• Weather Glossary (F)

    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
    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.

    ***

    F

    1) Fahrenheit- The standard scale used to measure temperature in the
    United States. On this scale, the freezing point of water is 32F and the boiling point is 212F. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32: F = (C * 9/5) + 32
    or

    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.

    F Corona
    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.

    F Region
    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.

    F Scale
    Abbreviation for Fujita Scale, a system of rating the intensity of
    tornadoes; for detailed information, see the definition for that term.

    FA
    Forecast Area

    FAA
    Federal Aviation Administration

    Face
    In hydrologic terms, the external surface of a structure, such as the
    surface of a dam.

    Facula
    In solar-terrestrial terms, a bright region of the photosphere seen in
    white light, seldom visible except near the solar limb.

    Fahrenheit
    (abbrev. F) The standard scale used to measure temperature in the United States. On this scale, the freezing point of water is 32F and the
    boiling point is 212F. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32:

    F = (C * 9/5) + 32

    Fair
    It is usually used at night to describe less than 3/8 opaque clouds, no precipitation, no extremes of visibility, temperature or winds. It
    describes generally pleasant weather conditions.

    Fall
    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.

    Fall Line
    A skiing term, indicating the line of steepest descent of a slope.

    Fall Wind
    A strong, cold, downslope wind.

    Fallstreak
    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.

    FAN
    AVN MOS Guidance (older version)

    Fanning
    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.

    FASTST
    Fastest

    Fathom
    Unit of water depth equal to 6 feet.

    FAWS
    Flight Advisory Weather Service

    FBO
    (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.

    FCST
    Forecast

    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.

    Feeder Bands
    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.

    FEMA
    Federal Emergency Management Agency. An agency of the federal government
    having responsibilities in hazard mitigation; FEMA also administers the National Flood Insurance Program.

    Ferrel Cell
    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.

    Fetch
    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.

    Few
    A National Weather Service convective precipitation descriptor for a 10
    percent chance of measurable precipitation (0.01 inch). Few is used interchangeably with isolated.

    Few Clouds
    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.

    FFG
    Flash Flood Guidance

    FG
    (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.

    Fibril
    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.

    Filament
    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.

    Filament Channel
    A broad pattern of fibrils in the chromosphere, marking where a filament
    may soon form or where a filament recently disappeared.

    Fill Dam
    In hydrologic terms, any dam constructed of excavated natural materials
    or of industrial wastes.

    Filling
    The opposite of deepening. A general increase in the central pressure of
    a low pressure system.

    Fire Wind
    A thermally driven wind blowing radially inward toward a fire, produced
    by horizontal temperature differences between the heated air above the
    fire and the surrounding cooler free atmosphere.

    Firebrand
    Any source of heat, natural or man made, capable of igniting wildland
    fuels; flaming or glowing fuel particles that can be carried naturally
    by wind, convection currents, or gravity into unburned fuels.

    Firn (Snow)
    In hydrologic terms, old snow on top of glaciers, granular and compact
    and not yet converted into ice. It is a transitional stage between snow
    and ice. Also called Neve.

    Firn Line
    In hydrologic terms, the highest level to which the fresh snow on a
    glacier's surface retreats during the melting season. The line separating
    the accumulation area from the ablation area.

    First Law of Thermodynamics
    The law of physics that states that the heat absorbed by a system either
    raises the internal energy of the system or does work on the environment.

    Flanking Line
    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.

    Flare
    In solar-terrestrial terms, a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.

    Flash
    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.

    Flash Flood
    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
    the tables.

    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.

    Flash Multiplicity
    The number of return strokes in a lightning flash.

    Flashboards
    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.

    FLG
    Falling

    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.

    Floc
    A cluster of frazil particles.

    Floe
    In hydrologic terms, an accumulation of frazil flocs (also known as a
    "pan") or a single piece of broken ice.

    Flood
    Any high flow, overflow, or inundation by water which causes or
    threatens damage.

    Flood Categories
    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
    public threat.

    *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
    damage.

    Flood Crest
    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.)

    Flood Plain
    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.

    Flood Prevention
    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.

    Flood Problems
    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.

    Flood Profile
    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.

    Flood Routing
    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.

    Flood Stage
    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.

    Flood Warning
    (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.

    Flood Wave
    In hydrologic terms, a rise in streamflow to a crest and its subsequent recession caused by precipitation, snowmelt, dam failure, or reservoir releases.

    Flooded Ice
    In hydrologic terms, ice which has been flooded by melt water or river
    water and is heavily loaded by water and wet snow.

    Floodproofing
    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.

    Floodwall
    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.

    Floodway
    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.

    Flow
    (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.

    Flow Separation
    The process by which a separation eddy forms on the windward or leeward
    sides of bluff objects or steeply rising hillsides.

    Flow Splitting
    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.

    Flowing 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 an Artesian well.

    FLRY
    Flurry

    FLS
    River Flood Statement

    Fluence
    Time integrated flux

    Fluid
    Matter which flows; gas or liquid.

    Flurries
    Snow flurries are an intermittent light snowfall of short duration
    (generally light snow showers) with no measurable accumulation (trace category).

    Flux
    The rate of transfer of fluids, particles or energy per unit area across
    a given surface (amount of flow per unit of time).

    FLW
    Follow (or) Flow- Wind

    FM
    From or Fathom

    fMin
    In solar-terrestrial terms, the lowest radiowave frequency that can be reflected from the ionosphere.

    FNT
    Front

    FNTGNS
    Frontogenesis

    FNTLYS
    Frontolysis

    Foehn
    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.

    Foehn Pause
    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.

    foEs
    In solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflec- tion from the sporadic E region of the ionosphere.

    foF2
    In solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflection from the F2 region of the ionosphere.

    Fog
    (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
    or less.

    Fogbow
    A rainbow that has a white band that appears in fog, and is fringed with
    red on the outside and blue on the inside.

    Forced Channeling
    Channeling of upper winds along a valley's axis when upper winds are
    diverted by the underlying topography. Compare pressure-driven channeling.

    Forebay
    In hydrologic terms, the water behind (upstream) of the dam.

    Forecast
    A statement of prediction.

    Forecast Crest
    In hydrologic terms, the highest elevation of river level, or stage,
    expected during a specified storm event.

    Forecast Guidance
    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.

    Forecast Periods
    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

    Forecast Point
    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.

    Foresight
    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
    instrument.

    Forward Flank Downdraft
    The main region of downdraft in the forward, or leading, part of a
    supercell, where most of the heavy precipitation is.

    Fountainhead
    In hydrologic terms, the upper end of a confined-aquifer conduit, where
    it intersects the land surface.

    FOUS
    Forecast Output United States

    FPO
    Flood Potential Outlook

    FPS
    Fujita-Pearson Scale

    FQT
    Frequent

    Fractocumulus
    A cumulus cloud presenting a ragged, shredded appearance, as if torn.

    Fractostratus
    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.

    Fracture
    In hydrologic terms, any break or rupture formed in an ice cover or floe
    due to deformation.

    Fracture Zone
    In hydrologic terms, an area which has a great number of fractures.

    Fracturing
    In hydrologic terms, deformation process whereby ice is permanently
    deformed, and fracture occurs.

    Fractus
    Ragged, detached cloud fragments; same as scud.

    Frazil Ice
    In hydrologic terms, fine spicules, plates, or discoids of ice suspended
    in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.

    Frazil Slush
    In hydrologic terms, an agglomerate of loosely packed frazil which floats
    or accumulates under the ice cover.

    Freak Wave
    A wave of much greater height and steepness than other waves in the
    prevailing sea or swell system. See Rogue Wave.

    Free Atmosphere
    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.

    Freeboard
    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
    confining structure.

    Freeze
    A freeze is when the surface air temperature is expected to be 32F 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.

    Freeze Warning
    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.

    Freezeup date
    In hydrologic terms, the date on which the water body was first observed
    to be completely frozen over.

    Freezing Drizzle
    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
    problems.

    Freezing Fog
    A suspension of numerous minute ice crystals in the air, or water
    droplets at temperatures below 0 Celsius, based at the Earth's surface,
    which reduces horizontal visibility; also called ice fog.

    Freezing Level
    The altitude at which the air temperature first drops below freezing.

    Freezing Rain
    Rain that falls as a liquid but freezes into glaze upon contact with
    the ground.

    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
    problems.

    Freezing Spray
    An accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and
    vessel movement.

    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.

    Freezup jam
    In hydrologic terms, ice jam formed as frazil ice accumulates and
    thickens.

    French Drain
    In hydrologic terms, an underground passageway for water through the interstices among stones placed loosely in a trench.

    Freshet
    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.

    Friction
    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
    surface.

    Friction Head
    In hydrologic terms, the decrease in total head caused by friction.

    Friction Layer
    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.

    FRMG
    Forming

    Front
    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.

    Frontal Inversion
    A temperature inversion that develops aloft when warm air overruns
    the cold air behind a front.

    Frontogenesis

    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.

    FROPA
    Frontal Passage

    FROSFC
    Frontal Surface

    Frost
    (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
    32F. 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.

    Frost Advisory
    Issued during the growing season when widespread frost formation is
    expected over an extensive area. Surface temperatures are usually in the
    mid 30s Fahrenheit.

    Frost Point
    Dew point below freezing.

    Frostbite
    Human tissue damage caused by exposure to intense cold.

    Frozen Dew
    When liquid dew changes into tiny beads of ice. This occurs when dew forms
    and temperatures later drop below freezing.

    FRST
    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 32F. 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.

    FRZ
    Freeze

    FRZN
    Frozen

    FSCBG
    A specific aerial spray dispersion model. The acronym comes from the
    names of the sponsor and developers (Forest Service, Cramer, Barry,
    Grim).

    FT
    Feet (or foot)

    FTHR
    Further

    FTPMAIL
    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 ftpmail@weather.noaa.gov with the word "help" in the body.

    Fugitive Dust
    Dust that is not emitted from definable point sources such as
    industrial smokestacks. Sources include open fields, roadways, storage
    piles, etc.

    Fujita Scale
    (or F Scale) - A scale of tornado intensity in which wind speeds are
    inferred from an analysis of wind damage:

    Rating Wind, Damage

    F0 (weak) 40-72 mph, light damage
    F1 (weak) 73-112 mph, moderate damage
    F2 (strong) 113-157 mph, considerable damage
    F3 (strong) 158-206 mph, severe damage
    F4 (violent) 207-260 mph, devestating damage
    F5 (violent) 260-318 mph (rare), incredible 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.

    Fujiwhara Effect
    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.

    Fumigation
    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.

    Funnel Cloud
    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.

    Funnelling
    The process whereby wind is forced to flow through a narrow opening
    between adjacent land areas, resulting in increased wind speed.

    FVT
    Forecast Verification Tool

    FWC
    NGM MOS Guidance

    FWD
    Forward

    FZRA
    freezing rain
    --- SBBSecho 3.11-Win32
    * Origin: The Thunderbolt BBS - tbolt.synchro.net (57:57/10)