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F

FAA
Federal Aviation Administration.

Fahrenheit temperature scale
A temperature scale on which the freezing point of water equals 32° and the boiling point equals 212° at standard atmospheric pressure (760 mm, Hg).

Feed
The source of illumination for an antenna reflector. Also called antenna feed.

Fiducial point
A point (or line) on a scale used for reference or comparison purposes. In calibration of meteorological thermometers, for example, the fiducial points are 100°C (212°F) and 0°C (32°F), which correspond to the boiling point and ice point at standard pressure (760 min Hg).

Fiducial temperature
That temperature at which, in a specified latitude, the reading of a particular barometer requires no temperature or latitude correction.

Field elevation
The officially designated elevation of an airport above mean sea level, taken as the highest point on any of the runways of the airport. Same as airport elevation.

Filling
An increase in the central pressure of a pressure system; opposite of a deepening. More commonly applied to a low rather than a high.

Fire-danger meter
A graphical aid used in fire weather forecasting to calculate the degree of forest-fire danger (or burning index). Commonly in the form of a circular slide rule, the firedanger meter relates numerical indices of (a) the seasonal stage of foliage, (b) the cumulative effect of past precipitation or lack thereof, (c) the measured fuel moisture, and (d) the speed of the wind in the woods.

Fire weather
The state of the weather with respect to its effect upon the kindling and spreading of forest fires.

Firmware
Programs or instructions which are stored in readonly memory.

Firn
Old snow that has become granular and compacted as a result of melting and refreezing.

Fixed-beam ceilometer
See ceilometer.

Flight forecast
An aviation weather forecast for a specific flight.

Float barograph
A type of recording siphon barometer. The mechanically magnified motion of a float resting on the lower mercury surface is used to record atmospheric pressure on a rotating drum.

Floating pan
An evaporation pan in which the evaporation is measured from water in a pan floating in a larger body of water.

Float-type rain gauge
A class of rain gauge in which the level of the collected rain water is measured by the position of a float resting on the surface of the water.

Flood
Overflowing by water of the normal confines of a stream or other body of water, or accumulation of water by drainage over areas which are not normally submerged.

Flood stage
That stage, on a fixed river gauge, at which overflow of the natural banks of the stream begins to cause damage in any portion of the reach for which the gauge is used as an index.

Flyoff
The total amount of water transferred to the atmosphere by evapotranspiration.

Foehn
A warm, dry wind on the lee side of a mountain range, the warmth and dryness due to adiabatic compression upon descent.

Fog
A hydrometeor consisting of a visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Fog differs from cloud only in that the base of fog is at the earth's surface while clouds are above the surface.

Fogbow
A faintly-colored circular arc similar to a rainbow but formed on fog layers containing drops whose diameters are 100 microns or less. Also called mistbow, white rainbow.

Fohn
See foehn.

Foot-candle
A unit of illuminance or illumination equal to one lumen per foot'. This is the illuminance provided by a light source of one candle at a distance of one foot.

Footlambert
A unit of luminance or brightness equal to 1/d candles per foot'.

Foot-pound
A unit of energy equal to 1.356joules.

Forel scale
A scale of yellows, greens, and blues for recording the color of sea water, as seen against the white background of a Secchi disk.

Fortin barometer
A type of cistern barometer in which the level of mercury in the cistern is adjusted to the zero point of the scale before each reading.

Free balloon
A buoyant balloon rising freely in the atmosphere, as opposed to a captive balloon.

Free lift
The actual lifting force of an inflated balloon, usually expressed in grams.

Freezing level
Lowest altitude in the atmosphere over a given location at which the air temperature is 0°C.

Freezing nucleus
Particle on which the freezing of water occurs.

Freezing point
Temperarure at which a liquid solidifies under any given set of conditions. It may or may not be the same as the melting point or the more rigidly defined true freezing point or (for water) ice point.

Fresh breeze
Wind with a speed between 17 and 21 knots (19 and 24 mph); Beaufort scale number 5.

Fresh gale
Wind with a speed between 34 and 40 knots (39 and 46 mph); Beaufort scale number 8.

Frost
Ice crystal deposits formed by sublimation (conversion of water vapor directly to ice) when temperature and dew point are below freezing.

Frequency modulation
A type of modulation in which the frequency of a continuous radio carrier wave is varied in accordance with the properties of a second (modulating) wave.

Frost-point hygrometer
An instrument for measuring the frost point of the atmosphere.

FSK
Frequency Shift Keying. A form of frequency modulation of a data signal performed by a modem for transmission over dedicated wire or phone lines.

F.S. output
The transducer's output when the maximum sensed value is applied to the transducer's input. For example, the F.S. output of a 4-20 mA transmitter is 20 mA, whereas its span is only 16mA.

Fuel-moisture
Determined by weighing a special type of wooden stick that has been exposed in the woods, its weight being proportional to its contained water.

Fujita tornado scale
Based upon damage patterns, classifies twisters into six categories of wind speed (F0 thru F5), ranging from 40 to 318 mph estimated wind speed.

Full duplex
Operation mode of a communication circuit in which each end can simultaneously transmit and receive.