F Scale - see Fujita scale

Fahrenheit Scale - a temperature scale on which the boiling point of water is at 212 degrees above the zero of the scale and the freezing point of water is at 32 degrees above zero

Far Infrared - the region of the electromagnetic spectrum with wavelengths greater than infrared but shorter than microwaves; radiation between about 4 microns and 0.1 millimeters

Ferrel Cell - in the general circulation of the atmosphere, the name given to the middle latitude cell marked by sinking motion near 30 degrees and rising motion near 60 degrees latitude

Fine Line - see thin line

First Law of Thermodynamics - the law of physics that states that the heat absorbed by a system either raises the internal energy of the system or does work on the environment

Flanking Line - a line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm.

Flash Flood - a local flood of great volume and short duration generally resulting from heavy rainfall in the immediate vicinity

Flash Flood Guidance - forecast guidance specific to flash flooding

Flood - the condition that occurs when water rises and overflows the natural or artificial confines of a body of water on to normally dry land, or accumulates in low-lying areas

Fluid - matter which flows; a gas or a liquid

Flux - the amount of flow per unit time (i.e. energy flux or radiation flux)

Fog - a cloud with its base in direct contract with the ground

Force - mass times acceleration; the ability to change an object's state of rest or motion

Forecast - a statement predicting an event will occur

Forecast Guidance - computer-generated forecast materials used to assist the preparation of a forecast

Forward Flank Downdraft - the main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation occurs

Freeze (verb) - to solidify as a result of removal of heat

Freeze (noun) - a condition which occurs over a widespread area when the surface air temperature remains below freezing long enough to damage certain agricultural crops

Freezing Drizzle - drizzle that falls as liquid water but freezes upon impact to form a coating of ice upon the ground and on exposed objects

Freezing Rain - rain that falls as liquid water but freezes upon impact to form a coating of ice upon the ground and on exposed objects

Frequency - the rate at which an event will reoccur; the reciprocal of the period. In radiation, the number of wave crests that pass by a point each second, described in hertz.

Friction - the mechanical resistive force of one object on another object's relative movement when in contact with the first object

Friction Layer - same as planetary boundary layer

Front - the boundary or transition zone between two dissimilar air masses

Frontal Inversion - a temperature inversion in the atmosphere resulting from the vertical boundary between two air masses along a sloping front

Frontal Passage - the movement of a front across an area; characterized by changes in temperature, and, generally in pressure, wind direction and speed, visibility, cloudiness and precipitation

Frost - the condition which exists ice is formed on the earth's surface when the temperature falls below freezing

Frostbite - the freezing or local effect of partial freezing of some part of the body

Fujita Scale - a scale for classifying tornadoes according to the damage they cause; the tornado's rotational wind speed is inferred from an analysis of the wind damage

Funnel Cloud - a rotating cloud column or inverted cloud cone extending downward from a cloud base that is not in contact with the ground