Aeronautical Terms beginning with F


The change of liquid water to ice.

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.


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.


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.


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.)


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 level

A level in the atmosphere at which the temperature is 0° C (32° F).


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.

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.


The initial formation of a front or frontal zone.


The dissipation of a front.

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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