NOAA's National Weather Service Weather Forecast Office

 
F

F scale:  See Fujita Scale.

Face:  The external surface of a structure, such as the surface of a dam.

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.

Family of Services (FOS):  Since 1983, the National Weather Service (NWS) has provided external user access to U.S. Government obtained or derived weather information through a collection of data communication line services called the Family of Services (FOS).  FOS is accessible via dedicated telecommunications access lines in the Washington, D.C., area. All FOS data services are driven by the NWS Telecommunication Gateway computer systems located at NWS headquarters in Silver Spring, MD.  Users may obtain any of the individual services from NWS for a one-time connection charge and an annual user fee. Several private companies subscriber to the FOS and then resell the data as received and/or provide value-added information services for their customers.

Fathom: A unit of length equal to six feet which is used to measure the depth of water.

FCEXEC:  A component of the NWSRFS FCST Program.
 
FCST:  NWSRFS Forecast Program to produce operational forecasts

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

Feeder Cloud: The flanking lines of developing cumulus congestus clouds that sometimes merge with and appear to intensify supercells.

Fetch:  1) An area from which waves are generated by a wind that is nearly constant in direction and speed. 2) The effective distance which waves have travelled 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 (FEW):  1) 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. 2) A National Weather Service convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch).  Few is used interchangeably with isolated.  See Precipitation Probability (PoP).

FFG:  An acronym for Flash Flood Guidance.  See Flash Flood Guidance.

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.

Fill Dam:  Any dam constructed of excavated natural materials or of industrial wastes.

Fire Behavior:  A complex chain-reaction process that describes the ignition, buildup, propagation, and decline of any fire in wildland fuels.

Fire Danger:  The result of both constant factors (fuels) and variable factors (primarily weather), which affects the ignition, spread, and difficulty of control of fires and the damage they cause.

Fire Danger Maps:  A Fire Danger Rating determination is the cumulative effort of the NFDRS (National Fire Danger Rating System), taking into account current and antecedent weather, fuel types, and the state of both live and dead fuel moisture.  Local managers have much flexibility in local application of NFDRS. They may select up to four fuel models from 20 (broadly covering grass, timber, brush, and slash). Staffing levels may be based on one of several NFDRS indexes, though about 90% use the Burning Index (BI) which is related to potential flame lengths for the selected fuel type. Staffing class breakpoints are set by local managers from historical fire weather climatology. The adjective class rating is NFDRS's method of normalizing rating classes across different fuel models and station locations. It is based on the primary fuel model cataloged for the station, the fire danger index selected to reflect staffing levels, and climatological class breakpoints. All this information is provided by local station managers.

Fire Danger Rating:  A fire control management system that integrates the effects of selected fire danger factors into one or more qualitative or numerical indices from which ease of ignition and probable fire behavior may be estimated.  This is also called a Burn Index.

Fire Weather District:   A fire weather district is the area of routine service responsibility as defined by the NWS.  This area is usually defined by climatological factors, but may be modified somewhat to administrative boundaries of the User Agencies.

Fire Weather Office Operating Plan:  A procedural guide which describes the services provided within the area of a fire weather office's responsibility.

Fire Weather Watch:  A NWS Fire Weather Forecaster will issue this product whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters within the next 48 hours are: 1) a sustained wind average 15 mph or greater 2) relative humidity less than or equal to 25 percent and 3) a temperature of greater than 75 degrees F.  See Red Flag Warning.

Firn Line:  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.

Firn Snow:  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.

Fischer & Porter Punched Tape Recorder Gage:  A precipitation gage which converts weight into a code disk position. The code disk position is then punched on paper tape in a binary decimal format suitable for automatic machine processing.

Flare Echo: This image will once in awhile appear on the WSR-88D reflectivity product. It shows up as a straight line along the radial pointing away from a very high reflectivity (usually over 60 dBZ) core of a thunderstorm and the radar site itself. This occurs as a result of the radar beam hitting a hail shaft (usually containing hail of greater than 1 inch in diameter). The hail shaft causes the radar beam to be reflected to the ground. When the radar beam hits the wet ground, it is reflected back up into the hail shaft and eventually arrives at the radar site. When it arrives back at the radar site, it assumes that the radar beam went out further than it actually did and it plots it along the radial beyond the high reflectivity core. This image tells that radar operator that the thunderstorm is likely producing hail of greater than
1 inch in diameter.

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, and generally coincides with the pseudo-cold front.

Flashboards:  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.

Flash Flood:  A flood which follows within a few hours (usually less than 6 hours) of heavy or excessive rainfall, dam or levee failure, or the sudden release of water impounded by an ice jam. This is a dangerous situation that threatens lives and property.
 
Flash Flood Guidance (FFG):  An internal product produced by the RFC's containing rainfall threshold values which must be exceeded in order for flooding to occur. These values change daily according to soil moisture, season of year, etc.
 
Flash Flood Statement (FFS):  This product is issued after either a Flash Flood Watch or a Flash Flood Warning has been issued  by a local National Weather Service Forecast Office (NWFO).  It will provide the latest information on the flash flooding situation or event.  It will also be used to remove parts of the geographical area covered by the original watch or warning when the flash flooding event is no longer a threat or has ended in a certain area.  It cannot be used to add a geographical area to either a watch or a warning.  A new watch or warning is required to do this.  Finally, this statement can be used to terminate the original watch or warning when it is no longer valid.  This is usually optional when either a watch or warning expires.
 
Flash Flood Table:  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 (FFW):  This warning signifies a short duration of intense flooding of counties, communities, streams, or urban areas with high peak rate of flow.  Flash floods may result from such things as torrential downpours, dam breaks, or ice jam breaks.

They are issued by the local National Weather Service Office (NWFO) for 4 hours or less.  Usually, the 3-hour Flash Flood Guidance values are exceeded for the specified county mentioned in the warning.  Since flash flooding can occur in severe thunderstorms, this type of warning can be combined with either a Tornado Warning or a Severe Thunderstorm Warning.

The warning will include the following:  1) the time period for which the warning is in effect 2) the communities, counties, or river basin which are being affected by the flash flooding 3) the location and movement of the flood-producing storm or storms 4) a site/event specific call-to-action statement highlighting the fact that the flash flooding poses a significant threat to life and property.

Flash Flood Watch (FFA):  This product is issued by the local National Weather Service office (NWFO) for events that have the potential for short duration (usually less than 6 hours) intense flooding of counties, communities, streams or areas for which the occurrence is neither certain nor imminent. This watch indicates that flash flooding is a possibility in or close to the watch area.  Those in the affected area are urged to be ready to take action if a Flash Flood Warning is issued or flooding is observed.

A Flash Flood Watch may be issued for potential flooding from either dam breaks, ice jam breaks, or torrential downpours.  They are usually issued up to 12 hours prior to a possible flash flood event.  They are normally issued for the first period of the forecast, but they may extend into the second period of the forecast.  Depending on the likelihood and the severity of the event, it may be issued for the second period of the forecast.  They can vary in size depending on the size of the meteorological event.

The watch will include the following:  1) the time period for which the watch is in effect 2) The communities, counties, or other geographical areas covered by the watch, 3) the definition of a "Flash Flood Watch" 4) The extent and severity (if possible) of the flash flooding conditions expected 5) A site/event specific call-to-action statement.

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

Float Recording Precipitation Gage:  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:  An accumulation of frazil flocs (also known as a "pan") or a single piece of broken ice.

Flood: The inundation of a normally dry area caused by high flow, or overflow of water in an established watercourse, such as a river, stream, or drainage ditch ; or ponding of water at or near the point where the rain fell. This is a duration type event with a slower onset than flash flooding, normally greater than 6 hours.

Flood Control Storage: Storage of water in reservoirs to abate flood damage.  

Flood Crest:  The Maximum height of a flood wave as it passes a location.

Flood Frequency Curve:  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:  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:  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: Lowland, bordering a river, that is usually dry, but which is subject to flooding.

Flood Plain Information Studies:  Reports usually prepared by the U.S. Army Corps of Engineers (USACE) following a survey of a flood-impacted community.
 
Flood Potential Outlook (ESF on AFOS) (FPO for Acronym):  This is a long range (36-72 hours) outlook issued by a local National Weather Service Office (NWFO) when forecast meteorological conditions indicate that a significant heavy rainfall event may occur that would either cause flooding or aggravate an existing flooding situation.  It is issued when successive numerical guidance model runs indicate that synoptic conditions are favorable for flooding; however, either uncertainty or the time frame precludes the forecaster from issuing either a Flood Watch or Flash Flood Watch.  This outlook includes:  1) the area that will be affected 2) the timing of the event 3) a discussion of the hydrometeorological conditions (such as synoptic conditions, Quantitative Precipitation Forecast (QPF), soil conditions, and River Forecast Center guidance and 4) information as to when an update will be issued.
 
Flood Prevention:  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:  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:  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:  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):  This product is issued after either a Flash Flood Watch or a Flash Flood Warning has been issued  by a local National Weather Service Forecast Office (NWFO).  It will provide the latest information on the flash flooding situation or event.  It will also be used to remove parts of the geographical area covered by the original watch or warning when the flash flooding event is no longer a threat or has ended in a certain area.  It cannot be used to add a geographical area to either a watch or a warning.  A new watch or warning is required to do this.  Finally, this statement can be used to terminate the original watch or warning when it is no longer valid.  This is usually optional when either a watch or warning expires.

Flood Watch (FFA):  This watch is issued by a local National Weather Service Office (NWFO) to indicate that there ia a potential of flooding in or close to the watch area.  Those in the affected area are urged to be ready to take action if a flood warning is issued or flooding is observed.  In flooding, the onset of flooding take place much slower (usually greater than 6 hours) than a flash flood.  This type of flooding usually occurs in Michigan during the convective season with "train echoes" or slow moving thunderstorms, and can also occur with synoptic scale systems that last a relatively long period of time and encompass a large area.  They are usually issued up to 12 hours prior to the possible flood event.  These watches can vary in size depending on the size of the meteorological event.

The watch will include the following:  1) the time period for which the watch is in effect 2) The communities, counties, or other geographical areas covered by the watch, 3) the definition of a "Flood Watch" 4) The extent and severity (if possible) of the flooding conditions expected, and 5) A site/event specific call-to-action.

Flood Warning (FLW):  This warning signifies a longer duration and more gradual flooding of counties, communities, streams, or urban areas.  Floods usually begin after 6 hours of excessive rainfall.  They are issued by the local National Weather Service Forecast Office (NWFO) for 6 hours or less.

The warning will include the following:  1) the time period for which the warning is in effect 2) the communities, counties, or river basin which are being affected by the flooding 3) the location and movement of the flood producing storm or storms 4) a site/event specific call-to-action statement highlighting the fact that the flooding poses a significant threat to life and property.

Flood Wave:  A rise in streamflow to a crest and its subsequent recession caused by precipitation, snowmelt, dam failure, or reservoir releases.
 
Flooded Ice:  Ice which has been flooded by melt water or river water and is heavily loaded by water and wet snow.
 
Floodproofing:  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:  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:  1) A part of the flood plain, otherwise levied, 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: Volume of water in a river or stream, passing a specific observation site, during a specific time period. It is typically expressed in units of cubic feet per second. It is also called "Streamflow", "Discharge", and "Flow Discharge".

Flow Duration Curve:  A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded..
 
Flowing Well:  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.

Flurries:  Light snowfall that generally does not produce a measurable accumulation.

Fog (FG): A visible aggregate of minute water particle (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than 5/8 statue mile, and unlike drizzle, it does not fall to the ground. It occurs most frequently in coastal regions because of the great water vapor content of the air. However, it can occur anywhere. The rapidity with which fog can form makes it especially hazardous. It forms by any atmospheric process that does one of the following: 1) Cools the air to its dew point 2) Raises the dew point to the air temperature. Names given to fog types identify their methods of formation. The principle types are radiational fog, ice fog, advection fog, upslope fog, rain induced fog, and steam fog. These types of fog are called "dense" when the surface visibility is equal to or less than 1/4 miles. A Dense Fog Advisory will be issued when the dense fog becomes widespread.

Fog Bow: A nebulous arc or circle of white or yellowish light sometimes seen in fog.

Folding:  Aliasing; applied to both velocity and range aliasing.

Forebay:  The water behind (upstream) of the dam.
 
Forecast Crest:  The highest elevation of river level, or stage, expected during a specified storm event.

Forecast Models:  Forecasters use numerical weather models to make their forecasts.  These numerical models are classified into four main classes. The first is global models, which focus on the entire northern hemisphere.  The second is national models, which focus on the USA.  The third is regional models.  These regional models have a finer grid than national models and are run out for smaller periods of time.  The final class of models is relocatable models, which do not focus on any permanent geographical location.  Relocatable models are very limited on the size of the geographical area for which they can forecast, but these models have very high resolutions, or very small forecast grid boxes.
 
Forecast Point:  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.
 
Foresight:  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.

Forestry Weather Interpretation System:  A computerized USDA Forest Service program which utilizes National Weather Service (NWS) forecasts, observations, warnings, and advisories for specific forestry operations and problems, such as smoke management, prescribed burning, planting, harvesting, etc.
 
Forward Flank Downdraft:  The main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is.  Compare with rear flank downdraft.  See pseudo-warm front and supercell.

Fountainhead:  The upper end of a confined-aquifer conduit, where it intersects the land surface.

Fracture:  Any break or rupture formed in an ice cover or floe due to deformation.
 
Fracture Zone:  An area which has a great number of fractures.
 
Fracturing:  Deformation process whereby ice is permanently deformed, and fracture occurs.

Frazil Ice:  Fine spicules, plates, or discoids of ice suspended in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.
 
Frazil Slush:  An agglomerate of loosely packed frazil which floats or accumulates under the ice cover.

Free Ground Water:  Unconfined ground water whose upper boundary is a free water table.
 
Freeboard:  The vertical distance between the normal maximum level of the water surface in a channel, reservoir, 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: It is when the surface air temperature is expected to be 32^oF 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. Adjectives such as "killing", "severe", or "hard" will be used when appropriate. "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 or plants.

Freezing Level: The lowest altitude in the atmosphere, or a given location, at which the air temperatures is 32 degrees.

Freezing Rain or Drizzle: This occurs when rain or drizzle freezes on surfaces (such as the ground, trees, power lines, motor vehicles, streets, highways, etc.) that have a temperature of 32^o F or below. Small accumulations of ice can cause driving and walking difficulties. Meanwhile, heavy accumulations of ice can pull down trees and utility lines. In this situation, it would be called an "Ice Storm".

Freezing Rain Advisory:  This product is issued by the National Weather Service when freezing rain or freezing drizzle causes significant inconveniences, but does not meet warning criteria (normally an ice accumulation of 1/4 inch or greater) and if caution is not exercised, it could lead to life-threatening situations.

Freezing Spray:  An accumulation of supercooled water droplets on a vessel or object which are below the freezing point of water. It usually develops in areas with winds of at least 25 knots.  The following table gives the Categories of Freezing Spray/Icing.
 

Freezeup Date:  Date on which the water body was first observed to be completely frozen over.
 
Freezup Jam:  Ice jam formed as frazil ice accumulates and thickens.

French Drain:  An underground passageway for water through the interstices among stones placed loosely in a trench.

Frequency:  The number of recurrences of a periodic phenomenon per unit time. Electromagnetic energy is usually specified in Hertz (Hz), which is a unit of
frequency equal to one cycle per second. Weather radars typically operate in the GigaHertz range (GHz). See also wavelength.

Frequency Band:   A range of frequencies, between some upper and lower limit.

Frequency Carrier:  The fundamental transmitted microwave frequency between 2700 and 3000 MHz. It is modulated so that it exists for a few microseconds each pulse.

Frequency Curve:  A curve that expresses the relation between the frequency distribution plot, with the magnitude of the variables as abscissas and the number of occurrences of each magnitude in a given period as ordinates. The theoretical frequency curve is a derivative of the probability curve.

Fresnel Reflection:  The reflection of a radar signal from a single, dominating discontinuity of the refractive index, usually with a large horizontal extent. Also called "partial reflection" because only a small fraction of the incident power is reflected, "specular reflection" if the horizontal surface discontinuity is assumed to be smooth, or "diffuse reflection" if the discontinuity is assumed to be corrugated or somewhat rough.

Fresnel Scatter:  Scatter which occurs if several or many refractive index discontinuities exist along the pointing direction. The difference between Fresnel reflection and scatter may be primarily a matter of resolution of the sampling volume compared to the size of the reflecting target.

Frigophobia:  The fear of the cold and cold things.
 
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.  See Cold Front, Occluded Front, Stationary Front, and Warm Front.

Frontal Inversion: A transition zone between 2 different air masses. The temperature curve is the basic reference for locating frontal zones aloft. If the front were a sharp discontinuity, the temperature curve should show a clear cut inversion separating the lapse rates typical of cold and warm air masses. However, a shallow isothermal or relatively stable layer is more usual indication of a well-defined front. Frequently, the frontal boundary is so weak or distorted by other discontinuities that frontal identification is difficult. In an ideal case, the dew point curve through the frontal zone will show an inversion or sharp change associated with that of the temperature curve. The frontal surface is considered to be located at the top of the inversion.

Frontogenesis: The process in which a front forms. This occurs when there is an increase in the temperature gradient across a front.

Frontolysis: The process in which a front dissipates. This occurs when the temperatures and pressures equalize across a front.

Frost: The formation of 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 fall below 32^oF. 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 temperature reading in the mid 30s.  Frost may also form on aircraft in flight during descent from subfreezing   into a warmer, moist layer below.  This may cause considerable consternation if the windshield glazes over or the windows frost over.

Frost/Freeze Advisory:  This product is issued by the National Weather Service when freezing temperatures or conditions conducive to the formation of frost occur during the growing season.

Frost Point: Dew point below freezing.

Fractus:  Ragged, detached cloud fragments; same as scud.

Friction Head:  The decrease in total head caused by friction.

FTP ( File Transfer Protocol):  A method of data transfer that can take place between Frame Relay Networks, and Workstations.

Fuel Moisture:  The water content of fuel particle expressed as a percent of the oven dried weight of the fuel particle.  Fuel moisture observations are generally for the 10-hour time lag fuels (medium-sized roundwood 1/4 to 1 inch in diameter).
 
Fujita Scale (F-scale): A scale used to classify the strength of a tornado. It was devised by Dr. Theodore Fujita from the University of Chicago. The F-scale gives tornadoes a numerical rating from F0 to F5. The following table shows the F scale in more detail:
 

 
Since the F-scale is based on tornado damage (primarily to buildings), there is some ambiguity in the scale. For example, a tornado which moves over open country will tend to receive a lower rating than a tornado which strikes a populated area. Since buildings have a wide variation in age, quality of design, and quality of building materials, more uncertainties are thrown into the mix. Tornadoes over open country will probably encounter varying type of vegetation, leading to uncertainties in these cases.

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.

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

Funnel Cloud (FC): 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.