FlurriesSnow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).
FluxThe amount of energy passing through a unit area (for example, 1m2) per time. On this website, we will be discussing the flux of light energy due to…
FoehnA 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…
Foehn PauseA 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.
foEsIn solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflec- tion from the sporadic E region of the ionosphere.
foF2In solar-terrestrial terms, the maximum ordinary mode radiowave frequency capable of reflection from the F2 region of the ionosphere.
FogFine particles of liquid suspended in the air, such as of water in a fog chamber used for acclimatizing recent ex vitro transplants. See: mist propagation.
FogbowA rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.
Forced ChannelingChanneling of upper winds along a valley's axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.
forcingfactors that drive or "force" a system to change. Forcings that affect our climate system include changes in solar radiation and in greenhouse gas…
Forcing MechanismA process that alters the energy balance of the climate system, i.e. changes the relative balance between incoming solar radiation and outgoing infrared…
Forecast CrestThe highest elevation of river level, or stage, expected during a specified storm event.
Forecast GuidanceComputer-generated forecast materials used to assist the preparation of a forecast, such as numerical forecast models.
Forecast Issuance StageThe 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…
Forecast PeriodsOfficial definitions for NWS products: Today...............................Sunrise to sunset This afternoon..................noon till 6 p.m. This evening.
Forecast PointA location that represents an area (reach of a river), where a forecast is made available to the public.
Forecast valid forThe 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.
ForesightA sighting on a point of unknown elevation from an instrument of known elevation.
ForestTerrestrial ecosystem (biome) with enough average annual precipitation (at least 76 centimeters or 30 inches) to support growth of various species of…
Forward Flank DowndraftThe main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is.
Fossil fuelSolid, liquid or gaseous fuels formed in the ground after millions of years by chemical and physical changes in plant and animal residues under high…
Fossil fuel combustionBurning of coal, oil (including gasoline), or natural gas. This burning, usually to generate energy, releases carbon dioxide, as well as combustion by…
FountainheadThe upper end of a confined-aquifer conduit, where it intersects the land surface.
FractocumulusA cumulus cloud presenting a ragged, shredded appearance, as if torn.
FractostratusA stratus cloud presenting a ragged, shredded appearance, as if torn. It differs from a fractocumulus cloud in having a smaller vertical extent and darker…
Fracture ZoneAn area which has a great number of fractures.
FracturingDeformation process whereby ice is permanently deformed, and fracture occurs.
FractusRagged, detached cloud fragments; same as scud.
Frazil IceFine spicules, plates, or discoids of ice suspended in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.
Frazil SlushAn agglomerate of loosely packed frazil which floats or accumulates under the ice cover.
Freak WaveA wave of much greater height and steepness than other waves in the prevailing sea or swell system. See Rogue Wave.
Free AtmosphereThe part of the atmosphere that lies above the frictional influence of the earth's surface.
Free Ground WaterUnconfined ground water whose upper boundary is a free water table.
FreeboardPortion of the dam above spillway level to protect the dam against washout.
FreezeA 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.
Freeze WarningIssued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time…
Freezeup dateDate on which the water body was first observed to be completely frozen over.
Freezing DrizzleA drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.
Freezing Drizzle AdvisoryIssued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected.
Freezing FogA suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0º Celsius, based at the Earth's surface, which…
Freezing LevelThe altitude at which the air temperature first drops below freezing.
Freezing RainRain that falls as a liquid but freezes into glaze upon contact with the ground.
Freezing Rain AdvisoryIssued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected.
Freezing SprayAn accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and vessel…
Freezing Spray AdvisoryAn advisory that may be issued within the Offshore Waters Forecast, the Coastal Waters Forecast, the Nearshore Marine Forecast, and the Open Lake Forecast…
Freezup jamIce jam formed as frazil ice accumulates and thickens.
French DrainA covered ditch containing a layer of fitted or loose stone or other pervious material.
FreonA registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.
Freshetthe 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.
freshwaterA term referring to bodies of water that do not have salt.
FrictionResistance to motion when one body is sliding or tending to slide over another.
Friction HeadThe energy lost from the movement of a fluid in a conduit (pipe) due to the disturbances created by the contact of the moving fluid with the surfaces of…
Friction LayerSame 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…
friction velocityA measure of the surface stress reflecting the effects of surface roughness and wind velocity.
Frontal InversionA temperature inversion that develops aloft when warm air overruns the cold air behind a front.
Frontogenesis1. The initial formation of a front or frontal zone. 2. In general, an increase in the horizontal gradient of an airmass property, principally density…
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 AdvisoryIssued during the growing season when widespread frost formation is expected over an extensive area.
FTPMAILAn Internet server application which provides access to Internet FTP server files via e-mail.
Fugitive DustThe dust released from any source other than a definable point source such as stack, chimney, or vent.
Fugitive emissionsUnintended gas leaks from the processing, transmission, and/or transportation of fossil fuels, CFCs from refrigeration leaks, SF6 from electrical power…
Fujita ScaleThe official classification system for tornado damage. The scale ranges from F0 (gale tornado, minor damage, winds up to 72 mph) to F5 (devastating…
Fujiwhara EffectA 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…
Full-Physics Numerical ModelA computer model used to calculate air pollution concentrations. A full-physics numerical model uses a full set of equations describing the thermodynamic…
FumigationA pattern of plume dispersion produced when a convective boundary layer grows upward into a plume trapped in a stable layer.
Funnel CloudA 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…
FunnellingThe process whereby wind is forced to flow through a narrow opening between adjacent land areas, resulting in increased wind speed.
FusionThe combining of lighter elements into heavier ones. For lighter elements (e.g., hydrogen, helium) this process releases energy.