FAHRENHEIT TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of +32°F and a boiling point of +212°F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.
FAIR
This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical location.
FATHOM
The common unit of depth in the ocean for countries using the English system of measurement. It is six feet or 1.83 meters. It can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile.
FEEDER BANDS
In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm.
FETCH
An area of the water surface over which waves are generated by a wind having a constant direction and speed. Also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. One of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture.
FEW
The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer.
FILLING
Used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The opposite of deepening.
FIREWHIRL
A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption.
FIRST GUST
Another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm.
FLANKING LINE
A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell.
FLASH FLOOD
A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.
FLOOD
High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.
FLOOD PLAIN
Level land that may be submerged by flood waters.
FLOOD STAGE
The level of a river or stream where overflow onto surrounding areas can occur.
FOEHN
A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Classified as a katabatic wind.
FOG
A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METAR.
FOG BANK
A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake, or along coastal areas.
FOGBOW
A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. An additional bow with reversed colors sometimes appears inside the first.
FORECAST
A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.
FRACTUS
The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been shred or torn. Never appears in cirrus clouds. Also known as scud.
FREEZING DRIZZLE
Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. It is reported as "FZDZ" in an observation and on the METAR.
FREEZING FOG
Used to describe the phenomena when fog is present and the air temperature is below 0°C. It is reported as "FZFG" in an observation and on the METAR.
FREEZING POINT/FREEZE
The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0°C or 32°F. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity.
FREEZING PRECIPITATION
Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces.
FREEZING RAIN
Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. It is reported as "FZRA" in an observation and on the METAR.
FRESH WATER
Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity.
FRICTION
In meteorology, it is the turbulent resistance of the earth on the atmosphere. Considered as the resistance of fluids (air and water) to the relative motion of a solid body. The amount is dependent on the size and shape of the body.
FRICTION LAYER
The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. Above this level, air tends to flow parallel to the isobars. Wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the underlying surface features.
FRONT
The transition zone or interface between two air masses of different densities, which usually means different temperatures. For example, the area of convergence between warm, moist air and cool, dry air.
FRONTAL PASSAGE
It is the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as "fropa."
FRONTOGENESIS
The birth or creation of a front. This occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. It could happen when either air mass, or both, move over a surface which strengthens their original properties. However, it occurs most often along the eastern coasts of North America and Asia, when the air mass moving out over the ocean has a weak or no distinct boundary. The opposite of frontolysis.
FRONTOLYSIS
The destruction or dying of a front where the transition zone is losing its contrasting properties. The opposite of frontogenesis.
FROST
The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.
FROZEN PRECIPITATION
Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.
FUJITA-PEARSON SCALE
A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most intense damage exhibited by a tornado. Developed by T. Theodore Fujita and Allen Pearson.
FUNNEL CLOUD
A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it. It is reported as "FC" in an observation and on the METAR.
FUSION
The change of state from a solid to a liquid at the same temperature. The heat of fusion is the number of gram calories of heat necessary to change one gram of a substance from the solid to the liquid state. It is the opposite of freezing.