F

Fahrenheit – the standard scale used to measure temperature in the United States; in which the freezing point of water is 32 degrees and the boiling point is 212 degrees.

Fair – describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions.

Fall wind – a strong, cold, downslope wind.

Fine – no rain or other precipitation (hail, snow etc.). The use of fine is generally avoided in excessively cloudy, windy, foggy or dusty conditions. In particular note that fine means the absence of rain or other precipitation such as hail or snow - not 'good' or 'pleasant' weather.

Flash flood – a flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure or water released from an ice jam.

Flood – a condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Flood crest – the highest stage or flow occurring in a flood.

Flurry – a flurry or snow shower is a snowfall that suddenly stops and starts and changes rapidly in intensity; the accumulation and extent of the snow are limited.

Fog – water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet). In Canada, fog is defined as a cloud at ground level, and occurs when air is cooled to its dew point and below, or when atmospheric moisture increases through evaporation from water that is warmer than the air. In the United Kingdom, fog is described to occur when visibility less than one kilometer. In Australia, it is defined as a dense mass of small water droplets or particles in the lower atmosphere.

Fogbow – a rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

Forecast – a forecast provides a description of the most significant weather conditions expected during the current and following days. The exact content depends upon the intended user, such as the Public or Marine forecast audiences.

Freeze – occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time. In Australia, the term “freeze” is defined as the change from a liquid to a solid.

Freezing – the change in a substance from a liquid to a solid state.

Freezing drizzle – drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing fog – a suspension of numerous minute ice crystals in the air, or water droplets at temperatures below zero degrees Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Freezing level – the altitude in the atmosphere where the temperature drops to 32F.

Freezing rain – rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. In Canada, freezing rain is described to occur when the air temperature is below zero Celsius near the ground but above zero Celsius higher up.

Front – the boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

Frost – the formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid.

Frost point – when the temperature to which air must be cooled to in order to be saturated is below freezing.

Fujita scale – system developed by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes.

Fujiwhara effect – the Fujiwhara effect describes the rotation of two storms around each other.

Funnel cloud – a rotating, cone-shaped column of air extending downward from the base of a thunderstorm but not touching the ground. When it reaches the ground it is called a tornado.