Glossary

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A Horizon -  Soil horizon normally found below the O horizon and above the B horizon. This layer is characterized by the following two features: (1) A layer in which humus and other organic materials are mixed with mineral particles. (2) A zone of translocation from which eluviation has removed finer particles and soluble substances.
Abiotic - 

Non-living thing. Usually refers to the physical and chemical components of an organism's environment. Also called inorganic.

Ablation - 

Surface removal of ice or snow from a glacier or snowfield by melting, sublimation, and/or calving.

Ablation Zone - 

Region in a glacier where there is a surface net removal of snow and/or ice by melting, sublimation, and/or calving.

Abrasion - 

Physical wearing and grinding of a surface through friction and impact by material carried in air, water, or ice.

Absolute Humidity - 

Measurement of atmospheric humidity. Absolute humidity is the mass of water vapor in a given volume of air (this measurement is not influenced by the mass of the air). Normally expressed in grams of water vapor per cubic meter of atmosphere at a specific temperature.

Absorption - 

Atmospheric absorption is defined as a process in which solar radiation is retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. In general, the absorption of solar radiation by substances in the Earth's atmosphere results in temperatures that get no higher than 1800° Celsius. According to Wien's Law, bodies with temperatures at this level or lower would emit their radiation in the longwave band.

Abstract Space - 

Geographic model or representation of the real world. For example, maps and globes are abstractions of the real world or concrete space.

Abyssal Fan - 

Fan shaped accumulation of sediment from rivers that is deposited at the base of a submarine canyon within a ocean basin.

Abyssal Plain - 

Another name for ocean floor.

Acceleration - 

Force resulting in the speed of a moving body to increase.

Accretion - 

The growth of the continental masses over geologic time via the addition of marine sediments. These sediments are added on to the edges of the continents through tectonic collision with other oceanic or continental plates.

Acid -  (1) Substance having a pH less than 7. (2) Substance that releases hydrogen ions (H+).
Acid Deposition - 

Atmospheric deposition of acids in solid or liquid form on the Earth's surface. Also see acid precipitation

Acid Precipitation - 

Atmospheric precipitation with a pH less than 5.6. Normal pH of precipitation is 5.6.

Acid Rain - 

Rain with a pH less than 5.6. Normal pH of precipitation is 5.6.

Acidic - 

Any substance with a pH below 7.

Acidic Solution - 

Any water solution that is acidic (pH less than 7) or has more hydrogen ions (H+) than hydroxide ions (OH-). Also see basic solution and neutral solution.

Active Layer - 

Upper zone of soil in higher latitude locations that experiences daily and seasonal freeze-thaw cycles.

Active Remote Sensing - 

Form of remote sensing where the sensor provides its own source of electromagnetic radiation to illuminate the object understudy. Radar is an example of an active remote sensing device.

Actual Evapotranspiration - 

Is the amount of water that is actually removed from a surface due to the processes of evaporation and transpiration.

Adiabatic - 

A process in which heat does not enter or leave a system. In the atmospheric sciences, adiabatic processes are often used to model internal energy changes in rising and descending parcels of air in the atmosphere. When a parcel of air rises it expands because of a reduction in pressure. If no other non-adiabatic processes occur (like condensation, evaporation and radiation), expansion causes the parcel of air to cool at a set rate of 0.98° Celsius per 100 meters. The opposite occurs when a parcel of air descends in the atmosphere. The air in a descending parcel becomes compressed. Compression causes the temperature within the parcel to increase at a rate of 0.98° Celsius per 100 meters.

Adiabatic Cooling - 

The cooling of a rising parcel of air due to adiabatic processes.

Advection - 

Advection involves the transfer of heat energy by means of horizontal mass motions through a medium.

Advection Fog - 

Fog generated when winds flow over a surface with a different temperature. Two types of advection fog exist. When warm air flows over a cold surface it can produce fog through contact cooling. Cold air blowing over a warm moist surface produces a form of advection fog know as evaporation fog.

Aeolian - 

Geomorphic process involving wind. Alternative spelling eolian.

Aeolian Landform - 

Is a landform formed from the erosion or deposition of weathered surface materials by wind. This includes landforms with some of the following geomorphic features: sand dunes, deflation hollows, and desert pavement. Alternative spelling eolian landform.

Aerial Photography - 

Form of remote sensing that captures images of objects using photographic cameras and film from platforms in the atmosphere.

Aerobic - 

(1) Presence of molecular oxygen.
(2) Occurring only in the presence of molecular oxygen.
(3) Growing in the presence of molecular oxygen.

Aftershock - 

Smaller earth tremors that occur seconds to weeks after a major earthquake event.

Aggradation - 

Readjustment of the stream profile where the stream channel is raised by the deposition of bed load.

Agronomy - 

Field of science that studies phenomena related to agriculture.

Air Mass - 

A body of air whose temperature and humidity characteristics remain relatively constant over a horizontal distance of hundreds to thousands of kilometers. Air masses develop their climatic characteristics by remaining stationary over a source region for a number of days. Air masses are classified according to their temperature and humidity characteristics.

Air Pollution - 

Toxification of the atmosphere through the addition of one or more harmful substances in the air. Substance must be in concentrations high enough to be hazardous to humans, other animals, vegetation, or materials. Also see primary pollutant and secondary pollutant.

Air Pressure - 

See atmospheric pressure.

Albedo - 

Is the reflectivity of a surface.

Earth Albedo:
Is the reflectivity of the Earth's atmosphere and surface combined. Measurements indicate that the average Earth albedo is approximately 30%.

Algae - 

A simple photosynthetic plant that usually lives in moist or aquatic environments. The bodies of algae can be unicellular or multicellular is design.

Alien Species - 

Species that is not naturally found in a region.

Alkaline - 

(1) Having a pH greater than 7.
(2) Substance that releases hydroxyl ions (OH-).

Alluvial Fan - 

Large fan shaped terrestrial deposit of alluvial sediment on which a braided stream flows over. Form as stream load is deposited because of a reduction in the velocity of stream flow.

Alluvial Terraces - 

Flat elevated benches composed of unconsolidated alluvium found either side of a stream channel. Formed when a stream down cuts into its floodplain.

Alluvium - 

Sediment that originates from a stream.

Alpine Glacier - 

Small glacier that occupies a U-shaped valley on a mountain. Also called a mountain glacier.

Alpine Permafrost - 

Form of permafrost that exists at high altitudes in mountainous environments.

Alpine Tundra - 

High altitude biome dominated by a few species of dwarf shrubs, a few grasses, sedges, lichens, and mosses. Productivity is low in this biome because of the extremes of climate. Quite similar to tundra.

Alternative Hypothesis (H1) - 

Is a hypothesis that has been suggested because it is believed to be false or because it is to be used as a starting point for scien 04/06/2010 12:19 ng to organize arguments.

Altitude - 

Vertical distance above sea-level.

Altocumulus Clouds - 

Middle altitude cloud that is colored from white to gray. This cloud is composed of a mixture of water droplets and ice crystals. It appears in the atmosphere as layers or patches that are well rounded and commonly wavelike. Found in an altitude range from 2,000 to 8,000 meters.

Altostratus Clouds - 

Gray-looking middle altitude cloud that is composed of water droplets and ice crystals. Appears in the atmosphere as dense sheet like layer. Can be recognized from stratus clouds by the fact that you can see the Sun through it. Found in an altitude range from 2,000 to 8,000 meters.

Anaerobic - 

(1) Absence of molecular oxygen.
(2) Occurring only in the absence of molecular oxygen.
(3) Growing in the absence of molecular oxygen.

Andesite - 

An extrusive igneous rock that develops from a magma that is chemically between felsic and mafic and whose mineral crystals are fine.

Anemometer - 

Mechanical instrument used to measure wind speed. These instruments commonly employee three methods to measure this phenomenon: 1) A device with three or four open cups attached to a rotating spinal. The speed of rotation is then converted into a measurement of wind speed; 2) A pressure plate that measures the force exerted by the moving wind at right angles; 3) An instrument consisting of a heated-wire where electrical resistance (temperature of the wire) is adjusted to account for heat lost by air flow. The faster the wind the greater the heat loss and thus the more energy that is required to keep the wire at a constant temperature. As a result, wind speed is measured through the drain of electrical current.

Aneroid Barometer - 

Barometer that measures atmospheric pressure via the expansion and contraction of a sealed hollow cell which is partially depleted of air.

Angle of Incidence - 

Angle at which the Sun's rays or insolation strike the Earth's surface. If the Sun is positioned directly over head or 90° from the horizon, the incoming insolation strikes the surface of the Earth at right angles and is most intense.

Angle of Repose - 

Measurement commonly used in civil engineering. It is the maximum angle at which a material can be inclined without failing. Geomorpologist use this measurement for determining the stability of slope to mass movements.

Antarctic Circle - 

Latitude of 66.5° South. The northern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year.

Antarctic High - 

A region of high pressure that occupies central Antarctic throughout the year. This pressure system is responsible for very cold temperatures and extremely low humidity.

Anticline - 

A fold in rock layers that forms an arch.

Anticyclone - 

An atmospheric pressure system consisting of an area of high pressure and outward circular surface wind flow. In the Northern Hemisphere winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise.

Aphelion - 

It is the point in the Earth's orbit when it is farthest from the Sun (152.5 million kilometers). Aphelion occurs on the 3rd or 4th of July.

Aquatic - 

With reference to water.

Aquiclude - 

Rock formations that are impermeable to groundwater water.

Aquifer - 

Rock formations that store groundwater water.

Aquifer Recharge Area - 

Surface area that provides water for an aquifer.

Archipelago - 

A group of islands that have an arc shaped distribution. These islands are usually of volcanic origin and are associated with subduction zones.

Arctic Circle - 

Latitude of 66.5° North. The southern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year.

Arête - 

Sharp topographic ridge that separates cirques on a mountain that is or has been glaciated.

Arkose - 

A type of sedimentary sandstone that contains a large quantity of weathered feldspar grains. This type of sedimentary rock forms in arid conditions.

Artesian Water - 

Groundwater that is confined by two impermeable layers beneath the Earth's surface.

Artesian Well - 

A well where the water rises and flows out to the surface because of hydrostatic pressure.

Aspect of Slope - 

Main compass direction (North, North East, East, South East, South, South West, West, and North West) that a slope faces.

Asthenosphere - 

Zone in the Earth's mantle that exhibits plastic properties. Located below the lithosphere at between 100 and 200 kilometres.

Astronomy - 

Field of knowledge that studies the nature, motion, origin, and constitution of celestial bodies.

Atmosphere - 

The atmosphere is the vast gaseous envelope of air that surrounds the Earth. Its boundaries are not easily defined. The atmosphere contains a complex system of gases and suspended particles that behave in many ways like fluids. Many of its constituents are derived from the Earth by way of chemical and biochemical reactions.

Atmospheric Pressure - 

Weight of the atmosphere on a surface. At sea-level, the average atmospheric pressure is 1013.25 millibars. Pressure is measured by a device called a barometer.

Atmospheric Stability - 

Relative stability of parcels of air relative to the atmosphere that surrounds them. Three conditions are generally described: stable, unstable, and neutral.

Atoll - 

A ring shaped reef composed largely of coral. These features are quite common in the tropical waters of the Pacific Ocean.

Aurora - 

Multicolored lights that appear in the upper atmosphere (ionosphere) over the polar regions and visible from locations in the middle and high latitudes. Caused by the interaction of solar wind with oxygen and nitrogen gas in the atmosphere. Aurora in the Northern Hemisphere are called aurora borelis and aurora australis in the Southern Hemisphere.

Autumn - 

Season between summer and winter. Astronomically it is the period from the autumnal equinox to the winter solstice in the Northern Hemisphere.

Autumnal Equinox - 

One of two days during the year when the declination of the Sun is at the equator. The autumnal equinox denotes the first day of the fall season. For the Northern Hemisphere, the date of autumnal equinox on either September 22 or 23 (changes yearly). March 20 or 21 is the date of the autumnal equinox in the Southern Hemisphere. During the autumnal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night.

Available Water - 

Portion of the capillary water that is available for plant root uptake.

Average Global Temperature - 

Average annual temperature of the Earth's entire surface atmosphere.

Azimuth - 

A system that measures direction clockwise from North over 360°.

B Horizon - 

Soil horizon normally found below the A horizon and above the C horizon. This layer is characterized by the following features:
(1) Enrichment of clay because of illuviation from the A horizon.
(2) Enrichment of iron and aluminum oxides because of illuviation of these materials from the A horizon. In some cases the precipitation of iron can cause the development of a hardpan.
(3) Accumulation of calcium carbonate, calcium sulfate, and other salts.
(4) Higher bulk density because of the illuvial deposition of clay particles.

Background Extinction - 

Normal extinction of species that occurs as a result of changes in local environmental conditions. Also see mass extinction.

Backscattering -  Portion of solar radiation directed back into space as a result of particle scattering in the atmosphere.
Backshore - 

Area behind the shore. This coastal feature is located between the beach berm and the backshore slope.

Backshore slope - 

Sloping bank landward of the shore. This coastal feature is composed of relatively non-mobile sediments.

Backswamp - 

Marshy low lying area in a stream's floodplain. Commonly found behind levees.

Backwash - 

The return water flow of swash. This sheet of water flows back to ocean because of gravity.

Bacteria - 

Simple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen.

Badlands - 

(1) Term used to describe a part of South Dakota.
(2) Term used to describe a semi-arid landscape that has been influenced by heavy fluvial erosion. Characterized by deep ravines and gullies, shape ridges, and a generally barren surface.

Bajada - 

Consecutive series of alluvial fans forming along the edge of a linear mountain range. Surface of this feature undulates in a rolling fashion as one moves from the center of one alluvial fan to another. Normally occurs in arid climates.

Bank-Caving - 

Collapse of stream bank material into a stream channel.

Bar - 

(1) Coarse grained deposit of sediment from a stream or ocean currents.
(2) A unit of measurement for quantifying force. Equivalent to 1,000,000 dynes per square centimeter.

 

Barchan Dune - 

Crescent shaped sand dune that has its long axis transverse to the wind and its crescent tips pointed downwind.

Barometer - 

Instrument that measures atmospheric pressure.

Barrier Beach - 

A long and narrow beach of sand and/or gravel that runs parallel to the coastline and is not submerged by the tide.

Barrier Island - 

Long, narrow islands of sand and/or gravel that are usually aligned parallel to the shore of some coasts.

Basal Sliding - 

The sliding of a glacier over the surface it rests on. Caused by the gradient of the slope and the weight of the glacier's mass.

Basalt - 

A dark colored fine grained igneous rock formed from mafic magma.

Basalt Plateau - 

Extensive continental deposits of basaltic volcanic rock.

Basaltic Magma - 

Mafic magma that forms basaltic igneous rocks.

Base - 

(1) Substance having a pH greater than 7.
(2) Substance that releases hydroxide ions (OH-).

Base Flow - 

Rate of discharge in a stream where only the throughflow and groundwater flow from subsurface aquifers contribute to the overall flow.

Base Level - 

The subterranean elevation below which a stream cannot vertically erode sediment. For many streams this hypothetical elevation is sea-level.

Basement Rock - 

Very old granite and metamorphic rocks found in continental crust. These rocks make up the continental shield.

Basin - 

A topographic rock structure whose shape is concave downwards.

Batholith - 

A large mass of subsurface intrusive igneous rock that has its origins from mantle magma.

Bay - 

A body of sheltered water found in a crescent shaped coastal configuration of land.

Bay-Mouth Bar - 

A narrow deposit of sand and/or gravel found across the mouth of a bay.

Bayhead Beach - 

An extensive deposit of sand and/or gravel in the form of a beach at the back of a bay.

Beach - 

The terrestrial interface area in between land and a water body where there are accumulations of unconsolidated sediments like sand and gravel. These deposits are laid down by the action of breaking waves.

Beach Drift - 

The lateral movement of sediments on a beach when the angles of swash and backwash differ.

Bearing - 

A system that measures in reference to the cardinal points of a compass in 90 degree quadrants.

Beaufort Wind Scale - 

Descriptive system that determines wind speed by noting the effect of the wind on the environment. Originally developed for use at sea by Admiral Beaufort of the British Navy in 1806.

Bed - 

Sedimentary structure that usually represents a layer of deposited sediment.

Bed Load - 

Portion of the stream load that is carried along the stream bed without being permanently suspend in the flowing water.

Bedding Plane - 

A layer in a series of sedimentary beds that marks a change in the type of deposits.

Bedrock - 

Rock at or near (beneath soil and regolith) the Earth's surface that is solid and relatively unweathered.

Bergschrund - 

A deep crevasse commonly found at the head of an alpine glacier. Forms when the glacial ice pulls away from the mountain side.

Berm - 

Low hill of sand that forms along coastal beaches.

Bifurcation Ratio - 

Quantitative ratio determined between the parts of systems that display branching. For example, trees have a main stem that bifurcates into smaller and smaller branches. The ratio between the branches that are derived from a larger branch or main stem is the bifurcation ratio.

Biodiversity - 

The diversity of different species (species diversity), genetic variability among individuals within each species (genetic diversity), and variety of ecosystems (ecosystem diversity). Abbreviation of biological diversity.

Biogeography - 

Field of physical geography that studies the spatial pattern of living organisms.

Biological Weathering - 

The disintegration of rock and mineral due to the chemical and/or physical agents of an organism.

Biome - 

Largest recognizable assemblage of animals and plants on the Earth. The distribution of the biomes is controlled mainly by climate.

Bioregion - 

A unique region on the Earth that has distinct soils, landforms, watersheds, climates, native plants, and animals, and/or other particular natural characteristics.

Biosphere - 

Part of the Earth where life is found. The biosphere consists of all living things, plant and animal. This sphere is characterized by life in profusion, diversity, and clever complexity. Cycling of matter in this biosphere involves not only metabolic reactions in organisms, but also many abiotic chemical reactions. Also called ecosphere.

Biotic - 

(1) Referring to life.
(2) Influences caused by living organisms.

Biotic Potential - 

Maximum rate that a population of a given species can increase in size (number of individuals) when there are no limits on growth rate.

Biotite - 

Rock forming mineral of the mica group.

Black Body - 

Is a body that emits electromagnetic radiation, at any temperature, at the maximum possible rate per unit surface area. This body also absorbs all electromagnetic radiation that is intercepted by it.

Blizzard - 

Winter severe weather condition characterized by strong wind, blowing snow, and cold temperatures.

Blowout Depression - 

Saucer shaped depressions created by wind erosion. At the leeward end of the feature there usually is a deposit of sand. Blowouts are found in coastal beach areas and in arid and semiarid regions of the world. These features are smaller than a deflation hollow.

Body Wave - 

Type of seismic wave that travels through the interior of Earth.

Bog - 

A habitat that consists of waterlogged spongy ground. Common vegetation are sedges and sphagnum moss. Bogs are common in Canada, Russia, and Scandinavia.

Bolson - 

Is a closed desert basin with no drainage outlet, surrounded by mountains.

Bora - 

Term used to describe a katabatic wind in Yugoslavia.

Bottomset Bed - 

Horizontal deltaic deposit of alluvial sediment composed of fine silt and clay.

Boulder - 

Large fragment of rock that has a diameter greater than 256 millimeters (200 millimeters in the United Kingdom).

Bowen Reaction Series - 

Model that explains the origin of the various types of igneous rocks. It suggests that the presence or absence of particular minerals in igneous rocks depends on the temperature of crystallization and on the magma's original chemical composition.

Brackish - 

Environment that is influenced by seawater with a salinity less than 35 parts per thousand (usually caused by the presence of an inflow of fresh water).

Braided Stream - 

Shallow stream channel that is subdivided into a number of continually shifting smaller channels that are separated by bar deposits.

Breaker - 

The quick collapse of an overextended water wave as it approaches the shoreline. The collapse occurs when the ratio of wave height to wavelength exceeds 1:7. This phenomenon also produces swash.

Breccia - 

Coarse grained sedimentary rock composed of cemented angular rock fragments.

Brine - 

Seawater with a salinity greater than 35 parts per thousand. Usually occurs in isolated bodies of seawater that have high amounts of water loss due to evaporation.

C Horizon - 

Soil horizon normally found below the B horizon and above the R horizon. This layer is composed of weathered bedrock that has not been yet significantly affected by the pedogenic processes.

Calcification - 

A dry environment soil-forming process that results in the accumulation of calcium carbonate in surface soil layers.

Calcite - 

Mineral formed from calcium carbonate. Common mineral found in limestone.

Caldera - 

A large circular depression in a volcano.

Caldera Volcano - 

Explosive type of volcano that leaves a large circular depression. Some of these depressions can be as large as 40 kilometers in diameter. These volcanoes form when wet granitic magma quickly rises to the surface of the Earth.

Calving - 

The loss of glacier mass when ice breaks off into a large water body like an ocean or a lake.

Cambrian - 

Geologic period that occurred from 570 to 505 million years ago. During this period, invertebrates become common in the oceans and the Burgess Shale was formed.

Canopy Drip - 

Redirection of a proportion of the rain or snow falling on a plant to the edge of its canopy.

Canyon - 

Steep-sided valley where depth is considerably greater than width. These features are the result of stream erosion.

Capillary Action - 

Movement of water along microscopic channels. This movement is the result of two forces: the adhesion and absorption of water to the walls of the channels; and cohesion of water molecules to each other.

Capillary Water - 

Water that moves horizontally and vertically in soils by the process of capillary action. This water is available for plant use.

Carbon Cycle - 

Storage and cyclic movement of organic and inorganic forms of carbon between the biosphere, lithosphere, hydrosphere, and atmosphere.

Carbon Dioxide - 

Common gas found in the atmosphere. Has the ability to selectively absorb radiation in the longwave band. This absorption causes the greenhouse effect. The concentration of this gas has been steadily increasing in the atmosphere over the last three centuries due to the burning of fossil fuels, deforestation, and land-use change. Some scientists believe higher concentrations of carbon dioxide and other greenhouse gases will result in an enhancement of the greenhouse effect and global warming. The chemical formula for carbon dioxide is CO2.

Carbon Monoxide - 

A colorless, odorless, and tasteless gas that is produced by the incomplete burning of fossil fuels. The chemical formula for carbon monoxide is CO.

Carbonate - 

Compound consisting of a single atom of carbon and three atoms of oxygen. Carbonate has the following chemical structure CO3.

Carbonation - 

Is a form of chemical weathering where carbonate and bicarbonate ions react with minerals that contain calcium, magnesium, potassium, and sodium.

Cardinal Points - 

The four main navigational directions (North, East, South, and West) found on a compass or a map.

Carnivore - 

Heterotrophic organism that consumes living animals or the parts of living animals for food. Examples of carnivores include lions, cheetahs, leopards, frogs, snakes, hawks, and spiders. A carinore can also be called a secondary consumer or tertiary consumer. Also see herbivore, detritivore, scavenger, and omnivore.

Carrying Capacity (K) - 

The maximum size of population of a single species that a certain habitat can support.

Cartography - 

Field of knowledge that studies map construction. The act of creating a map.

Cascading System - 

This is a system where we are primarily interested in the flow of energy and/or matter from one element to another and understand the processes that cause this movement. In a cascading system, we do not fully understand quantitative relationships that exist between elements related to the transfer of energy and/or matter.

Cave - 

A natural cavity or recess that is roughly positioned horizontally to the surface of the Earth.

Cavitation - 

Process of intense erosion due to the surface collapse of air bubbles found in constricted rapid flows of water. Causes the detachment of material from a surface.

Celsius - 

Scale for measuring temperature. In this scale, water boils at 100° and freezes at 0°.

Cenozoic - 

Geologic era that occurred from 65 million years ago to today.

Central Vent - 

The main passage way by which volcanic magma travels to the Earth's surface.

Centripetal Force - 

Force required to keep an object moving in a circular pattern around a center of rotation. This force is directed towards the center of rotation. Common in meteorological phenomena like tornadoes and hurricanes.

CFCs - 

Is an artificially created gas that has become concentrated in the Earth's atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of foams. The basic chemical formula for chlorofluorocarbons is CFx Clx .

Chalk - 

Form of limestone. This sedimentary rock is composed of the shells and skeletons of marine microorganisms.

Chemical - 

One of the millions of different elements and compounds found naturally and synthesized by humans.

Chemical Energy - 

Energy consumed or produced in chemical reactions.

Chemical Reaction - 

Reaction between chemicals where there is a change in the chemical composition of the elements or compounds concerned.

Chemical Weathering - 

Breakdown of rock and minerals into small sized particles through chemical decomposition.

Chinook Wind - 

The name of a North American wind that occurs on the leeward side of mountains. This wind is warm and has a low humidity.

Chlorofluorocarbons - 

Is an artificially created gas that has become concentrated in the Earth's atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of foams. The basic chemical formula for chlorofluorocarbons is CFx Clx .

Chlorofluorocarbons (CFCs) - 

Is an artificially created gas that has become concentrated in the Earth's atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of foams. The basic chemical formula for chlorofluorocarbons is CFx Clx .

Chlorophyll - 

Green pigment found in plants and some bacteria used to capture the energy in light through photosynthesis.

Chloroplast - 

Organelle in a cell that contains chlorophyll and produces organic energy through photosynthesis.

Cinder Cone Volcano - 

A small volcano, between 100 and 400 meters tall, made up of exploded rock blasted out of a central vent at a high velocity. These volcanoes develop from magma of basaltic to intermediate composition.

Circle of Illumination - 

A line that bisects areas on the Earth receiving sunlight and those areas in darkness. Cuts the spherical Earth into lighted and dark halves.

Circum-Pacific Belt - 

A zone circling the edge of the Pacific Ocean basin where tectonic subduction causes the formation of volcanoes and trenches. Also called the ring of fire.

Cirque - 

Glacially eroded rock basin found on mountains. Most alpine glaciers originate from a cirque.

Cirque Glacier - 

Small glacier that just occupies a cirque.

Cirrocumulus Clouds - 

Patchy white high altitude cloud composed of ice crystals. Found in an altitude range from 5,000 to 18,000 meters.

Cirrostratus Clouds - 

High altitude sheet like clouds composed of ice crystals. These thin clouds often cover the entire sky. Found in an altitude range from 5,000 to 18,000 meters.

Cirrus Clouds - 

High altitude cloud composed of ice crystals. The appearance of these clouds is white feather like patches, filaments or thin bands. Found in an altitude range from 5,000 to 18,000 meters.

Classification - 

Process of grouping things into categories.

Clastic Sedimentary Rock - 

Sedimentary rocks that are formed by the lithification of weathered rock debris that has been physically transported and deposited.

Clay - 

Mineral particle with a size less than 0.004 millimeters in diameter. Also see silt and sand.

Cleavage - 

The tendency of some minerals or rocks to break along planes of weakness. This weakness occurs because of the nature of the bonds between mineral grains.

Cliff - 

A tall steep rock face.

Climate - 

General pattern of weather conditions for a region over a long period time (at least 30 years).

Climatology - 

Scientific study of the Earth's climate over long time spans (greater than several days). May also involve the investigation of climate's influence on the biotic and the abiotic environment.

Climograph - 

Two dimensional graph that plots a location's air temperature and precipitation on times scales that range from a 24 hour period to a year.

Closed System - 

Is is a system that transfers energy, but not matter, across its boundary to the surrounding environment. Our planet is often viewed as a closed system.

Closed Talik - 

Is a form of localized unfrozen ground (talik) in an area of permafrost. It is completely enclosed by permafrost in all directions.

Cloud - 

A collection of tiny particles of liquid or solid water occurring above the Earth's surface. Clouds are classified accord to their height of occurrence and shape. The major types of clouds include: Cirrus, Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Nimbostratus, Stratocumulus, Stratus, Cumulus, and Cumulonimbus.

Coal - 

Sedimentary rock composed of the compacted, lithified and altered remains of plants. Coal is a solid, combustible mixture of organic compounds, hydrocarbons, with 30% to 98% carbon by weight, mixed with various amounts of water and small amounts of sulfur and nitrogen compounds. It is formed in several stages as the remains of plants are subjected to heat and pressure over millions of years.

Coalescence - 

Process where two or more falling raindrops join together into a single larger drop because of a midair collision.

Coastal Dune - 

Sand dune that forms in coastal areas. The sand for its formation is supplied from a beach.

Coastal Wetland - 

Wetland habitat found along a coastline and is covered with ocean salt water for all or part of the year. Examples of this type of habitat include tidal marshes, bays, lagoons, tidal flats, and mangrove swamps.

Coastal Zone - 

Relatively nutrient-rich, shallow part of the ocean that extends from the high-tide mark on land to the edge of the continental shelf.

Coastline - 

The line that separates a land surface from an ocean or sea.

Col - 

Saddle like depression found between two mountain peaks. Formed when two opposing cirque glaciers back erode an arête.

Cold Desert - 

Desert found in the high latitudes and at high altitudes where precipitation is low. Surface air temperatures are generally cold in these dry environments.

Cold Front - 

A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass.

Cold Glacier - 

Glacier in which the ice found from the its surface to base has a temperature as cold as -30° Celsius throughout the year. This is well below the pressure melting point. Pressure melting can cause the melting of ice at the base of these glaciers. One of the three types of glaciers: cold glacier; temperate glacier; and subpolar glacier.

Colonization - 

Movement of individuals or propagules of a species to a new territory.

Comet - 

A large mass of ice and dust that has an orbit around a star.

Community - 

Refers to all the populations of interacting species found in a specific area or region at a certain time.

Compass - 

Navigation instrument that uses the Earth's magnetic field to determine direction.

Competition - 

Interaction where two or more organisms in the same space require the same resource (e.g., food, water, nesting space, and ground space) which is in limiting supply to the individuals seeking it. Competition can occur at the interspecific or intraspecific biotic levels. Competition may also be the result of two different processes: exploitation or interference.

Competitive Exclusion - 

Situation where no two competitively interacting species can occupy exactly the same fundamental niche indefinitely because of resource limitations. The outcome of this process is the local extinction the species that is a poorer competitor.

Composite Volcano - 

Volcano created from alternate layers of flows and exploded rock. Their height ranges from 100 to 3,500 meters tall. The chemistry of the magma of these volcanoes is quite variable ranging from basalt to granite.

Concrete Space - 

Actual geographic space in the real world. Geographers approximate this space when they try to represent it in a model or map. This approximation is referred to as abstract space.

Condensation - 

The change in state of matter from vapor to liquid that occurs with cooling. Usually used in meteorology when discussing the formation of liquid water from vapor. This process releases latent heat energy to the environment.

Condensation Nuclei - 

Microscopic particle of dust, smoke or salt that allows for condensation of water vapor to water droplets in the atmosphere. Nucleus for the formation of a rain drop. Condensation normally occurs on these particles when relative humidity becomes 100%. Some condensation nuclei, like salt, are hygroscopic and water can condense on them at relative humidities lower than 100%.

Conduction - 

Conduction consists of energy transfer directly from atom to atom and represents the flow of energy along a temperature gradient.

Cone of Depression - 

Cone shaped depression occurring horizontally across a water table. Causes by excessive removal of groundwater by a surface well.

Confined Aquifer - 

Aquifer between two layers of relatively impermeable earth materials, such as clay or shale.

Confined Groundwater - 

Groundwater trapped between two impervious layers of rock.

Conglomerate - 

Coarse grained sedimentary rock composed of rounded rock fragments cemented in a mixture of clay and silt.

Coniferous Vegetation - 

Cone-bearing vegetation of middle and high latitudes that are mostly evergreen and that have needle-shaped or scale like leaves. Compare with deciduous vegetation.

Conservation Biology - 

Multidisciplinary science that deals with the conservation of genes, species, communities, and ecosystems that make up Earth's biodiversity. It generally investigates human effects on biodiversity and tries to develop practical approaches to preserving biodiversity and ecological integrity.

Consumer - 

An organism that receives the nutrients (food) required for maintenance, growth, and reproduction from the consumption of tissues of producers and/or other consumers. Also called a heterotroph. Several different kinds of consumers have been recognized including: carnivores, omnivores, scavengers, herbivores, detritivores, secondary consumers, and tertiary consumers.

Contact Metamorphism - 

Is the small scale metamorphic alteration of rock due to localized heating. It is usually cause by an igneous intrusion like a sill or a dyke.

Continental Arctic Air Mass - 

Air mass that forms over extensive landmass areas of the high latitudes. In the Northern Hemisphere, these system form only in winter over Greenland, northern Canada, northern Siberia, and the Arctic Basin. Continental Arctic air masses are very cold and extremely dry. These air masses are also very stable.

Continental Crust - 

Granitic portion of the Earth's crust that makes up the continents. Thickness of the continental crust varies between 20 to 75 kilometers. See sial layer.

Continental Divide - 

The elevated area that occurs on a continent that divides continental scale drainage basins.

Continental Drift - 

Theory that suggests that the Earth's crust is composed of several continental plates that have the ability to move. First proposed by A. Snider in 1858 and developed by F.B. Taylor (1908) and Alfred Wegener (1915).

Continental Effect - 

The effect that continental surfaces have on the climate of locations or regions. This effect results in a greater range in surface air temperature at both daily and annual scales. Also see maritime effect.

Continental Glacier - 

Largest type of glacier with a surface coverage in the order of 5 million square kilometres.

Continental Ice Sheet - 

Largest type of glacier with a surface coverage in the order of 5 million square kilometres.

Continental Margin - 

The area between a continent's shoreline and the beginning of the ocean floor. It includes the continental shelf, continental rise, and continental slope.

Continental Plate - 

A rigid, independent segment of the lithosphere composed of mainly granite that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's continental plates are an average 125 kilometers thick and were formed more than 3 billion years ago. Also see oceanic plate.

Continental Polar Air Mass - 

Air mass that forms over extensive landmass areas of middle to high latitudes. In North America, these system form over northern Canada. Continental Polar air masses are cold and very dry in the winter and cool and dry in the summer. These air masses are also atmospherically stable in both seasons.

Continental Rise - 

Thick layers of sediment found between the continental slope the ocean floor.

Continental Shelf - 

Shallow submerged margin of the continents that lies between the edge of the shoreline and the continental slope. This nearly level area of the continental crust has surface layers composed of sediment or sedimentary rock.

Continental Shelf Break - 

Boundary zone between the continental shelf and slope.

Continental Shield - 

A large stable area of exposed very old (more than 600 million years) igneous and metamorphic rock found on continents. This rock forms the nucleus of the continents.

Continental Slope - 

Steeply sloping portion of continental crust found between the continental shelf and continental rise.

Continental Tropical Air Mass - 

Air mass that forms over extensive landmasses areas of the low latitudes. In North America, these system form over southwestern United States and northern Mexico. Continental Tropical air masses are warm and dry in the winter and hot and dry in the summer. These air masses are also generally unstable in the winter and stable in the summer.

Continuous Permafrost - 

Form of permafrost that exists across a landscape as an unbroken layer.

Contour Interval - 

Difference in elevation between two successive contour lines. The interval at which contours are drawn on a map depends on the amount of the relief depicted and the scale of the map.

Contour Line - 

Line on a topographic map that connects all points with the same elevation.

Convection - 

Convection involves the transfer of heat energy by means of vertical mass motions through a medium.

Convection Current - 

The movement of a gas or a fluid in chaotic vertical mass motions because of heating.

Convectional Lifting - 

The vertical lifting of parcels of air through convective heating of the atmosphere. This process can initiate adiabatic processes inside the air parcel.

Convectional Precipitation - 

Is the formation of precipitation due to surface heating of the air at the ground surface. If enough heating occurs, the mass of air becomes warmer and lighter than the air in the surrounding environment, and just like a hot air balloon it begins to rise, expand and cool. When sufficient cooling has taken place saturation occurs forming precipitation. This process is active in the interior of continents and near the equator forming cumulus clouds and possible later thunderstorms. Rain is usually the precipitation type that is formed, and in most cases this moisture is delivered in large amounts over short periods of time in extremely localized areas.

Convergence - 

Horizontal inflow of wind into an area. Once at the area, the wind then travels vertically.

Convergence 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 called frontal precipitation.

Convergent Lifting - 

The vertical lifting of parcels of air through the convergence of opposing air masses in the atmosphere. This process can initiate adiabatic processes inside the air parcel.

Coordinated Universal Time - 

Current official world time reference for civil and scientific purposes. Coordinated Universal Time is measured from six standard atomic clocks at the International Bureau of Weights and Measures (BIPM) in Paris, France. Implemented in 1964.

Coral - 

Simple marine animals that live symbiotically with algae. In the symbiotic relationship, the algae provides the coral with nutrients, while the coral provide the algae with a structure to live in. Coral animals secrete calcium carbonate to produce a hard external skeleton.

Coral Bleaching - 

Situation where coral lose their colorful symbiotic algae. Thought to be caused by unusually warm water, changes in salinity of ocean seawater, or excessive exposure to ultraviolet radiation.

Coral Reef - 

Ridge of limestone found generally below the ocean surface. This marine feature is produced by numerous colonies of tiny coral animals, called polyps, that create calcium carbonate structures around themselves for protection. When the corals die, their vacant exterior skeletons form layers that cause the reef to grow. Coral reefs are found in the coastal zones of warm tropical and subtropical oceans.

Core - 

The core is a layer rich in iron and nickel found in the interior of the Earth. It is composed of two sub-layers: the inner core and outer core. The core is about 7,000 kilometers in diameter.

Coriolis Force - 

An apparent force due to the Earth's rotation. Causes moving objects to be deflected to the right in the Northern Hemisphere and to the left in the Southern hemisphere. Coriolis force does not exist on the equator. This force is responsible for the direction of flow in meteorological phenomena like mid-latitude cyclones, hurricanes, and anticyclones.

Correlation Coefficient - 

Statistic that measures the degree of linear association between two variables. Its values vary from between -1 and 1. Perfect positive (the dependent variable increases with an increase in the independent variable) linear association has a correlation coefficient of 1. Perfect negative (the dependent variable decreases with an increase in the independent variable) linear association has a correlation coefficient of -1. Absolutely no association between variables has a value of zero.

Coulee - 

(1) Steep-sided flow of volcanic lava that has solidified.
(2) Abandoned glacial meltwater channel.
(3) Term used in the United States to describe a steep-sided stream valley.

Counter-Radiation - 

Redirection of the Earth's longwave radiation back to the surface because of the greenhouse effect.

Crater - 

Circular depression in the ground surface created by volcanic activity or asteroid impact.

Craton - 

Stable foundation core of the Earth's various plates of continental crust. Composed of the shield and platform.

Creep - 

(1) Slow mass movement of soil downslope. Occurs where the stresses on the slope material are too small to create a rapid failure. See soil creep.
(2) Another term used to describe traction.

Cretaceous - 

Geologic period that occurred roughly 65 to 144 million years ago. During this period, the first flowering plant species appear and dinosaurs are at their greatest diversity. Dinosaurs die out at the end of this period.

Crevasse - 

(1) Opening on a levee that allows for the drainage of water from the floodplain to the stream channel.
(2) Fracture on the brittle surface of a glacier.

Critical Entrainment Velocity - 

Velocity required to entrain a particular sized particle into the moving medium of air or water.

Crust - 

Earth's outer most layer of solid rock. Between 7 to 70 kilometers thick. Two types of crust exist: oceanic crust and continental crust.

Cryostatic Pressure - 

Pressure exerted on a substance by ice at rest.

Cryotic - 

Something that is frozen.

Cumulonimbus Cloud - 

A well developed vertical cloud that often has top shaped like an anvil. These clouds are very dense with condensed and deposited water. Weather associated with this cloud includes: strong winds; hail; lightning; tornadoes; thunder; and heavy rain. When this weather occurs these clouds are then thunderstorms. Can extend in altitude from a few hundred meters above the surface to more than 12,000 meters.

Cumulus Cloud - 

Puffy clouds with relatively flat bases. Cumulus clouds form when moist warm air bubbles vertically escape from the Earth's surface. Found in an altitude range from 300 to 2,000 meters.

Cuspate Foreland - 

Is a triangular accumulation of sand and/or gravel located along the coastline. This feature is formed by the joining of two spits.

Cyclogenesis - 

Process of cyclone formation, maturation, and death.

Cyclone - 

Area of low pressure in the atmosphere that displays circular inward movement of air. In the Northern Hemisphere circulation is counterclockwise, while Southern Hemisphere cyclones have clockwise wind patterns.

DALR - 

The rate of decline in the temperature of a rising parcel of air before it has reached saturation. This rate of temperature decline is 9.8° Celsius per 1000 meters because of adiabatic cooling.

Day Length - 

Period of time for a location on the Earth when insolation from the Sun is being received.

Daylight Savings Time - 

The setting of time so it is one hour ahead starting in the spring and one hour back beginning in the fall in the Northern Hemisphere. In Canada and the United States the dates for these events is the first Sunday in April (spring ahead) and the last Sunday in October (fall back).

Debris Flow - 

A type of mass movement where there is a downslope flow of a saturated mass of soil, sediment, and rock debris.

December Solstice - 

Date during the year when the declination of the Sun is at 23.5° South of the equator. During the December solstice, locations in the Northern Hemisphere experience their shortest day. The December solstice is also the first day of winter in the Northern Hemisphere. Locations in the Southern Hemisphere have their longest day on the June solstice. This date also marks the first day of summer in the Southern Hemisphere.

Deciduous Vegetation - 

Type of vegetation that sheds its leaves during winter or dry seasons. Compare with coniferous vegetation.

Declination - 

Location (latitude) on the Earth where the Sun on a particular day is directly overhead (90° from horizon) at solar noon. This location is somewhere between 23.5° North and 23.5° South depending on the time of the year.

Decomposer - 

A type of detritivore. Decomposers play an important role in recycling organic matter back into inorganic nutrients in ecosystems. This recycling is done by decomposing complex organic matter and then coverting the less complex organic products into inorganic compounds and atoms. Much of the recycled inorganic nutrients are then consumed by producers. Bacteria and fungi are the most common decomposers found in most ecosystems. Also see detritus feeders.

Decomposition - 

(1) To chemically or physically breakdown a mass of matter into smaller parts or chemical elements.
(2) Breakdown of organic matter into smaller parts or inorganic constituents by decomposing organisms.

Deduction - 

Inference in which the conclusion about particulars follows necessarily from general theory. In a science like Physical Geography, deductive reasoning would involve stating a theory first and then trying to find facts that reject this idea.

Deflation - 

Process where wind erosion creates blowout depressions or deflation hollows by removing and transporting sediment and soil.

Deflation Hollow - 

A surface depression or hollow commonly found in arid and semiarid regions caused by wind erosion. Also see the related blowout depression.

Deforestation - 

Removal of trees from a habitat dominated by forest.

Degradation - 

Readjustment of the stream profile where the stream channel is lowered by the erosion of the stream bed. Usually associated with high discharges.

Delta - 

Large deposit of alluvial sediment located at the mouth of a stream where it enters a body of standing water.

Dendritic - 

Term used to describe the stream channel pattern that is completely random. Resembles the branching pattern of blood vessels or tree branches.

Denitrification - 

Conversion of nitrates into gaseous nitrogen and nitrous oxide.

Density - 

Refers to the quantity of mass per unit volume. For gases, density involves the number of atoms and molecules per unit volume.

Denudation - 

(1) The erosion or wearing down of a landmass.
(2) Removal of the vegetative cover from an area.

Deoxyribonucleic Acid - 

Form of nucleic acid that is organized into a double-helix molecule. DNA is used by most organisms to chemically code their genetics and to direct the development and functioning of cells. This direction requires RNA which represents a copy of a portion of DNA. Found in the nucleus of cells.

Deposition - 

(1) The change in state of matter from gas to solid that occurs with cooling. Usually used in meteorology when discussing the formation of ice from water vapor. This process releases latent heat energy to the environment.
(2) Laying down of sediment transported by wind, water, or ice.

Deposition Nuclei - 

Six-sided microscopic particle that allows for deposition of water as ice crystals in the atmosphere. Nucleus for the formation of snowflakes. Deposition normally occurs on these particles when relative humidity becomes 100%.

Depositional Landform - 

Is a landform formed from the deposition of weathered and eroded surface materials. On occasion, these deposits can be compressed, altered by pressure, heat and chemical processes to become sedimentary rocks. This includes landforms with some of the following geomorphic features: beaches, deltas, floodplains, and glacial moraines.

Depression - 

(1) Concave hollow found on the Earth's surface.
(2) Term used to describe a cyclone or an atmospheric low pressure system.

Deranged Drainage - 

Drainage pattern that is highly irregular. Areas that have experienced continental glaciation may have this type of drainage pattern.

Desert - 

(1) Biome that has plants and animals adapted to survive severe drought conditions. In this habitat, evaporation exceeds precipitation and the average amount of precipitation is less than 25 centimeters a year.
(2) Area that receives low precipitation. Also see cold desert and warm desert.

Desert Pavement - 

A veneer of coarse particles left on the ground after the erosion of finer particles by wind.

Desertification - 

Conversion of marginal rangeland or cropland to a more desert like land type. Desertification can be caused by overgrazing, soil erosion, prolonged drought, or climate change.

Detachment - 

One of three distinct processes involved in erosion. This process involves the disengagement of a particle from its surroundings.

Detrital Rock - 

Sedimentary rock that is composed of particles transported to their place of deposition by erosional processes. Examples of such rock include sandstone and shale.

Devonian - 

Geologic period that occurred roughly 360 to 408 million years ago. During this period, the first amphibians and trees appear.

Dew - 

Condensation of water on the Earth's surface because of atmospheric cooling.

Dew Point - 

Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%. If the dew point is below freezing, it is referred to as the frost point.

Diffused Solar Radiation - 

Solar radiation received by the Earth's atmosphere or surface that has been modified by atmospheric scattering.

Diffusion - 

(1) Molecular mixing of one substance into another substance.
(2) Redirection or refraction of solar insolation in many directions. Process cause the beam of traveling radiation to become less intense.

Diorite - 

A coarse grained igneous rock of intrusive origin that is darker and chemically more mafic than granite.

Dip - 

One of the directional properties of a geologic structure such as a fold or a fault. Dip is the inclination angle of the formation as measured at right angles to strike.

Direct Solar Radiation - 

Solar radiation received by the Earth's atmosphere or surface which has not been modified by atmospheric scattering.

Discharge - 

A river or stream's rate of flow over a particular period of time. Usually measured by a current meter and expressed in cubic meters per second. Stream discharge depends on the volume and velocity of the flow.

Discontinuous Permafrost - 

Form of permafrost that contains numerous scattered pockets of unfrozen ground.

Dissolution - 

The process of a substance dissolving and dispersing into a liquid.

Dissolved Load - 

Portion of the stream load that is in solution in the flowing water.

Distance Ratio - 

Method for measuring the gradient of a slope. Simply involves dividing the vertical change in distance (rise) by horizontal change in distance (run) or rise/run. The measurement is usually presented as a percentage or relative to some unit distance traveled in the horizontal.

Distributary - 

A smaller branching stream channel that flows away from a main stream channel. Common on deltas. Opposite of tributary.

Distributional Limit - 

Spatial boundary that defines the edge of a species geographical range.

Disturbance - 

(1) Partial or complete alteration of a community or an ecosystem by a biotic or abiotic factor.
(2) Cyclonic low pressure system.

Diurnal Tide - 

Tides that have one high and one low water per tidal period.

Divergence - 

Horizontal outflow of wind from an area. In a surface divergence, outflow originates from the upper atmosphere.

DNA - 

Form of nucleic acid that is organized into a double-helix molecule. DNA is used by most organisms to chemically code their genetics and to direct the development and functioning of cells. This direction requires RNA which represents a copy of a portion of DNA. Found in the nucleus of cells.

Doldrums - 

Area of low atmospheric pressure and calm westerly winds located at the equator. Similar to Intertropical Convergence Zone.

Dolomite - 

(1) Sedimentary rock formed from CaMg(CO3)2.
(2) Mineral with the chemical formula CaMg(CO3)2.

Downdraft - 

Downward movement of air in the atmosphere.

Downwelling Current - 

Ocean current that travels downward into the ocean because of the convergence of opposing horizontal currents or because of an accumulation of seawater.

Drainage Basin - 

Land surface region drained by a length of stream channel.

Drainage Density - 

Is the measure of the length of stream channel per unit area of drainage basin. Mathematically its is expressed as:
Drainage Density (Dd) = Stream Length / Basin Area

Drainage Divide - 

Topographic border between adjacent drainage basins or watersheds.

Drainage Network - 

System of interconnected stream channels found in a drainage basin.

Drainage Pattern - 

Geometric pattern that a stream's channels take in the landscape. These patterns are controlled by factors such as slope, climate, vegetation, and bedrock resistance to erosion.

Drought - 

Climatic condition where water loss due to evapotranspiration is greater than water inputs through precipitation.

Droughts - 

Climatic condition where water loss due to evapotranspiration is greater than water inputs through precipitation.

Drumlin - 

A hill shaped deposit of till. The shape of these features resembles an elongated teaspoon laying bowl down. The tapered end of the drumlin points to the direction of glacier advance. Drumlins come in assorted sizes. Lengths can range from 100 to 5,000 meters and heights can be as great as 200 meters.

Dry Adiabatic Lapse Rate - 

The rate of decline in the temperature of a rising parcel of air before it has reached saturation. This rate of temperature decline is 9.8° Celsius per 1000 meters because of adiabatic cooling.

Dry Deposition - 

The transport of gases and minute liquid and solid particles from the atmosphere to the ground surface without the aid of precipitation or fog. Compare with wet deposition.

Dry Line - 

A boundary the separates dry and moist air in the warm sector of a mid-latitude cyclone wave. Found ahead of the cold front.

Dry-Bulb Thermometer - 

Thermometer on a psychrometer used to determine current air temperature. This measurement and the reading from a wet-bulb thermometer are then used for the determination of relative humidity or dew point from a psychrometric table.

Dune - 

(1) Stream bed deposit found streams whose channel is composed mainly of sand and silt. Dunes are about 10 or more centimeters in height and are spaced a meter or more apart and are common in streams with high velocities.
(2) Terrestrial deposit of sand that resembles a mound or ridge that was formed from aeolian processes. Also see sand dune.

Dune Field - 

An extensive region covered by numerous sand dunes.

Dust Dome - 

Dome of air that surrounds a city created from the urban heat island effect that traps pollutants like particulate matter.

Dyke - 

Thin vertical veins of igneous rock that form when magma enters and cools in fractures found within the crust. Also see intrusive igneous rock.

Dynamic Equilibrium - 

A dynamic equilibrium occurs when a system displays unrepeated average states through time.

Dynamic Metamorphism - 

Form of metamorphism that causes only the structural alteration of rock through pressure. The minerals in the altered rocks do not change chemically. The extreme pressures associated with mountain building can cause this type of metamorphism.

Earth Albedo - 

Is the reflectivity of the Earth's atmosphere and surface combined. Measurements indicate that the average Earth albedo is approximately 30%.

Earth Revolution - 

Refers to the orbit of the Earth around the Sun. This celestial motion takes 365 1/4 days to complete one cycle. Further, the Earth's orbit around the Sun is not circular, but elliptical.

Earth Rotation - 

Refers to the spinning of the Earth on its polar axis.

Earthflow - 

A rapid type of downslope mass movement that involves soil and other loose sediments. Usually triggered by water saturation from rainfall.

Earthquake - 

Is a sudden motion or trembling in the Earth. The motion is caused by the quick release of slowly accumulated energy in the form of seismic waves. Most earthquakes are produced along faults, tectonic plate boundaries, or along the mid-oceanic ridges.

Earthquake Focus - 

Point of stress release in an earthquake.

Easterly Wave - 

Atmospheric disturbance in the tropical trade winds. Occasionally these systems intensify into hurricanes.

Ebb Tide - 

Time during the tidal period when the tide is falling. Compare with flood tide.

Eccentricity - 

Geometric shape of the Earth's orbit. This shape varies from being elliptical to almost circular.

Ecological Diversity - 

The variety of unique biological communities found on the Earth. A component of biodiversity. Also see genetic diversity and species diversity.

Ecological Niche - 

Is all of the physical, chemical and biological conditions required by a species for survival, growth and reproduction. Two further abstractions of this concept are the fundamental niche and the realized niche.

Ecology - 

The study of the factors that influence the distribution and abundance of species.

Ecosphere - 

Part of the Earth where life is found. The biosphere consists of all living things, plant and animal. This sphere is characterized by life in profusion, diversity, and clever complexity. Cycling of matter in this biosphere involves not only metabolic reactions in organisms, but also many abiotic chemical reactions. Also called biosphere.

Ecosystem - 

An ecosystem is a system where populations of species group together into communities and interact with each other and the abiotic environment.

Ecosystem Diversity - 

The variety of unique biological communities found on the Earth. A component of biodiversity. Also see genetic diversity and species diversity.

Ecotone - 

Boundary zone between two unique community types.

Eddy - 

A localized chaotic movement of air or liquid in a generally uniform larger flow.

Eddy Diffusion - 

Mixing of the atmosphere by chaotic air currents.

Edge Wave - 

A wave of water that moves parallel to the shore. This wave is usually a secondary wave of complex formation.

Effusive Eruption - 

Volcanic eruption where low-viscosity basaltic magma is released. This type of eruption is not explosive and tends to form shield volcanoes.

El Niño - 

Name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru. When this warming occurs the tropical Pacific trade winds weaken and the usual upwelling of cold, nutrient rich deep ocean water off the coast of Ecuador and Peru is reduced. The El Niño normally occurs around Christmas and lasts usually for a few weeks to a few months. Sometimes an extremely warm event can develop that lasts for much longer time periods.

Elastic Deformation - 

Change in the shape of a material as the result of the force of compression or expansion. Upon release of the force, the material returns to its original shape. Also called plastic deformation.

Elastic Limit - 

Maximum level of elastic deformation of a material without rupture.

Elastic Rebound Theory - 

Theory that describes how earthquakes arise from the horizontal movement of adjacent tectonic plates along a linear strike-slip fault. This theory suggests that the two plates moving in opposite directions become locked for some period of time because of friction. However, the accumulating stress overcomes the friction and causes the plate to suddenly move over a short time period which generates an earthquake.

Elastic Wave - 

An energy wave that causes elastic deformation in a material without its structure and shape being deformed.

Electrical Energy - 

Energy produced from the force between two objects having the physical property of electrical charge.

Electromagnetic Energy - 

Energy stored in electromagnetic waves or radiation. Energy is released when the waves are absorbed by a surface. Any object with a temperature above absolute zero (-273° Celsius) emits this type of energy. The intensity of energy released is a function of the temperature of the radiating surface. The higher the temperature the greater the quantity of energy released.

Electromagnetic Radiation - 

Emission of energy in the form of electromagnetic waves. All objects above the temperature of absolute zero (-273.15° Celsius) radiate energy to their surrounding environment. The amount of electromagnetic radiation emitted by a body is proportionally related to its temperature.

Electromagnetic Spectrum - 

Is a graph that describes the quantity of radiation that is emitted from a body at particular wavelengths. Also see spectrum.

Electron - 

A sub-particle of an atom that contains a negative atomic charge.

ELR - 

Environmental Lapse Rate:
The rate of air temperature increase or decrease with altitude. The average ELR in the troposphere is an air temperature decrease of 6.5° Celsius per 1000 meters rise in elevation.

Eluviation - 

Movement of humus, chemical substances, and mineral particles from the upper layers of a soil to lower layers by the downward movement of water through the soil profile. Compare with illuviation.

Emigration - 

Migration of an organism out of an area for the purpose of changing its residence permanently. Compare with immigration.

Endangered Species - 

A species found in nature that has so few surviving individuals that the it could soon become extinct in all or most of its natural range. Also see threatened species.

Endogenic - 

Refers to a system that is internal to the Earth.

Energy - 

Is defined as the capacity for doing work. Energy can exist the following forms: radiation; kinetic energy; potential energy; chemical energy; atomic energy; electromagnetic energy; electrical energy; and heat energy.

Energy Flux - 

The rate of energy flow from, into, or through a substance.

Entrainment - 

One of three distinct processes involved in erosion. More specifically, it is the process of particle lifting by an agent of erosion.

Entropy - 

Entropy is the measure of the disorder or randomness of energy and matter in a system.

Environment - 

(1) Abiotic and biotic factors that influence the life of an organism.
(2) Abiotic and biotic factors that influence the function of some nonliving natural system.

Environmental Gradient - 

Spatial gradient where abiotic and biotic factors vary.

Environmental Lapse Rate - 

The rate of air temperature increase or decrease with altitude. The average ELR in the troposphere is an air temperature decrease of 6.5° Celsius per 1000 meters rise in elevation.

Environmental Science - 

Field of knowledge that studies of how humans and other species interact with one another and with the nonliving environment. It is both a physical and social science that integrates knowledge from a wide range of disciplines, including physics, chemistry, biology, geology, geography, economics, political science, sociology, psychology, and philosophy.

Environmental System - 

A system where life interacts with the various abiotic components found in the atmosphere, hydrosphere, and lithosphere.

Enzyme - 

Are types of proteins that are used to facilitate and regulate chemical reactions within cells.

Eolian - 

Geomorphic process involving wind. Alternative spelling Aeolian.

Eolian Landform - 

Is a landform formed from the erosion or deposition of weathered surface materials by wind. This includes landforms with some of the following geomorphic features: sand dunes, deflation hollows, and desert pavement. Alternative spelling aeolian landform.

Eon - 

Longest geologic time unit.

Epicenter - 

Surface location of an earthquake's focus.

Epoch - 

Geologic time unit that is shorter than a period.

Equator - 

Location on the Earth that has a latitude of 0°.

Equilibrium - 

Equilibrium describes the average condition of a system, as measured through one of its elements or attributes, over a specific period of time.

Equinox - 

Two days during the year when the declination of the Sun is at the equator. The September equinox occurs on September 22 or 23. The March equinox occurs on March 20 or 21. On these days, all locations on our planet (except the poles) experience equal (12 hour) day and night.

Era - 

Geologic time unit that is shorter than an eon but longer than a period.

Erg Desert - 

A region in a desert where sand is very abundant.

Erosion - 

The removal of weathered sediment or rocks by the forces of wind, water, and ice.

Erosional Landform - 

Is a landform formed from the removal of weathered and eroded surface materials by wind, water, glaciers, and gravity. This includes landforms with some of the following geomorphic features: river valleys, glacial valleys, and coastal cliffs.

Erratic - 

A large rock boulder that has been transported by glaciers away from its origin and deposited in a region of dissimilar rock.

Esker - 

Long twisting ridges of sand and gravel found on the Earth's surface. Created when the deposits of subsurface glacial streams are placed on the ground after glacial melting.

Establishment - 

Subsequent growth and/or reproduction of a colonized species in a new territory.

Estimator - 

An estimator is any value calculated from the sample data For example, the sample mean is an estimator of the population mean.

Estuary - 

Somewhat enclosed coastal area at the mouth of a river where nutrient rich fresh water meets with salty ocean water.

Eustacy - 

Variations in sea-level that are related to changes in the volume of seawater in the oceans.

Eutrophic Lake - 

Lake that has an excessive supply of nutrients, mostly in the form of nitrates and phosphates. Also see mesotrophic lake and oligotrophic lake.

Eutrophication - 

Physical, chemical and biological changes in a water body as a result of the input nitrogen and phosphorus.

Evaporation - 

Evaporation can be defined as the process by which liquid water is converted into a gaseous state. Evaporation can only occur when water is available. It also requires that the humidity of the atmosphere be less than the evaporating surface (at 100% relative humidity there is no more evaporation). The evaporation process requires large amounts of energy. For example, the evaporation of one gram of water at a temperature of 100° Celsius requires 540 calories of heat energy (600 calories at 0° Celsius).

Evaporation Fog - 

A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called steam fog or sea smoke.

Evaporation Pan - 

Meteorological instrument that is used to measure evaporation rates.

Evaporite - 

Type of sedimentary rock that is formed from the concentration of dissolved salts through evaporation.

Evapotranspiration - 

Combined loss of water to the atmosphere via the processes of evaporation and transpiration.

Evergreen Vegetation - 

Vegetation that keeps a majority of their leaves or needles throughout the year. Also see deciduous vegetation and succulent vegetation.

Evolution - 

Is a process by which species come to possess genetic adaptations to their environment. Its mechanism is natural selection. It also requires genetic mutations.

Exfoliation Dome - 

A physical weathering feature associated with granite that is the result of the erosion of overburden material and pressure-release. With the release of pressure, layers of rock break off in sheets or shells leaving a dome-like bedrock feature.

Exogenic - 

Refers to a system that is external to the Earth.

Exosphere - 

The outermost zone in the Earth's atmosphere. This layer has an altitude greater than 480 kilometers and is primarily composed of hydrogen and helium gas.

Exotic Stream - 

A stream that has a course that begins in a humid climate and end in an arid climate. Because of reductions in precipitation and and increases in evaporation, the discharge of these streams deceases downslope. Examples of such streams are the Nile and Colorado Rivers.

Experiment - 

A controlled investigation designed to evaluate the outcomes of causal manipulations on some system of interest.

Exploitation - 

Form of competition where the indirect effects of the two or more species or individuals reduce the supply of the limiting resource or resources needed for survival.

Explosive Eruption - 

Volcanic eruption where high-viscosity granite-rich magma causes an explosion of ash and pyroclastic material. This type of eruption is common to composite and caldera volcanoes.

Extinction - 

Disappearance of a species from all or part of their geographic range. Also see background extinction and mass extinction.

Extrusive Igneous Rock - 

Igneous rock that forms on the surface of the Earth. Also called volcanic igneous rock.

Eye - 

Area in the center of a hurricane that is devoid of clouds.

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.

Gabbro - 

An intrusive igneous rock that develops from mafic magma and whose mineral crystals are coarse. Mineralogically this rock is identical to basalt.

Gaia Hypothesis - 

The Gaia hypothesis states that the temperature and composition of the Earth's surface are actively controlled by life on the planet. It suggests that if changes in the gas composition, temperature or oxidation state of the Earth are induced by astronomical, biological, lithological, or other perturbations, life responds to these changes by growth and metabolism.

Galaxy - 

An assemblage of millions to hundreds of billions of stars.

Gall-Peters Projection - 

Map projection system that reduces the area distortion found in Mercator projections.

Gamma Radiation - 

A type of ionizing, electromagnetic radiation that readily penetrates the body tissues of organisms. Has a wavelength less than 0.03 nanometers.

Gas - 

A state of matter where molecules are free to move in any direction they like. The state of matter where the substance completely fills any container that it occupies.

GCM - 

Computer-based climate model that produces future forecast of weather and climate conditions for regions of the Earth or the complete planet. Uses complex mathematical equations and physical relationships to determine a variety of climate variables in a three-dimensional grid.

Gelifluction - 

Form of mass movement in periglacial environment where a permafrost layer exists. It is characterized by the movement of soil material over the permafrost layer and the formation of lobe-shaped features. Also see solifluction.

Gene Pool - 

Sum total of all the genes found in the individuals of the population of a particular species.

General Circulation Model - 

Computer-based climate model that produces future forecast of weather and climate conditions for regions of the Earth or the complete planet. Uses complex mathematical equations and physical relationships to determine a variety of climate variables in a three-dimensional grid.

Generalist Species - 

Species that can survive and tolerate a broad range of environmental conditions.

Genetic Adaptation - 

Changes in the genetic makeup of organisms of a species due to mutations that allow the species to reproduce and gain a competitive advantage under changed environmental conditions.

Genetic Diversity - 

Genetic variability found in a population of a species or all of the populations of a species. Also see biodiversity, ecosystem diversity, and species diversity.

Genus - 

A group in the classification of organisms. Classification level above the species group. It consists of similar species. Similar genera (plural form of genus) are grouped into a family.

Geocoding - 

The conversion of features found on an analog map into a computer-digital form. In this process, the spatial location of the various features is referenced geographically to a coordinate system used in the computer's software system.

Geodesy - 

The science that measures the surface features of the Earth.

Geographic Cycle - 

Theory developed by William Morris Davis that models the formation of river-eroded landscapes. This theory suggests that landscapes go through three stages of development (youth, maturity, and old age) and argues that the rejuvenation of landscapes arises from tectonic uplift of the land.

Geographic Information System - 

A geographic information system merges information in a computer database with spatial coordinates on a digital map.

Geographic Isolation - 

Reproductive isolation of two or more populations of a species by distance or physical barriers. Over long periods of time geographic isolation leads to speciation through divergent evolution because of environmental heterogeneity. Also called spatial isolation.

Geographic Range - 

Spatial distribution of a species. The geographic ranges of species often fluctuate over time

Geographical Coordinate System - 

System that uses the measures of latitude and longitude to locate points on the spherical surface of the Earth.

Geography - 

The study natural and human constructed phenomena relative to a spatial dimension.

Geoid - 

True shape of the Earth, which deviates from a perfect sphere because of a slight bulge at the equator.

Geologic Time Scale - 

(1) Scale used to measure time relative to events of geological significance.
(2) Time scale that occurs over millions and billions of years.

Geology - 

The field of knowledge that studies the origin, structure, chemical composition, and history of the Earth and other planets.

Geomorphic Threshold - 

The amount of slow accumulated change a landform can take before it suddenly moves into an accelerated rate of change that takes it to a new system state.

Geomorphology - 

The field of knowledge that investigates the origin of landforms on the Earth and other planets.

Geostationary Operational Environmental Satellite - 

Series of geostationary meteorological satellites launched by the United States starting in 1968. The main purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting and environmental monitoring.

Geostationary Orbit - 

Satellite that has an orbit that keeps it over the same point on the Earth at all times. This is accomplished by having the satellite travel in space at the same angular velocity as the Earth.

Geostrophic Wind - 

Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.

Geothermal Energy - 

Heat energy derived from the Earth's interior.

Germination - 

The beginning of vegetative growth of a plant from a seed.

GIS - 

A geographic information system merges information in a computer database with spatial coordinates on a digital map.

Glacial Drift - 

A generic term applied to all glacial and glaciofluvial deposits.

Glacial Ice - 

A very dense form frozen water that is much harder than snow, névé, or firn.

Glacial Lake - 

A natural impoundment of melt-water at the front of a glacier.

Glacial Milk - 

Term used to describe glacial melt-water which has a light colored or cloudy appearance because of clay-sized sediment held in suspension.

Glacial Polish - 

The abrasion of bedrock surfaces by materials carried on the bottom of a glacier. This process leaves these surfaces smooth and shiny.

Glacial Retreat - 

The backwards movement of the snout of a glacier.

Glacial Surge - 

A rapid forward movement of the snout of a glacier.

Glacial Trough - 

A deep U-shaped valley with steep valley walls that was formed from glacial erosion. At the base of many of these valleys are cirques.

Glacial Uplift - 

Upward movement of the Earth's crust following isostatic depression from the weight of the continental glaciers.

Glacial Valley - 

Valley that was influenced by the presence of glaciers. The cross-section of such valleys tends to be U-shaped because of glacial erosion. Similar to glacial trough.

Glaciation - 

(1) Period of time during an ice age when glaciers advance because of colder temperatures.
(2) Involving glaciers and moving ice. Usually pertaining to processes associated with glaciers.

Glacier - 

A large long lasting accumulation of snow and ice that develops on land. Most glaciers flow along topographic gradients because of their weight and gravity.

Glaciofluvial - 

Geomorphic feature whose origin is related to the processes associated with glacial meltwater.

Glaze - 

Coating of ice that forms when rain falls on a surface with a temperature below freezing.

Gleization - 

A soil formation process that occurs in poorly drained environments. Results in the development of extensive soil organic layer over a layer of chemically reduced clay that takes on a blue color.

Gleysol Soil - 

Soil order (type) of the Canadian System of Soil Classification. This soil type is found in habitats that are frequently flooded or permanently waterlogged. Its soil horizons show the chemical signs of oxidation and reduction.

Global Dimming - 

Global Dimming is a decrease in the amount of sunlight reaching the surface of the earth, believed to be caused by pollution in the atmosphere

Global Positioning System - 

System used to determine latitude, longitude, and elevation anywhere on or above the Earth's surface. This system involves the transmission of radio signals from a number of specialized satellites to a hand held receiving unit. The receiving unit uses triangulation to calculate altitude and spatial position on the Earth's surface.

Global Warming - 

Warming of the Earth's average global temperature because of an increase in the concentration of greenhouse gases. A greater concentration in greenhouse gases in the atmosphere is believed to result in an enhancement of the greenhouse effect.

GMT - 

Former standard world time as measured at Greenwich, England (location of the Prime Meridian). Replace in 1928 with Universal Time (UT).

Gneiss - 

A metamorphosed coarse grained igneous rock. In this rock you get the recrystallization of quartz, feldspars, micas and amphiboles into bands.

GOES - 

Series of geostationary meteorological satellites launched by the United States starting in 1968. The main purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting and environmental monitoring.

GPS - 

System used to determine latitude, longitude, and elevation anywhere on or above the Earth's surface. This system involves the transmission of radio signals from a number of specialized satellites to a hand held receiving unit. The receiving unit uses triangulation to calculate altitude and spatial position on the Earth's surface.

Graben Fault - 

This fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys.

Graded Stream - 

A stream that has a long profile that is in equilibrium with the general slope of the landscape. A graded profile is concave and smooth. Stream's maintain their grade through a balance between erosion, transportation, and deposition. Erosion removes material from bumps in the profile and deposition fills in dips.

Gradient - 

The steepness of a slope as measured in degrees, percentage, or as a distance ratio (rise/run).

Gradient Wind - 

Horizontal wind in the upper atmosphere that moves parallel to curved isobars. Results from a balance between pressure gradient force, Coriolis force, and centripetal force.

Granite - 

Medium to coarse grained igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.

Granitic Magma - 

Felsic magma that generates mainly granitic rocks.

Graphic Scale - 

Way of expressing the scale of a map with a graphic.

Grass - 

Type of plant that has long slender leaves that extend from a short stem or the soil surface.

Grassland - 

Ecosystem whose dominant species are various types of grass. Found in regions where average precipitation is not great enough to support the growth of shrublands or forest.

Graupel - 

A type of precipitation that consists of a snow crystal and a raindrop frozen together. Also called snow pellets.

Gravel - 

A term used to describe unconsolidated sediments composed of rock fragments. These rock fragments have a size that is greater than 2 millimetres.

Gravitational Water - 

Water that moves through soil due to gravitational forces. Soil water in excess of hygroscopic water and capillary water.

Gravity - 

Is the process where any body of mass found in the universe attracts other bodies with a force proportional to the product of their masses and inversely proportional to the distance that separates them. First proposed by Sir Issac Newton in 1686.

Grazing Food Chain - 

Model describing the trophic flow of organic energy in a community or ecosystem.

Great Circle - 

An imaginary circle drawn on the Earth's surface that has its center synchronize to the center of the planet. The equator is a great circle.

Greenhouse Effect - 

The greenhouse effect causes the atmosphere to trap more heat energy at the Earth's surface and within the atmosphere by absorbing and re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth's average global temperature has already increased by 0.3 to 0.6° Celsius, since the beginning of this century, because of this enhancement.

Greenhouse Gases - 

Gases responsible for the greenhouse effect. These gases include: water vapor (H2O), carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFxClx); and tropospheric ozone (O3).

Greenwich Mean Time - 

Former standard world time as measured at Greenwich, England (location of the Prime Meridian). Replace in 1928 with Universal Time (UT).

Grid North - 

The direction north as measured on the Universal Transverse Mercator grid system.

Grid South - 

The direction south as measured on the Universal Transverse Mercator grid system.

Gross Sediment Transport - 

The total amount of sediment transported along a shoreline in a specific time period.

Ground Fog - 

A type of fog that is also called Radiation fog. Ground fog is generated by near surface cooling by radiation loss during the evening hours. For the fog to develop, the overnight cooling must cause saturation occur. This type of fog is normally quite shallow.

Ground Frost - 

Frost that penetrates the soil surface in response to freezing temperatures.

Ground Ice - 

General term used to describe all bodies of ice in the ground surface of the permafrost layer. Also called anchor ice. Some forms of ground ice include: pore ice, needle ice, ice wedge, segregated ice, sand wedge, and ice lenses.

Ground Moraine - 

A thick layer of till deposited by a melting glacier.

Groundwater - 

Water that occupies the pore spaces found in some types of bedrock.

Groundwater Flow - 

Underground topographic flow of groundwater because of gravity.

Groundwater Recharge - 

The replenishment of groundwater with surface water.

Gulf Stream - 

Warm ocean current that originates in and around the Caribbean and flows across the North Atlantic to northwest Europe.

Gust Front - 

A boundary found ahead of a thunderstorm that separates cold storm downdrafts from warm humid surface air. Winds in this phenomenon are strong and fast.

Gypsum - 

Sedimentary rock created by the chemical precipitation of calcium, sulfur, and oxygen.

Gyre - 

Arrangement of surface ocean currents into a large macro-scale circular pattern of flow.

Habitat - 

Location where a plant or animal lives.

Hadean - 

Geologic eon that occurred from 3800 to 4600 million years ago. The Earth's oldest rocks date to the end of this time period.

Hadley Cell - 

Three-dimensional atmospheric circulation cell located at roughly 0 to 30° North and South of the equator. The Hadley cell consists of rising air (intertropical convergence zone) at the equator and descending air (subtropical highs) at 30° North and South.

Hail - 

Hail is a solid form of precipitation that has a diameter greater than 5 millimeters. Occasionally, hailstones can be the size of golf balls or larger. Hailstones of this size can be quite destructive. The intense updrafts in mature thunderstorm clouds are a necessary requirement for hail formation.

Hair Hygrometer - 

Hygrometer that uses the expansion and contraction of hair to determine atmospheric humidity.

Halite - 

Sedimentary rock created by the chemical precipitation of sodium and chlorine.

Hamada - 

A very flat desert area of exposed bedrock.

Hanging Valley - 

A secondary valley that enters a main valley at an elevation well above the main valley's floor. These features are result of past erosion caused by alpine glaciers. Hanging valleys are often the site of spectacular waterfalls.

Hanging Wall - 

The topmost surface of an inclined fault.

Hardpan - 

Impervious layer found within the soil. It can result from the precipitation of iron, illuviation of clay or the cementing of sand and gravel by calcium carbonate precipitates.

Headlands - 

A strip of land that juts seaward from the coastline. This feature normally bordered by a cliff.

Headwaters - 

Upper portion of stream's drainage system.

Heat - 

Heat is defined as energy in the process of being transferred from one object to another because of the temperature difference between them. In the atmosphere, heat is commonly transferred by conduction, convection, advection, and radiation.

Heat Capacity - 

Is the ratio of the amount of heat energy absorbed by a substance compared to its corresponding temperature rise.

Heat Energy - 

A form of energy created by the combined internal motion of atoms in a substance.

Heat Island - 

The dome of relatively warm air which develops over the center of urbanized areas.

Helical Flow - 

Movement of water within a stream that occurs as spiral flows.

Heterosphere - 

The upper layer in a two part classification of the atmosphere based on the general homogeneity of chemical composition. In this layer, oxygen atoms and nitrogen molecules dominate and remain constant in their relative quantities. The heterosphere extends upward from a height of 80 to 100 kilometers depending on latitude. Below this layer is the homosphere.

High Pressure - 

An area of atmospheric pressure within the Earth's atmosphere that is above average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars it is called an anticyclone.

HIV - 

Short for human immunodeficiency virus. Any of various strains of a retrovirus of the genus Lentivirus that cause AIDS by infecting the body's immune system.

Holocene Epoch - 

Period of time from about 10,000 years ago to today. During this period glaciers retreated because of a warmer global climate. Time of modern humans.

Homeostasis - 

A constant or non-changing state of equilibrium in a system despite changes in external conditions.

Homeostatic - 

A constant or non-changing state of equilibrium in a system despite changes in external conditions.

Homosphere - 

The lower layer in a two part classification of the atmosphere based on the general homogeneity of chemical composition. In this layer, nitrogen, oxygen, argon, carbon dioxide, and the trace gases dominate and remain constant in their relative proportions. The homosphere extends from the Earth's surface to a height of 80 to 100 kilometers depending on latitude. Above this layer is the heterosphere.

Horizon - 

(1) A surface separating two beds in sedimentary rock.
(2) A layer within a soil showing unique pedogenic characteristics. Four major horizons are normally found in a soil profile: A, B, C, and O.
(3) Point at which the visible edge of the Earth's surface meets the sky.

Horn - 

Pyramidal peak that forms when several cirques erode a mountain from three or more sides.

Horst Fault - 

A fault that is produced when two reverse faults cause a block of rock to be push up.

Host - 

Organism that develops disease from a pathogen or is being feed on by a parasite.

Hot Spot - 

A volcanic area on the surface of the Earth created by a rising plume of magma.

HP - 

An area of atmospheric pressure within the Earth's atmosphere that is above average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars it is called an anticyclone.

Human Geography - 

Field of knowledge that studies human-made features and phenomena on the Earth from a spatial perspective. Sub-discipline of Geography.

Human-Land Tradition - 

Academic tradition in modern Geography that investigates human interactions with the environment.

Humidity - 

A general term used to describe the amount of water vapor found in the atmosphere.

Humus - 

Dark colored semi-soluble organic substance formed from decomposition of soil organic matter.

Hurricane - 

An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour.

Hydration - 

A form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

Hydraulic Gradient - 

The slope of the water table or aquifer. The hydraulic gradient influences the direction and rate of groundwater flow.

Hydrograph - 

A graph describing stream discharge over time.

Hydrologic Cycle - 

Model that describes the movement of water between the hydrosphere, lithosphere, atmosphere, and biosphere.

Hydrology - 

Field of physical geography that studies the hydrosphere.

Hydrolysis - 

Chemical weathering process that involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involve through the release of the hydroxide ions.

Hydrosphere - 

The hydrosphere describes the waters of the Earth. Water exists on the Earth in various stores, including the: atmosphere, oceans, lakes, rivers, glaciers, snowfields and groundwater. Water moves from one store to another by way of: evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, and groundwater flow.

Hydrostatic Pressure - 

Force caused by water under pressure.

Hygrometer - 

An instrument for measuring atmospheric humidity.

Hygroscopic - 

Substances that have the ability to absorb water and therefore accelerate the condensation process.

Hypothesis - 

A tentative assumption that is made for the purpose of empirical scientific testing. A hypothesis becomes a theory when repeated testing and evidence suggests the hypothesis has a strong chance of being correct.

Hypothesis Testing - 

Process where an alternative and a null hypothesis are statistically tested for the purpose of falsifying a hypothesis.

Ice Age - 

Period of time when glaciers dominate the landscape of the Earth. The last major Ice Age was during the Pleistocene epoch.

Ice Age - 

Period of time from about 2 million years ago to 10,000 years ago. During this period areas of land at higher and middle latitudes where covered with glacial ice.

Ice Cap - 

Large dome-shaped glacier found covering a large expanse of land. Smaller than an ice sheet.

Ice Fall - 

An area of crevassed ice on a glacier. Caused when the base of the glacier flows over steep topography.

Ice Field - 

Large level area of glacial ice found covering a large expanse of land. Similar in size to an ice cap but does not have a dome-shape.

Ice Fog - 

A fog that is composed of small suspended ice crystals. Common in Arctic locations when temperatures are below -30° Celsius and a abundant supply of water vapor exists.

Ice Sheet - 

A dome-shaped glacier covering an area greater than 50,000 square kilometers. Greenland and Antarctica are considered ice sheets. During the glacial advances of the Pleistocene ice sheets covered large areas of North America, Europe, and Asia. Larger than an ice cap.

Ice Shelf - 

Large flat layer of ice that extends from the edge of the Antarctic ice cap into the Antarctic Ocean. Source of icebergs.

Iceberg - 

A mass of ice found floating in the ocean or a lake. Often icebergs form when ice calves from land-based glaciers into the water body. Icebergs can be dangerous to shipping in high and mid-latitude regions of the ocean because 90 percent of their mass lies below the ocean surface.

Ideal Gas Law - 

This law describes the physical relationships that exist between pressure, temperature, volume, and density for gases. Two mathematical equations are commonly used to describe this law:
Pressure x Volume = Constant x Temperature
and
Pressure = Density x Constant x Temperature

Igneous Rock - 

Rocks formed by solidification of molten magma either beneath (intrusive igneous rock) or at (extrusive igneous rocks) the Earth's surface.

Illuviation - 

Deposition of humus, chemical substances, and fine mineral particles in the lower layers of a soil from upper layers because of the downward movement of water through the soil profile. Compare with eluviation.

Immigration - 

Migration of an organism into an area for the purpose of changing its residence permanently. Compare with emigration.

Index Contour - 

Contour line that is accentuated in thickness and is often labeled with the appropriate measure of elevation. Index contours occur every four or fifth contour interval and help the map user read elevations on a map.

Industrial Smog - 

Form of air pollution that develops in urban areas. This type of air pollution consists of a combination of sulfur dioxide, suspended droplets of sulfuric acid, and a variety of suspended solid particles. Also see photochemical smog.

Infiltration - 

The absorption and downward movement of water into the soil layer.

Infiltration Capacity - 

The ability of a soil to absorb surface water.

Infiltration Rate - 

Rate of absorption and downward movement of water into the soil layer.

Inner Core - 

Inner region of the Earth's core. It is thought to be solid iron and nickel with a density of about 13 grams per cubic centimeter. It also has a diameter of about 1220 kilometres.

Inorganic - 

Non-living thing. Usually refers to the physical and chemical components of an organism's environment. Some times called abiotic.

Inselberg - 

A German term used to describe a steep-sided hill composed of rock that rises from a pediplain.

Insolation - 

Direct or diffused shortwave solar radiation that is received in the Earth's atmosphere or at its surface.

Insolation Weathering - 

Form of physical weathering. Involves the physical breakdown of minerals and rock due to thermal expansion and contraction.

Instability - 

Atmospheric condition where a parcel of air is warmer that the surrounding air in the immediate environment. This condition causes the parcel to rise in the atmosphere. Also see unstable atmosphere.

Interception - 

Is the capture of precipitation by the plant canopy and its subsequent return to the atmosphere through evaporation or sublimation. The amount of precipitation intercepted by plants varies with leaf type, canopy architecture, wind speed, available radiation, temperature, and the humidity of the atmosphere.

Interglacial - 

Period of time during an ice age when glaciers retreated because of milder temperatures.

Intermittent Stream - 

A stream that flows only for short periods over a year. Flow events are usually initiated by rainfall.

International Date Line - 

A line drawn almost parallel to the 180 degree longitude meridian that marks the location where each day officially begins. The location of the International Date Line was decided upon by international agreement.

Intertropical Convergence Zone - 

Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Location of the thermal equator.

Intrusive Igneous Rock - 

A mass of igneous rock that forms when magma from the mantle migrates upward and cools and crystallizes near, but not at, the Earth's surface. Also called plutonic igneous rock. Also see dyke, sill, and batholith.

Inversion - 

Situation where a layer of warmer air exists above the Earth's surface in a normal atmosphere where air temperature decreases with altitude. In the warmer layer of air, temperature increases with altitude. Also see temperature inversion.

Ionosphere - 

A region in the atmosphere above 50 kilometers from the surface where relatively large concentrations of ions and free electrons exist. The ionosphere is important for human communications because it re-directs AM radio transmissions. This process extends the distance that radio transmissions can travel.

Island Arc - 

A line of volcanic islands found of the ocean that have been created by the convergence of two tectonic plates and the subsequent subduction of one of the plates beneath the other. Subduction cause magma plumes to rise to the Earth's surface creating the volcanic islands.

Isobar - 

Lines on a map joining points of equal atmospheric pressure.

Isoline - 

Lines on a map joining points of equal value.

Isostacy - 

The buoyant condition of the Earth's crust floating in the asthenosphere. The greater the weight of the crust the deeper it floats into the asthenosphere. When weight is removed the crust rises higher.

Isostatic Depression - 

Large scale sinking of the crust into the asthenosphere because of an increase in weight on the crustal surface. Common in areas of continental glaciation where the crust was depressed by the weight of the ice.

Isostatic Rebound - 

The upward movement of the Earth's crust following isostatic depression.

Isotherm - 

Lines on a map joining points of equal temperature.

Isothermal Layer - 

Vertical layer in the atmosphere where temperature remains unchanged. In the Earth's atmosphere, three isothermal layers are found in the lower regions of the stratosphere, mesosphere, and the thermosphere.

Isotopic Dating - 

Dating technique used to determine the age of rock and mineral through the decay of radioactive elements.

ITCZ - 

Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Location of the thermal equator.

Jet Stream - 

Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. A number of jet streams have been identified in the atmosphere. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers. This jet stream flows from west to east at average speeds, depending on the time of year, between 110 to 185 kilometers per hour. Another strong jet stream occurs above the sub-tropical highs at an altitude of 13 kilometers. This jet stream is commonly called the subtropical jet stream. The subtropical jet stream's winds are not as strong as the polar jet stream.

Joint - 

A fracture in a rock where no movement has taken place or where no movement has taken place perpendicular to the surface of the fracture. Important in rock weathering because it increases the exposed surface area.

June Solstice - 

Date during the year when the declination of the Sun is at 23.5° North of the equator. During the June solstice, locations in the Northern Hemisphere experience their longest day. The June solstice is also the first day of summer in the Northern Hemisphere. Locations in the Southern Hemisphere have their shortest day on the June solstice. This date also marks the first day of winter in the Southern Hemisphere.

Jurassic - 

Geologic period that occurred roughly 144 to 208 million years ago. During this period, the first birds and mammals appear and large areas of the continents are covered by shallow seas.

Kame - 

A steep conical hill composed of glaciofluvial sediments. This feature develops when glacial crevasses and depressions in stagnant glacial ice are filled with sand and gravel deposits from sediment loaded meltwater.

Kame Terrace - 

A long flat ridge composed of glaciofluvial sediment. This feature forms along the margin of a valley glacier where the glacial ice meets the valley's slope. Sediment is deposited by laterally flowing meltwater streams.

Kaolinite - 

A type of clay that is widespread in tropical and subtropical regions. It is produced from the weathering of granite.

Karst - 

Landform type with limestone bedrock and dominated by geomorphic features created from solution chemical weathering.

Katabatic - 

A wind common to mountainous regions that involves heavy cold air flowing along the ground from high to low elevations because of gravity. Also called mountain wind.

Katabatic Wind - 

Any wind blowing down the slope of a mountain.

Kelvin Scale - 

Scale for measuring temperature. In this scale, absolute zero is 0 Kelvins, water boils at 373.15 Kelvins and freezes at 273.15 Kelvins.

Kettle Hole - 

Depression found in glacial deposits. Created when a piece of ice from a retreating glacier becomes embedded in soft glacial till or glacial drift deposits. Many are filled with water to form a small lake or pond.

Kettle Moraine - 

An area of glaciofluvial influenced moraine deposits pitted with kames and kettle holes.

Kilopascal - 

A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 10,000 dynes per square centimetre.

Kirchoff's Law - 

This law suggests that good emitters of radiation are also good absorbers of radiation at specific electromagnetic radiation wavelength bands. It also suggests that poor emitters of radiation are also poor absorbers of radiation at specific wavelength bands.

Knickpoint - 

Point on the long profile of a stream where the gradient is broken sudden drop in elevation. Knickpoints are the locations of rapids and waterfalls.

Köppen Climate Classification - 

System that uses monthly precipitation and temperature data and total annual precipitation data to classify a location's climate into one of five main categories: Tropical Moist Climates; Dry Climates; Moist Mid-latitude Climates with Mild Winters; Moist Mid-Latitude Climates with Cold Winters; and Polar Climates. These categories are further divided into number of subcategories. First developed in 1918 by German biologist W. Köppen, this system has undergone a number of modifications.

kPa - 

A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 10,000 dynes per square centimetre.

La Niña - 

Condition opposite of an El Niño. In a La Niña, the tropical Pacific trade winds become very strong and an abnormal accumulation of cold water occurs in the central and eastern Pacific Ocean.

Lagoon - 

(1) A body of seawater that is almost completely cut off from the ocean by a barrier beach.
(2) The body of seawater that is enclosed by an atoll.

Lahar - 

A very rapid type of downslope mass movement that involving mudflows from volcanic ash.

Lake - 

A body standing water found on the Earth's continental land masses. The water in a lake is normally fresh. Also see eutrophic lake, mesotrophic lake, and oligotrophic lake.

Laminar Flow - 

Movement of water within a stream that occurs as uninterrupted parallel flows. Laminar flow generally occurs in areas where friction is low.

Land Breeze - 

Local thermal circulation pattern found at the interface between land and water. In this circulation system, surface winds blow from land to water during the night.

Landfall - 

The coastline location where a tropical storm or hurricane moves from ocean onto land.

Landsat - 

Series of satellites launched by NASA for the purpose of remotely monitoring resources on the Earth. The first Landsat satellite was launched by the United States in 1972. Landsat uses two types of sensors to monitor the Earth: Thematic Mapper and Multispectral Scanner. See the following website for more information - Landsat Program.

Landslide - 

Term used to describe the downslope movement of soil, rock, and other weathered materials because of gravity.

Landward - 

Positioned or located away from a water body but towards the land.

Latent Heat - 

Is the energy required to change a substance to a higher state of matter (solid > liquid > gas). This same energy is released from the substance when the change of state is reversed (gas > liquid > solid).

Latent Heat Flux - 

Latent heat flux is the global movement of latent heat energy through circulations of air and water. Atmospheric circulation moves latent heat energy vertically and horizontally to cooler locations where it is condensed as rain or is deposited as snow releasing the heat energy stored within it.

Latent Heat of Condensation - 

The amount of heat energy release to the environment when a gas changes its state to a liquid. For one gram of water, the amount of heat energy released is 540 calories at a temperature of 100° Celsius.

Latent Heat of Vaporization - 

The amount of heat energy required from the environment to change the state of a liquid to a gas. For one gram of water, the amount of heat energy required is 540 calories at a temperature of 100° Celsius.

Lateral Moraine - 

Moraine that is found along the sides of a glacier. Commonly found on glaciers that occupy a valley.

Laterite - 

Hard subsurface deposit of oxides of aluminum and iron found in tropical soils where the water table fluctuates with seasonal changes in precipitation.

Latitude - 

Latitude is a north-south measurement of position on the Earth. It is defined by the angle measured from a horizontal plane located at the Earth's center that is perpendicular to the polar axis. A line connecting all places of the same latitude is termed a parallel. Latitude is measured in degrees, minutes, and seconds. Measurements of latitude range from equator (0°) to 90° North and South from this point.

Laurasia - 

Northern section of Pangaea.

Lava - 

Molten magma released from a volcanic vent or fissure.

Lava Flow - 

Stream of lava flowing from a volcanic vent.

LDC - 

Country characterized by minimal industrialization, low technological development, low per capita income, and high population growth rates. Many of these countries are found in Asia, Africa, and Central and South America. Also see more developed country.

Leaching - 

Process in which water from precipitation removes plant nutrients from the surface of leaves.

Leaching - 

Process in which water removes and transports soil humus and inorganic nutrients in solution.

Lee - 

Side of a slope that is opposite to the direction of flow of ice, wind, or water. Opposite of stoss.

Leeward - 

Downwind side of an elevated area like a mountain. Opposite of windward.

Less Developed Country - 

Country characterized by minimal industrialization, low technological development, low per capita income, and high population growth rates. Many of these countries are found in Asia, Africa, and Central and South America. Also see more developed country.

Levee - 

Ridge of coarse deposits found alongside the stream channels and elevated above the floodplain. Forms from the deposition of sediment during floods.

Light Year - 

Distance that light travels in the vacuum of space in one year. Approximately 9.7 trillion kilometres.

Lightning - 

Visible discharge of electricity created by thunderstorms.

Lignite - 

Low grade coal. Also called brown coal.

Limestone - 

Sedimentary rock composed of carbonate minerals, especially calcium carbonate. Limestone can be created by clastic and non-clastic processes. Clastic limestones are formed from the break up and deposition of shells, coral and other marine organisms by wave-action and ocean currents. Non-clastic limestones can be formed either as a precipitate or by the lithification of coral reefs, marine organism shells, or marine organism skeletons.

Liquefaction - 

Temporary transformation of a soil mass of soil or sediment into a fluid mass. Occurs when the cohesion of particles in the soil or sediment is lost. Often triggered by seismic waves from an earthquake. For this condition to take place the pore spaces between soil particles must be at or near saturation.

Liquid - 

A state of matter where molecules have the ability to flow and the surface of this mass displays the property of surface tension.

Lithification - 

Process by which sediments are consolidated into sedimentary rock.

Lithosphere - 

Is the solid inorganic portion of the Earth (composed of rocks, minerals, and elements). It can be regarded as the outer surface and interior of the solid Earth.

Litter - 

Accumulation of leaves, twigs and other forms of organic matter on the soil surface. In most soils, the surface layer of litter is at various stages of decomposition.

Litterfall - 

Movement of leaves, twigs and other forms of organic matter from the biosphere to the litter layer found in soil.

Little Climatic Optimum - 

Time period from 900 - 1200 AD. Warmest period since the Climatic Optimum.

Little Ice Age - 

Time period from 1550 to 1850 AD. During this period, global temperatures were at their coldest since the beginning of the Holocene.

Littoral Drift - 

The sediment that is transported by waves and currents through beach drift and longshore drift along coastal areas.

Littoral Transport - 

The process of sediment moving along a coastline. This process has two components: longshore transport and onshore-offshore transport.

Littoral Zone - 

The zone along a coastline that is between the high and low-water spring tide marks.

Loam - 

A soil that contains a roughly equal mixture of clay, sand, and silt. Good for growing most crops.

Location - 

A term used in geography that deals with the relative and absolution spatial position of natural and human-made phenomena.

Long Wave - 

A large wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called Rossby waves.

Longitude - 

Longitude is a west-east measurement of position on the Earth. It is defined by the angle measured from a vertical plane running through the polar axis and the prime meridian. A line connecting all places of the same longitude is termed a meridian. Longitude is measured in degrees, minutes, and seconds. Measurements of longitude range from prime meridian (0°) to 180° West and East from this point.

Longshore Current - 

A water current that moves parallel to the shoreline.

Longshore Drift - 

The movement and deposition of coastal sediments because of longshore currents.

Longshore Transport - 

The transport of sediment in water parallel to a shoreline.

Longwave Radiation - 

Form of electromagnetic radiation with a wavelength between 0.7 and 100 micrometers (µm). Also called longwave radiation.

Low Pressure - 

An area of atmospheric pressure within the Earth's atmosphere that is below average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars it is called a cyclone.

Lower Mantle - 

Layer of the Earth's interior extending from 670 to 2,900 kilometers below the surface crust. Composed of ultramafic rock. This layer is hot and plastic and part of the mantle layer.

Lysimeter - 

Meteorological instrument used to measure potential and actual evapotranspiration.

Mafic Magma - 

Magma that is relative poor in silica but rich in calcium, magnesium, and iron content. This type of magma solidifies to form rocks relatively rich in calcium, magnesium, and iron but poor in silica.

Magma - 

Molten rock originating from the Earth's interior.

Magma Plume - 

A rising vertical mass of magma originating from the mantle.

Magnetic Declination - 

The horizontal angle between true north and magnetic north or true south and magnetic south.

Magnetic Field - 

The space influence by magnetic force. The Earth's magnetic field is believed to be generated by the planet's core.

Magnetic North - 

Location in the Northern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.

Magnetic Reversal - 

A change in the polarity of the Earth's magnetic field. In the past 4 million years there have been nine reversals.

Magnetic South - 

Location in the Southern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.

Magnetosphere - 

Zone that surrounds the Earth that is influenced by the Earth's magnetic field.

Magnitude - 

(1) The quantifiable size of a natural event.
(2) A quantitative measure of the size of an earthquake using the Richter scale.

Mangrove - 

Treed wetlands located on the coastlines in warm tropical climates.

Mantle - 

Layer of the Earth's interior composed of mostly solid rock that extends from the base of crust to a depth of about 2,900 kilometres.

Map Projection - 

Cartographic process used to represent the Earth's three-dimensional surface onto a two-dimension map. This process creates some type of distortion artifact on the map.

Map Scale - 

Ratio between the distance between two points found on a map compared to the actual distance between these points in the real world.

Maps - 

An abstraction of the real world that is used to depict, analyze, store, and communicate spatially organized information about physical and cultural phenomena.

Marble - 

Metamorphic rock created by the recrystallization of calcite and/or dolomite.

March Equinox - 

One of two days during a year when the declination of the Sun is at the equator. The March equinox denotes the first day of spring in the Northern Hemisphere. In the Southern Hemisphere, this date marks the first day of fall. During the March equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night. The March equinox occurs on either March 20 or 21.

Marine - 

With reference to ocean environments and processes.

Maritime Effect - 

The effect that large ocean bodies have on the climate of locations or regions. This effect results in a lower range in surface air temperature at both daily and annual scales. Also see Continental Effect.

Maritime Polar Air Mass - 

Air mass that forms over extensive ocean areas of the middle to high latitudes. Around North America, these air mass system form over the Atlantic and Pacific oceans at the middle latitudes. Maritime Polar air masses are mild and humid in summer and cool and humid in winter. In the Northern Hemisphere, maritime polar air masses are normally unstable during the winter. In the summer, atmospheric stability depends on the position of the air mass relative to a continent. Around North America, Maritime Polar air masses found over the Atlantic are stable in summer, while Pacific systems tend to be unstable.

Maritime Tropical Air Mass - 

Air mass that forms over extensive ocean areas of the low latitudes. Around North America, these system form over the Gulf of Mexico and the eastern tropical Pacific. Maritime Tropical air masses are warm and humid in both winter and summer. In the Northern Hemisphere, maritime tropical air masses can normally stable during the whole year if they have form just west of a continent. If they form just east of a continent, these air masses will be unstable in both winter and summer.

Mass Balance - 

The relative balance between the input and output of material within a system.

Mass Extinction - 

A catastrophic, widespread perturbation where major groups of species become extinct in a relatively short time compared to normal background extinctions.

Mass Movement - 

General term that describes the downslope movement of sediment, soil, and rock material.

Mass Wasting - 

General term that describes the downslope movement of sediment, soil, and rock material.

mb - 

A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 1000 dynes per square centimetre.

MDC - 

A highly industrialized country characterized by significant technological development, high per capita income, and low population growth rates. Examples of such countries include the United States, Canada, Japan, and many countries in Europe. Also see less developed country.

Mean - 

Statistical measure of central tendency in a set of data. The mean is calculated by adding all of the data values and dividing this quantity by the total number of data values.

Mean Sea-Level - 

The average height of the ocean surface as determined from the mean of all tidal levels recorded at hourly intervals.

Mean Solar Day - 

Time it takes to complete one Earth rotation relative to the position of the Sun (for example, from midnight to midnight). This measurement takes 24 hours and is longer than a sidereal day because it includes the effect of the Earth's movement (Earth revolution) around the Sun.

Meander - 

Sinuous shaped stream channel. Usually found in streams flowing over a very shallow elevation grade.

Medial Moraine - 

Deposit of material found down the center of a glacier. Created when two glacier and their lateral moraines merge.

Median - 

Statistical measure of central tendency in a set of data. The median is the value halfway through a data set where the values have been ordered from lowest to highest. In an even data set, the median is the average of the two halfway values.

Melting - 

The physical process of a solid becoming a liquid. For water, this process requires approximately 80 calories of heat energy for each gram converted.

Meltwater - 

Water produced from the melting of snow and/or glacial ice.

Mercalli Scale - 

A scale for rating the power of an earthquake.

Mercator Projection - 

Map projection system that presents true compass direction. Distortion is manifested in terms of area. Area distortion makes continents in the middle and high latitudes seem larger than they should be. Specifically designed for nautical navigation.

Mercury Barometer -  Type of barometer that measures changes in atmospheric pressure by the height of a column of mercury in a U-shaped tube which has one end sealed and the other end immersed in an open container of mercury. The force of the pressure exerted by the atmosphere on the mercury in the open container pushes mercury up the other end of the tube. The height of this level is then used as a measure of atmospheric pressure relative to the surface level of the mercury in the container.
Meridian -  A circular arc that meets at the poles and connects all places of the same longitude.
Meridional -  Movement of wind or ocean waters in a direction that is roughly perpendicular to the lines of latitude.
Meridional Transport -  Transport of atmospheric and oceanic energy from the equator to the poles.
Mesa -  A flat topped hill that rises sharply above the surrounding landscape. The top of this hill is usually capped by a rock formation that is more resistant to weathering and erosion.
Mesocyclone -  A cylinder of cyclonically flowing air that form vertically in a severe thunderstorm. They measure about 3 to 10 kilometers across. About 50% of them spawn tornadoes.
Mesopause -  Thin boundary layer found between the mesosphere and the thermosphere. It is usually found at an average altitude of 80 kilometers. Coldest temperatures in the atmosphere are found at the mesopause.
Mesoscale Convective Complex -  A cluster of thunderstorms covering an area of 100,000 kilometers or more. Convective circulation within this system encourages the growth of new thunderstorms for up to 18 hours.
Mesosphere -  Atmospheric layer found between the stratosphere and the thermosphere. Usually located at an average altitude of 50 to 80 kilometers above the Earth's surface. Air temperature within the mesosphere decreases with increasing altitude.
Mesozoic -  Geologic era that occurred from 245 to 65 million years ago.
Metamorphic Rock -  A rock that forms from the recrystallization of igneous, sedimentary or other metamorphic rocks through pressure increase, temperature rise, or chemical alteration.
Metamorphism -  Process that creates metamorphic rocks.
Metasomatic Metamorphism -  Form of metamorphism that causes the chemical replacement of elements in rock minerals when gases and liquids permeate into bedrock.
Meteor -  A body of matter that enters the Earth's atmosphere from space. While traveling through the atmosphere, these objects begin to burn because of friction and are sometimes seen as luminous streaks in the sky by ground observers. Many of these objects burn up completely and never reach the Earth's surface.
Meteorology -  The scientific study of the atmosphere and its associated phenomena.
Methane -  Methane is very strong greenhouse gas found in the atmosphere. Methane concentrations in the atmosphere have increased by more than 140% since 1750. The primary sources for the additional methane added to the atmosphere (in order of importance) are: rice cultivation, domestic grazing animals, termites, landfills, coal mining, and oil and gas extraction. Chemical formula for methane is CH4.
Mica -  Silicate mineral that exhibits a platy crystal structure and perfect cleavage. Common forms of mica are biotite and muscovite.
Microwave Radiation -  Form of electromagnetic radiation with a wavelength between 0.1 to 100 centimeters.
Mid-Latitude Cyclone -  Cyclonic storm that forms primarily in the middle latitudes. Its formation is triggered by the development of troughs in the polar jet stream. These storms also contain warm, cold and occluded fronts. Atmospheric pressure in their center can get as low as 970 millibars. Also called wave cyclones or frontal cyclones.
Mid-Oceanic Ridge -  Chain of submarine mountains where oceanic crust is created from rising magma plumes and volcanic activity. Also associated with this feature is plate divergence which creates a rift zone.
Migration -  Movement of organisms in an intentional way between two points in space. Many migrations are seasonal.
Milankovitch Theory -  Theory proposed by Milutin Milankovitch that suggests that changes in the Earth's climate are cause by variations in solar radiation received at the Earth's surface. These variations are due to cyclical changes in the geometric relationship between the Earth and the Sun.
Military Grid Reference System -  A simplified subset of the Universal Transverse Mercator (UTM) Grid System. This rectangular coordinate system used to find location of points on the Earth's surface. Based on the Universal Transverse Mercator projection system.
Milky Way Galaxy -  Aggregation of about 400 billion stars in a flattened, disk-shaped structure in space. Our solar system is found in this structure.
Miller Cylindrical Projection -  Map projection that mathematically projects the Earth's surface onto a cylinder that is tangent at the equator. Directions and distances are only true at the equator. Distance, area, and shape distortion increases as one moves towards the poles. Very popular projection used in world maps.
Millibar -  A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 1000 dynes per square centimetre.
Mineral -  Component of rocks. A naturally occurring inorganic solid with a crystalline structure and a specific chemical composition. Over 2,000 types of minerals have been classified.
Mineralization -  Decomposition of organic matter into its inorganic elemental components.
Mississippian -  Geologic period that occurred roughly 320 to 360 million years ago. During this period, insects undergo major speciation and ferns first appear. Trees become a dominant plant form on continents.
Mistral -  Term used to describe a katabatic wind in southern France.
Mixed Tide -  Tides that have a higher high water and lower high water as well as higher low water and lower low water per tidal period.
Mode -  Statistical measure of central tendency in a set of data. The mode is the most frequently occurring value in a data set. Data sets can contain two or more mode values that occur with the same frequency.
Model -  (1) Generalization of reality. (2) System describing how a phenomenon functions. (3) Mathematical representation of a system from which predictions or inferences can be made.
Moho Discontinuity -  The lower boundary of the crust. At this boundary seismic wave velocities show an increase in speed as they enter the upper mantle.
Molecule -  Minute particle that consists of connected atoms of one or many elements.
Mollweide Projection -  Map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Monocline -  A fold in layered rock that creates a slight bend.
Monsoon -  A regional scale wind system that predictably change direction with the passing of the seasons. Monsoon winds blow from land to sea in the winter, and from sea to land in the summer. Summer monsoons are often accompanied with precipitation.
Montmorillonite -  A type of clay that has a large capacity to shrink and expand with wetting and drying.
Montreal Protocol -  Treaty signed in 1987 by 24 nations to cut the emissions of chlorofluorocarbons (CFCs) into the atmosphere. Since 1987 the treaty has been amended to quicken the reduction in CFC production and use.
Moraine -  A hill of glacial till deposited directly by a glacier.
More Developed Country -  A highly industrialized country characterized by significant technological development, high per capita income, and low population growth rates. Examples of such countries include the United States, Canada, Japan, and many countries in Europe. Also see less developed country.
Moss -  About 9,500 species of plants that belong to the division bryophyta. These low growing plants are common in moist habitats.
Mountain Breeze -  Local thermal circulation pattern found in areas of topographic relief. In this circulation system, surface winds blow from areas of higher elevation to valley bottoms during the night.
Mountain wind. -  A wind common to mountainous regions that involves heavy cold air flowing along the ground from high to low elevations because of gravity. Also called katabatic wind.
Mouth -  End of a stream. Point at which a stream enters a lake, sea, or ocean.
Movement -  A term used in geography that deals with the migration, transport, communication, and interaction of natural and human-made phenomena across the spatial dimension
MSS -  Remote sensing device found on Landsat satellites that acquires images in four spectral bands from visible to reflected infrared.
Mudflow -  Form of mass movement where fine textured sediments and soil mix with water to create a liquid flow.
Mudstone -  Fine grained sedimentary rock composed of lithified silt and clay particles.
Multispectral Scanner -  Remote sensing device found on Landsat satellites that acquires images in four spectral bands from visible to reflected infrared.
Muscovite -  Rock forming mineral of the mica group.
Muskeg -  Poorly drained marshes or swamps found overlying permafrost.
Natural Gas -  Hydrocarbon based gas, mainly composed of methane, commonly found in the pores of sedimentary rocks of marine origin.
Natural Hazards -  (1) Natural phenomena that produce negative effects on life. (2) The study of the hazards of natural phenomena.
Neap Tide -  Tide that occurs every 14 to 15 days and coincides with the first and last quarter of the moon. This tide has a small tidal range because the gravitational forces of the moon and Sun are perpendicular to each other. Contrasts with spring tide.
Needle Ice -  A form of periglacial ground ice that consists of groups ice slivers at or immediately below the ground surface. Needle ice is about a few centimeters long.
Neutral -  Any substance with a pH around 7.
Neutral Atmosphere -  Condition in the atmosphere where isolated air parcels do not have a tendency to rise or sink. The parcels of air tend to be same temperature as the air that surrounds them.
Névé -  Partially melted and compacted snow that has a density of at least 500 kilograms per cubic meter.
Niche -  Describes the total range of environmental conditions that are suitable for a species existence without the effects of inter-specific competition and predation from other species.
Niche -  Adaptive role that a species has in a habitat. This includes its behavior and interactions with other species.
Nimbostratus Clouds -  Dark, gray low altitude cloud that produces continuous precipitation in the form of rain or snow. Found in an altitude range from the surface to 3,000 meters.
Nivation -  Process where snow patches initiate erosion through physical weathering, meltwater flow, and gelifluction.
Nivation Hollow -  Ground depression found in periglacial areas that is created by nivation.
Noctilucent Clouds -  High altitude clouds composed of ice crystals that appear to glow silver or bright blue shortly after sunset.
Non-Clastic Sedimentary Rock -  Sedimentary rocks that are created either from chemical precipitation and crystallization, or by the lithification once living organic matter.
Non-Renewable Resource -  Resource that is finite in quantity and is being used faster than its ability to regenerate itself.
Normal Distribution -  A common probability distribution displayed by population data. If the values of the distribution are plotted on a graph's horizontal axis and their frequency on the vertical axis the pattern displayed is symmetric and bell-shaped. The central value or mean represents the peak or the most frequently occurring value.
Normal Fault -  Vertical fault where one slab of the rock is displaced up and the other slab down. It is created by tensional forces acting in opposite directions.
Normal Lapse Rate -  Average rate of air temperature change with altitude in the troposphere. This value is approximately a decrease of 6.5° Celsius per 1000 meters rise in elevation.
North Magnetic Pole -  Location in the Northern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.
North Pole -  Surface location defined by the intersection of the polar axis with Earth's surface in the Northern Hemisphere. This location has a latitude of 90° North.
Nuclear Energy -  Energy released when the nucleus of an atom experiences a nuclear reaction like the spontaneous emission of radioactivity, nuclear fission, or nuclear fusion.
Nuclear Fission -  Process where the mass of an atomic nucleus is made smaller by the removal of subatomic particles. This process releases atomic energy in the form of heat and electromagnetic radiation.
Nuclear Fusion -  Process where the mass of an atomic nucleus is made larger by the addition of subatomic particles. This process releases atomic energy in the form of heat and electromagnetic radiation.
Nuee Ardente -  A glowing cloud of dense hot volcanic gas and ash that moves downslope at high speeds, incinerating the landscape.
Null Hypothesis -  Is a hypothesis that has been suggested because it is believed to be true or because it is to be used as a starting point for scientific argument. Used in statistical testing to organize arguments.
Nutrient -  Any food, chemical element or compound an organism requires to live, grow, or reproduce.
Nutrient Cycle -  The cycling of a single element by various abiotic and biotic processes through the various stores found in the biosphere, lithosphere, hydrosphere, and atmosphere.
O Horizon -  Topmost layer of most soils. It is composed mainly of plant litter and humus.
Oblique Aerial Photograph -  Photograph taken from a non-perpendicular angle from a platform in the atmosphere.
Obliquity -  Tilt of the Earth's polar axis as measured from the perpendicular to the plane of the Earth's orbit around the Sun. The angle of this tilt varies from 22.5 to 24.5° over a 41,000 year period. Current obliquity is 23.5°
Obsidian -  Glassy dark colored volcanic rock. Usually composed of rhyolite.
Occluded Front -  A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.
Occlusions -  A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.
Ocean Basin -  Part of the Earth's outer surface that is comprised of the ocean floor, mid-oceanic ridges, continental rise, and continental slope. The ocean basins are filled with saline water that makes up the oceans.
Ocean Currents -  Large scale horizontal flow of ocean water that is persistent and driven by atmospheric circulation.
Ocean Floor -  Flat plain found at the bottom of the ocean. The ocean floor represents the surface of the oceanic crust. The ocean floor lies between the mid-oceanic ridges and the trenches, usually 5,000 to 7,000 meters below the ocean surface. Also called the abyssal plain.
Ocean Trench -  Deep depression found at the edge of the ocean floor. Represents area of tectonic plate subduction.
Oceanic Crust -  Basaltic portion of the Earth's crust that makes up the ocean basins. Approximately 5 to 10 kilometers thick. See sima layer.
Oceanic Plate -  A rigid, independent segment of the lithosphere composed of mainly basalt that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's oceanic plates are an average 75 kilometers thick and were formed less than several hundred million years ago at one of the Earth's mid-oceanic ridges. Also see continental plate.
Oceanography -  The scientific study of phenomena found in the world's oceans.
Oceans -  A body of saline water found occupying all or part of the Earth's ocean basins. There are five recognized oceans: the Atlantic, the Southern Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.
Oil -  Hydrocarbon based liquid commonly found in the pores of sedimentary rocks of marine origin.
Oligotrophic Lake -  Lake with a low supply of nutrients in its waters. Also see eutrophic lake and mesotrophic lake.
Onshore-Offshore Transport -  The up and down movement of sediment roughly perpendicular to a shoreline because of wave action.
Open Sea -  That part of the ocean that extends from the continental shelf. Compare with coastal zone.
Open Talik -  Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface but enclosed to permafrost below and at its sides.
Opisometer -  Mechanical device for measuring non-linear distances on maps.
Ordovician -  Geologic period that occurred roughly 438 to 505 million years ago. During this period, the first fish and fungi species appear.
Organic -  (1) Relating to an organism. (2) Derived from an organism.
Organic Matter -  Mass of matter that contains living organisms or non-living material derived from organisms. Sometime refers to the organic constituents of soil. Also see soil organic matter.
Orogenesis -  The process of mountain building through tectonic forces of compression and volcanism.
Orogenic Belt -  A major range of mountains on the continents.
Orographic Precipitation -  Is precipitation that forms when air is forced to rise because of the physical presence of elevated land. As the parcel rises it cools as a result of adiabatic expansion at a rate of approximately 10° Celsius per 1,000 meters until saturation. The large amounts of precipitation along the west coast of Canada are due mainly to this process.
Orographic Uplift -  Uplift of an air mass because of a topographic obstruction. Uplift also causes the cooling of the air mass. If enough cooling occurs condensation can occur and form into orographic precipitation.
Orthographic Projection -  Map projection that presents the Earth's surface in two-dimensions as if it were being observed from a great distance in space. Distortion of areas and angles becomes greater as you move from the center of the projection to its edges.
Outcrop -  Area of exposed bedrock at the Earth's surface with no overlying deposits of soil or regolith.
Outer Core -  Outer region of the Earth's core. It is believed to be liquid nickel and iron and has a density of about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of about 2,250 kilometres.
Outgassing -  The release of gas from cooling molten rock or the interior of the Earth. Much of the atmosphere's gaseous constituents, like water vapor, nitrogen, and argon, came from outgassing.
Outwash -  Glaciofluvial sediments deposited by meltwater streams at the edge of a glacier.
Outwash Plain -  A flat or gentle sloping surface of glaciofluvial sediments deposited by meltwater streams at the edge of a glacier. Usually found in close spatial association with moraines.
Overbank Flow -  Movement of flood waters outside a stream channel during period of high discharge.
Overland Flow -  The topographic movement of a thin film of water from precipitation to lower elevations. With time, this water will begin to organizing its flow into small channels called rills. The rills converge to form progressively larger channels until stream channels are formed. Occurs when the infiltration capacity of an area's soil has been exceeded. Also called sheet flow or runoff.
Overthrust Fault -  Fault produced by the fracturing of rock in a fold because of intense compression.
Overturned Fold -  A fold in rock layers where one limb is pushed past the perpendicular. This results in both limbs having dips in the same direction.
Oxbow Lake -  Is portion of abandoned stream channel filled with stagnant water and cut off from the rest of the stream. Oxbow lakes are created when meanders are cut off from the rest of the channel because of lateral stream erosion.
Oxidation -  (1) Chemical attachment of free oxygen to other elements and compounds. One of the types of chemical weathering. (2) Loss of an electron during a chemical reaction from one atom to another.
Ozone -  Tri-atomic oxygen that exists in the Earth's atmosphere as a gas. Ozone is highest in concentration in the stratosphere (10-50 kilometers above the Earth's surface) where it absorbs the Sun's ultraviolet radiation. Stratospheric ozone is produced naturally and helps to protect life from the harmful effects of solar ultraviolet radiation. Over the last few decades levels of stratospheric ozone have been declining globally, especially in Antarctica. Scientists have determined that chlorine molecules released from the decomposition of chlorofluorocarbons are primarily responsible for ozone destruction in the stratosphere.It is also abundant near the the Earth's surface in highly polluted urban centers. In these areas, it forms as a by product of photochemical smog, and is hazardous to human health.
Ozone Hole -  Is a sharp seasonal decrease in stratospheric ozone concentration that occurs over Antarctica in the spring. First detected in the late 1970s, the ozone hole continues to appear as a result of complex chemical reaction in the atmosphere that involves CFCs.
Ozone Layer -  Atmospheric concentration of ozone found at an altitude of 10 to 50 kilometers above the Earth's surface. This layer is important to life on the Earth because ozone absorbs harmful ultraviolet radiation.
Ozonosphere -  Another name for the ozone layer. Atmospheric concentration of ozone found at an altitude of 10 to 50 kilometers above the Earth's surface. This layer is important to life on the Earth because ozone absorbs harmful ultraviolet radiation.
P-Wave -  A seismic wave that moves material in push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.
Pangaea -  Hypothetical super continent that existed in the geological past. Its break-up created the current configuration of landmasses found on the Earth.
Parallel -  A line parallel to the equator and connecting all places of the same latitude.
Parent Material -  The mineral material from which a soil forms.
Passive Remote Sensing -  Form of remote sensing where the sensor passively captures electromagnetic radiation reflected or emitted by an object.
Paternoster Lakes -  A linear series of mountain valley lakes that are formed from glacial erosion. They form behind glacial moraines or in glacially carved out rock basins. The name of this feature is related to the series of lakes looking like a string of beads.
Patterned Ground -  Term used to describe a number of surface features found in periglacial environments. These features can resemble circles, polygons, nets, steps, and stripes. The development of some of these shapes is thought to be the result of freeze-thaw action.
Peak Annual Flow -  The largest discharge produced by a stream during a one year period.
Peat -  Partially decomposed remains of plants that once flourished in a waterlogged environment.
Pebble -  A rounded piece of rock that is larger than gravel.
Pediment -  A gradually sloping bedrock surface located at the base of fluvial-eroded mountain range. Found in arid locations and normally covered by fluvial deposits.
Pediplain -  An arid landscape of little relief that is occasionally interrupted by the presence of scattered inselbergs. Formed by the coalescence of several pediments.
Perched Water Table -  Water table that is positioned above the normal water table for an area because of the presence of a impermeable rock layer.
Percolation -  Vertical movement or infiltration of water from the Earth's surface to its subsurface. Movement usually stops when the flowing water reaches the water table.
Peridotite -  Coarse grained ultramafic igneous rock composed mainly of olivine and pyroxene. The mantle is though to be composed primarily of this rock type.
Periglacial -  Landforms created by processes associated with intense freeze-thaw action in an area high latitude areas or near an alpine or continental glacier.
Perihelion -  It is the point in the Earth's orbit when it is closest to the Sun (147.5 million km). Perihelion occurs on the 3rd or 4th of January.
Period -  Geologic time unit that is shorter than an era but longer than a epoch.
Permafrost -  Zone of permanently frozen water found in high latitude soils and sediments. Five types of permafrost have been recognized: continuous permafrost, discontinuous permafrost, sporadic permafrost, alpine permafrost, and subsea permafrost.
Permeability -  A measure of the ability of soil, sediments, and rock to transport water horizontally and vertically. Permeability is dependent on the porosity of the medium the water is flowing through. Some rocks like granite have very poor permeability, while rocks like shale are actually quite pervious. As for soils, sand is the most pervious, while clay has the lowest permeability. Silt usually is somewhere in the middle.
Permian -  Last geologic period in the Paleozoic era. Occurred from 286 to 245 million years ago. This period saw the mass extinction of many corals, brachiopods, and trilobites. It also saw the diversification and growing dominance of the reptiles.
PGF -  Force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance (meters or kilometers). This force is primarily responsible for the formation of wind.
pH -  Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity.
Phanerozoic -  Geologic eon that occurs from 2500 million years ago to today. During this time period, life becomes more diversified and complex.
Phase Change -  Reorganization of a substance at the atomic or molecular level resulting in a change of the physical state of matter. For example, a change from solid to liquid to a gas.
Photochemical Smog -  Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds created from fossil fuel combustion) interact under the influence of sunlight to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. Also see industrial smog.
Photogrammetry -  The science of using aerial photographs and other remote sensing imagery to obtain measurements of natural and human-made features on the Earth.
Photoperiod -  The duration of the daylight period.
Photosphere -  Visible surface of Sun from which radiant energy is release.
Photosynthesis -  Is the chemical process where plants and some bacteria can capture and organically fix the energy of the Sun. This chemical reaction can be described by the following simple equation: 6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2
Physical Geography -  Field of knowledge that studies natural features and phenomena on the Earth from a spatial perspective. Subdiscipline of Geography.
Physical Weathering -  Breakdown of rock and minerals into small sized particles through mechanical stress.
Phytoplankton -  Small photosynthetic organisms, mostly algae and bacteria, found inhabiting aquatic ecosystems. Also see plankton and zooplankton.
Piedmont Glacier -  A large glacier formed from the merger of several alpine glaciers.
Pingo -  A large conical mound that contains an ice core. This feature can be up to 60 to 70 meters in height. Form in regions of permafrost. Common in the Mackenzie Delta region of Canada. Also see the related palsa.
Pitted Topography -  Landscape characterized by numerous kettle holes on a glacial outwash plain.
Place -  A term used in geography that describes the factors that make the location of natural and human-made phenomena unique.
Plagioclase Feldspar -  A type of feldspar that is rich in sodium and calcium. Common rock forming mineral.
Plane of the Ecliptic -  Hypothetical two-dimensional surface in which the Earth's orbit around the Sun occurs.
Planet -  (1) Any one of the nine primary celestial bodies that orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (2) A similar body orbiting another star.
Plankton -  Minute plant (phytoplankton) and animal organisms (zooplankton) that are found in aquatic ecosystems.
Plant -  Organisms belonging to the kingdom Plantae. These organisms have the following general characteristics: lack of locomotion, lack of a nervous system, and cellulose cell walls. Most plants can photosynthesize.
Plastic Deformation -  Irreversible change in the shape of a material without fracture as the result of the force of compression or expansion.
Plate Tectonics -  Theory suggesting that the Earth's surface is composed of a number of oceanic and continental plates. Driven by convection currents in the mantle, these plates have the ability to slowly move across the Earth's plastic asthenosphere. This theory is very important to geology and geomorphology because it helps to explain the occurrence and formation of mountains, folds, faults, volcanoes, earthquakes, ocean trenches, and the mid-oceanic ridges.
Plateau Basalt -  An accumulation of horizontal flows of basaltic lava. Also called flood basalts.
Platform -  Horizontal sedimentary deposits found on top of continental shield deposits.
Playa -  A dry lake bed found in a desert.
Pleistocene Epoch -  Period of time from about 2 million years ago to 10,000 years ago. During this period areas of land at higher and middle latitudes where covered with glacial ice.
Plucking -  Erosive process of particle detachment by moving glacial ice. In this process, basal ice freezes in rock surface cracks. As the main body of the glacial ice moves material around the ice in the cracks is pulled and plucked out. Also called quarrying.
Pluton -  Any mass of intrusive igneous rock.
Point Bar -  Stream bar deposit that is normally located on the inside of a channel bend.
Polar Axis -  Is a line drawn through the Earth around the planet rotates. The point at which the polar axis intercepts the Earth's surface in the Northern Hemisphere is called the North Pole. Likewise, the point at which the polar axis intercepts the Earth's surface in the Southern Hemisphere is called the South Pole.
Polar Cell -  Three-dimensional atmospheric circulation cell located at roughly 60 to 90° North and South of the equator. Vertical air flow in the Polar cell consists of rising air at the polar font and descending air at the polar vortex.
Polar Easterlies -  Winds that originate at the polar highs and blow to the subpolar lows in a east to west direction.
Polar Front -  Weather front located typically in the mid-latitudes that separates arctic and polar air masses from tropical air masses. Along the polar front we get the development of the mid-latitude cyclone. Above the polar front exists the polar jet stream.
Polar High -  Surface area of atmospheric high pressure located at about 90° North and South latitude. These high pressure systems produced by vertically descending air currents from the polar vortex.
Polar Jet Stream -  Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers. This jet stream flows from west to east at speeds between 110 to 185 kilometers per hour. Also see jet stream and subtropical jet stream.
Polar Stratospheric Clouds -  High altitude clouds found in the stratosphere where the temperature is less than -85° Celsius. Commonly found over Antarctica. Have a role in the creation of the ozone hole over Antarctica.
Polar Vortex -  High pressure system located in the upper atmosphere at the polar regions. In this system, air in the upper troposphere moves into the vortex center and then descends to the Earth's surface to create the polar highs.
Pollutant -  A substance that has a harmful effect on the health, survival, or activities of humans or other living organisms.
Pollution -  Physical, chemical, or biological change in the characteristics of some component of the atmosphere, hydrosphere, lithosphere, or biosphere that adversely influences the health, survival, or activities of humans or other living organisms.
Polycyclic Landform -  Landform that shows the repeated influence of one or more major geomorphic processes over geological time. Major geomorphic processes are: weathering, erosion, deposition, and massive Earth movements caused by plate tectonics.
Polygenetic Landform -  Landform that shows the influence of two or more major geomorphic processes. Major geomorphic processes are: weathering, erosion, deposition, and massive earth movements caused by plate tectonics.
Pool -  Scoured depression found on the bed of streams. Associated with riffles.
Population -  (1) Refers to all the individuals of a given species in a specific area or region at a certain time. Its significance is more than that of a number of individuals because not all individuals are identical. Populations contain genetic variation within themselves and between other populations. (2) A statistical population is the entire collection of people, animals, plants or things from which we may collect data from.
Population Density -  Number of individuals of a particular species found in a specified area.
Population Parameter -  A value used to represent a certain quantifiable characteristic of a population. For example, the population mean is a parameter that is often used to indicate the central value of quantity.
Pore Ice -  A form of periglacial ground ice that is found in the spaces that exist between particles of soil.
Porosity -  The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of void space in a volume of substance.
Porous -  The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of void space in a volume of substance.
Precambrian -  Span of geologic time that dates from 4.6 billion to 570 million years ago. Made up of three geologic eras: Hadean, Archean, and Proterozoic.
Precambrian Shield -  Another term for shield.
Precession of the Equinox -  Wobble in the Earth's polar axis. This motion influences the timing aphelion and perihelion over a cyclical period of 23,000 years.
Precipitable Water -  Amount of water potentially available in the atmosphere for precipitation. Usually measured in a vertical column that extends from the Earth's surface to the upper edge of the troposphere.
Precipitation -  (1) Is any aqueous deposit, in liquid or solid form, that develops in a saturated atmosphere (relative humidity equals 100%) and falls to the ground generally from clouds. Most clouds, however, do not produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome natural updrafts found in the atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere as clouds. (2) The state of being precipitated from a solution.
Prediction -  Forecast or extrapolation of the future state of a system from current or past states.
Pressure -  Is defined as the force acting on a surface from another mass per unit area.
Pressure Gradient Force -  Force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance (meters or kilometers). This force is primarily responsible for the formation of wind.
Pressure Melting Point -  Temperature at which minerals deep within the Earth and ice below the surface of a glacier are caused to melt because of the introduction of pressure.
Prevailing Wind -  Dominant direction that a wind blows from for a location or region.
Primary Pollutant -  Air pollutants that enter the atmosphere directly. Also see secondary pollutant.
Primary Wave -  A seismic wave that moves material in push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.
Prime Meridian -  The location from which meridians of longitude are measured. Has the measure of 0° of longitude. The prime meridian was selected by international agreement to run through Greenwich, England.
Probability -  Statistical chance that an event will occur.
Progradation -  The natural extension of a shoreline seaward.
Proterozoic -  Geologic eon that occurred from 570 to 2500 million years ago. During this time period, the first single-celled and multi-celled eukaryotic organisms evolved and developed.
Proxy Data -  Data that measures the cause and effect relationship between two variables indirectly.
Psychrometer -  Instrument used to measure atmospheric humidity. It consists of two thermometers (wet-bulb and a dry-bulb) one of which has its bulb covered by a moistened wick. Humidity is determined by the difference in readings between the two thermometers after air has passed over both of them for a specific time period.
Psychrometric Table -  Table of values that allows for the determination of relative humidity and dew point from dry-bulb and wet-bulb temperatures recorded on a psychrometer.
Pyroclastic Material -  Pieces of volcanic rock thrown out in a volcanic explosion.
Quantitative Revolution -  Time in the history (after 1950) of physical geography when measurement became the central focus of research. This measurement was used primarily for hypothesis testing. With measurement came mapping, models, statistics, and mathematics. Researchers began investigating process rather than mere description of the environment.
Quarrying -  Erosive process of particle detachment by moving glacial ice. In this process, basal ice freezes in rock surface cracks. As the main body of the glacial ice moves material around the ice in the cracks is pulled and plucked out. Also called plucking.
Quartz -  Mineral with the chemical formula SiO2. Quartz is common in continental crust but rare in oceanic crust.
Quartzite -  Metamorphic rock rich in quartz created by the recrystallization of sandstone.
Quaternary -  Geologic period that occurred roughly 1.6 million years ago to today. During much of this period continental glaciers in the Northern Hemisphere covered large regions of land surface in the high and mid-latitudes. Homo sapiens appear about 200,000 years BP (before present) and become the first species to alter the Earth's environment on a large-scale.
R Horizon -  Soil horizon found beneath the C horizon. Consists of consolidated rock showing little sign of weathering or pedogenesis.
Radiant Energy -  Energy in the form of electromagnetic waves and photons. In some cases it refers to the radiation emitted from the Sun.
Radiation -  The emission of energy from an object in the form of electromagnetic waves and photons.
Radiation Fog -  A type of fog that is also called ground fog. Radiation fog is generated by near surface cooling by radiation loss during the evening hours. For the fog to develop, the overnight cooling must cause saturation occur. This type of fog is normally quite shallow.
Rain -  A form of precipitation. It is any liquid deposit that falls from clouds in the atmosphere to the ground surface. Rain normally has a diameter between than 0.5 and 5.0 millimetres.
Rain Gauge -  Instrument that measures the rain that falls at a location over a period of time.
Raindrop Impact -  Force exerted by a falling raindrop on a rock, sediment, or soil surface.
Rainshadow Effect -  Reduction of precipitation commonly found on the leeward side of a mountain. The reduction in precipitation is the result of compression warming of descending air.
Rainsplash -  Soil erosion caused from the impact of raindrops.
Rainwash -  The erosion of soil by overland flow. Normally occurs in concert with rainsplash.
Random -  Process or event that occurs by chance.
Range -  A statistical measure of the dispersion of observation values in a data set. Determined by taking the difference between the largest and the smallest observed value.
Rangeland -  Land-use type that supplies vegetation for consumption by grazing and browsing animals. This land-use type is normally not intensively managed.
Re-Entrants -  A prominent indentation in an escarpment, ridge or shoreline.
Reach -  An expanse of a stream channel.
Recessional Moraine -  Moraine that is created during a pause in the retreat of a glacier. Also called a stadial moraine.
Recharge Area -  The area on the Earth's surface that receives water for storage into a particular aquifer.
Rectangular Coordinate System -  System that measures the location of points on the Earth on a two-dimensional coordinate plane. See the Universal Transverse Mercator (UTM) Grid System.
Recumbent Fold -  A fold in which the axial plane is almost horizontal.
Recurrence Interval -  The average time period that separates natural events of a specific magnitude. For example, floods of a specific stream discharge level.
Reduction -  (1) Chemical process that involves the removal of oxygen from a compound. (2) A form of chemical weathering.
Reef -  A ridge of rocks found in the tidal zone along a coastline. One common type of reef is the coral reef.
Reference Map -  Map that shows natural and human-made objects from the geographical environment with an emphasis on location. Compare with thematic map.
Reflected Infrared Radiation -  Form of electromagnetic radiation with a wavelength between 0.7 to 3.0 micrometers (µm).
Reflected Wave -  A water wave that reflects off the shore or another obstacle and is redirected towards the sea or lake.
Reflection -  Process of returning sound or light waves back to their source.
Reflection (Atmospheric) -  Process where insolation is redirect by 180° after striking a particle. This redirection causes 100% loss. Most of the reflection in the Earth's atmosphere occurs in clouds because of light's interception with particles of liquid and frozen water. The reflectivity of a cloud can range from 40-90%.
Refraction -  Process where insolation is redirect to a new direction of travel after entering another medium.
Reg -  A rocky desert landscape. See desert pavement.
Region -  A term used in geography that describes an area of the Earth where some natural or human-made phenomena display similar traits.
Regional Metamorphism -  Large scale metamorphic modification of existing rock through the heat and pressure of plutons created at tectonic zones of subduction.
Regolith -  Loose layer of rocky material overlying bedrock.
Relative Humidity -  The ratio between the actual amount of water vapor held in the atmosphere compared to the amount required for saturation. Relative humidity is influenced by temperature and atmospheric pressure.
Relief -  The range of topographic elevation within a specific area.
Remote Sensing -  The gathering of information from an object or surface without direct contact.
Remote Sensor -  Mechanical devices used to remotely sense an object or phenomenon.
Representative Fraction -  The expression of map scale as a mathematical ratio.
Reptile -  Group of terrestrial vertebrate animals that includes turtles, tortoises, snakes, lizards, crocodiles, and alligators.
Resource -  Anything obtained from the environment to meet the needs of a species.
Reverse Fault -  This vertical fault develops when compressional force causes the displacement of one block of rock over another.
Revolution -  Refers to the orbit of the Earth around the Sun. This celestial motion takes 365 1/4 days to complete one cycle. Further, the Earth's orbit around the Sun is not circular, but elliptical. Also see Earth revolution.
RF -  The expression of map scale as a mathematical ratio.
Rhumb Line -  A line of constant compass direction or bearing which crosses the meridians at the same angle. A part of a great circle.
Rhyolite -  A fine grained extrusive igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.
Ria Coast -  An extensively carved out coast with conspicuous headlands and deep re-entrants.
Ribbon Falls -  Spectacular narrow waterfalls that occur at the edge of a hanging valley.
Richter Scale -  A logarithmic measurement scale of earthquake magnitude. This scale measures the energy released by the largest seismic wave associated with the earthquake.
Riffle -  Bar deposit found on the bed of streams. Associated with these deposits are pools.
Rift -  Zone between two diverging tectonic plates. The mid-oceanic ridge is an area where such plate divergence is occurring.
Rift Valley -  Steep sided valley found on the Earth's surface created by tectonic rifting.
Rill -  A very small steep sided channel carrying water. This landscape feature is intermittent and forms for only a short period of time after a rainfall.
Rime -  Deposit of ice crystals that occurs when fog or super cooled water droplets comes in contact with an object with a temperature below freezing (0° Celsius). This deposit develops outward on the windward side of the object.
Ring of Fire -  A zone circling the edge of the Pacific Ocean basin where tectonic subduction causes the formation of volcanoes and trenches. Also called the Circum-Pacific Belt.
Rip Current -  A strong relatively narrow current of water that flows seaward against breaking waves.
Ripple -  Stream bed deposit found streams. Ripples are only a few centimeters in height and spacing and are found in slow moving streams with fine textured beds.
River -  A long narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Many rivers empty into lakes, seas, or oceans.
Robinson Projection -  Map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Roche Moutonnee -  A feature of glacial erosion that resembles an asymmetrical rock mound. It is smooth and gently sloping on the side of ice advance. The lee-side of this feature is steep and jagged.
Rock -  A compact and consolidated mass of mineral matter. Three types of rock are recognized: igneous, sedimentary, and metamorphic.
Rock Cycle -  General model describing the geomorphic and geologic processes involved in the creation, modification and recycling of rocks.
Rock Flour -  Very finely ground rock fragments that form between the base of a glacier and the underlying bedrock surface.
Rock Slide -  Large scale mass movement of rock materials downslope.
Rockfall -  Type of mass movement that involves the detachment and movement of a small block of rock from a cliff face to its base. Normally occurs when the rock has well defined bedding planes that are exaggerated by freeze-thaw action or thermal expansion and contraction.
Roll Cloud -  A dense, cigar shaped cloud found above the gust front of a thunderstorm. Air within the cloud rotates around the long axis.
Rossby Wave -  A large wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called long wave.
Rotation -  Is a line drawn through the Earth around the planet rotates. The point at which the polar axis intercepts the Earth's surface in the Northern Hemisphere is called the North Pole. Likewise, the point at which the polar axis intercepts the Earth's surface in the Southern Hemisphere is called the South Pole. Also see Earth rotation.
Rotational Slip -  Form of mass movement where material moves suddenly along a curvilinear plane. Also called a slump.
Runoff -  The topographic flow of water from precipitation to stream channels located at lower elevations. Occurs when the infiltration capacity of an area's soil has been exceeded. It also refers to the water leaving an area of drainage. Also called overland flow.
S-Wave -  A seismic wave that moves material it encounters perpendicular to its direction of travel. This type of seismic wave causes shear stress in the material it moves through. Also called a secondary wave or a shear wave.
Salinity -  Concentration of dissolved salts found in a sample of water. Measured as the total amount of dissolved salts in parts per thousand. Seawater has an average salinity of about 34 parts per thousand.
Salinization -  Pedogenic process that concentrates salts at or near the soil surface because evapotranspiration greatly exceeds water inputs from precipitation.
SALR -  The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.
Salt -  (1) The mineral sodium chloride. (2) Compounds that are produced as the result of a metal atom replacing a hydrogen atom in an acid.
Salt Marsh -  Coastal wetland ecosystem that is inundated for some period of time by seawater. Plants that exist in this community have special adaptation to survive in the presence of high salinities in their immediate environment. Generally, found poleward of 30° North and South latitude.
Saltation -  Transport of sediment initiated by moving air or water where particles move from a resting surface to the transport medium in quick continuous repeated cycles.
Saltwater Intrusion -  The invasion of saltwater into freshwater aquifers in coastal and inland areas. This condition can be cause when groundwater, which charges the aquifer, is withdrawn faster than it is recharged by precipitation.
Sample -  A sample is a subset group of data selected from a larger population group. Most samples are drawn at random to guaranty equal representation in the data.
Sand -  Mineral particle with a size between 0.06 and 2.0 millimeters in diameter. Also see clay and silt.
Sand Dune -  A hill or ridge of aeolian sand deposits with a minimum height of less than one meter and a maximum height of about 50 meters. Found in hot deserts and along sandy coastlines.
Sand Ripples -  Another term used for wind ripples.
Sand Sea -  A large region of sand and sand dunes in a desert. Common to erg deserts.
Sand Sheet -  Deposit of sometimes stratified less well sorted sand that almost resemble dunes. Common in northern Europe. Believed to form when windblown materials settle on areas of patchy snow.
Sand Wedge -  A form of ice wedge that contains accumulations of wind blown sand in long vertical layers. A form of periglacial ground ice.
Sandstone -  A type of sedimentary rock that contains a large quantity of weathered quartz grains.
Santa Ana -  A warm, dry chinook like wind that occurs in southern California. Originates from the east off an elevated desert plateau.
Santa Ana Wind -  A warm, dry chinook like wind that occurs in southern California. Originates from the east off an elevated desert plateau.
Saturated Adiabatic Lapse Rate -  The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.
Saturation -  Atmospheric condition where water is changing its phase to liquid or solid. At saturation, relative humidity is 100% unless there is a shortage of deposition nuclei or condensation nuclei. Generally, this process is caused by the cooling of the atmosphere.
Saturation Mixing Ratio -  Mass of water vapor that a kilogram of dry air can hold at saturation. Measured in grams.
Savanna -  A tropical or sub-tropical plant community characterized by trees and shrubs scattered among a cover of grasses, herbs and forbs. The climate of a savanna is tropical with a dry season occurring in the low Sun period of the year.
Scale -  A specific relative or proportional size or extent of a phenomena as measured through space and/or time.
Scarification -  Extensive movements of soil, sediment, and rock material caused by humans.
Scattering -  Is an atmospheric process where small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy. Scattering does, however, reduce the amount of incoming radiation reaching the Earth's surface. A significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering that takes place is dependent on two factors: wavelength of the incoming radiation and the size of the scattering particle or gas molecule. In the Earth's atmosphere, the presence of a large number of particles with a size of about 0.5 µm results in shorter wavelengths being preferentially scattered. This factor also causes our sky to look blue because this color corresponds to those wavelengths that are best diffused.
Schist -  A medium to coarse grained metamorphic rock with well developed bedding planes derived from the foliated recrystrallization of platy like minerals like mica.
Science -  Science is a way of acquiring knowledge. To do science, one must follow a specific universal methodology. The central theme in this methodology is the testing of hypotheses and the ability to make predictions. The overall goal of science is to better understand nature and our Universe.
Scientific Method -  The approach science uses to gain knowledge. This method tries to be unbias and neutral. Involves inductive and deductive reasoning, hypothesis testing and falsification, and predictive model testing.
Sclerophyllous Vegetation -  Term used to describe drought resistant vegetation common in Mediterranean climates. Some common adaptations present in this type of vegetation include: deep roots, reduced leaf area exposed to the atmosphere, and waxy thick leaves with closing stomata which resist water loss.
Scour -  (1) Refers to the erosive power of water. (2) Abrasive effects of rocks and sediments incorporated in the ice base of a glacier.
Scree -  An accumulation of weathered rock fragments at the base of a steep rock slope or cliff.
Sea -  (1) A body of saline water found on the Earth's continental surface. (2) A portion of a ocean that is in close proximity to a continent.
Sea Arch -  A coastal landform composed of rock that resembles an arch. These landforms are created when waves erode through a thin headland from both sides.
Sea Breeze -  Local thermal circulation pattern found at the interface between land and water. In this circulation system, surface winds blow from water to land during the daytime.
Sea Stack -  A steep pillar of rock located in the ocean a short distance from the coastline. These landforms are created when waves erode through a thin headland from both sides.
Sea-Floor Spreading -  The process of oceanic crust creation and sea-floor movement that occurs at the mid-oceanic ridge.
Sea-Level -  The average surface elevation of the world's oceans.
Sea-Level Pressure -  Average atmospheric pressure at sea-level. This value is 1013.2 millibars.
Seamount -  A volcanic mountain found on an ocean basin that has an origin not related to a mid-oceanic ridge or a tectonic subduction zone.
Seasons -  Time periods generally based on the changes in the intensity and duration of sunlight as received in the middle and high latitudes. Four seasons are normally recognized: Spring; Summer; Fall; and Winter. The astronomical definition is more precise and suggests the following time periods for the four seasons: Spring - March 22 to June 21; Summer - June 22 to September 22; Fall - September 23 to December 22; and Winter - December 23 to March 21.
Seaward -  Positioned or located away from land but towards an ocean or sea.
Seawater -  The mixture of water and various dissolved salts found in the world's oceans and seas.
Second Law of Thermodynamics -  This law states that heat can never pass spontaneously from a colder to a hotter body. As a result of this fact, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible. This law also predicts that the entropy of an isolated system always increases with time.
Secondary Pollutant -  Atmospheric pollutants that are created chemically in the atmosphere when primary pollutants and other components of the air react. Also see primary pollutant.
Secondary Wave -  A seismic wave that moves material it encounters perpendicular to its direction of travel. This type of seismic wave causes shear stress in the material it moves through. Also called a S-wave or a shear wave.
Sediment -  Solid material that has been or is being eroded, transported, and deposited. Transport can be due to fluvial, marine, glacial or aeolian agents.
Sediment Rating Curve -  Numerical expression or graphical curve that describes the quantitative relationship between stream discharge and the sediment transported by a particular stream.
Sedimentary Rock -  Rocks formed by the deposition, alteration and/or compression, and lithification of weathered rock debris, chemical precipitates, or organic sediments. Also see clastic vs non-clastic sedimentary rocks.
Seepage -  (1) The gradual movement of water into the soil layer. (2) Slow movement of sub-surface water to the surface. This flow is not great enough to call it a spring.
Seepage Lake -  A lake that gets its water primarily from the seepage of groundwater.
Segregated Ice -  A form of periglacial ground ice that consists of almost pure ice that often exists as an extensive horizontal layer. The ice layer grows because of the active migration of water from around the feature. These features are found just below the active layer.
Seif -  (1) A large sand dune that is elongated in the general direction of the dominant winds. (2) A sand dune formed by winds from multiple directions.
Seismic -  Shaking displacement usually caused by an earthquake.
Seismic Wave -  Successive wave-type displacement of rock usually caused by an earthquake.
Seismograph -  Instrument that measures the energy contained in seismic waves from an earthquake or other type of ground displacement.
Seismology -  A branch of science focused on the study of earthquakes and seismic activity.
Self-Regulation -  The ability of some systems to maintain a steady state equilibrium through positive and negative feedbacks.
Semi-Diurnal Tide -  Tides that have two high and two low waters per tidal period.
Sensible Heat -  Heat that can be measured by a thermometer and thus sensed by humans.
Sensible Heat Flux -  Process where excess heat energy is transferred into the atmosphere. The process first involves the movement of heat energy from the Earth's surface to the atmosphere by conduction and convection. The heat energy then can move horizontally advection (atmospheric circulation).
September Equinox -  One of two days during a year when the declination of the Sun is at the equator. The September equinox denotes the first day of fall in the Northern Hemisphere. In the Southern Hemisphere, this date marks the first day of spring. During the September equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night. The September equinox occurs on either September 22 or 23.
Shale -  Fine grained sedimentary rock composed of lithified clay particles.
Shear Stress -  Stress caused by forces operating parallel to each other but in opposite directions.
Shear Wave -  A seismic wave that creates wave-like motion perpendicular to the direction of seismic energy propagation. Also called S-wave or Secondary Wave.
Sheet Flow -  The topographic movement of a thin film of water from precipitation to lower elevations. With time, this water will begin to organizing its flow into small channels called rills. The rills converge to form progressively larger channels until stream channels are formed. Occurs when the infiltration capacity of an area's soil has been exceeded. Also called overland flow or runoff.
Sheeting -  A form of physical weathering of rock where surface sheets of material fracture and exfoliate because of pressure release. Also see exfoliation dome.
Sheetwash -  The removal of loose surface materials by overland flow. Process of erosion.
Shield -  A large stable area of exposed very old (more than 600 million years) igneous and metamorphic rock found on continents. This rock forms the nucleus of the continents.
Shield Volcano -  Volcano created from alternate layers of lava flows. Shield volcanoes are slightly sloping having a gradient between 6 and 12°. Their height can be as high as 9000 meters. The chemistry of the magma of these volcanoes is basaltic.
Shore -  The land area bordering a relatively large water body like a lake or ocean.
Shoreline -  The line that separates a land surface from a water body. Also see coastline.
Short Wave -  A small wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with long waves.
Shortwave Radiation -  Electromagnetic radiation with a wavelength between 0.1 and 0.7 micrometers (µm). Commonly used to describe the radiation emitted from the Sun.
Sial Layer -  The part of the crust that forms the continents and is composed of relatively light, granitic rocks.
Sidereal Day -  Time it takes to complete one Earth rotation relative to the position of a fixed star. This measurement takes 23 hours, 56 minutes, and 4.09 seconds. Compare with mean solar day.
Silcretes -  Sedimentary rock created by the chemical precipitation of silica.
Silica -  Mineral that is composed of silicon dioxide, SiO2.
Silicate -  Group of minerals that have crystal structures based on a silica tetrahedron (SiO4).
Silicate Magma -  Magma that is felsic in composition.
Sill -  Horizontal planes of igneous rock that run parallel to the grain of the original rock deposits.They form when magma enters and cools in bedding planes found within the crust. Also see intrusive igneous rock.
Silt -  Mineral particle with a size between 0.004 and 0.06 millimeters in diameter. Also see clay and sand.
Siltstone -  Fine grained sedimentary rock composed of lithified silt particles.
Silurian -  Geologic period that occurred roughly 408 to 438 million years ago. During this period, the first plant and insect species appear.
Sima Layer -  The part of the crust that forms the ocean basins and lower layers in the crust and is composed of relatively heavy, basaltic rocks.
Sink -  (1) Site of the storage of some material. (2) Another name for sinkhole.
Sinkhole -  A pit like hole in found in areas of karst. These features are caused by the weathering of limestone or dolomite by subsurface drainage. Also called a sink or doline.
Sinusoidal Equal-Area Projection -  Map projection that represents areas in their true form on a two-dimensional map. Distances are only correct along parallels and central meridian. Shapes become more distorted away from the central meridian and close to the poles.
Slate -  A fine grained metamorphic rock with well developed bedding planes derived from the slight recrystrallization of shale.
Sleet -  A type of precipitation. Ice pellets or sleet are transparent or translucent spheres of frozen water that fall from clouds. Ice pellets have a diameter less than 5 millimeters. To form, this type of precipitation requires an environment where raindrops develop in an atmosphere where air temperature is above freezing. These raindrops then fall into a lower layer of air with freezing temperatures. In this lower layer of cold air, the raindrops freeze into small ice pellets. Like freezing rain, an air temperature inversion is required for development of ice pellets. Also see ice pellets.
Sling Psychrometer -  Psychrometer that uses a rotating handle and a whirling motion to ventilate its wet-bulb thermometer.
Slip-Face -  The lee side of a dune where material accumulates and slides or rolls downslope.
Slope Aspect -  Main compass direction (North, North East, East, South East, South, South West, West, and North West) that a slope faces.
Slope Failure -  The downslope movement of soil and sediment by processes of mass movement.
Slump -  Form of mass movement where material moves suddenly along a curvilinear plane. Also called a rotational slip.
Small Circle -  A circle on the globe's surface that does not bisect the center of the Earth. Parallels of latitude are examples of small circles.
Smog -  Generic term used to describe mixtures of pollutants in the atmosphere. Also see industrial smog and photochemical smog.
Snout -  Front end of a glacier. Also called the terminus.
Snow -  A type of solid precipitation that forms in clouds with an air temperature below freezing. Snow forms when water vapor deposits directly as a solid on a deposition nuclei. Snowflakes begin their life as very tiny crystals developing on a six-sided hexagonal deposition nuclei. The developing snowflak, then grows fastest at the six points of the nuclei as these surfaces are more exposed to atmosphere's water vapor. Snowfall is most common with the frontal lifting associated with mid-latitude cyclones during fall, winter, and spring months when air temperatures are below freezing.
Snow Line -  Altitudinal or latitudinal limit separating zones where snow does not melt during the summer season from areas in which it does. Similar to the concept of firm limit except that it is not limited to glaciers.
Snow Melt -  Conversion of snow into runoff and groundwater flow with the onset of warmer temperatures.
Snow Pellets -  A form of precipitation also known as graupel. Snow pellets are white, spherical bits of ice with a maximum diameter of 5 millimeters. Snow pellets develop when supercooled droplets freeze on snowflakes. Snow pellets often fall for a brief time period when precipitation transforms from ice pellets to snow. Snow pellets can be easily distinguished from packed snowflakes as they tend to bounce when they strike the ground. Packed snowflakes are not dense enough to cause them to bounce.
Snowfield -  An area of permanent snow accumulation. Usually at high altitudes or latitudes.
Soil -  Layer of unconsolidated material found at the Earth's surface that has been influenced by the soil forming factors: climate, relief, parent material, time, and organisms. Soil normally consists of weathered mineral particles, dead and living organic matter, air space, and the soil solution.
Soil Creep -  Slow mass movement of soil downslope. Occurs where the stresses on the slope material are too small to create a rapid failure.
Soil Erosion -  Transport of soil mineral particles and organic matter by wind, flowing water, or both. Human activities that disturb the soil surface or remove vegetation can enhance this natural process.
Soil Fertility -  The ability of a soil to provide nutrients for plant growth.
Soil Horizon -  Layer within a soil profile that differs physically, biologically or chemically from layers above and/or below it.
Soil Moisture Recharge -  The process of water filling the pore space found in a soil (storage).
Soil Organic Matter -  Organic constituents of soil.
Soil Permeability -  The rate at which water and air move vertically through a soil.
Soil Porosity -  The volume of water that can be held in a soil. Also refers to the ratio of the volume of voids to the total volume of the soil.
Soil Profile -  Vertical arrangement of layers or horizons in a soil.
Soil Science -  The study of soils from an interdisciplinary perspective.
Soil Solution -  Aqueous liquid found within a soil. This liquid normally contains ions released from mineral particles, organic matter or plant roots and leaves.
Soil Structure -  General term that describes how mineral and particles organic matter of are organized and clumped together in a soil.
Soil Taxonomy -  The classification of a soil in a hierarchical system based on its various properties. Grouping goes from general to specific.
Soil Texture -  The relative quantities of the different types and sizes of mineral particles in a soil.
Soil Water -  The water found occupying the pore spaces between soil particles.
Soil-Heat Flux -  The rate of flow of heat energy into, from, or through the soil.
Solar Altitude -  Height of the Sun above the horizon from either True North or True South.
Solar Constant -  A term used to describe the average quantity of solar insolation received by a horizontal surface at the edge of the Earth's atmosphere. This value is approximately 1370 Watts per square meter.
Solar Day -  Time required for the Earth to complete one rotation relative to the Sun.
Solar Energy -  Direct or diffused shortwave solar radiation that is received in the Earth's atmosphere or at its surface. Also see insolation.
Solar Noon -  Point of time during the day when the Sun is aligned with True North and True South.
Solar Radiation -  Electromagnetic radiation that originates from the Sun. Most of the Sun's radiation is emitted at wavelengths between 1.0 and 0.1 microns (µm). Also see insolation, direct solar radiation, and diffused solar radiation.
Solar System -  The collection of celestial bodies that orbit around the Sun.
Solar Wind -  Mass of ionized gas emitted to space by the Sun. Plays a role in the formation of auroras.
Solar Year -  The time it takes the Earth to make one orbit around the Sun. This is approximately 365.2422 days.
Solid -  A state of matter where molecules where the mass of the substance does not have the property of flow.
Solifluction -  Form of mass movement in environments that experience freeze-thaw action. It is characterized by the slow movement of soil material downslope and the formation of lobe-shaped features. Also see gelifluction.
Solstice -  Dates when the declination of the Sun is at 23.5° North or South of the equator. For the Northern Hemisphere this date falls on June 21 or 22 (Summer Solstice). In the Southern Hemisphere the date is December 21 or 22 (Winter Solstice).
Solum -  Part of the soil that is capable of supporting life.
Solution -  (1) Form of chemical weathering where rocks and minerals are dissolved by water. Materials entering the mixture can alter the chemical nature of the solution and can increase the strength of this weathering agent. For example, the mixing of carbon dioxide and water can form carbonic acid. (2) The dissolving of a substance into a liquid.
Source Region -  Area where air masses originate and come to possess their moisture and temperature characteristics.
South Magnetic Pole -  Location in the Southern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.
South Pole -  Surface location defined by the intersection of the polar axis with Earth's surface in the Southern Hemisphere. This location has a latitude of 90° South.
Southern Oscillation -  Reversal of atmospheric circulation in tropical Pacific Ocean that triggers the development of an El Niño.
Space -  (1) A distance, area, or volume. (2) An infinite three-dimensional area in which objects have relative coordinates to each other. (3) The region beyond the outer limits of the Earth's atmosphere.
Spatial Analysis -  The examination of the spatial pattern of natural and human-made phenomena using numerical analysis and statistics.
Spatial Tradition -  Academic tradition in modern Geography that investigates geographic phenomena from a strictly spatial perspective.
Specific Heat -  Is the heat capacity of a unit mass of a substance or heat needed to raise the temperature of 1 gram (g) of a substance 1 degree Celsius.
Specific Humidity -  Measurement of atmospheric humidity. Specific humidity is the mass of water vapor in a given mass of air. Normally expressed in grams of water vapor per kilogram of air at a specific temperature.
Spectrum -  Is a graph that describes the quantity of radiation that is emitted from a body at particular wavelengths.
Speed of Light -  Velocity of light in a vacuum. This velocity is approximately 3 x 108 meters per second. It takes light from the Sun 8 minutes and 20 seconds to reach the Earth.
Spheroidal Weathering -  A type of below ground chemical weathering where the corners of jointed rocks become rounded over time. Rock changes from a rectangular to more round shape.
Spit -  A long and narrow accumulation of sand and/or gravel that projects into a body of ocean water. These features form as the result of the deposition of sediments by longshore drift.
Sporadic Permafrost -  Form of permafrost that exists as small islands of frozen ground in otherwise unfrozen soil and sediments.
SPOT (Centre National d'Etudes Spatiales) -  Series of satellites developed by the French Space Agency, with the cooperation with Belgium and Sweden for the purpose of remotely monitoring resources on the Earth. The first SPOT satellite was launched in 1986. See the following website for more information - SPOT Image.
Spring -  (1) Season between winter and summer. Astronomically it is the period from the vernal equinox to the summer solstice in the Northern Hemisphere. (2) A natural flow of water from the sub-surface to the surface. Usually occurs when the water table intersects the Earth's surface.
Spring Tide -  Tide that occurs every 14 to 15 days and coincides with the new and full moon. This tide has a large tidal range because the gravitational forces of the moon and Sun are complementary to each other. Contrasts with neap tide.
Squall Line -  A band of thunderstorm development found ahead of a cold front.
Stability -  The capability of a system to tolerate or recover from disturbance or an environmental stress.
Stable Atmosphere -  Condition in the atmosphere where isolated air parcels have a tendency to sink. The parcels of air tend to be cooler than the air that surrounds them.
Stable Equilibrium -  In a stable equilibrium the system displays tendencies to return to the same equilibrium after disturbance.
Stadial Moraine -  Moraine that is created during a pause in the retreat of a glacier. Also called a recessional moraine.
Stage -  The elevation of the water surface in a stream channel.
Standard Atmospheric Pressure -  A pressure of 101.32 kilopascals or 1013.2 millibars.
Standard Deviation -  A statistical measure of the dispersion of observation values in a data set. Calculated by determining the square root of the variance.
Star -  A large and very massive, self-luminous celestial body of gas that illuminates via the radiation derived from its internal source of energy.
State of Matter -  Form of matter. Matter can exist in three different forms gas, liquid, and solid.
Static Equilibrium -  Static equilibrium occurs where force and reaction are balanced and the properties of the system remain unchanged over time.
Stationary Front -  A transition zone in the atmosphere where there is little movement of opposing air masses and winds blow towards the front from opposite directions.
Steady State Equilibrium -  In this type of equilibrium the average condition of the system remains unchanged over time.
Steam Fog -  A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called evaporation fog or sea smoke.
Stefan-Boltzmann Law -  This radiation law suggests the amount of radiation given off by a body is proportional to the 4th power of its temperature as measured in Kelvin units. This law can be expressed by the following simple equation: E* = sT 4 where E* is the amount of radiation emitted by the body in Watts per square meter, s is a constant equal to 0.0000000567, and T is the temperature of the body in Kelvins.
Stemflow -  Is the process that directs precipitation down plant branches and stems. The redirection of water by this process causes the ground area around the plant's stem to receive additional moisture. The amount of stemflow is determined by leaf shape and stem and branch architecture. In general, deciduous trees have more stemflow than coniferous vegetation.
Storm Surge -  Relatively rapid rise in the height of the ocean along a coastline. Often caused by the storm winds pushing water towards land.
Storm Track -  The path taken by a storm (thunderstorm, mid-latitude cyclone or hurricane) or the average path taken by storms.
Stoss -  Side of a slope that faces the direction of flow of ice, wind, or water. Opposite of lee.
Strata -  The layers or beds found in sedimentary rock.
Stratified Drift -  A type of glacial drift that has been partially sorted by glaciofluvial meltwater.
Stratigraphy -  Subdiscipline of geology that studies sequence, spacing, composition, and spatial distribution of sedimentary deposits and rocks.
Stratocumulus Clouds -  Low altitude gray colored cloud composed of water droplets that has a patchy appearance. Each cloud patch consists of a rounded mass. This cloud has a somewhat uniform base and normally covers the entire sky. Between the patches blue sky can be seen. Found in an altitude range from the surface to 3,000 meters.
Stratopause -  The stratopause is a relatively thin atmospheric transition layer found between the stratosphere and the mesosphere. The height of this layer is about 50 kilometers above the Earth's surface.
Stratosphere -  Atmospheric layer found at an average altitude of 11 to 50 kilometers above the Earth's surface. Within the stratosphere exists the ozone layer. Ozone's absorption of ultraviolet sunlight causes air temperature within the stratosphere to increase with altitude.
Stratovolcano -  Volcano created from alternate layers of flows and exploded rock. Their height ranges from 100 to 3,500 meters tall. The chemistry of the magma of these volcanoes is quite variable ranging from basalt to granite. Also see composite volcano.
Stratus Clouds -  Low altitude gray colored cloud composed of water droplets. This cloud has a uniform base and normally covers the entire sky. It is also quite thick and can obscure the Sun. Light precipitation is often found falling from it. Found in an altitude range from the surface to 3,000 meters.
Stream -  A long narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Many streams empty into lakes, seas or oceans.
Stream Bank -  Sides of the stream channel.
Stream Bed -  Bottom of the stream channel.
Stream Channel -  Long trough-like depression that is normally occupied by the water in a stream.
Stream Discharge -  A river or stream's rate of flow over a particular period of time. Usually measured by a current meter and expressed in cubic meters per second. Stream discharge depends on the volume and velocity of the flow.
Stream Flow -  The flow of water in a river or stream channel.
Stream Gradient -  The change in elevation from a stream's headwaters to its mouth expressed in degrees, percentage, or as a distance ratio (rise/run).
Stream Load -  Refers to the material or sediment carried by a stream. In normally consists of three components: bed load (pebbles and sand which move along the stream bed without being permanently suspend in the flowing water), suspended load (silts and clays in suspension) and dissolved load (material in solution).
Stream Long Profile -  Vertical and horizontal profile of the stream. Most streams have a profile that is concave shaped.
Stream Order -  The relative position, or rank, of a stream channel segment in a drainage network.
Striations -  Grooves of scratches found in surface rock that are the result of glacial abrasion.
Strike -  One of the directional properties of a geologic structure such as a fold or a fault. Strike is the horizontal directional taken by an imaginary line drawn on the plane of the formation. Also see dip.
Strike-Slip Fault -  Fault that primarily displays horizontal displacement.
Structural Landform -  Is a landform created by massive Earth movements due to plate tectonics. This includes landforms with some of the following geomorphic features: fold mountains, rift valleys, and volcanoes.
Subduction -  Process of plate tectonics where one lithospheric plate is pushed below another into the asthenosphere.
Subduction Zone -  Linear area where tectonic subduction takes place.
Sublimation -  Process where ice changes into water vapor without first becoming liquid. This process requires approximately 680 calories of heat energy for each gram of water converted.
Submarine Canyon -  V-shaped canyons cut into the continental slope to a deep of up to 1200 meters. These features are normally associated with major rivers
Subpolar Glacier -  Glacier in which the ice found from the its surface to base has a temperature as cold as -30° Celsius throughout the year. This is well below the pressure melting point. However, melting does occur in the accumulation zone in the summer. One of the three types of glaciers: cold glacier; temperate glacier; and subpolar glacier.
Subpolar Lows -  Surface zone of atmospheric low pressure located at about 60° North and South latitude. These low pressure systems are produced by the frontal lifting of subtropical air masses over polar air.
Subsea Permafrost -  Form of permafrost that exists beneath the sea in ocean sediments.
Subsidence -  Lowering or sinking of the Earth's surface.
Subsolar Point -  The location on the Earth where the Sun is directly overhead. Also see declination.
Subtropical High Pressure Zone -  Surface zone of atmospheric high pressure located at about 30° North and South latitude. These high pressure systems produced by vertically descending air currents from the Hadley cell.
Subtropical Jet Stream -  Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. The subtropical jet stream exists in the subtropics at an altitude of approximately 13 kilometers. This jet stream flows from west to east and has a speed that is somewhat slower that the polar jet stream. Also see jet stream.
Succession -  Directional cumulative change in the types plant species that occupy a given area, through time.
Succulent Vegetation -  Group of plants that have the ability to survive in deserts and other dry climates by having no leaves. Instead their branches and stems that are photosynthetic. This adaptation reduces the surface area for evaporation thus reducing the loss of scarce water.
Sulfate Aerosol -  Type of solid compound commonly found in the atmosphere. These particles play an important role in reflecting, absorbing, and scattering incoming insolation. The source of these compounds is both natural and human-made. Most of the human-made particles come from the combustion of fossil fuels.
Sulfur Dioxide -  A gas produced from volcanic eruptions, ocean spray, organic decomposition and the burning of fossil fuels. Sulfur dioxide is a component in the creation of acid precipitation. This colorless gas has the chemical formula SO2.
Sulfuric Acid -  Acid with the chemical formula H2SO4.
Summer -  Season between spring and fall. Astronomically it is the period from the summer solstice to the autumnal equinox in the Northern Hemisphere.
Summer Solstice -  The summer solstice denotes the first day of the summer season. For the Northern Hemisphere, the date of summer solstice is either on June 21 or 22 (changes yearly). December 21 or 22 is the date of the summer solstice for the Southern Hemisphere. During the summer solstice, locations in their respective hemispheres experience the longest day of the year.
Sun -  Luminous star around which the Earth and other planets revolve around. The Sun emits 63,000,000 Watts per square meter of electromagnetic radiation. The Sun has an average distance from the Earth of about 150,000,000 kilometers. The Earth's orbit is not circular but elliptical.
Sunrise -  Moment of time when the Sun's edge first appears above the Earth's horizon.
Sunset -  Moment of time when the Sun's edge completely disappears below the Earth's horizon.
Sunspot -  Dark colored region on the Sun that represents an area of cooler temperatures and extremely high magnetic fields.
Super-Saturation -  Atmospheric condition where saturation occurs at a relative humidity greater than 100% because of a shortage of deposition or condensation nuclei.
Supercooled Water -  Cooling of water below 0° Celsius without freezing. Common in clouds where there is a deficiency of condensation nuclei.
Surface Creep -  The sliding and rolling movement of soil particles on the Earth's surface because of wind. Eolian process of soil particle movement.
Surface Heat Flux -  Process where heat energy is transferred into land and ocean surfaces on the Earth. Much of this transfer takes place when solar radiation absorbed at the land or ocean surface is converted into heat energy. On land surfaces, surface heat is transfered down into the ground by conduction. Heat energy is transfered to greater depths in ocean surfaces because liquids have the ability mix by convection. Heat energy stored in ocean waters can also move quickly over large horizontal distances in a poleward direction through ocean currents.
Surface Tension -  Tension of a liquid's surface. Due to the forces of attraction between molecules.
Surface Wave -  Type of seismic wave that travels across the Earth's surface. These earthquake generated waves cause the Earth's surface to roll or sway like waves on the ocean.
Surge -  A large, destructive ocean wave caused by very low atmospheric pressure and strong winds. Hurricanes often cause a surge of the ocean surface.
Suspended Load -  Portion of the stream load that is carried almost permanently suspended in flowing water.
Suspension -  Erosional movement of sediment continually held in the transport medium of air, water or ice.
Sustainable Development -  Forms of economic growth and other human activities that meet the requirements of the present without jeopardizing the ability of future generations of individuals to meet their own needs.
Swash -  A thin sheet of water that moves up the beach face after a wave of water breaks on the shore.
Swell -  A relatively smooth ocean wave that travels some distance from the area of its generation.
Symbiotic -  Mutual relationship between two organisms which is necessary for either to survive.
Symbiotic Mutualism -  Mutualistic interaction where the species interact physically and their relationship is biologically essential for survival.
Syncline -  A fold in rock layers that forms a trough-like bend.
synoptic chart -  A weather chart reflecting the state of the atmosphere over a large area at a given moment.
Synoptic Scale -  Scale of geographic coverage used on daily weather maps to describe large scale atmospheric phenomenon (for example, mid-latitude cyclone, air masses, fronts, and hurricanes).
synoptic weather map -  A weather chart reflecting the state of the atmosphere over a large area at a given moment.
System -  A system is a set of interrelated components working together towards some kind of process.
System Attribute -  A system attribute is the perceived characteristic of a system element. For example, number, size, color, volume, and temperature may be some of the perceived characteristics of clouds in the atmospheric system.
System Boundary -  Outer edge of system. Zone between one system and another system.
System Element -  System elements are the kinds of things or substances composing the system. They may be atoms or molecules, or larger bodies of matter-sand grains, rain drops, plants, or cows.
System Relationship -  Is the association that exist between the elements and attributes of a system based on cause and effect.
System State -  Current value of a system's elements, attributes, and/or relationships.
Taku -  Name for a katabatic type of cold wind that occurs in Alaska.
Talik -  An unfrozen section of ground found above, below, or within a layer of discontinuous permafrost. These layers can also be found beneath water bodies in a layer of continuous permafrost. A number of different types of talik have been distinguished: closed talik, open talik, and through talik.
Talus -  An accumulation of angular rock debris from rockfalls.
Talus Slope -  A slope that is composed of talus.
Tarn -  A small mountain lake that occurs inside a cirque basin.
Tectonic Plate -  An extensive layer of lithosphere that moves as a discrete unit on the surface of the Earth's asthenosphere.
Temperate Deciduous Forest -  Forested biome found in the mid-latitudes and dominated by deciduous vegetation.
Temperate Glacier -  Glacier in which the ice found below 10 to 20 meters from its surface is at the pressure melting point. One of the three types of glaciers: cold glacier; temperate glacier; and subpolar glacier.
Temperate Rain Forest -  An ecosystem that is dominated by large and very tall evergreen trees. This biome occurs along the Pacific Northwest coast of North America where annual precipitation is high and temperatures are mild.
Temperature -  Temperature is defined as the measure of the average speed of atoms and molecules. The higher the temperature the faster they move.
Temperature Inversion -  Situation where a layer of warmer air exists above the Earth's surface in a normal atmosphere where air temperature decreases with altitude. In the warmer layer of air, temperature increases with altitude.
Tephra -  Fragmented rock material ejected by a volcanic explosion. Also called pyroclastic material.
Terminal Fall Velocity -  Velocity at which a particle being transported by wind or water falls out of the moving medium. This velocity is dependent on the size of the particle.
Terminal Moraine -  Moraine that marks the maximum advance of a glacier.
Terminal Velocity -  Maximum speed that can be achieve by a body falling through a fluid like water or air.
Terminus -  End or snout of a glacier.
Terrace -  An elevated surface above the existing level of a floodplain or shore that is created by stream or ocean wave erosion.
Tertiary -  Geologic period that occurred roughly 1.6 to 65 million years ago. During this period, mammals become a dominant species on the planet.
Tertiary Consumer -  Organisms that occupy the fourth trophic level in the grazing food chain. These organisms are carnivores. Also known as a secondary carnivore.
Tetrahedron -  Silicon atom joined by four oxygen atoms (SiO4). The atomic properties of this molecule cause it to develop a unique three dimensional crystal lattice that is pyramid shaped.
Texture -  The relative quantities of the different types and sizes of mineral particles in a deposit of sediment. Also see the related soil texture.
Thalweg -  Line of deepest water in a stream channel as seen from above. Normally associated with the zone of greatest velocity in the stream.
Thematic Map -  Map that displays the geographical distribution of one phenomenon or the spatial associations that occur between a few phenomena. Compare with reference map.
Thematic Mapper -  Remote sensing device found on Landsat satellites that scans images in seven spectral bands from visible to thermal infrared.
Theory -  Proposed explanation for the causal mechanisms responsible for a phenomenon or a set of facts. Also see hypothesis.
Thermal Circulation -  Atmospheric circulation caused by the heating and cooling of air.
Thermal Equator -  Continuous area on the globe that has the highest surface temperatures because of the presence of the Intertropical Convergence Zone.
Thermal High -  Area of high pressure in the atmosphere caused by the area having warmer temperatures relative to the air around it.
Thermal Infrared Radiation -  Form of electromagnetic radiation with a wavelength between 3 to 14 micrometers (µm).
Thermal Low -  Area of low pressure in the atmosphere caused by the area having cooler temperatures relative to the air around it.
Thermal Metamorphism -  Is the metamorphic alteration of rock because of intense heat released from processes related to plate tectonics.
Thermocline -  Boundary in a body of water where the greatest vertical change in temperature occurs. This boundary is usually the transition zone between the layer of warm water near the surface that is mixed and the cold deep water layer.
Thermodynamic Equilibrium -  This type of equilibrium describes a condition in a system where the distribution of mass and energy moves towards maximum entropy.
Thermodynamic Laws -  Laws that describe the physical processes, relationships, and phenomena associated with heat.
Thermokarst -  Landscape dominated by depressions, pits, and caves that is created by the thawing of ground ice in high latitude locations. Resembles karst landscape but is not created by chemical weathering.
Thermometer -  Device used to measure temperature.
Thermosphere -  Atmospheric layer above the mesosphere (above 80 kilometers) characterized by air temperatures rising rapidly with height. The thermosphere is the hottest layer in the atmosphere. In the thermosphere, gamma, X-ray, and specific wavelengths of ultraviolet radiation are absorbed by certain gases in the atmosphere. The absorbed radiation is then converted into heat energy. Temperatures in this layer can be greater than 1200° Celsius.
Third Law of Thermodynamics -  This law states if all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would result and all energy would be randomly distributed.
Threatened Species -  Species that is still plentiful in its natural range but is likely to become endangered because of declining population numbers.
Threshold -  The level of magnitude of a system process at which sudden or rapid change occurs.
Threshold Velocity -  Velocity required to cause entrainment in the erosional agents of wind, water or ice. Threshold velocity is usually higher than the velocity required for transport because factors like particle cohesion. Also see critical entrainment velocity.
Through Talik -  Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface and to an area of unfrozen ground beneath it. Permafrost encases it along the sides.
Throughfall -  Describes the process of precipitation passing through the plant canopy. This process is controlled by factors like: plant leaf and stem density, type of the precipitation, intensity of the precipitation and duration of the precipitation event. The amount of precipitation passing through varies greatly with vegetation type.
Throughflow -  The roughly horizontal flow of water through soil or regolith.
Thrust Fault -  A geologic fault where the hanging wall is forced over the foot wall.
Thunder -  Sound created when lightning causes the rapid expansion of atmospheric gases along its strike path.
Thunderstorm -  A storm several kilometers in diameter created by the rapid lifting of moist warm air which creates a cumulonimbus cloud. Thunderstorms can have the following severe weather associated with them: strong winds; hail; lightning; tornadoes; thunder; and heavy rain.
Tidal Current -  Regional scale ocean current that is created the tidal rise and fall of the ocean surface.
Tidal Period -  Time it takes for one tidal cycle.
Tidal Zone -  Area along the coastline that is influence by the rise and fall of tides.
Tide -  Cyclical rise and fall of the surface of the oceans. Caused by the gravitational attraction of the Sun and moon on the Earth.
Till -  Heterogeneous sediment deposited directly by a glacier. The particles within this deposit have not been size sorted by the action of wind or water.
Till Plain -  Extensive flat plain of till that forms when a sheet of ice becomes detached from the main body of the glacier and melts in place depositing the sediments it carried.
Time -  Measurable period in which cause and effect occurs and systems function.
TIROS (Television and Infrared Observation Satellite) -  Series of meteorological satellites launched by the United States starting in 1960. The main purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting. TIROS program was very successful, providing the first accurate weather forecasts based on data gathered from space. TIROS began continuous monitoring of the Earth's weather in 1962.
Tombolo -  A coastal feature that forms when a belt sand and/or gravel is deposited between an island and the mainland. This feature is above sea-level for most of the time.
Topographic Map -  Map that displays topography through the use of elevation contour lines. Base elevation on these maps is usually sea-level.
Topographic Profile -  A two-dimensional diagram that describes the landscape in vertical cross-section
Topography -  The relief exhibited by a surface.
Topset Bed -  Horizontal deltaic deposit composed of coarse alluvial sediment. Represents current or past surface of the delta.
Tornado -  A vortex of rapidly moving air associated with some severe thunderstorms. Winds within the tornado funnel may exceed 500 kilometers per hour.
Tornado Alley -  Region in North America which receives a extraordinary high number of tornadoes. This region stretches from central Texas to Illinois and Indiana.
Tornado Warning -  A warning issued to the public that a tornado has been observed by an individual in a specified region. This warning can also be issued if meteorological information indicates a high probability that a tornado will develop in a specified region.
Tornado Watch -  A forecast issued to the public that a tornado may occur in a specified region.
Total Column Ozone -  A measurement of ozone concentration in the atmosphere.
Traction -  Erosional movement of particles by rolling, sliding and shuffling along the eroded surface. Occurs in all erosional mediums (air, water, and ice).
Trade Winds -  Surface winds that generally dominate air flow in the tropics. These winds blow from about 30° North and South latitude (subtropical high pressure zone) to the equator (intertropical convergence zone). Trade winds in the Northern Hemisphere have northeast to southwest direction and are referred to as the Northeast Trades. Southern Hemisphere trade winds have southeast to northwest direction but are called the Southeast Trades.
Transform Fault -  Massive strike-slip fault continental in size. Examples of such faults occur along tectonic plate boundaries and at the mid-oceanic ridge.
Transparency -  The ability of a medium to allow light to pass through it.
Transpiration -  Transpiration is the process of water loss from plants through stomata. Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. Some dry environment plants do have the ability to open and close their stomata. Transpiration is a passive process largely controlled by the humidity of the atmospheric and the moisture content of the soil. Of the transpired water passing through a plant only 1% is used in the growth process. Transpiration also transports nutrients from the soil into the roots and carries them to the various cells of the plant.
Transport -  One of three distinct processes involved in erosion. It is the movement of eroded material in the medium of air, water or ice.
Triassic -  Geologic period that occurred roughly 208 to 245 million years ago. During this period, the first dinosaurs appeared.
Tributary -  A smaller branching stream channel that flows into a main stream channel. Opposite of distributary.
Trophic Level -  Level of organization in the grazing food chain.
Trophic Pyramid -  A graphic model describing the distribution of energy, biomass, or some other measurable quantity between the different trophic levels found in an ecosystem.
Tropic of Cancer -  Latitude of 23.5° North. Northern limit of the Sun's declination.
Tropic of Capricorn -  Latitude of 23.5° South. Southern limit of the Sun's declination.
Tropical Cyclone -  An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour. Another name for hurricane.
Tropical Depression -  An organized group of thunderstorms often found over a tropical ocean that generates a cyclonic flow of between 37 and 63 kilometers per hour. Can develop into a hurricane.
Tropical Disturbance -  An organized group of thunderstorms often found over a tropical ocean that generates a slight cyclonic flow of less than 37 kilometers per hour. Can develop into a hurricane.
Tropical Rainforest -  Forested biome found near the equator and dominated by evergreen vegetation.
Tropical Storm -  An organized group of thunderstorms often found over a tropical ocean that generates a cyclonic flow of between 64 and 118 kilometers per hour. Often develops into a hurricane.
Tropopause -  The tropopause is a relatively thin atmospheric transition layer found between the troposphere and the stratosphere. The height of this layer varies from 8 to 16 kilometers above the Earth's surface.
Troposphere -  Layer in the atmosphere found from the surface to a height of between 8 to 16 kilometers of altitude (average height 11 kilometers). The troposphere is thinnest at poles and gradually increases in thickness as one approaches the equator. This atmospheric layer contains about 80% of the total mass of the atmosphere. It is also the layer where the majority of our planet's weather occurs. Maximum air temperature occurs near the Earth's surface in this layer. With increasing altitude air temperature drops uniformly with increasing height at an average rate of 6.5° Celsius per 1000 meters (commonly called the Environmental Lapse Rate), until an average temperature of -56.5° Celsius is reached at the top of the troposphere.
Trough -  An elongated area of low pressure in the atmosphere.
True North -  Direction of the North Pole from an observer on the Earth.
True South -  Direction of the South Pole from an observer on the Earth.
Tsunami -  Large ocean wave created from an earthquake or volcanic eruption. Open ocean wave height may be as high as 1 meter. When entering shallow coastal waters, land configuration can amplify waves to heights of over 15 meters.
Tundra -  High latitude biome dominated by a few species of dwarf shrubs, a few grasses, sedges, lichens, and mosses. Productivity is low in this biome because of the extremes of climate.
Turbulent Flow -  Movement of water within a stream that occurs as discrete eddies and vortices. Turbulent flow is caused by channel topography and friction.
Two-Tailed Statistical Test -  Is an inferential statistical test where the values for which one can reject the null hypothesis are located either side of the center of the probability distribution.
Typhoon -  An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour. Another name for hurricane.
Ultramafic -  Rock that is rich in magnesium and iron content.
Ultraviolet Radiation -  Electromagnetic radiation with a wavelength between 0.1 and 0.4 micrometers (µm).
Unconfined Aquifer -  Aquifer that is not restricted by impervious layers of rock.
Unconfined Groundwater -  Groundwater that is not restricted by impervious layers of rock.
Unconformity -  A break in the sequence of sedimentary strata. Often the unconformity surface is the result of erosion.
Undercut Bank -  Steep bank found on the inside of stream meanders. Formed by the erosion that occurs when a stream channel moves horizontally.
Uniformitarianism -  Is a theory that rejects the idea that catastrophic forces were responsible for the current conditions on the Earth. The theory suggested instead, that continuing uniformity of existing processes were responsible for the present and past conditions of this planet.
Universal Time -  (UT) The mean solar time of the meridian at the Prime Meridian. Universal Time replaced the time standard known as Greenwich Mean Time in 1928. Universal Time is commonly used to denote solar time.
Universal Transverse Mercator -  (UTM) Grid System: Rectangular coordinate system used to find location of points on the Earth's surface. Based on the Universal Transverse Mercator projection system.
Universe -  All of the observable phenomena in the celestial cosmos.
Unloading -  The releasing of downward pressure on rocks because of removal of overlying material by erosion. Unloading can cause the development of horizontal bedding in once solid rock.
Unstable Atmosphere -  Condition in the atmosphere where isolated air parcels have a tendency to rise. The parcels of air tend to be warmer than the air that surrounds them.
Unstable Equilibrium -  In an unstable equilibrium the system returns to a new equilibrium after disturbance.
Updraft -  Upward movement of air.
Upper Air Westerlies -  Consistent winds that exist in the upper troposphere that flow east to west from about 20° of latitude to the poles.
Upper Mantle -  Layer of the Earth's interior extending from the base of the crust to 670 kilometers below the surface. Part of the Earth's mantle layer. The upper mantle is composed of peridotite, an ultramafic magma primarily made up of the minerals olivine and pyroxene. The top layer of the upper mantle, 100-350 km below surface, is called the asthenosphere.
Upslope Fog -  Fog produced by air flowing over topographic barriers. As the air is forced to rise, it is cooled by adiabatic expansion. Upslope fog is most common on the windward slopes of hills or mountains.
Upwelling -  The movement of nutrient-rich deep seawater to the ocean's surface.
Urban Area -  Geographic area with a high density of people over a limited area. Homes and other types of buildings tend to be close together. Urban systems also tend to differentiate themselves spatially into particular types of human activities.
Urban Heat Island -  Observed condition that urban areas tend to be warmer than surrounding rural areas.
Urbanization -  Expansion of cities into rural regions because of population growth. In most cases, population growth is primarily due to the movement of rural based people to urban areas. This is especially true in Less Developed Countries.
Vacuum -  (1) Space devoid of atoms or molecules. (2) Emptying of air.
Valley -  A linear depression in the landscape that slopes down to a stream, lake or the ocean. Formed by water and/or ice erosion.
Valley Breeze -  Local thermal circulation pattern found in areas of topographic relief. In this circulation system, surface winds blow from the valley bottom to areas of higher elevation during the daytime.
Valley Fog -  Fog formed by the movement of cooler, more dense air from higher elevations to the warm valley bottom.
Valley Train -  A linear accumulation of glaciofluvial outwash sediments found in a once glaciated valley.
Valley Wall -  The side slope of a stream or glacial valley.
Vapor Pressure -  Pressure exerted by water vapor molecules in a given quantity of atmosphere.
Variance -  A statistical measure of the dispersion of observation values in a data set. The variance of a sample is the sum of the square of each value in the data set subtracted from the mean divided by one less than the total number of observations in the data set.
Varve -  A thin yearly deposit of sediment found on the bottom of a lake. Within each yearly varve, there are variations in the color and the texture of the material deposited. The thickness of the varve and its associated layers can be used to reconstruct past environmental conditions influencing the lake.
Velocity -  The speed of movement of an object in one direction.
Ventifact -  A loose piece of rock that has been polished smooth by wind transported particles. Common in arid environments.
Venturi -  An increase in the velocity of a fluid or gas due to the constriction of flow.
Vernal Equinox -  One of two days during the year when the declination of the Sun is at the equator. The vernal equinox denotes the first day of the spring season. For the Northern Hemisphere, the date of vernal equinox on either March 20 or 21 (changes yearly). September 22 or 23 is the date of the vernal equinox in the Southern Hemisphere. During the vernal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night.
Vertical Aerial Photograph -  Photograph taken from a overhead or near overhead angle from a platform in the atmosphere.
Viscosity -  The amount of the resistance to flow in a fluid due to intermolecular friction.
Volcanic Ash -  Small sized particles ejected from explosive volcanoes.
Volcanic Pipe -  A dyke reaches the surface of the Earth. Also called volcanic neck.
Volcanic Vent -  An opening on a volcano through which lava is released and rock fragments and ash are ejected.
Volcano -  An elevated area of land created from the release of lava and ejection of ash and rock fragments from and volcanic vent.
Volume -  The occupation of space in three dimensions. Measured in cubic units.
Vortex -  A rapid spiraling motion of air or liquid around a center of rotation.
Warm Desert -  Desert found in the subtropics or interiors of continents at the middle latitudes where precipitation is low and surface air temperatures are high.
Warm Front -  A transition zone in the atmosphere where an advancing warm air mass displaces a cold air mass.
Wash -  (1) Coarse alluvial sediments. (2) The downslope movement of small particles of soil by overland flow. Also called sheetwash. (3) A term used in the United States for a shallow intermittent stream channel found in arid and semi-arid regions.
Water Consumption -  The complete removal of water from some type of source, like groundwater, for some use by humans. This water is not returned to the source. Compare with water withdrawal.
Water Table -  Top surface of groundwater.
Water Withdrawal -  The removal of water from some type of source, like groundwater, for some use by humans. The water is subsequently returned some period of time later after its is used. The quality of the returned water may not be the same as when it was originally removed. Compare with water consumption.
Waterfall -  (1) A location in the long profile of a stream where water flows vertically. A nickpoint. (2) Verical drop in elevation that causes a stream's dischange to flow vertically.
Watershed -  Catchment area of a drainage basin.
Watershed -  The topographic ridge that separates drainage basins.
Waterspout -  A vortex of rapidly moving air over water that is associated with some thunderstorms.
Wave -  A moving swell or ridge on the surface of a solid or liquid or within the medium of a gas. Electromagnetic radiation also travels in waves.
Wave Crest -  The curved tops or ridges of an oscillating wave.
Wave Cyclone -  Another name for mid-latitude cyclone.
Wave Height -  Vertical distance between a wave's trough and crest.
Wave Period -  The time elapsed for a wave to travel the distance of one wavelength.
Wave Refraction -  The re-orientation of a wave so that it approaches a shoreline at a more perpendicular angle. This process is caused by the differential reduction of water depth as a linear wave approaches a curved shoreline. A reduction in water depth causes a wave to slow down causing the waves approaching a nonlinear shoreline to curve with the shore's shape.
Wave Trough -  Area in between wave crests.
Wave-Cut Notch -  A rock recess at the foot of a sea cliff where the energy of water waves is concentrated.
Wave-Cut Platform -  A flat or slightly sloping bedrock surface that forms in the tidal zone. Caused by wave erosion.
Wavelength -  Distance between two successive wave crests or troughs.
Weather -  The state of the atmosphere at a specific time and place.
Weather Map -  Map that displays the condition of the physical state of the atmosphere and its circulation at a specific time over a region of the Earth.
Weathering -  Physical, chemical or biological breakdown of rocks and minerals into smaller sized particles.
Weathering Landform -  Is a landform created by the physical or chemical decomposition of rock through weathering. Weathering produces landforms where rocks and sediments are decomposed and disintegrated. This includes landforms with some of the following geomorphic features: karst, patterned ground, and soil profiles.
Westerlies -  Dominant winds of the mid-latitudes. These winds move from the subtropical highs to the subpolar lows from west to east.
Wet Adiabatic Lapse Rate -  The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.
Wet Deposition -  The transport of gases and minute liquid and solid particles from the atmosphere to the ground surface with the aid of precipitation or fog. Compare with dry deposition.
Wet-Bulb Depression -  The value calculated by subtracting a wet-bulb thermometer reading from a dry-bulb thermometer reading. Used to determine the air's relative humidity or dew point from a psychrometric table.
Wet-Bulb Thermometer -  Thermometer on a psychrometer that has a moisten wick on its reservoir bulb. When ventilated this thermometer records a temperature that is modified by the cooling effects of evaporation. This measurement and the temperature reading from a dry-bulb thermometer are then used to determine the air's relative humidity or dew point from a psychrometric table.
Wetland -  Natural land-use type that is covered by salt water or fresh water for some time period. This land type can be identified by the presence of particular plant species or characteristic conditions.
Wetting and Drying -  Physical weathering process where rocks are mechanically disintegrated by the accumulation of successive layers of water molecules in between the mineral grains of a rock. Sometimes called slaking.
Wind -  Air moving horizontally and/or vertically.
Wind Ripples -  Wind ripples are miniature sand dunes between 5 centimeters and 2 meters in length and 0.1 to 5 centimeters in height. They are created by saltation when the sand grains are of similar size and the wind has a constant speed. Also called sand ripples.
Wind Vane -  A mechanical device used to measure the direction of wind flow. Usually consists of a horizontal bar with a fin at one end and a aerodynamic pointer at the other end. The center of horizontal is attached to a vertical spindle which is connected to a mechanical device that records direction.
Windward -  Upwind side or side directly influenced to the direction that the wind blows from. Opposite of leeward.
Winter -  Season between fall and spring. Astronomically it is the period from the winter solstice to the vernal equinox in the Northern Hemisphere.
Winter Solstice -  The winter solstice denotes the first day of the winter season. For the Northern Hemisphere, the date of winter solstice is either on December 21 or 22 (changes yearly). June 21 or 22 is the date of the winter solstice for the Southern Hemisphere. During the winter solstice, locations in their respective hemispheres experience the shortest day of the year.
X-axis -  Horizontal axis on a graph.
X-Ray Radiation -  Form of electromagnetic radiation with a wavelength between 0.03 to 30 nanometers.
Xerophyte -  Plant that have adaptations to survive prolonged periods of soil drought.
Y-axis -  Vertical axis on a graph.
Yardang -  Rock that has developed a streamline form because of wind erosion. The long axis of these features is aligned with the dominant wind direction.

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