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Fahrenheit - 

Scale for measuring temperature. In this scale, water boils at 212° and freezes at 32°.

Fault - 

A fracture in rock caused by stress.

Fault Plane - 

The plane that represents the fracture surface of a fault.

Fault Scarp - 

The section of the fault plane exposed in a fault. Also called an escarpment.

Feedback Loop - 

Process where the output of a system causes positive or negative changes to some measured component of the system.

Feldspar - 

A group of common aluminum silicate minerals that contains potassium, sodium, or calcium.

Felsic Magma - 

Magma that is relatively rich in silica, sodium, aluminum, and potassium. This type of magma solidifies to form rocks relatively rich in silica, sodium, aluminum, and potassium.

Fen - 

A habitat composed of woodland and swamp.

Fermentation - 

Decomposition and breakdown of organic matter by anaerobic means.

Ferrel Cell - 

Three-dimensional atmospheric circulation cell located at roughly 30 to 60° North and South of the equator.

Ferricretes - 

Sedimentary rock created by the chemical precipitation of iron.

Fertilizer - 

Substance that adds inorganic or organic nutrients to soil for the purpose of increasing the growth of crops, trees, or other vegetation.

Fetch -  The distance of open water in one direction across a body of water over which wind can blow.
Field Capacity - 

The water remaining in a soil after the complete draining of the soil's gravitational water.

Firn - 

Névé on a glacier that survives the year's ablation season. With time much of the firn is transformed into glacial ice.

Firn Limit - 

The lower boundary of the zone of accumulation on a glacier where snow accumulates on an annual basis. Also called the Firn Line.

Firn Line - 

The lower boundary of the zone of accumulation on a glacier where snow accumulates on an annual basis. Also called the Firn Limit.

Fish - 

Group of vertebrate animals that inhabit aquatic habitats.

Fissure - 

Opening or crack in the Earth's crust.

Fitness - 

A measure of the health of a species in terms of physiology and future reproductive success.

Fixed Energy - 

A process, like photosynthesis, where organisms repackage inorganic energy into organic energy.

Fjord - 

A glacial valley or glacial trough found along the coast that is now filled with a mixture of fresh water and seawater.

Flash Flood - 

A rapid and short-lived increase in the amount of runoff water entering a stream resulting in a flood.

Flocculation - 

Chemical processes where salt causes the aggregation of minute clay particles into larger masses that are too heavy to remain suspended water.

Flood - 

Inundation of a land surface that is not normally submerged by water from quick change in the level of a water body like a lake, stream, or ocean.

Flood Tide - 

Time during the tidal period when the tide is rising. Compare with ebb tide.

Floodplain - 

Relatively flat area found alongside the stream channel that is prone to flooding and receives alluvium deposits from these inundation events.

Fluid - 

Substance, gas or liquid, that has the property of flow.

Fluid Drag - 

Reduction in the flow velocity of a fluid by the frictional effects of a surface.

Fluvial - 

Involving running water. Usually pertaining to stream processes.

Fog - 

Fog exists if the atmospheric visibility near the Earth's surface is reduced to 1 kilometer or less. Fog can be composed of water droplets, ice crystals or smoke particles. Fogs composed primarily of water droplets are classified according to the process that causes the air to cool to saturation. Common types of this type of fog include: radiation fog; upslope fog; advection fog; evaporation fog; ice fog; and frontal fog.

Föhn Wind - 

A warm dry wind coming off the lee slopes of a mountain range, especially off the northern slopes of the Alps.  European equivalent of Chinook wind.

Fold - 

Wavelike layers in rock strata that are the result of compression.

Folding - 

The deformation of rock layers because of compressive forces to form folds.

Foliation - 

Process where once randomly distributed platy minerals in a rock become reoriented, because of metamorphism, in a parallel manner.

Food Chain - 

Movement of energy through the trophic levels of organisms. In most ecosystems, this process begins with photosynthetic autotrophs (plants) and ends with carnivores and detritivores.

Food Web - 

A model describing the organisms found in a food chain. Food webs describe the complex patterns of energy flow in an ecosystem by modeling who consumes who.

Foot Wall - 

The bottom-most surface of an inclined fault.

Force - 

Process that changes the state of rest or motion of a body.

Foreset Bed - 

Deltaic deposit of alluvial sediment that is angled 5 to 25° from horizontal. Most of the delta is made up of these deposits.

Foreshock - 

Small earth tremors that occur seconds to weeks before a significant earthquake event.

Forest - 

Ecosystem dominated by trees. Major forest biomes include tropical evergreen forest, tropical savanna, deciduous forest, and boreal forest.

Fossil - 

Geologically preserved remains of an organism that lived in the past.

Fossil Fuel - 

Carbon based remains of organic matter that has been geologically transformed into coal, oil and natural gas. Combustion of these substances releases large amounts of energy. Currently, humans are using fossil fuels to supply much of their energy needs.

Freeze Thaw - 

Processes associated with daily and seasonal cycles of freezing and melting.

Freezing - 

The change in state of matter from liquid to solid that occurs with cooling. Usually used in meteorology when discussing the formation of ice from liquid water.

Freezing Rain - 

A type of precipitation. Occurs when liquid rain hits a cold surface and then immediately freezes into ice. For this to occur, a surface temperature inversion is usually required. In such an inversion, the surface must have a temperature below freezing, while the temperature of the atmosphere where the precipitation forms is above freezing.

Fresh Water - 

Water that is relatively free of salts.

Friction - 

Resistance between the contact surfaces of two bodies in motion.

Frictional Force - 

Force acting on wind near the Earth's surface due to frictional roughness. Causes the deceleration of wind.

Front - 

Transition zone between air masses with different weather characteristics.

Frontal Fog - 

Is a type of fog that is associated with weather fronts, particularly warm fronts. This type of fog develops when frontal precipitation falling into the colder air ahead of the warm front causes the air to become saturated through evaporation.

Frontal Lifting - 

Lifting of a warmer or less dense air mass by a colder or more dense air mass at a frontal transitional zone.

Frontal Precipitation - 

The formation of precipitation due to the convergence of two air masses. In most cases, the two air masses have different climatological characteristics. One is usually warm and moist, while the other is cold and dry. The leading edge of the latter air mass acts as an inclined wall or front causing the moist warm air to be lifted. Of course the lifting causes the warm moist air mass to cool due to expansion resulting in saturation. This precipitation type is common at the mid-latitudes where cyclones form along the polar front. Also see convergence precipitation.

Frost - 

Deposition of ice at the Earth's surface because of atmospheric cooling.

Frost Creep - 

Slow mass movement of soil downslope that is initiated by freeze-thaw action. Occurs where the stresses on the slope material are too small to create a rapid failure.

Frost Point - 

Is the temperature at which water vapor saturates from an air mass into solid usually forming snow or frost. Frost point normally occurs when a mass of air has a relative humidity of 100%.

Frost Shattering - 

A process of physical weathering in which water freezes in a crack and exerts force on the rock causing further rupture.

Fujita Tornado Intensity Scale - 

Tornado classification system developed by T. Theodore Fujita. This system six levels from F0 to F5. These levels are based on the estimated speed of the tornado's winds from proxy information like property damage.

Fungi - 

Group, at the kingdom level, in the classification of life. Multicellular organisms that have a eukaryotic cell type, mitochondria, and a cell wall composed of chitin and other noncellulose polysaccharides.

Funnel Cloud - 

A tornado which is beginning its descent from the base of a cumulonimbus cloud. This severe weather event may or may not reach the ground surface.