Weather Glossary - F

F scale

See Fujita Scale.

Fahrenheit scale

The temperature scale where 32°F is the freezing point of water and 212°F is the boiling point (at sea level).

Fahrenheit temperature scale

Thermodynamic scale of temperature. Temperature in degrees Fahrenheit can be obtained from value in degrees Celsius by the following formula: F=(9C/5)+32

Feeder bands

Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the north through east (i.e., parallel to the inflow). Same as inflow bands. This term also is used in tropical meteorology to describe the spiral-shaped bands of convection surrounding and moving toward the centre of a tropical cyclone.

Feels Like

The 'Feels Like' Index displayed in the Observations pages on Weatherzone is a combination of the heat index and wind chill factor.

Fetch

Distance, measured in the upward wind direction. Fetch is important in determining swell size. The longer the fetch the higher the swell.

Few

Describes cloud cover when between 1/8th and 2/8th of the sky is obscured by cloud.

Filling

Describes an increase in the central pressure of a low pressure system.

Flang

[Slang] FLash bANG, a very close lightning strike followed immediately by thunder.

Flanking line

A line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a thunderstorm, normally on the northwest 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. See HP storm and supercell.

Flash flood

Heavy rainfall and localised flooding of short duration with a relatively high peak discharge. Often caused by severe thunderstorms or intense rainfall events such as an East Coast Low. For severe thunderstorms, a flash flood event is considered to be a 1 in 10 year rainfall event.

Flash flooding

Short duration localised flooding caused by heavy rainfall with a relatively high peak discharge. Often caused by severe thunderstorms or intense rainfall events such as an East Coast Low. For severe thunderstorms, a flash flood event is considered to be a 1 in 10 year rainfall event.

Flood

A flood occurs when water inundates (covers) land which is normally dry.

Foehn effect

The warming effect of air flowing down the leeward side of a mountain range. The heating and drying of the air is due to adiabatic compression of the air as it flows down the slope.

As air rises up the western side of the Great Dividing Range it cools at the dry adiabatic lapse rate (DALR). If there is sufficient moisture, then this mositure will condense and the air will then cool at the saturated adiabatic lapse rate (SALR) which is less than the DALR. As the air descends the eastern side of the Great Dividing Range, the air will warm at the DALR. If condensation does occur, then the air will warm faster as it descends than it cooled as it ascended. Thus the air temperature is greater on the lee side of the Range.

The Foehn effect is one of the reasons Sydney can reach very warm temperatures in summer (due to northwest winds from the Blue Mountains), and rarely gets extremely cold in winter (southwest winds off the Southern Tablelands).

Fog

A dense mass of small water droplets suspended in the air near the ground. Visibility is reduced to less than 1 kilometre. See also Radiation fog, Advection Fog, Upslope fog and Mist.

Forced convection

Motion forced by mechanical forces such as deflection or friction. A cold front will cause forced convection as warmer is forced vertically above advancing cooler air.

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.

Fractus

Latin - to break or fracture
Ragged, detached cloud fragments.

Free convection

Motion caused only by density differences in a fluid. See also Level of Free Convection

Freeze

The phase change of a liquid to solid.

Freezing rain

Rain that becomes supercooled and freezes on impact with the ground or with objects on the earth's surface. Freezing rain can cause great damage, due to the weight of ice accumulating on structures in extreme events. Freezing rain is not a common event in Australia.

Front

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. Fronts can be a region of localised lifting, leading to rainfall or thunderstorms.

Frontogenesis

The process where a front is either created or intensifying. Occurs when two adjacent air masses exhibiting different characteristics are brought together by prevailing winds.

Frost

Deposits of white ice crystals or frozen dew drops on objects on or near the ground. Formed when the surface temperature falls below freezing (0°).

Fujita Scale

(or F Scale) A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:

Funnel cloud

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.

Fusion

The phase from a solid to a liquid.