Fabric - A three-dimensional representation of the shape and spatial orientation of particles (clasts) in a sediment. This measurement is useful for interpreting and understanding the environmental conditions associated with particle deposition.

Facet - (1) A flat face on a rock caused by wind, water, or ice abrasion. (2) A natural or cut and polished flat surface on a gemstone. (3) A flat surface on a crystal.

Facies - (1) A trait found in sediment or rock that provides information about the environmental conditions associated with its deposition. (2) A trait found in sediment or rock that can be used to distinguish it from other adjacent sediments or rocks.

Facilitation - A modification to a system that makes subsequent modifications easier to accomplish.

Facilitation Model of Succession - This model of plant succession suggests that changes in plant species dominance over time are driven by modifications in the abiotic environment imposed by the developing community. Thus, the entry and growth of the later species depend on earlier species preparing the ground.

Factor - In statistics, a factor is a causal agent that is responsible for some measured effect.

Factorial Experiment - An experiment that has been set up to determine how several factors influence a measured effect, individually and in combination, via a statistical technique like Analysis of Variance.

Fahrenheit Scale - Scale used in the measurement of temperature. In this scale, water boils at 212° and freezes at 32°. It is used in only a few countries, most notably the United States, where it is used for weather forecasting and other non-scientific purposes. Compare with Celsius and Kelvin temperature scales.

Failure - (1) A situation that causes an object or a structure to fail in some way. (2) The deformation or fracture of soil, sediment, or rock because of the addition of stress.

Fall - The season between summer and winter. Astronomically, it is the period from the September Equinox to the December Solstice in the Northern Hemisphere and from the March Equinox to the June Solstice in the Southern Hemisphere. 

Fall Line - A linear boundary in the surface landscape that separates relatively harder igneous or metamorphic rock from softer sedimentary rock. Fall lines are readily apparent when intersected by streams, as they form waterfalls or rapids. 

Falling Dune - A stationary accumulation of well-sorted sand found on the leeward side of a large vertical obstruction in the landscape, like a hill, butte, mesa, or mountain. Falling dunes can often form some distance downwind of the obstacle because of the existence of localized back eddies. For this dune to form, there must be a source of sand and consistent winds from mainly one direction. A large falling dune can, in some situations, act as a sand source for dune fields further downwind. Compare with a climbing dune.

Falling Tide - See ebb tide.

False Bedding - See cross-bedding.

False Color Film  - A type of film used in remote sensing that was created to image near infrared and visible light. 

False Origin - The location of the starting coordinates, picked to the south and west of the true origin of a rectangular coordinate system. False origins are used to avoid negative coordinates.

Falsification - Is a procedure used in science to test the validity of a hypothesis or theory. It involves explicitly stating a logical output from theory and looking for counterexamples in observations, experiments, or the predictive output of models.

Family - The third category in the classification of organisms. This classification level is above the genus category. A family consists of all the genera with similar morphological and physiological characteristics and related genetics. Similar families are grouped into a category called an order.

Fan - A cone-shaped accumulation of sediments once transported by a stream or a debris flow. Fans develop when the agents of transport have a significant reduction in their travel speed, leading to deposition. See alluvial fan.

Fanglomerate - A type of sedimentary rock formed from the cementation and compaction of particles associated with an ancient alluvial fan.

Far Infrared - (1) A form of electromagnetic radiation with a wavelength between 1.5 and 40 micrometers (µm). (2) A form of electromagnetic radiation with a wavelength between 15 and 1000 micrometers (µm). (3) Astronomers typically define it as electromagnetic radiation with a wavelength between 25-40 and 200-350 micrometers (µm).

Fast Ice - A type of sea ice that forms and grows attached to a coastline or along the edge of an iceberg. Fast ice requires calm wind and air temperatures falling below -2° Celsius (28° F) to develop. 

Fathogram - A profile chart of the ocean floor from echo sounder data.

Fathom - A nautical unit of water depth where one fathom = 6 feet = 1.829 meters. 100 fathoms = 1 cable and 1,000 fathoms = 1 nautical mile.

Fatigue Failure - The fracturing of some substance because of many repeated cycles of stress. This process is well understood and studied in metals by engineers. Their observations suggest that repeated stress fracturing of metals occurs at levels well below the material's instantaneous maximum strength. This process has been applied to rock disintegration through wetting and drying, insolation weathering, haloclasty, and freeze-thaw action.

Fault - A fracture in rock or rock strata caused by stress forces.

Fault Creep - The very slow and more or less continuous movement of opposing rock slabs on either side of a fault.

Faulting - The fracturing of rock or rock strata because of stress forces.

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.

Fauna - A general term used to describe animals.

Faunal Realms - An evolutionary grouping of the various animals (fauna) found on our planet based on a spatial scale at the continental level. This classification suggests the existence of eight faunal realms: Paleoarctic; Nearctic; Afrotropic; Neotropic; Australasian; Indomalaya; Oceania; and Antarctic. Also called ecozones or zoogeographic regions. The concept of faunal realms has recently been updated by a study (2013) that analyzed the geographical distributions and phylogenetic relationships of 21,037 species of amphibians, birds, and mammals. In this work, eleven zoogeographical realms were recognized. See http://www.sciencemag.org/content/339/6115/74.abstract

Feedback Loop - A process where the output of a system causes positive or negative changes to some measured component of the system. Also see positive feedback and negative feedback.

Feldspar - A group of common aluminum silicate minerals that contain potassium, sodium, or calcium.

Felsic - A rock that is quite rich in silica, sodium, aluminum, and potassium.

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

Fen - A low-lying, flooded habitat dominated with grasses, sedges, mosses, and sometimes shrub and tree species like willow (Salix spp.) and alder (Alnus spp.). Surface water and groundwater of fens tend to be nutrient-rich and neutral to alkaline in pH.  Fens often have accumulations of non-acidic peat. Compare with bog.

Fen Soil - An organic rich soil that forms above fen peat.

Feral Relief - A landscape that experiences intense runoff and, as a result, has deep dissection of valley sides by insequent streams.

Fermentation - The metabolic decomposition and breakdown of organic matter by anaerobic processes. This process essentially converts sugars into gases, acids, and/or alcohol to release energy for metabolism. Yeasts and bacteria are two organisms that employ this process.

Fern - A group of about 11,000 species of vascular seedless plants that belong to the division Pteridophyta. Ferns have stems, leaves, and roots like other vascular plants, but reproduce with spores instead of seeds. About 75 percent of the various species of ferns are found in the tropics. Some ferns grow on the branches of trees as epiphytes.

Ferrallitization - A pedogenic process in tropical soils that occurs under moist and warm conditions and involves chemical weathering and intense leaching. In this process, iron and aluminum oxides, silica, and soluble bases are released from the parent material through weathering. These products are then translocated down the soil profile due to leaching. 

Ferrel Cell - A three-dimensional atmospheric circulation cell located at roughly 30 to 60° North and South of the equator. The Ferrel Cell consists of descending air (Subtropical Highs) at 30° North and South and rising air at 60° North and South (polar front). On the Earth's surface, this global-scale circulation system generates the Westerlies. Compare with the Hadley Cell and the Polar Cell.

Ferricrete - A type of sedimentary rock that is formed by the chemical precipitation of iron.

Ferrisols - See ultisols.

Ferrous - Containing or composed of iron, often in a chemically reduced form.

Fertilizer - A substance that adds inorganic or organic nutrients to soil or water, resulting in the increased growth of algae, aquatic plants, crops, trees, or other vegetation.

Fetch - The distance of open water in one direction across a lake, sea, ocean or other water body over which wind can travel, and waves can develop. 

Fibric Layer - A near-surface layer in soil dominated by poorly decomposed fibrous organic matter. Compare with mesic and humic layers.

Field Capacity - The water remaining in a soil after the complete draining of gravitational water. This quantity varies from soil to soil due to factors such as texture, organic matter content, structure, and compaction.

Field Drainage - The artificial process of increasing soil drainage in an area by employing trenches and pipes to drain away excess water. This technique is used to increase the fertility of agricultural fields that are prone to waterlogging. 

Field Intensity - The measured strength of a magnetic or electric field at some spatial point.

Film Water - An extremely thin layer of water found surrounding soil particles. Film water is between 1 and 100 molecules thick and is not available for plant uptake.

Finger Lakes - Long, narrow lakes that occupy deep linear basins eroded by glaciers. Lakes of this type occur in several different locations around the world. 

Firn - Névé on a glacier that survives the year's ablation season. Over time, most firn is transformed into glacial ice.

Firn Field - An area where firn accumulates. This area does not need to be associated with a glacier.

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 - See firn limit.

First Law of Motion -  A motionless object will continue to be motionless, while an object that is in motion will stay moving with the same velocity, in the same direction, unless it is acted upon by another force. This law is sometimes called the Law of Inertia. First suggested by Sir Isaac Newton in 1687. See Second Law of Motion.

First Law of Thermodynamics - See Law of Conservation of Energy.

Fish - Are a group of vertebrate animals that inhabit aquatic habitats. Fish are abundant on our planet, with more than 32,000 recognized species. Fish can be found in nearly all aquatic environments, from streams and lakes on our planet's mountains to the bottom of the ocean floor. As a group of organisms, fish characteristically have gills, an internal skeletal structure composed of bone or cartilage, and lack limbs. Most fish are ectothermic, and as a result, their body temperature varies with ambient environmental temperatures. Fish consist of several classes, including: spiny rayed fishes (Actinopterygii); lampreys (Cephalaspidomorphi); sharks, skates, and rays (Elasmobranchii); chimaeras (Holocephali); hagfish (Myxini); and lobe-finned fishes (Sarcopterygii).

Fissility - Planes of weakness found in some sedimentary rocks and metamorphic rocks that cause them to split into thin, flat layers. Fissility is common in rocks formed from argillaceous sediments such as mudstones, shales, and slate.

Fissionable Isotope - An isotope that can undergo nuclear fission when hit by a neutron at the right speed. Examples include uranium-235 and plutonium-239.

Fissure - A geologic term used to describe an opening or crack in the Earth's crust.

Fissure Eruption - A volcanic eruption that occurs at a surface fissure or fault, releasing ash, gases, lava, and pyroclastic material. Often associated with very fluid basaltic magma. Common in Iceland and the mid-oceanic ridge areas.

Fitness - A measure of the health of a species relative to physiology and future reproductive success.

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

Fjord - (Fiord) - A steep and long, U-shaped valley that connects to a sea or ocean and contains a mixture of freshwater and seawater. Most fjords are in relict glacial valleys or glacial troughs that formed during past episodes of glacial erosion. The coasts of Canada, Alaska, Norway, Iceland, Russia, and Greenland have many fjords..

Flagstone - A type of limestone that has a high degree of fissility. They are commonly used as a building material.

Flash Flood - A rapid and short-lived increase in the amount of runoff water entering a stream, resulting in a flood. The National Weather Service in the United States defines a flash flood as a flood event that occurs within 6 hours of rainfall. Flash floods are often caused by heavy rain from a severe thunderstorm, tropical storm, or hurricane. Compare with a flood.

F-Layer - A layer found in the ionosphere located at a height above 250 kilometers (155 miles) from the Earth's surface. This atmospheric layer can reflect high-frequency radio transmissions from the ground back to the Earth's surface.

Flint - A microcrystalline silica-rich (quartz) sedimentary rock that is chemically similar to chert. Occurs typically in masses embedded in limestone and chalk.

Float Recorder - A mechanical instrument employing a float, used to make a constant recording of the water level in a well, stream, or lake.

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

Flood - The inundation of a land surface not usually submerged by water from a quick change in the water level of a lake, stream, or ocean. Floods last from days to weeks and are often the result of multiple factors. Compare with a flash flood.

Flood Basalt - See plateau basalt.

Flood Current - The flow of seawater from a tidal current that is moving from the sea to the shore.  When its flow finishes, a high tide will occur. Compare with an ebb current.

Flood Frequency - A probabilistic analysis of past records of flow for a stream to determine the frequency of having a stream discharge over a particular threshold value. This calculation is expressed as a ratio or a percentage and is often used to predict the likelihood of a flood.

Flood Geomorphology - A sub-discipline of Geomorphology that studies the causes of floods and how these events influence landscape processes and morphology.

Flood Tide - The time during a tidal period when the tide is on the rise. Compare with an ebb tide.

Floodplain - The relatively flat boundary area found alongside the stream channel that is prone to flooding and receives frequent deposits of alluvium from these inundation events.

Flora - Refers to plants or a collection of plants.

Floristic Realms - An evolutionary grouping of the various plants (flora) found on our planet based on a continental-level spatial scale. This classification suggests the existence of eight floristic realms: Paleoarctic; Nearctic; Afrotropic; Neotropic; Australasian; Indomalaya; Oceania; and Antarctic. Plant species in these realms generally share some common morphological and physiological characteristics. Also called an ecozone.

Flow Regimes - This refers to the classification of water flow in an open channel into four types associated with channel geometry. The classification is based on calculated Reynolds and Froude numbers. A Reynolds number that is greater than 2000 suggests turbulent flow, while laminar flow occurs when the value is less than 500. A Froude number that is greater than 1.0 indicates supercritical flow, while subcritical flow occurs when the value is less than 1.0. Based on these calculations, the four flow types are laminar/supercritical, laminar/subcritical, turbulent/supercritical, and turbulent/subcritical.

Flow Till - A type of glacial till deposited from the flow of supraglacial sediments at the ice front.

Flower - A specialized reproductive structure found in some plants, like the angiosperms. Flowers consist of two parts: modified vegetative tissues, often used to attract potential pollinators, and reproductive tissues.

Flowstone - Flat sheetlike deposits of calcite and other carbonate minerals are commonly found on the walls and floors of karst caves. Occur where mineral-rich water regularly flows.

Fluid - A substance, gas, liquid, or plasma that has the property of movement.

Fluid Drag - The reduction in flow velocity of a fluid caused by the frictional effects of a surface.

Fluid Mechanics - A field of physics that studies fluids and the forces that act upon them.

Flume - (1) A human-constructed channel used to measure the water flow of a much larger stream channel. (2) A human-constructed channel used to move water from a stream channel to some intended use. Some uses include simple water transfer to a storage reservoir, irrigation, moving logs, and generating power. 

Flute - Cone-shaped grooves found on the floor of a stream channel created by the scouring action of turbulent flow.

Fluted Moraine -  A type of linear ground moraine that is oriented parallel to the direction of glacial ice movement.

Fluvial - Something that involves running water. The term is normally used in association with stream processes.

Fluviokarst - A karst landscape that is strongly influenced by fluvial processes.

Fluvial Landform - Is a landform formed from erosional and depositional processes associated with streams. This includes landforms with the following geomorphic features: alluvial fans, deltas, terraces, stream valleys, floodplains, levees, and oxbow lakes.

Flux - The rate of input or output of energy and/or matter from some object.

Flysch - The deposition of sedimentary strata in a marine environment found in a basin adjacent to a developing orogeny. Such deposits often show numerous repeated sedimentary cycles, with strata becoming coarser with depth. First deposits tend to be conglomerates or breccias, followed by sandstones, and finally mudstones and shales.

Focal Depth - The minimal distance between an earthquake focus and the Earth's ground surface.

Focus - See earthquake focus. 

Fog - An atmospheric condition near the Earth's surface where minute water droplets, ice crystals, or smoke particles reduce visibility.  Fog exists if the visibility near the Earth's surface is reduced to 1 kilometer (0.62 miles) or less. Fogs composed primarily of water droplets are classified according to the process that cools the air to saturation. Common types of fog include radiation fog, upslope fog, advection fog, evaporation fog, ice fog, and frontal fog. Compare with mist.

Fog Drip / Fog Precipitation - The production of falling precipitation by the interception of moving fog by trees and other surfaces.

Föhn Wind - A regional wind system that occurs on the leeward side of mountains. This wind is warm, has low humidity, and tends to form in winter. The European equivalent of a Chinook wind. 

Fold - Wavelike layers in rock strata that are the result of large-scale compression of the Earth's crust. Also see anticline, syncline, monocline, overturned fold, and recumbent fold.

Folding - The deformation of rock strata because of compressive forces that form folds.

Foliar Leaching - A process in which water from precipitation removes plant nutrients from the surface of leaves.

Foliation - A process where once randomly distributed platy minerals in a rock become reoriented, because of metamorphism, in an aligned manner.

Food Chain - The 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 intricate patterns of organic energy flow in an ecosystem by modeling who consumes whom.

Foothills - A line of hills that runs parallel to a much higher mountain range.

Foot Wall - The bottommost surface of an inclined fault.

Foraminifera - Microscopic organisms that belong to the group protozoa and are found living mainly in marine environments. These organisms produce shells rich in calcium carbonate. Sedimentation and lithification of these shells produce the sedimentary rock chalk. Scientists have used these organisms in several different ways to describe changes in their surrounding environment.

Force - A process that can alter the state of rest or motion of a body.

Force of Acceleration - A force that increases the speed of a moving body.

Forecast - A prediction into the future based on available information and an understanding of the cause-and-effect processes operating in a system of interest. For example, in meteorology, we can make forecasts using current weather data and computer climate models to numerically predict future weather.

Foredeep - A structural basin found on the ocean floor that is adjacent to or in front of an island arc or coastal mountain range.

Foredune - (1) A sand dune that develops windward of an obstruction. (2) The sand dunes found in a coastal dune field that are closest to a sea or ocean.

Foreland - (1) An area of land adjacent to or in front of a particular landscape feature. (2) A structural basin that is adjacent to or in front of a mountain range.

Foreset Bed - A deltaic deposit of alluvial sediment that is angled 5 to 25° from horizontal. Most of the delta is composed of this type of deposit.

Foreshock - Small ground tremors that take place seconds to weeks before a significant earthquake event. 

Foreshore - A zone found below, adjacent to, and parallel to a beach that extends from the low water spring tide level to the high water spring tide level.

Forest - An ecosystem dominated by trees. Major forest biomes include tropical broadleaf forest, tropical deciduous forest, tropical savanna, deciduous forest, and boreal forest.

Forest Hydrology - A field of hydrology that studies specifically the transfer and storage of water in forest environments.

Forestry - The field of knowledge that studies forests. This is done from the perspective of several disciplines, including biology, botany, genetics, management, engineering, and agronomy.

Form Ratio - A calculation made for a stream that finds the ratio between channel width and channel depth.

Fossil - The preserved remains of an organism that lived sometime in the past.

Fossil Fuel - Carbon-based remains of organic matter that have been geologically transformed into coal, oil, and natural gas. Combustion of these substances releases significant amounts of energy. Currently, humans use fossil fuels to meet much of their energy needs. However, the use of fossil fuels is creating a number of environmental problems, including acid deposition, air and water pollution, and climate change. 

Fossil Record - The evolutionary history of life as chronicled by fossils commonly found in sedimentary rocks. 

Fossil Water - The long-term trapping and storage of water in some below-ground space, like groundwater in an aquifer, for more than a thousand years.

Fourier Analysis - A mathematical procedure that is used to analyze temporal or spatial data that exhibits patterns that are cyclical and wave-like. The end result of this analysis is the mathematical expression of this pattern using trigonometric functions.

Fracking - See hydrofracturing.

Fractal - Something that contains a detailed pattern that repeats itself (self-similarity) at any scale of space or time. In 1975, mathematics professor Benoit B. Mandelbrot coined the term fractal to describe these phenomena. 

Fracture - The breaking or failure of a solid object into two or more fragments.

Fracture Zone - A linear area composed of numerous side-by-side transform faults found along a mid-oceanic ridge.  

Fractus Cloud - Relatively small cloud fragments that are usually associated with much larger clouds. They form when winds break off pieces from a much larger cloud. The edge of fractus clouds often appears jagged.

Fragipan - Is a subsurface layer in a soil that reduces the downward flow of water and restricts root penetration. Fragipan layers are brittle and dense because of interparticle cementation by clay, silica, iron, aluminum, and organic matter.  

Frazil Ice - A collection of randomly oriented fine ice slivers that are suspended in water. Commonly occurs with the freezing of seawater.

Free Face - (1) Rock outcrop surface that is too steep for loose sediment and rocks to accumulate on it. (2) Part of a slope profile that is most vertical. 

Freeze-Thaw Action - (1) The repeated fluctuation of temperature above and below 0° Celsius over a period. (2) A physical weather process associated with daily and seasonal cycles of freezing and melting of water. This process can cause disintegration when water seeps into fractures on the surface of a rock or mineral. With freezing, water expands approximately 9% in volume, and this change exerts a significant rupturing effect. 

Freezing - The change in the state of matter from liquid to solid that occurs with cooling. The term freezing is used in meteorology when discussing the formation of ice from liquid water.

Freezing Drizzle - A type of precipitation. Freezing rain occurs when liquid drizzle hits a cold surface and then immediately freezes into ice. For this to occur, a near-surface atmospheric temperature inversion is 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.

Freezing Front - A subsurface zone found in high and middle-latitude soils that separates frozen and unfrozen soil. Typically, the freezing front first forms at the ground surface and then moves downward as temperatures drop seasonally. In soils with permafrost, the freezing front moves downward from the ground surface and upward from the permafrost below ground as temperatures cool. 

Freezing Rain - A type of precipitation. Freezing rain occurs when rain hits a cold surface and then immediately freezes into ice. For this to occur, a near-surface atmospheric temperature inversion is 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.

Freons - See chlorofluorocarbons.

Freshet - A sudden flood of water in a stream because of heavy rain and/or snowmelt. Streams located in the mid-latitudes often have a freshet in the spring.

Freshwater - Water that is relatively free of salts.

Friable - The ability of soil to crumble between one's fingers when it is wet or dry.

Friction - The resistance that occurs between the contact surfaces of two bodies in motion.

Friction Layer - The layer in the lower atmosphere where surface friction has aerodynamic effects, causing the vertical mixing of air. This layer usually extends from the Earth's surface to about 1,000 meters (3,300 feet). However, the thickness of the friction layer varies significantly with surface roughness.

Frictional Force - Force acting on wind near the Earth's surface because of frictional roughness. This force reduces wind speed.

Fringing Reef - A coral reef that is attached to an adjacent shore. 

Front - A transition zone found between air masses with different air densities and weather characteristics.

Front Slope - The slope on the side of a cuesta that is steep, forming an escarpment. Compare with a back slope. 

Frontal Cyclone - Another name for a mid-latitude cyclone.

Frontal Fog - 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 - The lifting of a warmer or less dense air mass by a colder or more dense air mass at a frontal transitional zone. This process causes the water vapor in the warmer air to cool and then condense or freeze, forming clouds and precipitation.

Frontal Precipitation - See convergence precipitation.

Frontal Zone - A transition area that exists between two air masses with different air temperature and/or humidity characteristics. Differences in air temperature and/or humidity cause the lower-density air mass to be pushed over the denser one. This process, known as frontal lifting, can lead to the development of clouds and precipitation.

Frontogenesis - The intensification of the thermal gradient located at a frontal zone because of cyclonic airflow, convergence, jet stream dynamics, and frontal lifting. Frontogenesis is caused when two air masses with different weather characteristics come together. Frontogenesis can create either a warm front or a cold front.

Frost - The deposition of ice crystals on the surface of an object because of atmospheric cooling. Frost occurs when moist air is cooled to its frost point.

Frost Action - A physical weathering process where cycles of freezing and thawing of water in the surface pores, cracks, and other openings of rocks cause them to break apart into smaller pieces. Frost action produces an angular appearance on the rock's surface.

Frost Crack - A soil fissure produced by a wedge of ice.

Frost Creep - The slow mass movement of soil downslope initiated by freeze-thaw action. Frost creep occurs when the stresses on the slope material are too small to cause rapid failure.

Frost Heave - The mainly upward movement of soil at and near the ground surface because of the freezing of subsurface water. Water expands by 9% in volume when frozen. Frost heave is common where the ground becomes seasonally frozen and where a below-ground permafrost layer exists. Further, the first ice crystals that are produced can grow into an extensive ice lense up to 30 cm (1 foot) thick if supplied more water via capillary action. 

Frost Point - The temperature at which water vapor saturates in an air mass into solid, usually forming snow or frost. The frost point occurs normally at a temperature at 0°C or less and when a mass of air has a relative humidity of 100%.

Frost Hollow - A topographic depression that is susceptible to the development of frost. The development of frost occurs because these depressions receive flows of dense cold air from nighttime radiative cooling.

Frost Smoke - A form of evaporation fog that is composed mainly of minute ice crystals suspended near the ground surface. Also called Arctic smoke.

Frost Weathering - See frost action.

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

Frozen Ground - Soil, sediments, or rock occurring in air temperatures that seasonally or permanently freeze the pore space water. Permanently frozen ground is also called permafrost.

Fujita Tornado Intensity Scale - Tornado classification system developed by T. Theodore Fujita. This system has 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.

Fulgurite - A subsurface feature produced by lightning that resembles a hollow glass tube. Often found in sandy soils, these features form when the high temperature (+ 1,800°C / 3,270°F) associated with a lightning strike melts silica, causing sand grains to melt and fuse. Fulgurite tubes can be many meters (feet) long and have a diameter of several centimeters (one inch).

Fulje - The hollowed-out area found in between barchan sand dunes or barchaniod sand ridges.

Fumarole - A small opening in the Earth's crust that emits steam and other hot gases into the atmosphere. Fumaroles are often associated with volcanoes.

Fundamental Niche - Describes the entire range of environmental conditions that are suitable for the existence of a species, ignoring the effects of interspecific competition and predation from other species.

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 non-cellulose polysaccharides. See the  Encyclopedia of Life for more information on this group of organisms. https://eol.org/pages/5559

Fungicide - A chemical substance or organism that causes reduced growth and/or reproduction or the death of fungi and fungal spores.

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