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FC

(abbr.) Funnel Cloud - (in METAR/TAF, this includes tornado/waterspout, so differs from the classical distinction between a funnel cloud not touching down, and one that does . See entry for Funnel cloud).

FEW

1 or 2 oktas cloud amount, used in Aviation/METAR reports etc. (see also SCT)

FG

(abbr) Fog (vis < 1000m, except when qualified by MI, BC, PR, VC); used in METAR/TAF reports etc.

FL

(abbr) Flight level (e.g. FL240 ... 24000ft amsl/standard atmosphere); used in aviation reports, forecasts etc.

Foehn (or Föhn) effect

Mechanisms which give rise to a warm, dry wind on the leeward side of mountains or significant hills. Broadly, there are two: (i) the 'subsidence' type where air at & just above the hill/mountain crest descends by lee-wave action, becoming even drier & warmer than when it started out; (ii) all air in a moist airstream on the upstream side of the hill/mountain rises, leading to cloud/precipitation formation, thence lowering the humidity content, this air then descending/warming adiabatically on the leeside.

Fog

Reduction in visibility to under 1 km caused by suspension of minute water droplets (water fog) or ice crystals (Ice fog - q.v.). Water fogs are further sub-divided according to the process by which the fog forms, e.g. Radiation fog (caused mainly due to loss of surface heat from the ground at night in conditions of near-calm wind and high relative humidity); Advection fog (caused by movement of humid air over a relatively colder surface); Upslope fog (adiabatic cooling of air having high relative humidity as it climbs over high ground) and Evaporation fog (caused by evaporation into cold air which lies over a relatively warm water surface).
( If the visibility is below 200M but greater than ~50M then it is usually referred to as 'Thick Fog' (& colloquially in the UK as 'motoring fog') & if below 50M then 'Dense Fog'. However, there are different criteria for climatological stations, and other services will have different rules - treat this note as a guide only. )

Fog point

(strictly fog-point temperature) The air temperature (as measured in a standard thermometer screen) at which fog is expected/does form. Its calculation (before an event) is usually based on empirical work which employs either the surface air temperature/dew point at some time earlier in the day, or by construction on a thermodynamic diagram. The fog point is lower than the air-mass dew point, because as air cools through the evening and night, moisture is condensed out on contact with the chilled land surface, and this lowers the dew point from afternoon values.

Freezing fog

As for the definition of fog (above), but the droplets are super-cooled (i.e. temperature below zero), and strictly, the fog should be depositing rime-ice. However, in METAR/TAF coding, as long as the air temperature is below zero degC, then fog is coded as 'freezing' irrespective of whether rime is observed.

Freezing level

Taken as the altitude where the air temperature is 0 deg.C. However, it should be carefully noted that in the free atmosphere, liquid water does not necessarily freeze at this level, or indeed at altitudes some way above this value -- it should more correctly be called the melting level, or as in operational /aviation meteorology, the level (or altitude) of the zero degree isotherm. (see ZDL and Wet Bulb Freezing Level).

Front

A boundary separating two air masses such as warm, moist air and cold, dry air. If the cold air pushes into a region of warm air, a cold front occurs and if the warm air advances relative to the cold, a warm front occurs.

Frontal fracture

During rapid cyclogenesis events, a weakness appears along the portion of the cold-front nearest to the depression centre, thought to be due to a combination of subsidence in this region, plus differential thermal advection, as, unlike in the 'Norwegian' model, cold air is not advected so quickly eastwards to maintain the baroclinicity in this region. (See "What is the Shapiro-Keyser cyclone model?")

Frontogenetic

Any atmospheric process which leads to frontal formation, or an existing weak frontal zone to become enhanced, is termed frontogenetic. On charts issued by some national meteorological services, such fronts are shown with the normally solid line defining the front broken by spaces and large dots.

Frontolysis

When fronts weaken markedly due to, for example, marked anticyclonic subsidence across the front, then the feature is undergoing frontolysis. On charts issued by some national meteorological services, such fronts are shown with the line defining the front 'struck-through' by short inclined strokes.

Frost

See "What is the difference between a ground frost and an air frost?"

Frost point

(strictly frost-point temperature) The temperature (of an air sample that contains water vapour), to which that sample must be cooled (Pressure and humidity content being held constant) to achieve saturation with respect to an ice surface. (see also dew point).

FRQ

(abbr) Frequent (hardly or not separated, as in FRQ CB in aviation forecasts).

FU

(abbr) Smoke (as used in METAR/TAF reports etc.)

Funnel cloud (FC)

A visible rotating 'tube' of condensation particles, formed as the pressure falls in an intensifying vortex (extending below a cumulonimbus cloud) - perhaps reaching the ground/sea. It should be noted that the funnel cloud simply betrays the zone where the pressure is low enough and humidity high enough for cloud to form - the vortical circulation may well be in contact with the ground, but of such relatively weak intensity that it either causes little or no damage, or is detected only by surface dust / soil disturbance. There are well documented cases where tornadoes (as defined elsewhere) do not have cloudy funnels all the way to the surface. (based on Doswell, C.A. III, 2001)

FZ

(abbr) Freezing (used in connection with rain, drizzle, fog, all giving rise to ice deposition due to supercooled water droplets impacting upon surfaces with temperatures below 0.0degC, OR when temperature < 0.0degC anyway, whether or not ice deposits are observed.)