F

1) Fahrenheit- The standard scale used to measure temperature in the United States. On this scale, the freezing point of water is 32°F and the boiling point is 212°F. 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 ¼ 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 32°F and the boiling point is 212°F. 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.
• 2. 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 ¼ 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

(FFG) Forecast guidance produced by the River Forecast Centers, 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

An established gage height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull 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 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.

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’s 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.

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 reflection 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 ¼ 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

A location along a river or stream for which hydrologic forecast and warning services are provided by a WFO. The observed/forecast stage or discharge for a given forecast point can be assumed to represent conditions in a given reach (see /reach/).

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 32°F 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 fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze.

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 for 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 32°F. 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 32°F. 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@ftpmail.nws.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, devastating 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.

Funneling

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

 

 

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