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; and (2) A zone of translocation where finer solid particles and soluble substances are removed by the process of eluviation.
Abiotic - A nonliving thing. The term often refers to the physical and chemical components of an organism's environment. Also called inorganic.
Ablation - The surface loss of ice or snow from a glacier or snowfield by melting, sublimation, and/or calving.
Ablation Zone - The region on a glacier where there is a surface net removal of snow and/or ice by melting, sublimation, and/or calving.
Abney Level - An instrument used in surveying to determine slope angle and elevation. It uses a sighting tube that is fastened to a movable pointer and protractor. From the protractor, one then reads off the angle of the sighting tube.
Abrasion - The physical wearing and grinding of a surface through friction and collision by solid material carried in wind, water, or ice.
Abrasion Platform - A level and smooth area of land surface that is normally exposed only at low tide. It forms when abrasive wave action cuts into land formations along the coastline.
Absolute Age - The age of a geologic event, deposit, mineral, rock, or fossil measured in years. Compare with relative age.
Absolute Coordinates - A phrase used in geographic information systems that implies the coordinates of points are measured relative to an assigned origin in a two-dimensional computerized mapping system.
Absolute Drought - A climatological term used to technically suggest that drought is occurring. In the United Kingdom, it is defined as a period of at least 15 consecutive days without rainfall. In the USA, it is defined as an uninterrupted period of at least 14 days without rainfall.
Absolute Humidity - A measurement of humidity in the atmosphere. 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). This measurement is normally expressed in grams of water vapor per cubic meter (g/m3 or g m-3) of atmosphere at a specified temperature. This measure of humidity is less commonly used than mixing ratio or specific humidity because air volume changes with temperature and pressure.
Absolute Instability - A situation in the atmosphere in which a rising parcel of air becomes warmer than its surroundings due to adiabatic expansion. This causes an unstable atmosphere, usually leading to the development of cumulus and cumulonimbus clouds.
Absolute Stability - A situation in the atmosphere where a rising parcel of air becomes colder than the air environment around it because of adiabatic expansion. This causes a stable weather pattern.
Absolute Temperature - Refers to a system of temperature measurement of an object where absolute zero defines a state of lowest possible heat energy. See Kelvin temperature scale.
Absolute Vorticity - A meteorological term related to the vorticity of air moving clockwise or counterclockwise in the atmosphere. Absolute vorticity is the sum of two components: the vorticity caused by the Earth's rotation or the Coriolis effect; and the vorticity of air's circulation caused by the change in wind speed and wind direction over some distance.
Absolute Zero - A temperature of -273.15°C or -459.67°F or 0 Kelvin. At this temperature, atomic motion essentially stops, and the kinetic energy of atoms is at a minimum.
Absorbed Water - Water that is electrochemically held by the solid particles and other water molecules found in soil.
Absorption - (1) Process of taking in and being made part of an existing quantity of matter. (2) The interception of electromagnetic radiation or sound.
Absorption (Atmospheric) - Atmospheric absorption is a process in which solar radiation is retained by a substance found in the atmosphere and converted into heat energy. The creation of heat energy also causes atmospheric substances to emit their own electromagnetic radiation. In general, the absorption of solar radiation by substances in the Earth's atmosphere results in temperatures no higher than 1800°C (3300°F). According to Wien's Law, bodies at this temperature or lower would emit radiation in the longwave band.
Absorptivity - The fraction of incident electromagnetic radiation absorbed by a surface or substance, ranges from 0 (no absorption) to 1 (complete absorption). According to Kirchhoff’s law, absorptivity equals emissivity at the same wavelength and temperature for a body in thermal equilibrium. However, absorptivity varies with wavelength, which explains why the atmosphere is selectively transparent in the atmospheric window but opaque at wavelengths absorbed by water vapor, carbon dioxide, and ozone.
Abstract Space - A geographic model or conceptual representation of the Earth or some other celestial object. For example, the maps and globes we construct to represent space on the Earth are approximations of the real world. Compare with concrete space.
Abundance - Total count of the number of individuals for one or more species found in a certain area for a particular time.
Abyss - (1) The deep part of an ocean or a sea. (2) A very deep canyon or gorge.
Abyssal Fan - A fan-shaped accumulation of sediment that is deposited by a stream at the base of a submarine canyon, within an ocean basin.
Abyssal Plain - Another name for the ocean floor.
Abyssal Zone - A vertical zone in the oceans that has a depth of between 4,000 and 6,000 meters (13,100 to 19,700 feet). The abyssal zone has no light penetration from the surface. Seawater at this depth is between 2 and 3°C (35-37°F) and generally lacks nutrients. Life is rare in this environment, and the organisms that do exist here are adapted to survive in crushing pressures. Also see (in order of depth in a water body) pelagic zone, photic zone, aphotic zone, mesopelagic zone, bathyal zone, hadal zone, demersal zone, and benthic zone.
Accidented Relief - A landscape with a highly dissected, uneven relief pattern.
Acclimation - The slow physiological adjustment of an organism to new conditions in its surrounding environment.
Accordant Drainage - A pattern of stream drainage that is primarily determined by the underlying geologic structure of the landscape. Compare with discordant drainage.
Accretion - (1) The gradual growth of the continental masses over geologic time through the addition of marine sediments. These sediments are added to the edges of continents through tectonic collisions with other oceanic or continental plates. (2) The growth of an ice particle occurs through collision and freezing of supercooled water droplets upon contact. Accretion is the primary process responsible for the growth of graupel and hailstones within cumulonimbus clouds. The extent of accretion is influenced by the cloud’s liquid water content, updraft speed, and temperature.
Accumulated Cyclone Energy (ACE) - Is an index that quantifies the total energy of a tropical cyclone season. It does this by summing the squares of the maximum sustained wind speed (in knots) at six-hour intervals for all named storms. ACE takes into account the intensity, duration, and frequency of storms, providing a single metric for comparing the overall activity of different hurricane seasons.
Accumulation - The surface addition of snow to a glacier or snowfield.
Accumulation Zone - (1) Region in a glacier where there is a surface net addition of snow. (2) The part of a hill slope that has a net gain of material leading to a progressive raising of the slope's surface.
Acid - (1) Substance having a pH less than 7. (2) Substance that releases hydrogen ions (H+).
Acid Deposition - The surface accumulation of acids, originally from the atmosphere, in solid or liquid form. Also see acid precipitation.
Acid Precipitation - A form of precipitation with a pH less than 5.6. On average, the normal pH of precipitation falling to the Earth's surface is 5.6.
Acid Rain - Rain with a pH less than 5.6. On average, the normal pH of precipitation falling to the Earth's surface is 5.6.
Acid Rocks - An igneous rock that contains more than 66% free silica or silica in combination with highly siliceous minerals. Compare with basic rocks.
Acid Shock - A sudden acidification of runoff waters from the spring melting of accumulated snow in the middle latitudes because of the winter accumulation of acid precipitation.
Acidic - Any substance with a pH below 7. Compare with alkaline or basic.
Acidic Solution - Any water-based 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.
Acidophile - A type of plant that is adapted to survive on acidic soils.
Acre - A unit of area, two-dimensional measurement, with no predetermined shape, used in many countries, including the United States, United Kingdom, Canada, and Australia. One acre is equivalent to 43,560 square feet or 0.0015625 square miles. Converting one acre to metric units equals 4,047 square meters or 0.4047 hectares.
Actinometer - A meteorological instrument used to measure the intensity of solar radiation either by chemical or physical processes. Chemical actinometers use a chemical reaction to determine the intensity of sunlight. Physical actinometers can use a thermopile, bolometer, or photodiode to measure solar radiation strength.
Actinomycetes - A group of filamentous microorganisms. From an evolutionary perspective, these organisms are between bacteria and fungi.
Active Fault - A fault that regularly has periodic movement.
Active Glacier - Is a glacier that is in motion, flowing downslope or across a landscape because of gravity and the weight of the accumulating ice. Also see passive glacier and dead glacier.
Active Layer - Upper zone of the soil layer in higher latitude locations with permafrost that experience daily and seasonal freeze-thaw cycles. The depth of the active layer varies from a few centimeters to several meters, depending on factors like latitude, soil composition, vegetation, and snow cover. Climate change is causing the active layer to deepen, which contributes to the formation of thermokarst and the release of stored carbon dioxide and methane.
Active Movement - The movement of an organism by its own means. This may involve the use of morphological appendages such as wings, legs, and fins. Compare with passive movement.
Active Remote Sensing - A form of remote sensing where the sensor provides its own source of electromagnetic radiation to illuminate the object being viewed. Radar is an example of an active remote sensing device.
Actual Evapotranspiration - The amount of water that is actually removed from a surface due to the processes of evaporation and transpiration.
Actual Mixing Ratio - Another term used to describe mixing ratio.
Actual Resource - Is a substance or thing that is currently being consumed, and there is a measured estimate of how much of this resource exists in the environment.
Adaptation - (1) Evolutionary adaptation - a genetically based characteristic expressed by an organism. Particular adaptations found in populations become frequent and dominant if they enhance an individual's ability to survive in the environment. (2) Physiological adaptation - change in an organism's physiology as a result of exposure to some environmental condition. (3) In climate science, the process of adjusting natural or human systems to actual or anticipated climate change and its consequences. Adaptation strategies encompass flood defenses, drought-resistant crops, early-warning systems, and revised building codes. These strategies differ from mitigation, which focuses on reducing greenhouse gas emissions.
Adaptive Radiation - The evolution of a number of new species from one or a few ancestor species over many thousands or millions of years. Normally occurs after a mass extinction creates many vacant ecological niches, or after a radical environmental change produces new ones.
Adhesion - The intermolecular attraction that occurs between molecules of different substances.
Adiabatic - A process where heat does not enter or leave a system. In atmospheric sciences, adiabatic processes are often used to model internal energy changes in a rising or descending parcel 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 (such as condensation, evaporation, and radiation), expansion causes the parcel of air to cool at a set rate of 0.98°C per 100 meters (0.54°F per 100 feet). The opposite occurs when a parcel of air descends in the atmosphere. The air in a descending parcel becomes compressed. Compression increases the parcel's temperature by 0.98°C per 100 meters (0.54°F per 100 feet).
Adiabatic Compression - In our planet's atmosphere, adiabatic compression occurs when a parcel of air lowers its altitude. The increase in atmospheric pressure with lower altitude causes the parcel to decrease in size and for the gas molecules in the parcel to move closer together, which in turn increases the parcel's temperature at a rate of 0.98°C per 100 meters (0.54°F per 100 feet).
Adiabatic Cooling - The cooling of a rising air parcel because of adiabatic expansion.
Adiabatic Expansion - In our planet's atmosphere, adiabatic expansion occurs when a parcel of air rises in altitude. The decrease in atmospheric pressure with higher altitude causes the parcel to increase in size and the gas molecules in the parcel to move further apart from each other, which in turn causes the temperature in the parcel to decrease at a rate of 0.98°C per 100 meters (0.54°F per 100 feet) if saturation has not occurred.
Adiabatic Warming - The warming of a sinking air parcel because of adiabatic compression.
Adret - The side of a hill that faces the Sun most directly. As a result, this area receives more heat energy and light than other parts of the hill. The term is frequently used in the Alps.
Advection - A process that involves the transfer of mass and heat energy using horizontal motions through a fluid substance like air or water. Warm and cold advection are crucial concepts in synoptic-scale weather analysis and forecasting. Also see convection.
Advection Fog - A type of fog that develops when wind flows over a surface with a different temperature, causing the air to cool below its dew point through conduction. Two types of advection fog exist: (1) When warm air flows over a cold surface, it can produce fog through contact cooling. (2) Cold air blowing over a warm, moist surface produces a form of advection fog known as evaporation fog.
Adventitious - Refers to the buds, shoots, and roots that grow from unusual locations on a plant. For example, a root that develops from stem tissue.
Adventive Cone - A small volcanic cone or crater that forms on the sides of a much larger volcano.
Advisory - Weather statements issued by national meteorological services (such as the National Weather Service in the United States) to alert the public to weather conditions that may cause inconvenience or pose a moderate risk, but are not expected to be life-threatening. They are issued at a lower level of urgency compared to watches and warnings.
Aeolian - A geomorphic process involving wind. Alternative spelling is 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.
Aeolianite - A sedimentary rock consisting of aeolian sediment cemented together by substances like calcium carbonate. The degree of cementation can vary widely, and this factor influences rock porosity.
Aeration Zone - See zone of aeration.
Aerial Camera - A specially designed camera used for aerial photography.
Aerial Photography - A form of remote sensing that captures images of objects using photographic cameras and film from platforms in the atmosphere or space.
Aerobic - (1) Presence of molecular oxygen. (2) Occurring only in the presence of molecular oxygen. (3) Being able to exist in the presence of molecular oxygen.
Aerography - Scientific study of the geographic range of populations, species, genera, families, etc.
Aerosol - A term used to describe suspended solid and liquid particles found in the atmosphere. These particles include bacteria, sea salt, water droplets, ice crystals, smoke, condensation nuclei, deposition nuclei, various types of pollutants, dust, spores, and pollen.
Aerosol Direct Effect - The impact on the Earth’s energy balance by aerosols scattering and absorbing incoming solar radiation and outgoing longwave radiation. Sulfate aerosols tend to cool the planet by reflecting sunlight, while black carbon absorbs radiation and warms the atmosphere. The overall effect of all aerosols is estimated to be a modest cooling, which partially offsets the warming caused by greenhouse gases.
Aerosol Indirect Effect - The influence of aerosols on Earth's climate through their function as cloud condensation nuclei. Higher aerosol concentrations lead to clouds with more numerous but smaller droplets. This results in increased cloud cover and albedo (the first indirect, or Twomey, effect), as well as the potential suppression of precipitation and the prolongation of cloud lifetime (the second indirect effect). These effects introduce considerable uncertainty into projections from computer models of human-caused climate change.
Aerosol Optical Depth (AOD) - Is a measure of the reduction incoming solar radiation by aerosols in a vertical column of the atmosphere because of scattering and absorption. A zero AOD indicates a perfectly clean atmosphere, while values above 0.4 suggest heavy aerosol loading, such as during dust storms or wildfire smoke events. Usually measured by ground-based pyranometers and satellite sensors.
Aerosphere - Another term used to describe the atmosphere surrounding a planet or other large celestial body.
Aestivation - A state in some animals where there is summer season inactivity and metabolic reduction. Usually coincides with annual cycles of drought.
Affluent - A smaller stream flowing into a larger stream.
Afforestation - The establishment of forest cover on a site that previously did not support trees. Compare with reforestation.
Afrotropic - One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm covers all of Africa south of the Sahara Desert, the southern Arabian Peninsula, Madagascar, southern Iran, southwestern Pakistan, and various islands of the western Indian Ocean. The climate of this realm is mainly tropical to subtropical.
Afterglow - Curved area of white to orange to red light appearing for some time after sunset along the horizon. Afterglow occurs near the point where the Sun descends below the horizon. Afterglow is created by the Mie scattering of sunlight in the atmosphere.
Aftershock - Small earthquakes that occur seconds to weeks after a major earthquake event.
Agate - A semi-precious gemstone consisting normally of fine-grained chalcedony quartz that occurs in different colored alternating bands when seen in cross-section. Commonly found in metamorphic and volcanic rocks. Image Source: Wikimedia Commons.
Age - Shortest geologic time unit. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.
Ageostrophic Wind - The component of the actual wind that differs from the geostrophic wind. Ageostrophic flow, resulting from accelerations like curvature effects, friction, and changes in wind speed, is responsible for cross-isobar flow near the surface and the transverse circulations near jet streams. These circulations drive vertical motion and influence cyclogenesis.
Agglomerate - A coarse-grained igneous rock made up of different-sized fragments of lava that have been lithified with volcanic ash and dust by heat. Associated with explosive volcanic eruptions.
Aggradation - (1) Readjustment of the stream long profile where the stream channel is raised by the deposition of bed load. (2) The buildup of the land surface because of the deposition of sediments from aeolian, fluvial, and glacial processes. (3) The process of ice crystals colliding and sticking together to form larger snowflakes. Aggregation is most efficient at temperatures near 0°C, where a thin layer of liquid water on the surfaces of ice crystals acts promote bonding.
Aggradational Ice - Horizontal below-ground layers of ice that form in the active layer in permafrost environments.
Aggregate - An accumulation of soil particles that bind together to form a much larger mass. Aggregates change the physical nature of a soil by increasing air flow and permeability.
Aggregation Ratio - A soil science term that describes the proportion of different types of clay minerals to the total weight of clay particles in a sample of soil.
Aggressivity - The ability of water to dissolve calcium carbonate in limestone bedrock.
Agricultural Revolution - The transition from nomadic hunting and gathering human societies to a more advanced society where agriculture generates most of the food required for consumption. During this transformation, the practice of agriculture becomes increasingly reliant on tools, machines, and technology.
Agroclimatology (Agricultural Climatology) - A subfield of climatology that examines the longterm effect of climate on the components of agricultural systems.
Agroforestry (Agricultural Forestry) - Is a hybrid approach to land-use that combines the knowledge and technologies of agriculture and forestry to create a more efficient, productive, and sustainable forest land-use system.
Agrometerology (Agricultural Meteorology) - A subfield of meteorology that examines the short-term effect of weather on the components of agricultural systems.
Agronomy - A field of science that studies various phenomena associated with agriculture.
Air Density - The mass of air per unit volume, usually expressed in kilograms per cubic meter (kg/m3). At sea level under standard atmospheric conditions, dry-air density is approximately 1.225 kg/m3. Air density decreases as altitude increases, increases with pressure, and decreases with temperature and moisture content. These relationships are governed by the Ideal Gas Law, which connects pressure and temperature.
Air Mass - A large body of air whose temperature and humidity characteristics remain relatively constant over a horizontal distance of hundreds to thousands of kilometers (miles). Air masses develop their climatic characteristics by remaining stationary over a source region for many days. Air masses are classified according to their temperature and humidity characteristics.
Air Mass Classification - A system used to group air masses by their source-region properties. The major types are continental arctic (cA), continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT).
Air Mass Modification - Changes in the temperature, moisture, and stability of an air mass as it travels away from its source region and moves over surfaces with different temperature and moisture characteristics.
Air Mass Source Region - Large, relatively uniform areas of the Earth’s land or ocean surface where an air mass gains its temperature and humidity characteristics. These regions typically experience light winds and persistent high-pressure systems, leading to extended residence times. Examples of source regions include the subtropical oceans, which produce maritime tropical air, and the snow-covered interior of Russia, which generates continental polar air.
Air Mass Thunderstorm - Are common single-celled thunderstorms that develop from a mass of unstable air. Compare with a severe thunderstorm.
Air Parcel - A hypothetical volume of air used for modeling basic dynamic and thermodynamic properties of the atmosphere. The air parcel concept is often used to simulate adiabatic processes in the atmosphere and to predict atmospheric stability.
Air Pollution - The addition of one or more noxious or harmful substances to the atmosphere. These substances must be in concentrations high enough to be hazardous to humans, other animals, vegetation, or human-made materials. Also see primary pollutant and secondary pollutant. Also called atmospheric pollution.
Air Pressure - See atmospheric pressure.
Air Quality Index (AQI) - Is a standardized numerical scale used by government agencies to convey the current level of air pollution or the predicted pollution level to the public. The AQI usually ranges from 0 (good) to 500 (hazardous). AQI is determined from concentrations of ozone, particulate matter (PM2.5), carbon monoxide, sulfur dioxide, and nitrogen dioxide.
Aircraft Icing - The accumulation of ice on aircraft surfaces during flight from clouds containing supercooled water droplets. Icing increases drag, reduces lift, and can detrimentally affect the readings of aircraft instruments. Types of aircraft ice include rime ice (rough, opaque, formed from small droplets), glaze ice (smooth, clear, formed from larger droplets), and mixed icing.
Airglow - A dim emission of visible light by excited gas atoms in the upper atmosphere at night. In the Earth's upper atmosphere, airglow involves excited oxygen atoms and molecules and often has an emerald green color. Also called nightglow.
Airshed (Air Shed) - A defined geographic region within the atmosphere. This concept is often used to manage the effects of pollution and other emissions on air quality, as they relate to human and other forms of life's health.
Airuille - A pointed mountain peak or outcrop of rock that has developed its form because of the erosive forces of frost action.
Alas - A relatively large flat-floored depression with steep sides found in areas of permafrost and thermokarst. These features may fill with water, forming a lake or pond.
Albedo - The reflectivity of a surface with respect to visible light.
Albers Equal Area Map Projection - A type of Conic Map Projection system that accurately depicts area but distorts map scale and the shape of map features, such as coastlines. This projection system is commonly used in British Columbia, Canada, for provincial-scale mapping and by the United States Geological Survey.
Alcove - A deep, steep-sided recess on the face of a cliff that was produced by fluvial erosion.
Alcrete - An aluminum-rich soil deposit that forms as a hard crust at or near the ground surface. Forms mainly in the tropics, where high temperatures and high rainfall cause the chemical weathering of soil and rock, leaving behind deposits of relatively insoluble Al3+ ions. If the aluminum content is high enough, the ore is called bauxite, which is a very important source of aluminum for surface mining.
Aleutian Low - Subpolar low pressure system that commonly forms near the Aleutian Islands. Most develop during the winter season. This large-scale atmospheric pressure system spawns many mid-latitude cyclones and influences precipitation patterns across western North America. Its intensity is influenced by ENSO and the Pacific Decadal Oscillation.
Alfisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. Soil associated with forest vegetation. Upper layers of this soil are relatively rich in organic matter. A whitish layer is found in the A horizon because of eluviation. The illuvial layer forms in the B horizon.
Algae - A simple photosynthetic plant that commonly lives in moist or aquatic environments. Algal species can be unicellular or multicellular.
Algal Bloom - A rapid increase in the population size of algae in a fresh or saline aquatic system. Most blooms contain several hundred to millions of algae cells per milliliter of water. However, there is no official threshold for the number of algae cells to define an algae bloom. Algal blooms can have negative effects on other species in aquatic systems. Algal blooms can occur naturally or be caused by human activities, such as nutrient fertilization.
Alien Species - See invasive species. Also called exotic species and an introduced species.
Alimentation - The accumulation of snow and ice in a firn field, adding to the mass of a glacier.
Alkali - A term used to describe a chemical substance that is basic and has the ability to dissolve in water. Some chemical elements that have this property include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
Alkali Flat - A flat, extensive ground surface deposit of salts combined with other fine-grained sediments. Commonly form when a lake dries out because of a reduction in precipitation or an increase in evaporation or both. Also called a dry lake.
Alkali Soil - A soil that has a pH greater than 8.5 because of the presence of salts. Soils with this level of alkalinity deter the growth of most plants.
Alkaline - (1) Having a pH greater than 7. (2) A substance that releases hydroxyl ions (OH-). Compare with acidic.
Allele - Alternative forms of a gene. Each form produces a unique inheritable characteristic in an organism.
Allelopathy - A particular form of amensalism found in plants. In this interaction, one species produces and releases chemical substances that inhibit the growth of another species.
Allerød Interstadial - A relatively short interstadial that occurred from 11,000 to 12,000 BP (before present) in Europe during the last major glacial advance of the Pleistocene Epoch.
Allogenic Stream - A stream whose discharge is generated from locations outside its immediate area. For example, the Nile River's discharge does not come from the Egyptian desert areas but from sources to the south of Egypt. Allogenic streams can also receive much of their discharge from groundwater flow through permeable rocks.
Allogenic Succession - A plant succession caused by a change in environmental conditions that is unrelated to the activities of the developing plant community.
Allometric Growth - A concept applied to organic and inorganic systems that suggests a change in the size of the whole is correlated to changes in the size of the parts that make up the whole. For example, when we compare the morphology of mammals, changes in body size are generally proportional to changes in skull size, leg length, heart volume, etc. Or in an inorganic system, morphological analysis of streams of different sizes suggests the proportion of segments that fall into order classes remains proportionally the same.
Allopatric Speciation - The evolution of a new species because of the isolation of a small group of individuals from the other members of a population. Compare with sympatric speciation.
Allopatry - Two or more things that occur in distinctly different geographical areas. Often used by biologists to refer to species or populations. Compare with sympatry.
Allophane - A hydrated aluminosilicate substance commonly found associated with clay minerals.
Alluvial - Things related to a stream and its various associated landscape modifying processes.
Alluvial Channel - Refers to a stream channel that is incised into alluvial deposits.
Alluvial Fan - Large fan-shaped terrestrial deposit of alluvial sediment (alluvium) on which a braided stream flows over. These landscape features form when stream load is deposited due to a reduction in stream velocity.
Alluvial Soil - A soil that is the product of alluvial deposits from a stream. These soils are often fine-grained and composed of clay and silt particles. An alluvial soil can also be composed of coarse-grained deposits.
Alluvial Terrace - A flat, elevated bench composed of unconsolidated alluvium found on either side of a stream channel. Formed when a stream downcuts into its floodplain.
Alluvium - Sediment that originates from a stream. Compare with colluvium.
Alpha Particle - A particle of matter that is positively charged. Alpha particles consist of two neutrons and two protons and are emitted as a form of radioactivity from the nuclei of some radioisotopes. Also see beta particle and gamma rays.
Alp - (1) Any mountain with a relatively high elevation. (2) A summer pasture area found below the snow line in a mountainous region.
Alpha Diversity - A measure of species diversity that focuses on the mean number of different species within smaller subunits located in the broader landscape. Introduced by ecologist R.H. Whittaker. Compare with beta diversity and gamma diversity.
Alpine - Zone above the tree line but below the region of permanent snow on a mountain.
Alpine Glacier - A small glacier that occupies a U-shaped valley on a mountain. Also called a mountain glacier and a valley glacier. Compare with cirque glacier, piedmont glacier, and continental glacier.
Alpine Orogeny - A period of mountain building in southern Europe. This orogeny produced the Alps Mountains in Europe. This event took place from the late Mesozoic to the Tertiary.
Alpine Permafrost - A form of permafrost that is found at high altitudes in mountainous environments.
Alpine Tundra - A high-altitude biome or ecosystem dominated by a few species of dwarf shrubs, grasses, sedges, lichens, and mosses. Biological productivity is low in this biome because of the extremes of climate. Quite similar to a tundra.
Alteration - A term used in Geology to describe a change in the minerals that make up the composition of a rock. Often associated with chemical metamorphism.
Alternative Hypothesis (H1) - A hypothesis that has been suggested by a researcher because it is believed to be false or because it is being used as a starting point for a logical scientific argument. Used in statistical testing to help organize the testing of logical arguments. Compare with the null hypothesis.
Altimeter - An instrument used to measure the altitude of an object above a defined surface, like sea level.
Altiplanation - A type of solifluction that creates flat-topped terraces composed of loose rock. Also called cryoplanation.
Altithermal - A phase during the Holocene when global temperatures were 1 to 3°C (2 to 5°F) warmer than today. The date for this warm period is about 9,000 to 5,000 years ago. Also called Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, and Hypsithermal.
Altitude - The vertical distance above a fixed reference starting point. Most altitude measurements use mean sea level as the base datum.
Altocumulus Cloud (Ac) - 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 (6,500 to 26,250 feet).
Altostratus Cloud (As) - Gray-looking middle altitude cloud that is composed of water droplets and ice crystals. Appears in the atmosphere as a dense sheet-like layer. It can be recognized from stratus clouds by the fact that you can see the Sun through them. Found in an altitude range from 2,000 to 8,000 meters (6,500 to 26,250 feet).
Alveole - A network of small depressions found on the surface of a rock because of chemical weathering.
Ambient Temperature - The temperature of the environment surrounding or within some object or thing.
Amensalism - Interspecific interaction between different species where one species suffers in terms of fitness, while the fitness of the other species does not change. See allelopathy.
Amino Acid - A complex organic molecule containing mainly containing atoms of carbon, hydrogen, oxygen, and nitrogen. Used by organisms to construct proteins.
Ammonia - Chemical compound composed of nitrogen and hydrogen (NH3). A major component of the nitrogen cycle. Released from organic matter when decomposed.
Ammonium - Chemical compound composed of nitrogen and hydrogen (NH4+). A major component of the nitrogen cycle. Product of organic matter decomposition. Ammonium can be fixed to clay minerals found in soil and later chemically exchanged.
Ammonium Fixation - A soil process where ammonium ions (NH4+) are absorbed onto negatively charged sites found in a clay particle. Once absorbed, these ammonium ions are not readily available to plants.
Amphibian - A group of vertebrate animals that can inhabit both terrestrial and aquatic habitats. This group of animals contains frogs, newts, and salamanders. These organisms live at the land/water interface and spend most of their life in water. From an evolutionary perspective, these organisms are descended from fish and are ancestors to reptiles.
Amphibole - A group of double-chained inosilicate minerals whose basic chemical unit is the tetrahedron (SiO4). Amphiboles are common rock-forming minerals and are found in most igneous rocks and metamorphic rocks. They form at low temperatures when water is present in the crystallization environment. There are about 60 recognized mineral types in this group.
Amphidromic System - The circular movement of near-surface water within an ocean basin driven by a tidal cycle and Coriolis effect. Embedded within this flow are high tide wave crests and low tide wave troughs. Such systems move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Amplitude - A vertical measure of a wave from its center or equilibrium to the next wave crest.
Anabatic Wind - A local type of wind that is generated by surface heating and travels upslope. A valley breeze is an example of an anabatic wind.
Anabranch - A segment of a stream that branches away from the main stream channel and rejoins with the main stream channel somewhere downstream. Anabranching creates small islands in the stream system.
Anabranching - A type of stream channel pattern. Similar to a braided stream channel pattern, except that islands tend to be much wider than the width of the stream channel.
Anadromy - A migration behavior found in some fish species where individuals move from seawater to freshwater for reproduction.
Anaerobic - (1) Absence of molecular oxygen. (2) Occurring only in the absence of molecular oxygen. (3) Being able to exist without the presence of molecular oxygen.
Anaerobic Respiration - See fermentation.
Anafront - A situation of strong frontal activity where warm air rises up and over the surface of a cold or warm front to high altitudes. The moisture in this rising warm air usually produces precipitation that falls a considerable distance from the surface edge of the front. Associated with mid-latitude cyclones and cyclogenesis. Compare with katafront.
Analemma - (1) Is a curve drawn on a globe to describe the daily declination of the Sun. This curve allows one to find the line of latitude where the Sun will be directly overhead at solar noon for a specific date during the year. (2) A curve that plots the position of the Sun in the daytime sky at the same time each day, over a calendar year, for a particular Earth location.
Anamolistic Tidal Cycle - A tidal cycle caused by the repeated change in the distance between the Earth and Moon over 27.5 days.
Anaseism - The vertical movement of seismic waves away from an earthquake focus.
Anastomosing - The rejoining of two stream channels that were created by the division of a single channel somewhere upstream. Commonly seen in braided stream systems.
Anchor Ice - Submerged ice attached to the bottom of a water body like a stream, lake, or sea.
Andesite - An extrusive igneous rock that develops from magma that is chemically between felsic and mafic and whose mineral crystals are fine.
Andisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. These soils develop from volcanic parent materials.
Andosols - A common soil type that develops from volcanic deposits. These soils are dark in color and very fertile.
Anemograph - A meteorological instrument that constantly records wind speed and direction.
Anemometer - A meteorological instrument used to measure wind speed. These instruments commonly employ four methods to measure this weather phenomenon: (1) A device with three or four open cups attached to a rotating spindle. The speed of rotation is then converted into a measurement of wind speed; (2) A device that correlates propellor spin speed with wind speed; (3) A pressure plate that measures the force exerted by the moving wind at right angles; and (4) An instrument consisting of a heated-wire where electrical resistance (temperature of the wire) is adjusted to account for heat lost by airflow. 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 by the drain of electrical current.
Aneroid Barometer - A commonly used type of barometer that measures atmospheric pressure by way of the expansion and contraction of a sealed hollow cell, which is partially depleted of air.
Angiosperms - A large group of species of vascular plants that cover their seeds in a mature ovary or fruit.
Angle of Declination - (1) The angle from the direction produced by a magnetic needle in a compass to True North. (2) The angle to some object in space from a reference horizontal plane.
Angle of Incidence - Angle at which the Sun's rays or insolation strike the Earth's surface. If the Sun is directly overhead or 90° from the horizon, the incoming insolation strikes the Earth's surface 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 structurally failing. Geomorphologists use this measurement to determine the stability of a slope to mass movement.
Angular Momentum - Abstract concept in physics that models an object's speed of travel at a distance from an axis of rotation. Mathematically, it is the product of an object's mass, its velocity and its distance from the center of rotation. Also see momentum.
Angular Unconformity - Is a sedimentary deposit where horizontal layers of younger sedimentary rock are deposited on older layers that are tilted and eroded, creating an angular discordance.
Angular Velocity - The velocity at which a rotating object spins, measured in angular displacement per unit time. Normally measured in radians or degrees per second or per hour.
Anhydrite - A mineral composed of anhydrous calcium sulfate (CaSO4). Anhydrite minerals are commonly found in gypsum.
Animal - Organisms that belong to the kingdom Animalia. General characteristics of these organisms include eukaryotic cells, mitochondria, and a complex nervous system. This group of life includes organisms like sponges, jellyfishes, arthropods (insects, spiders, shrimp, and lobsters), mollusks (snails, clams, oysters, and octopuses), fish, amphibians (frogs, toads, and salamanders), reptiles (turtles, lizards, alligators, crocodiles, and snakes), birds, and mammals (kangaroos, bats, cats, rabbits, elephants, whales, porpoises, monkeys, apes, and humans).
Animal Territory - The spatial area defended by an individual of a particular animal species against other members of its species or animals of other species.
Animalia - Group, at the kingdom level, in the classification of life. Multicellular organisms that have a eukaryotic cell type, mitochondria, and a complex nervous system. See the term animal for more information.
Anion - An ion carrying a negative atomic charge. Examples of anions include nitrite (NO2-) and nitrate (NO3-).
Anisotropy - An object or thing that has an attribute whose measurement changes value with direction. Compare with isotropy.
Annual Mean Global Temperature - It is normally the annual mean temperature calculated from all of the Earth's surface (terrestrial and ocean) meteorological stations for a particular year. The World Meteorological Organization (WMO) calculated the annual mean global temperature for 2011 to be 14.41°C (57.94°F). This value can also be calculated for a specific number of years. For the 30-year period 1961-1990, the annual mean global temperature was about 14.0°C (57.2°F). NASA uses the period 1951-1980 for comparative purposes and suggests the average annual mean global temperature for this 30-year period was 14.0°C. Also called the annual average global temperature.
Annual Mean Temperature - The average surface temperature as recorded at a meteorological station for a location for a particular year. Usually, it is calculated by summing all of the monthly mean temperatures and then dividing by 12.
Annual Plant - A species of plant that completes its life in one growing season.
Annual Temperature Range - The numerical difference between the warmest and coldest monthly mean temperatures for a particular year. Two factors that strongly influence this measure are latitude and distance from large bodies of water. Locations near the equator generally have a smaller annual temperature range when compared to locations closer to the poles. Locations in the center of continents, away from the moderating effects of oceans, also tend to have larger annual temperature ranges.
Annular Drainage - A stream drainage pattern that is circular or ring-like in appearance (see figure). Annular drainage occurs when streams erode into softer horizontal rock layers within a dome or basin landscape feature.
Antarctic - (1) A region of the Earth found south of 66.5° South latitude. The Antarctic consists of the Southern Ocean and the continent of Antarctica. (2) One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm covers all of Antarctica. The climate of this realm is polar.
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 Antarctica throughout the year. This pressure system is responsible for creating surface air masses with very cold temperatures and extremely low humidity.
Antecedent Drainage - A stream drainage system that has maintained its particular pattern despite changes in the landscape because of localized landscape uplift.
Antecedent Moisture - Refers to the moisture conditions found in an area or within a soil before a rainfall event.
Anthracite - A type of coal that has the highest carbon content and the least impurities. Anthracite has a carbon content between 92.1% and 98%.
Anthromorphic Soil - A type of soil that has formed recognizable characteristics because of the activities of humans.
Anthropochore - A plant species that has been introduced to a habitat directly (for example, as a crop plant) or indirectly (for example, as a weed) by the direct or indirect action of humans.
Anthropogeomorphology - The geomorphological study of humans as an agent in landscape processes and forms.
Anticline - A fold in layers of rock that forms an upward arch or has a convex shape. Anticlines can also be asymmetrical. Compare with syncline, monocline, overturned fold, and recumbent fold.
Anticyclone - An atmospheric pressure system consisting of an area of high pressure and outward circular wind flow near the ground surface. In the Northern Hemisphere, winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise. Compare with a cyclone.
Antidune - A stream bed alluvial deposit which migrates upstream against the direction of water flow. Found in the shallow areas of streams that have a relatively high flow rate. Antidunes look very similar to ripples. However, these stream bed features develop in low-flow conditions. Also see bedforms.
Antiform - A landscape feature that is made of folded layers of rock that arch upward (convex shape). Antiforms differ from anticlines in that the oldest rocks may not be exposed in the middle of the feature.
Antipodes - Two locations on the Earth's surface that, if you draw a straight line between them, the line will pass through the planet's center.
Antitrades - An extensive layer of westerly wind in the troposphere that exists above the easterly trade winds.
Anvil Cloud - A cumulonimbus cloud that has reached the bottom of the stratosphere and has subsequently stopped its vertical development, forming an anvil-shaped top. Also called cumulonimbus incus.
Aphelion - The point in the Earth's orbit when it is farthest from the Sun. This distance is about 152.1 million kilometers (94.5 million miles). Aphelion occurs on the 3rd or 4th of July. Compare with perihelion.
Aphotic Zone - The layer of water in a lake, sea, or ocean where little or no sunlight penetrates the water column. Technically, this layer begins when less than 1% of the sunlight is available in the water column. Many organisms found at this depth create their own light via bioluminescence. In an ocean, this layer starts at a depth of between 200 and 1,000 meters (660 to 3,300 feet) and extends to the ocean floor. The aphotic zone is further subdivided into the bathyal zone, abyssal zone, and hadal zone. Also see (in order of depth in a water body) pelagic zone, photic zone, mesopelagic zone, demersal zone, and benthic zone.
Aphytic Zone - The portion of the floor of a lake, sea, or ocean that is devoid of plant growth because it is too deep for significant light penetration.
Apogean Tide - A type of tide that occurs when the Moon is farthest from the Earth (apogee). Apogean tides have a smaller range between successive high tide and low tide events because of reduced tide-producing power from the Moon.
Apogee - The point along a celestial body's orbit (for example, the Moon) when it is farthest from the Earth.
Apparent Dip - The perceived angle of dip of a geologic layer based on information from a two-dimensional vertical cross-section. If the cross-section is perpendicular to the strike of the layer, the apparent dip will be equal to the true dip.
Apparent Temperature - A composite index that quantifies how hot or cold the air feels to the human body. It considers the effects of air temperature, humidity, wind speed, and sometimes solar radiation. Common apparent temperature indices include the heat index, wind chill, humidex, and wet-bulb globe temperature. Also called feels-like temperature.
Applied Physical Geography - A field of applied science that uses theoretical information from the various sub-disciplines of Physical Geography to manage and solve problems related to natural phenomena found in the real world.
Apposed Glacier - A glacier produced by the meeting and joining of two separate glaciers.
Apron - The outer edge of shallow-angle alluvial sediment associated with an alluvial fan.
Aquaculture - The human activity of raising freshwater and seawater populations of fish, crustaceans, mollusks, and aquatic plants under controlled and often artificial conditions for resource use and consumption. Also called aquafarming.
Aquatic - With reference to water.
Aquatic Macrophyte - A plant that grows in an aquatic environment and has parts of its body involved in photosynthesis that are emergent, submerged, or floating on the water surface for at least part of the time.
Aquatic Plant - See hydrophyte.
Aquiclude - Rock formations that are impermeable to groundwater. These formations do not absorb or transmit significant amounts of water vertically or horizontally. Also called aquifuge.
Aquifer - Rock formations that store groundwater.
Aquifer Recharge Area - The surface area that provides water for an aquifer.
Aquifuge - See aquiclude.
Arboreal - Something related to or living in a tree.
Arch - A natural formation in rock that contains a curved arch. Such landscape features are common along coastlines and form because of wave erosion. Arches can also be found in arid landscapes. In this environment, the process responsible for the formation is wind erosion.
Archaea - A group of recently discovered organisms that visually resemble bacteria. However, these organisms are biochemically and genetically very different from bacteria. Some species of the domain Archaea live in the most extreme environments found on our planet.
Archaeological Dating - The dating of a thing or event using a chronological calendar associated with the known historical accomplishments of humans.
Archaebacteria - A term used to describe organisms that belong to the biological domain Archaea.
Archean - Geologic eon that occurred from 2.5 to 3.8 billion years ago. During this period, the first single-celled prokaryotic organisms evolved. Also see Hadean, Proterozoic, and Phanerozoic Eons. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Archipelago - A group of islands arranged in an arc shape. These islands are usually of volcanic origin and are associated with subduction zones.
Arctic - The region of the Earth found north of 66.5° North latitude. The Arctic consists of the Arctic Ocean and all or parts of the United States, Canada, Russia, Greenland, Norway, Sweden, Finland, and Iceland.
Arctic Air Mass - See Continental Arctic Air Mass.
Arctic Amplification - The situation where the Arctic region experiences a warming rate approximately two to four times higher than the global average in response to increased greenhouse gas concentrations. This warming is driven by positive feedback mechanisms, including the ice-albedo feedback. As reflective sea ice and snow melt, darker ocean and land surfaces absorb more solar radiation. Additionally, changes in cloud cover, increased water vapor, and poleward heat transport contribute to this amplification. Arctic amplification leads to the decline of sea ice and may influence mid-latitude weather patterns through changes in the jet stream.
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.
Arctic Front - Is a weather front (line with triangles on it) that separates a Continental Arctic (cA) air mass from a Continental Polar (cP) air mass. Arctic fronts are not very active in terms of the weather they create because they involve the interaction of air masses with similar temperature and moisture characteristics. Compare with polar front.
Arctic Haze - A reddish-brown haze that develops in the springtime at high latitude locations. This form of air pollution results from human activities. Tends to persist for an extended period because of a lack of atmospheric mixing and limited precipitation to wash out pollution.
Arctic Ocean - The smallest of the world's five ocean regions, covering about 3% of the Earth's total surface area, has a total area of about 14,056,000 square kilometers (5,426,000 square miles). Most of this nearly landlocked ocean region lies north of the Arctic Circle. The Arctic Ocean is also the shallowest ocean region with an average depth of 1,050 meters (3,450 feet). The Arctic Ocean is connected to the Atlantic Ocean by the Greenland Sea, and the Pacific Ocean via the Bering Strait. Much of the Arctic Ocean is covered by sea ice.
Arctic Oscillation (AO) - A large-scale atmospheric pattern found in the Northern Hemisphere. It is characterized by opposing pressure anomalies between the Arctic and mid-latitudes. It significantly influences winter weather across North America, Europe, and Asia. During its positive phase, lower-than-normal pressure over the Arctic and higher-than-normal pressure at mid-latitudes strengthen the polar vortex, confining cold air to higher latitudes. Conversely, during its negative phase, the weakened vortex allows cold Arctic air to penetrate southward. The Arctic Oscillation is directly connected to the North Atlantic Oscillation.
Arctic Smoke - A type of evaporation fog that commonly forms over water bodies in the Arctic. This fog can be composed of minute water droplets and/or ice crystals. Also called frost smoke when it is mainly composed of ice crystals.
Arcuate Delta - A delta with an edge that is convex outward into its adjacent water body. The Nile Delta would be an example of an arcuate delta.
Arcus - Are low, horizontal, wedge-shaped cloud formations that occur at the leading edge of a thunderstorm’s outflow boundary. They consist of two sub-types: shelf clouds, which are attached to the parent cumulonimbus, and roll clouds, which are detached and rotate around a horizontal axis. Arcus clouds often precede strong and potentially gusty winds.
Area Studies Tradition - Academic tradition in modern Geography that investigates an area on the Earth from a geographic perspective at either the local, regional, or global scale.
Areic - A landscape without any significant surface drainage or streams.
Arena - A surface landscape feature that consists of a wide circular shallow basin surrounded by an edge of higher land.
Arenaceous - Any type of sedimentary rock that contains sand, for example, sandstone.
Arête - A sharp topographic ridge that separates cirques on a mountain that is or has been glaciated.
Argillaceous - Something that contains, resembles, or is associated with clay. For example, an argillaceous rock is composed of lithified clay particles.
Argon (Ar) - A noble gas that constitutes approximately 0.93% of the dry atmosphere by volume, making it the third most abundant atmospheric gas after nitrogen and oxygen. Chemically inert, argon does not participate in atmospheric chemical reactions or radiative processes.
Arid Zone - A region of the Earth that is roughly found between 15 to 30° N and S latitude and is characterized by very low rainfall, high temperatures, and desert or semi-desert vegetation.
Aridisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. Aridisols are commonly found in dry environments with low organic matter and high levels of deposited salts.
Arkose - A type of sedimentary sandstone that contains a significant quantity of weathered feldspar grains. This type of sedimentary rock tends to form under arid climate conditions.
Armouring - A situation where a layer of coarse-grained sediments found at the top of the stream bed inhibits the transport of smaller-sized particles.
Arroyo - A stream bed found in an arid climate that is rectangular in shape and is filled with stream flow occasionally or seasonally, usually after rainfall events. A stream bed may completely lack any stream flow during extended periods of drought.
Artesian Water - Groundwater that is confined by two impermeable layers beneath the Earth's surface.
Artesian Well - A well where hydrostatic pressure forces groundwater to rise and flow out to the ground surface.
Arthropod - Is an invertebrate animal that has an external skeleton, segmented body, and jointed appendages (legs, wings, mouthparts, antennae, etc.). Insects, arachnids, and crustaceans are organisms common to this group.
Asbestos - A general term used to describe the following six fibrous minerals: Chrysotile, tremolite, anthophyllite, amosite (grunerite), crocidolite (riebeckite), and actinolite. Shown in the image is a close-up of the fibers found in a sample of chrysotile. Asbestos became widely used starting in the 19th century for a variety of applications. However, medical science determined that asbestos fibers can accumulate in the lungs of humans and can cause deadly health conditions like mesothelioma.
Asexual Reproduction - Any process of reproduction where the genes used to create a new individual come from just a single parent. Compare with sexual reproduction.
Ash - See volcanic ash.
Ash Cone - A volcanic cone composed mostly of ash. Most ash cones are small in size relative to volcanic cones composed of lava and tephra. Elevation from the crater floor to the rim of the cone is typically between 100 and 800 meters. Koko Crater on Oahu, Hawaii, is an example of an ash cone.
Ash Fall - The downward movement of ejected volcanic ash from an ash cloud to the ground surface.
Aspect - The direction that a slope faces. See term slope aspect.
Asphalt - A thick, sticky, black liquid or semi-solid form of petroleum that will only flow if diluted or heated. In its unrefined form, asphalt is used in road construction and as a roof sealant. The largest reserves of asphalt are found in Alberta, Canada covering about 140,000 square kilometers. Also called bitumen.
Assemblage Zone - A unique rock stratum that is identified by its particular collection of three or more fossil taxa.
Assimilation - (1) The creation and absorption of food resources by an organism. (2) The organic metabolic byproducts of food digestion (the various organic constituents of an organism).
Association - (1) A grouping of things. (2) A collection of plants with similar physiological and structural characteristics that occur in a specific type of habitat.
Asteroid - A small, often irregularly shaped, celestial body made of rock that orbits around a star. Our Solar System has millions of asteroids, many of which have orbits between Mars and Jupiter.
Asteroid Belt - A region in our Solar System located between the planets of Mars and Jupiter where more than 700,000 asteroids larger than 1 kilometer in diameter orbit around the Sun. The total mass of the asteroid belt is approximately 4% of the Moon. About half of this mass is found in just four very large asteroids named Ceres, Vesta, Pallas, and Hygiea.
Asthenosphere - An upper layer in the Earth's mantle where rock is highly viscous and exhibits plastic properties. The asthenosphere is located just below the lithosphere at a depth of between 100 and 200 kilometers (62 and 124 miles) below the surface. The lower boundary of the asthenosphere is not well defined, and in some areas of plate subduction, the bottom of the asthenosphere may be much deeper than 200 kilometers. The asthenosphere has a role in plate tectonics, isostatic depression, and isostatic rebound.
Astronomy - A field of science that studies the nature, motion, origin, and composition of celestial bodies in the Universe.
Asymmetrical Fold - An anticline or syncline fold that is not symmetrical.
Atlantic Multidecadal Oscillation (AMO) - A pattern of natural variability in North Atlantic sea surface temperatures with a period of about 60–80 years. The warm phase, when North Atlantic sea surface temperatures are warmer than normal, is associated with higher hurricane activity in the Atlantic, wetter summers in the Europe and North America, and reduced Arctic sea ice.
Atlantic Meridional Overturning Circulation (AMOC) - A system of ocean currents in the Atlantic that transports warm surface water northward and returns cold, deep water southward. The Atlantic Meridional Overturning Circulation encompasses the Gulf Stream and the deep-water formation regions in the Labrador and Nordic Seas. It plays a crucial role in thermohaline circulation, redistributing heat towards the poles and significantly influencing European and global climate patterns. There are concerns about its potential weakening due to climate change, which could have profound effects on global patterns of temperature and precipitation.
Atlantic Ocean - A relatively narrow body of ocean water that snakes between nearly parallel continental masse. North and South America are on its western side, while Europe and Africa are on its eastern side. The average depth of the Atlantic Ocean is about 3,300 meters (10,800 feet). The Atlantic Ocean is the second-largest of the five oceans recognized on our planet. It covers about 21% of the Earth's total surface area and has a total area of about 76,762,000 square kilometers (29,630,000 squaremiles). The Mid-Atlantic Ridge, running roughly down the center of this ocean region, separates the Atlantic Ocean into two large basins.
Atmometer - A scientific instrument designed to measure the rate of evaporation. Also called an evaporimeter.
Atmosphere - (1) The atmosphere is a gaseous envelope of air, along with suspended liquid and solid particles, that surrounds large celestial bodies like the Earth. This air zone is held to the object by gravitational forces. Layers of differing physical and chemical characteristics, defined by altitude, often exist in the atmosphere of celestial bodies. The atmosphere contains a complex system of gases that behave in many ways like fluids. Most of the constituents of the atmosphere are derived from the celestial body by way of chemical and biochemical reactions. (2) A unit of pressure defined as exactly 101,325 pascals (Pa), equivalent to 1013.25 hectopascals or millibars, 760 mm Hg, or 29.9213 inches of mercury. It represents the average atmospheric pressure at sea level under standard atmospheric conditions and is primarily used in chemistry and older meteorological literature.
Atmospheric Mass - The total mass of Earth's atmosphere, approximately 5.15 × 1018 kg. Understanding the atmospheric mass is crucial for calculating the total load of greenhouse gases, aerosols, and other components. It also helps in comprehending the atmospheric pressure at the surface.
Atmospheric Pollution - See air pollution.
Atmospheric Pressure - The weight of the atmosphere on a surface. At sea level, the average atmospheric pressure is 1013.25 millibars (29.92 inches of mercury). Atmospheric pressure can be measured with a barometer.
Atmospheric River - A long, narrow zone of concentrated water vapor in the lower troposphere, typically 300 to 500 kilometers (200 to 300 miles) wide and thousands of kilometers long. Atmospheric rivers, which can carry moisture fluxes comparable to the Amazon River, are capable of producing heavy orographic precipitation upon landfall, especially along western coastlines. Atmospheric rivers serve as a significant source of both beneficial water supply and flood hazards.
Atmospheric Stability - The term refers to the relative stability of parcels of air relative to the atmosphere that surrounds them. Three conditions of atmospheric stability are generally described: stable, unstable, and neutral.
Atmospheric Transmission - The amount of sunlight that passes through the atmosphere and is received at the Earth's surface. Losses of sunlight travelling through the atmosphere can occur by way of atmospheric absorption, reflection, and scattering.
Atmospheric Window - Refers to the various wavelengths of electromagnetic radiation that are transmitted through Earth's atmosphere without being greatly absorbed. Three atmospheric windows are recognized: 11 meters to 1.0 centimeter (radio window); 14 micrometers to 8 micrometers, 4.0 micrometers to 3.0 micrometers, 2.4 micrometers to 2.0 micrometers, and 1.8 micrometers to 1.1 micrometers (infrared window); and 1100 nanometers to 300 nanometers (optical window).
Atoll - A ring-shaped reef composed largely of coral. Atolls are quite common in the tropical waters of the Pacific Ocean.
Atom - Smallest unit of an element that still maintains its chemical characteristics.
Atomic Energy - Energy released from an atomic nucleus because of a change in its subatomic mass.
Atomic Mass Number - Combined number of protons and neutrons in an atom.
Atomic Number - Number of protons in the nucleus of an atom.
Atomic Weight - Combined weight of an atom's electrons, protons, and neutrons.
Atterberg Limits - A test used in Civil Engineering, Geomorphology, and Soil Science to determine the strength and settlement characteristics of fine-grained soils. In Civil Engineering, these tests are commonly used when building structures to assess soil shear strength and determine the volume change in soil under different moisture conditions. This test can also be used to determine whether a soil is mainly composed of silt or clay particles. Also called the liquid limit.
Attribute (System) - See system attribute.
Attribution Science - An area of climate research that employs statistical methods and climate models to quantify the impact of human activities on the likelihood or severity of specific extreme weather events, such as heatwaves, heavy rainfall, or droughts.
Attrition - A process where the individual particles of moving matter are reduced in size because of friction in the transport mediums of wind, water, or ice.
Augite - A common inosilicate mineral rich in aluminum, calcium, magnesium, sodium, titanium, and iron found in mafic igneous rocks.
Aurora - Multicolored lights that appear in the upper atmosphere (ionosphere) over the polar regions and are 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 is called Aurora Borealis, and Aurora Australis in the Southern Hemisphere.
Aurora Australis - Aurora located in the Southern Hemisphere.
Aurora Borealis - Aurora located in the Northern Hemisphere.
Australasian - One of eight biogeographic realms found on the Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm covers all of Australia, New Guinea, New Zealand, and nearby islands. This realm is separated from the Indomalaya realm by Wallace's Line. The climate of this realm is mainly tropical, subtropical, and mid-latitudinal.
Autecology - The ecological study of a single individual or a specific species with the biotic and abiotic components of its environment.
Autocorrelation - This term refers to the correlation of a measured temporal variable with its past and future values or the correlation of a measured spatial variable with values adjacent to it in space. Applying parametric statistics to autocorrelated data violates the independence assumptions of these tests, potentially rendering their interpretations invalid.
Autoecology - A field of Ecology that studies individual organisms and specific species.
Autogenic Succession - A type of plant succession where the plant community causes the environment to change, and this modification drives the succession.
Autotroph - An organism that produces food molecules inorganically by using visible light or chemical-based sources of external energy. This organism does not require outside sources of organic food energy for its survival. Also see chemical autotroph and photosynthetic autotroph.
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. In the Northern Hemisphere, the autumnal equinox occurs on either September 22 or 23 (changing each year). 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 periods (12 hours) of day and night.
Available Nutrients - Nutrients found in the soil solution that are easily absorbed by roots for plant nutrition.
Available Water - The portion of the soil capillary water that is available for plant root uptake.
Avalanche - (1) The instantaneous and swift movement of snow and/or ice down a slope. Shown is an avalanche in dry powder snow. (2) Can also refer to the instantaneous and swift movement of material (like soil and sediment) down a slope.
Avalanche Wind - The fast-moving air created in front of an avalanche by moving snow and ice. The destructive force of such winds is directly related to the size of the avalanche.
Average Global Temperature - Average annual temperature of the Earth's atmosphere as measured over land and ocean just above the surface. Scientists calculate this value using thousands of land weather stations, ship and buoy measurements from the ocean, and satellite data. 2024 was the warmest year in the 20th and 21st centuries, with global average surface temperature 1.29°C above the 20th‑century average and 1.46°C above pre‑industrial levels.
Avulsion - The process where a section of stream channel becomes used in terms of stream flow in favor of another section of channel located at a lower elevation.
Azimuth - Is the angular measurement of direction between a projected vector and a reference vector on a reference plane. Examples include the angular measurement of a star with reference to the horizon, and the measurement of wind direction in relation to the cardinal points of a compass and the 360° of a circle. Compare with bearing.
Azimuthal Map Projection - Is a two-dimensional map projection system that projects the Earth onto a flat surface or plane. The Earth's surface touches this plane at only one point or tangent. These maps are plagued by area and shape distortion that increases from the tangent point. The ancient Greeks developed the first planar maps around 600 BCE. Modern versions of this projection are often used to illustrate the polar regions of our planet. An alternative name for this system is Planar Map Projection.
Azoic - (1) Without life. (2) A time before life existed on the Earth.
Azonal Soil - Any young soil that does not show well-developed soil profile characteristics because of insufficient time for pedogenesis to occur. Compare with intrazonal soil and zonal soil.
Azores High - See Bermuda High.
Azotobacter - Is a genus of bacteria that are aerobic and commonly found free-living in soil and aquatic environments. They play an important role in the nitrogen cycle by fixing atmospheric nitrogen (N2) for their nutrition. This nitrogen is released into the surrounding soil upon their death and decay as ammonium ions.
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