Aeronautical Terms beginning with F
FA
Area Forecast. (Discontinued, replaced by Graphical Forecasts for Aviation)
Fahrenheit temperature scale (abbreviated F)
A temperature scale with 32 degrees as the melting point of pure ice and 212 degrees as the boiling point of pure water at standard sea level atmospheric pressure (29.92 inches or 1013.2 millibars).
Fall wind
A cold wind blowing downslope. Fall wind differs from foehn in that the air is initially cold enough to remain relatively cold despite compressional heating during descent.
Filling
An increase in the central pressure of a pressure system; opposite of deepening; more commonly applied to a low rather than a high.
First gust
The leading edge of the spreading downdraft, plow wind, from an approaching thunderstorm.
Flow line
A streamline.
Foehn
A warm, dry downslope wind; the warmness and dryness being due to adiabatic compression upon descent; characteristic of mountainous regions. Also known as a Chinook in the Rockies and Santa Ana winds in Southern California and Northern Baja.
Fog
A hydrometeor consisting of numerous minute water droplets and based at the surface; droplets are small enough to be suspended in the earth’s atmosphere indefinitely. (Unlike driv:.le, it does not fall to the surface; differs from cloud only in that a cloud is not based at the surface; distinguished from haze by its wetness and gray color.)
Forecast Icing Conditions
Environmental conditions expected by an NWS or an FAA-approved weather provider to be conducive to the formation of in-flight icing on aircraft.
Forecast Icing Product (FIP)
The FIP examines numerical weather prediction model output to calculate the probability and severity of icing conditions, plus SLD potential. This product is automatically produced with no human modification. More information can be found on the AWC ADDS Web site.
Fractus
Clouds in the form of irregular shreds, appearing as if torn; have a clearly ragged appearance; applies only to stratus and cumulus, i.e., cumulus fractus and stratus fractus.
Freezing
The change of liquid water to ice.
Freezing Drizzle
Drizzle is precipitation at ground level or aloft in the form of liquid water drops that have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle is water that remains in a liquid form at air temperatures less than 0 °C (supercooled) and can freeze upon contact with objects on the ground or in the air.
Freezing level
A level in the atmosphere at which the temperature is 0° C (32° F).
Freezing Rain
Rain is precipitation at ground level or aloft in the form of liquid water drops which have diameters greater than 0.5 mm. Freezing rain is rain that exists at air temperatures less than 0 °C, remains in liquid form (supercooled), and freezes upon contact with objects on the ground or in the air.
Front
A surface, interface, or transition zone of discontinuity between two adjacent air masses of different densities; more simply the boundary between two different air masses.
Front
The boundary between two air masses. A front can be classified as cold, warm, occluded, or stationary.
Frontal zone
A front or zone with a marked increase of density gradient; used to denote that fronts are not truly a “surface” of discontinuity but rather a “zone” of rapid transition of meteorological elements.
Frontogenesis
The initial formation of a front or frontal zone.
Frontolysis
The dissipation of a front.
Frost
Ice crystal deposits formed by sublimation when temperature and dewpoint are below freezing.
Frost (also hoarfrost)
Ice crystal deposits formed by sublimation when temperature and dew point are below freezing.
Funnel cloud
A tornado cloud or vortex cloud extending downward from the parent cloud but not reaching the ground.
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