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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°.