dust) in inter-planetary space.
In solar-terrestrial terms, the upper layer of the ionosphere,
approximately 120 to 1500 km in altitude. The F region is subdivided
into the F1 and F2 regions. The F2 region is the most dense and peaks
at altitudes between 200 and 600 km. The F1 region is a smaller peak in electron density, which forms at lower altitudes in the daytime.
Abbreviation for Fujita Scale, a system of rating the intensity of
tornadoes; for detailed information, see the definition for that term.
Federal Aviation Administration
In hydrologic terms, the external surface of a structure, such as the
surface of a dam.
In solar-terrestrial terms, a bright region of the photosphere seen in
white light, seldom visible except near the solar limb.
(abbrev. F) The standard scale used to measure temperature in the United States. On this scale, the freezing point of water is 32øF and the
boiling point is 212øF. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32:
øF = (øC * 9/5) + 32
It is usually used at night to describe less than 3/8 opaque clouds, no precipitation, no extremes of visibility, temperature or winds. It
describes generally pleasant weather conditions.
The season of the year which is the transition period from summer to
winter occurring as the sun approaches the winter solstice. In the
Northern Hemisphere, fall customarily includes the months of September,
October and November.
A skiing term, indicating the line of steepest descent of a slope.
A strong, cold, downslope wind.
Same as Virga; streaks or wisps of precipitation falling from a cloud
but evaporating before reaching the ground. In certain cases, shafts of
virga may precede a microburst.
AVN MOS Guidance (older version)
A pattern of plume dispersion in a stable atmosphere, in which the plume
fans out in the horizontal and meanders about at a fixed height.
Unit of water depth equal to 6 feet.
Flight Advisory Weather Service
(Great Lakes Freeze-Up/Break-Up Outlook) - A National Weather Service
product to keep mariners informed of the projected freeze-up date or
break-up date of ice on the Great Lakes.
Federal Snow Sampler
In hydrologic terms, a snow sampler consisting of five or more sections
of sampling tubes, one which has a steel cutter on the end. The combined snowpack measuring depth is 150 inches. This instrument was formerly the
Mount Rose Type Snow Sampling Set.
Lines or bands of low-level clouds that move (feed) into the updraft
region of a thunderstorm, usually from the east through south (i.e.,
parallel to the inflow). Same as inflow bands. This term also is used
in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.
Federal Emergency Management Agency. An agency of the federal government
having responsibilities in hazard mitigation; FEMA also administers the National Flood Insurance Program.
In the general circulation of the atmosphere, the name given to the middle latitude cell marked by sinking motion near 30 degrees and rising motion
near 60 degrees latitude.
1. The area in which ocean waves are generated by the wind. Also refers to
the length of the fetch area, measured in the direction of the wind.
2. In hydrologic terms,
The effective distance which waves have traversed in open water, from
their point of origin to the point where they break.
3. The distance of the water or the homogenous type surface over which the
wind blows without appreciable change in direction.
A National Weather Service convective precipitation descriptor for a 10
percent chance of measurable precipitation (0.01 inch). Few is used interchangeably with isolated.
An official sky cover classification for aviation weather observations, descriptive of a sky cover of 1/8 to 2/8. This is applied only when
obscuring phenomenon aloft are present--that is, not when obscuring
phenomenon are surface-based, such as fog.
Flash Flood Guidance
(Also abbrev. F) - Fog - water droplets suspended in the air at the
Earth's surface. Fog is often hazardous when the visibility is reduced to
¬ mile or less.
In solar-terrestrial terms, a linear pattern in the H-alpha chromosphere
of the sun, as seen through an H-alpha filter, occurring near strong
sunspots and plage or in filament channels.
Field (Moisture) Capacity
The amount of water held in soil against the pull of gravity.
Field Moisture Deficiency
The quantity of water, which would be required to restore the soil
moisture to field moisture capacity.
A mass of gas suspended over the photosphere by magnetic fields and seen
as dark lines threaded over the solar disk. A filament on the limb of the
sun seen in emission against the dark sky is called a prominence.
A broad pattern of fibrils in the chromosphere, marking where a filament
may soon form or where a filament recently disappeared.
In hydrologic terms, any dam constructed of excavated natural materials
or of industrial wastes.
The opposite of deepening. A general increase in the central pressure of
a low pressure system.
A thermally driven wind blowing radially inward toward a fire, produced
by horizontal temperature differences between the heated air above the
fire and the surrounding cooler free atmosphere.
Any source of heat, natural or man made, capable of igniting wildland
fuels; flaming or glowing fuel particles that can be carried naturally
by wind, convection currents, or gravity into unburned fuels.
In hydrologic terms, old snow on top of glaciers, granular and compact
and not yet converted into ice. It is a transitional stage between snow
and ice. Also called Neve.
In hydrologic terms, the highest level to which the fresh snow on a
glacier's surface retreats during the melting season. The line separating
the accumulation area from the ablation area.
First Law of Thermodynamics
The law of physics that states that the heat absorbed by a system either
raises the internal energy of the system or does work on the environment.
A line of cumulus or towering cumulus clouds connected to and extending
outward from the most active part of a supercell, normally on the
southwest side. The line normally has a stair-step appearance, with the
tallest clouds closest to the main storm.
In solar-terrestrial terms, a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.
A sudden, brief illumination of a conductive channel associated with
lightning, which may contain multiple strokes with their associated
stepped leaders, dart leaders and return strokes.
A rapid and extreme flow of high water into a normally dry area, or a
rapid water level rise in a stream or creek above a predetermined flood
level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may
vary in different parts of the country. Ongoing flooding can intensify
to flash flooding in cases where intense rainfall results in a rapid
surge of rising flood waters.
Flash Flood Guidance
Forecast guidance, often model output, specific to the potential for
flash flooding (e.g., how much rainfall over a given area will be
required to produce flash flooding).
Flash Flood Statement
(FFS) In hydrologic terms, a statement by the NWS which provides
follow-up information on flash flood watches and warnings.
Flash Flood Table
In hydrologic terms, a table of pre-computed forecast crest stage values
for small streams for a variety of antecedent moisture conditions and
rain amounts. Soil moisture conditions are often represented by flash
flood guidance values. In lieu of crest stages, categorical
representations of flooding, e.g., minor, moderate, etc. may be used on
Flash Flood Warning
Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.
Flash Flood Watch
Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the
occurrence is neither certain or imminent.
The number of return strokes in a lightning flash.
In hydrologic terms, a length of timber, concrete, or steel placed on
the crest of a spillway to raise the retention water level but which
may be quickly removed in the event of a flood by a tripping device, or
by deliberately designed failure of the flashboard or its supports.
Float Recording Precipitation gage
In hydrologic terms, a rain gage where the rise of a float within the instrument with increasing rainfall is recorded. Some of these gages
must be emptied manually, while others employ a self-starting siphon to
empty old rainfall amounts.
A cluster of frazil particles.
In hydrologic terms, an accumulation of frazil flocs (also known as a
"pan") or a single piece of broken ice.
Any high flow, overflow, or inundation by water which causes or
Terms defined for each forecast point which describe or categorize the
severity of flood impacts in the corresponding river/stream reach. Each
flood category is bounded by an upper and lower stage (see Example 1).
The severity of flooding at a given stage is not necessarily the same at
all locations along a river reach due to varying channel/bank
characteristics or presence of levees on portions of the reach.
Therefore, the upper and lower stages for a given flood category are
usually associated with water levels corresponding to the most
significant flood impacts somewhere in the reach. The flood categories
used in the NWS are:
*Minor Flooding* - minimal or no property damage, but possibly some
*Moderate Flooding* - some inundation of structures and roads near
stream. Some evacuations of people and/or transfer of property to higher elevations.
*Major Flooding* - extensive inundation of structures and roads.
Significant evacuations of people and/or transfer of property to higher elevations.
*Record Flooding* - flooding which equals or exceeds the highest stage
or discharge at a given site during the period of record keeping.
Note: all three of the lower flood categories (minor, moderate, major)
do not necessarily exist for a given forecast point. For example, at the
level where a river reaches flood stage, it may be considered moderate flooding. However, at least one of these three flood categories must
start at flood stage.
Flood Control Storage
In hydrologic terms, storage of water in reservoirs to abate flood
Maximum height of a flood wave as it passes a certain location.
Flood Frequency Curve
In hydrologic terms,
(1) A graph showing the number of times per year on the average, plotted
as abscissa, that floods of magnitude, indicated by the ordinate, are
equaled or exceeded.
(2) A similar graph but with recurrence intervals of floods plotted as abscissa.
Flood Loss Reduction Measures
In hydrologic terms, the strategy for reducing flood losses. There are
four basic strategies. They are prevention, property protection,
emergency services, and structural projects. Each strategy incorporates different measures that are appropriate for different conditions. In
many communities, a different person may be responsible for each strategy.
Flood of Record
In hydrologic terms, the highest observed river stage or discharge at a
given location during the period of record keeping. (Not necessarily the highest known stage.)
In hydrologic terms, the portion of a river valley that has been
inundated by the river during historic floods.
Flood Potential Outlook
(ESF on AFOS) (FPO for Acronym): In hydrologic terms, An NWS outlook that
is issued to alert the public of potentially heavy rainfall that could
send area rivers and streams into flood or aggravate an existing flood.
In hydrologic terms, measures that are taken in order to keep flood
problems from getting worse. Planning, land acquisition, river channel maintenance, wetlands protection, and other regulations all help modify development on floodplains and watersheds to reduce their susceptibility
to flood damage. Preventive measures are usually administered by the
building, zoning, planning and/ or code enforcement offices of the local government.
In hydrologic terms, problems and damages that occur during a flood as a
result of human development and actions. Flood problems are a result from:
1) Inappropriate development in the floodplain (e.g., building too low,
too close to the channel, or blocking flood flows);
2) Development in the watershed that increases flood flows and creates a
larger floodplain, or;
3) A combination of the previous two.
In hydrologic terms, a graph of elevation of the water surface of a river
in flood, plotted as ordinate, against distance, measured in the
downstream direction, plotted as abscissa. A flood profile may be drawn
to show elevation at a given time, crests during a particular flood, or
to show stages of concordant flows.
In hydrologic terms, process of determining progressively the timing,
shape, and amplitude of a flood wave as it moves downstream to successive points along the river.
A gage height at which a watercouse overtops its banks and begins to
cause damage to any portion of the defined reach. Flood stage is usually
higher than or equal to bankful stage.
Flood Statement (FLS)
In hydrologic terms, a statement issued by the NWS to inform the public
of flooding along major streams in which there is not a serious threat
to life or property. It may also follow a flood warning to give later information.
(FLW) In hydrologic terms, a release by the NWS to inform the public of flooding along larger streams in which there is a serious threat to life
or property. A flood warning will usually contain river stage (level) forecasts.
In hydrologic terms, a rise in streamflow to a crest and its subsequent recession caused by precipitation, snowmelt, dam failure, or reservoir releases.
In hydrologic terms, ice which has been flooded by melt water or river
water and is heavily loaded by water and wet snow.
In hydrologic terms, the process of protecting a building from flood
damage on site. Floodproofing can be divided into wet and dry
floodproofing. In areas subject to slow-moving, shallow flooding,
buildings can be elevated, or barriers can be constructed to block the
water' approach to the building. These techniques have the advantage of
being less disruptive to the neighborhood. It must be noted that during
a flood, a floodproofed building may be isolated and without utilities
and therefore unusable, even though it has not been damaged.
In hydrologic terms, a long, narrow concrete, or masonry embankment
usually built to protect land from flooding. If built of earth the
structure is usually referred to as a levee. Floodwalls and levees
confine streamflow within a specified area to prevent flooding. The
term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.
In hydrologic terms,
(1) A part of the flood plain, otherwise leveed, reserved for emergency diversion of water during floods. A part of the flood plain which, to facilitate the passage of floodwater, is kept clear of encumbrances.
(2) The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and
discharge the floodwater or floodflow of any river or stream.
Flood\/Flash Flood Warning
Issued to inform the public that flooding is imminent or in progress.
Flood\/Flash Flood Watch
Issued to inform the public and cooperating agencies that current and developing hydrometeorological conditions are such that there is a threat
of flooding, but the occurrence is neither certain nor imminent.
(abbrev. FLW) Wind. In meteorology, a qualitative reference of an air
parcel(s) with respect to its direction of movement, sometimes specified
at a certain height or pressure elevation, e.g. westerly flow at 500 mb.
In hydrology, the volumetric flow of water past a given point on a stream
or river, usually in cubic feet per second (cfs)
Flow Duration Curve
In hydrologic terms, a cumulative frequency curve that shows the
percentage of time that specified discharges are equaled or exceeded.
The process by which a separation eddy forms on the windward or leeward
sides of bluff objects or steeply rising hillsides.
The splitting of a stable airflow around a mountain barrier, with
branches going around the left and right edges of the barrier, often at accelerated speeds.
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 an Artesian well.
River Flood Statement
Time integrated flux
Matter which flows; gas or liquid.
Snow flurries are an intermittent light snowfall of short duration
(generally light snow showers) with no measurable accumulation (trace category).
The rate of transfer of fluids, particles or energy per unit area across
a given surface (amount of flow per unit of time).
Follow (or) Flow- Wind
From or Fathom
In solar-terrestrial terms, the lowest radiowave frequency that can be reflected from the ionosphere.
A warm, dry wind on the lee side of a mountain range, the warmth and
dryness of the air being due to adiabatic compression as the air descends
the mountain slopes. In the United States, the term chinook is used for
Foehn winds in the Rocky and Sierra mountains.
A temporary cessation of the foehn at the ground due to the formation or intrusion of a cold air layer which lifts the foehn off the ground.
In solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflec- tion from the sporadic E region of the ionosphere.
In solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflection from the F2 region of the ionosphere.
(abbrev. F) Fog is water droplets suspended in the air at the Earth's
surface. Fog is often hazardous when the visibility is reduced to ¬ mile
A rainbow that has a white band that appears in fog, and is fringed with
red on the outside and blue on the inside.
Channeling of upper winds along a valley's axis when upper winds are
diverted by the underlying topography. Compare pressure-driven channeling.
In hydrologic terms, the water behind (upstream) of the dam.
A statement of prediction.
In hydrologic terms, the highest elevation of river level, or stage,
expected during a specified storm event.
Computer-generated forecast materials used to assist the preparation of a forecast, such as numerical forecast models.
Forecast Issuance Stage
The stage which, when reached by a rising stream, represents the level
where RFCs need to begin issuing forecasts for a non-routine (flood-only) forecast point. This stage is coordinated between WFO and RFC personnel
and is not necessarily the same as action or alert stage. The needs of
WFO/RFC partners and other users are considered in determining this stage.
Official definitions for NWS products:
Today...............................Sunrise to sunset
This afternoon..................noon till 6 p.m.
This evening.....................6 p.m. till sunset Tonight.............................sunset till sunrise Tomorrow.........................sunrise to sunset of the following day
In hydrologic terms, a location that represents an area (reach of a
river), where a forecast is made available to the public. Each NWS river forecast point has an associated E-19a, Abridged Report on River Gage
Station, and E-19, Report on River Gage Station.
Forecast valid for
The period of time the forecast is in effect beginning at a given day,
date and time, and ending at a given day, date and time.
In hydrologic terms, a sighting on a point of unknown elevation from an instrument of known elevation. To determine the elevation of the point
in question, the foresight is subtracted from the height of the
Forward Flank Downdraft
The main region of downdraft in the forward, or leading, part of a
supercell, where most of the heavy precipitation is.
In hydrologic terms, the upper end of a confined-aquifer conduit, where
it intersects the land surface.
Forecast Output United States
Flood Potential Outlook
A cumulus cloud presenting a ragged, shredded appearance, as if torn.
A stratus cloud presenting a ragged, shredded appearance, as if torn. It differs from a fractocumulus cloud in having a smaller vertical extent
and darker color.
In hydrologic terms, any break or rupture formed in an ice cover or floe
due to deformation.
In hydrologic terms, an area which has a great number of fractures.
In hydrologic terms, deformation process whereby ice is permanently
deformed, and fracture occurs.
Ragged, detached cloud fragments; same as scud.
In hydrologic terms, fine spicules, plates, or discoids of ice suspended
in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.
In hydrologic terms, an agglomerate of loosely packed frazil which floats
or accumulates under the ice cover.
A wave of much greater height and steepness than other waves in the
prevailing sea or swell system. See Rogue Wave.
The part of the atmosphere that lies above the frictional influence of
the earth's surface.
Free Ground Water
In hydrologic terms, unconfined ground water whose upper boundary is a
free water table.
In hydrologic terms, the vertical distance between the normal maximum
level of the water surface in a channel, resrvoir, tank, canal, etc.,
and the top of the sides of a levee, dam, etc., which is provided so
that waves and other movements of the liquid will not overtop the
A freeze is when the surface air temperature is expected to be 32øF or
below over a widespread area for a climatologically significant period
of time. Use of the term is usually restricted to advective situations
or to occasions when wind or other conditions prevent frost. "Killing"
may be used during the growing season when the temperature is expected
to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops.
Issued during the growing season when surface temperatures are expected
to drop below freezing over a large area for an extended period of time, regardless whether or not frost develops.
In hydrologic terms, the date on which the water body was first observed
to be completely frozen over.
A drizzle that falls as a liquid but freezes into glaze or rime upon
contact with the cold ground or surface structures.
Freezing Drizzle Advisory
Issued when freezing rain or freezing drizzle is forecast but a
significant accumulation is not expected. However, even small amounts
of freezing rain or freezing drizzle may cause significant travel
A suspension of numerous minute ice crystals in the air, or water
droplets at temperatures below 0ø Celsius, based at the Earth's surface,
which reduces horizontal visibility; also called ice fog.
The altitude at which the air temperature first drops below freezing.
Rain that falls as a liquid but freezes into glaze upon contact with
Freezing Rain Advisory
Issued when freezing rain or freezing drizzle is forecast but a
significant accumulation is not expected. However, even small amounts
of freezing rain or freezing drizzle may cause significant travel
An accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and
Freezing Spray Advisory
An advisory that may be issued within the Offshore Waters Forecast,
the Coastal Waters Forecast, the Nearshore Marine Forecast, and the
Open Lake Forecast (GLF). An accumulation of freezing water droplets
on a vessel at a rate of less than 2 centimeters (cm) per hour caused
by some appropriate combination of cold water, wind, cold air
temperature, and vessel movement.
In hydrologic terms, ice jam formed as frazil ice accumulates and
In hydrologic terms, an underground passageway for water through the interstices among stones placed loosely in a trench.
The annual spring rise of streams in cold climates as a result of snow
melt; freshet also refers to a flood caused by rain or melting snow.
The mechanical resistive force of one object on another object's
relative movement when in contact with the first object. In meteorology, friction affects the motion of air (wind) at and near the Earth's
In hydrologic terms, the decrease in total head caused by friction.
Same as Planetary Boundary Layer; the layer within the atmosphere
between the earth's surface and 1 km above the surface; this is the
layer where friction affects wind speed and wind direction.
A boundary or transition zone between two air masses of different
density, and thus (usually) of different temperature. A moving front
is named according to the advancing air mass, e.g., cold front if
colder air is advancing.
A temperature inversion that develops aloft when warm air overruns
the cold air behind a front.
1. The initial formation of a front or frontal zone.
2. In general, an increase in the horizontal gradient of an airmass
property, principally density, and the development of the accompanying
features of the wind field that typify a front.
(Abbrev. FRST) - Frost describes the formation of thin ice crystals on
the ground or other surfaces in the form of scales, needles, feathers,
or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects falls below
32øF. As with the term "freeze," this condition is primarily significant
during the growing season. If a frost period is sufficiently severe to
end the growing season or delay its beginning, it is commonly referred to
as a "killing frost." Because frost is primarily an event that occurs as
the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s.
Issued during the growing season when widespread frost formation is
expected over an extensive area. Surface temperatures are usually in the
mid 30s Fahrenheit.
Dew point below freezing.
Human tissue damage caused by exposure to intense cold.
When liquid dew changes into tiny beads of ice. This occurs when dew forms
and temperatures later drop below freezing.
Frost- Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects falls below 32øF. As with the term "freeze," this condition is primarily significant during the growing
season. If a frost period is sufficiently severe to end the growing
season or delay its beginning, it is commonly referred to as a "killing
frost." Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level
temperature in the mid-30s.
A specific aerial spray dispersion model. The acronym comes from the
names of the sponsor and developers (Forest Service, Cramer, Barry,
Feet (or foot)
An Internet server application which provides access to Internet FTP
server files via e-mail. The National Weather Service operates an
FTPMAIL server which provides e-mail access to any product available
on the tgftp.nws.noaa.gov FTP server including marine text and graphic forecasts. For further information see: http://weather.noaa.gov/pub/fax/ftpmail.txt, or send an e-mail to email@example.com with the word "help" in the body.
Dust that is not emitted from definable point sources such as
industrial smokestacks. Sources include open fields, roadways, storage
(or F Scale) - A scale of tornado intensity in which wind speeds are
inferred from an analysis of wind damage:
All tornadoes, and most other severe local windstorms, are assigned a
single number from this scale according to the most intense damage
caused by the storm.
A binary interaction where tropical cyclones within a certain distance
(300-750 nm depending on the sizes of the cyclones) of each other begin
to rotate about a common midpoint.
Full-Physics Numerical Model
A computer model used to calculate air pollution concentrations. A
full-physics numerical model uses a full set of equations describing
the thermodynamic and dynamic state of the atmosphere and can be used
to simulate atmospheric phenomena.
A pattern of plume dispersion produced when a convective boundary layer
grows upward into a plume trapped in a stable layer. The elevated plume
is suddenly brought downward to the ground, producing high surface concentrations.
A condensation funnel extending from the base of a towering cumulus or
Cb, associated with a rotating column of air that is not in contact
with the ground (and hence different from a tornado). A condensation
funnel is a tornado, not a funnel cloud, if either
a) it is in contact with the ground or
b) a debris cloud or dust whirl is visible beneath it.
The process whereby wind is forced to flow through a narrow opening
between adjacent land areas, resulting in increased wind speed.