National Weather Service United States Department of Commerce

Weather Glossary: F's

F scale
The old Fujita tornado intensity scale. It was replaced by the Enhanced Fujita Scale (EF).
The external surface of a structure, such as the surface of a dam.
Fahrenheit (F)
The standard scale used to measure temperature in the United States; in which the freezing point of water is 32°F (0°C) and the boiling point is 212°F (100°C).
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). 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 subscribe to the FOS and then resell the data as received and/or provide value-added information services for their customers.

A unit of length equal to six feet (two meters) which is used to measure the depth of water.
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 (380 centimeters) . 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.
  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 traveled 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% chance of measurable precipitation (0.01" / 0.25 mm). Few is used interchangeably with isolated. See Precipitation Probability (PoP).
An acronym for 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 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 (24 km/h) or greater,
  2. relative humidity less than or equal to 25%, and
  3. a temperature of greater than 75°F (24°C).
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 a while 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" 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" 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.
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 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. 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. 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 for 4 hours or less.

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.

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 act 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.

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.
A cluster of frazil particles.
An accumulation of frazil flocs (also known as a "pan") or a single piece of broken ice.
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 six 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 Potential Outlook

This is a long range (36-72 hours) outlook issued by a local National Weather Service Office 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.

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

This watch is issued by a local National Weather Service Office 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 act if a flood warning is issued or flooding is observed.

In flooding, the onset of flooding take place much slower (usually greater than six hours) than a flash flood. This type of flooding usually occurs 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.

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.

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.

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.
  • 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.
  • 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.
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.
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 (1 km), 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:

  • Cools the air to its dew point, or
  • Raises the dew point to the air temperature.
Names given to fog types identify their methods of formation. The principle types are radiation 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 (400 meters). 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.
Aliasing; applied to both velocity and range aliasing.
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.

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

It is when the surface air temperature is expected to be 32°F (0°C) 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°F (0°C).
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°F (0°C) 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" or greater) and if caution is not exercised, it could lead to life-threatening situations.
Freezing Spray
Intensity Accumulation
Heavy > 2.0 cm/hr > 0.8 in/hr
Moderate 0.7 - 2.0 cm/hr 0.3 - 0.8 in/hr
Light < 0.7 cm/hr < 0.3 in/hr
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.
Freezup Jam
Ice jam formed as frazil ice accumulates and thickens.
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 wavelength.
Frequency Band
A range of frequencies, between some upper and lower limit.
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.
The fear of the cold and cold things.
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 two 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.

The process in which a front forms. This occurs when there is an increase in the temperature gradient across a front.
The process in which a front dissipates. This occurs when the temperatures and pressures equalize across a front.

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°F (0°C).

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 30'sF (0-2°C). 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.
Ragged, detached cloud fragments; same as scud.
Friction Head
The decrease in total head caused by friction.
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" in diameter).
Fujita Scale (F-scale)
The old scale used to classify the strength of a tornado. It was devised by Dr. Theodore Fujita from the University of Chicago. The F-scale gave tornadoes a numerical rating from F0 to F5. It was replaced by the < Enhanced Fujita Scale (EF).
Fujiwhara Effect
A binary interaction where tropical cyclones within a certain distance (300-750 nm / 480-1,200 km; depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint.
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:
  • It is in contact with the ground, or
  • A debris cloud or dust whirl is visible beneath it.