Abyssal plain: Flat, cold, sediment-covered ocean floor between the continental rise and the oceanic ridge at a depth of 3700 to 5500 meters. Abyssal plains are more extensive in the Atlantic and Indian oceans than in the Pacific Ocean.
Acid precipitation: Precipitation that is more acidic than natural precipitation, which has an average pH of about 5.5.
Acid rain: Rain containing acids and acid forming compounds such as sulfur dioxide and oxides of nitrogen.
Actual Evapotranspiration: Actual amount of evaporation and transpiration that occurs; derived in the water-balance equation by subtracting the deficit from potential evapotranspiration.
Adaptation: An inheritable structural or behavioral modification. A favourable adaptation gives a species an advantage in survival and reproduction. An unfavourable adaptation lessens a species ability to survive and reproduce.
Advection fog: Active condensation formed hen warm, moist air moves laterally over cooler water or land surfaces, causing the lower layers of the overlying air to be chilled to the deq-point temperature.
Aerology: The study of the free atmosphere through its vertical extent.
Aeroplankton: Spores and other microscopic organic particles and organisms that drift in the air because they weigh so little that they fall only slowly and are likely to be carried aloft again by raising air currents before settling on a surface.
Afforestation: It is a type of social forestry. If the trees are planted on a piece of land which was formerly not under plant cover for commercial or any other purpose is known as afforestation.
Agro-biodiversity: This is subset of biodiversity related to agriculture. It could be defined as diversity of agricultural systems, all agricultural related species and individuals within species.
Agroclimatology: The scientific study of the effects of climate upon crops.
Agroforestry: This is a syetem of land use where woody perennials are deliberately used on the same land management units as annual agriculture crops .
Albedo: The reflective quality of a surface, expressed as the relationship of incoming to reflected insolation and stated as the percentage; a function of surface colour, angle of incidence, and surface texture.
Algae: Collective term for nonvascular plants processing chlorophyll and capable of photosynthesis (singular, alga).
Algal bloom: The rapid and excessive growth of algae. It may deoxygenate the water leading to the loss of wildlife.
Alien species: When plants and animals get introduced in new areas crossing their own physical boundaries on land and sea, these are known as alien species. Alien species could either get introduced accidentally or deliberately. Their deliberate introduction could be for the sake of food, fodder, raw material, sports, aesthetics or any other necessity.
Alkaline: Basic. See base.
Alluvial fan: Fan shaped fluvial landform at the mouth of a canyon, generally occurs in arid landscapes where streams are intermittent.
Alluvium: General descriptive term for clay, silt, and sand, transported by running water and deposited in sorted or semi-sorted sediment on a floodplain, delta or stream bed.
Alpine Glacier: A glacier confined in a mountain valley or walled basin, consisting of three subtypes: valley glacier (within a valley), piedmont glacier (coalesced at the base of mountain, spreading freely over nearby lowlands), and outlet glacier (flowing outward from a continental glacier).
Alternation of generation: A reproductive cycle in which plant alternates between sexual and asexual stages.
Anemometer: A device to measure wind velocity.
Angiosperm: A flowering vascular plant that reproduces by means of a seed-bearing fruit. Examples are sea grasses and mangroves.
Angle of incidence: In meteorology, the angle of Sun above the horizon.
Animal: A multicellular organism unable to synthesize its own food often capable of movement.
Anroid barameter: A devise to measure air pressure using a partially emptied, sealed cell.
Antarctic bottom water: The densest ocean water (1.0279 g/cm3),, formed primarily in Antarctica’s Weddell Sea during Southern Hemisphere winters.
Antarctic Convergence: Convergence zone encircling Antarctica between about 50o and 60o, marking the boundary between Antarctic Convergence and the south by Antarctica.
Anticyclone: A dynamically or thermally caused area of high atmospheric pressure with descending and diverging air flows.
Aphelion: The most distant point in Earth’s elliptical orbit about the Sun; reached on July 4 at a distance of 152,083,000 km (94.5 million miles), variable over a 100,000 year cycle.
Aphotic zone: The dark ocean below the depth to which light can penetrate.
Aquaculture: The growing or farming of plants and animals in a water environment under controlled conditions.
Aquatic ecosystem: These are the ecosystems that occur in water and are also known as marine ecosystems.
Aquifer: Rock strata permeable to groundwater flow. Aquifer recharge area: The surface area where water enters an aquifer to recharge the water-bearing strata in a ground water system.
Arctic Circle: The imaginary line around the Earth, parallel to the equator at 66o 33’N marking the northernmost limit of sunlight at the December solstice. The Arctic Circle marks the southern limit of the area within which, for one day or more each year, the Sun does not set (around 21st June) or rise (around 21st December).
Arctic hurricanes: An intense low pressure system that develops along the edge of the pack ice in the Arctic. Arctic hurricanes produce high winds and heavy precicipitation and represent a major hazard to shipping along their path.
Arctic Ocean: An ice-covered ocean north of the continents of North America and Eurasia.
Arctic Tundra: A biome in the northern most portion of North America, Europe and Russia, featuring low ground level herbaceous plants as well as some woody plants.
Artesian water: Pressurised groundwater that rises in a well or rock structure above the local water table; may flow out onto the ground.
Asthenosphere: Regions of the upper mantle just below the lithosphere known as plastic layer; the least rigid portion of Earth’s interior; shattered if struck yet flows under extreme heat and pressure.
Astronomical twilight: The period after sunset or before sunrise ending or beginning when the Sun is 18o below the horizon.
Atmosphere: The thin veil of gases surrounding Earth, that forms a protective boundary between outer space and the biosphere, generally considered to be below 480 km (300 miles).
Atmospheric circulation cell: Large circuit of air driven by uneven solar heating and the Coriolis effect,. Three circulation cells form in each hemisphere (Hadley cell, Ferrel cell and Polar cell).
Atoll: A ring-shaped island of coral reefs and coral debris enclosing, or almost enclosing, a shallow lagoon from which on land protrudes. Atolls often form over sinking, inactive volcanoes.
Atom: The smallest particle of an element that exhibits the characteristics of that element.
Autotroph: An organism that makes its own food by photosynthesis or chemosynthesis.
Aurora: A display of coloured light seen in the polar skies; called aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere.
Australian Biome: Eucalyptus dominates the landscape in the Australian biome. Karri is the tallest species of eucalyptus. The height of Australian eucalyptus reaches 70 metres or more.
Authogenic succession: When a set of habitats paves way for another, it is known as authogenic succession.
Authomorphic succession: It refers to the biotic changes on a predominantly inorganic site.
Autotrophs: The primary producers (plants) are known as autotrophs.
Autumnal (September) equinox: The time around September 22-23 when the Sun’s declination crosses the equatorial parallel; all places on Earth experience days and nights of equal length. The Sun rises at the South Pole and sets at the North Pole.
Aves: A class of birds.
Backwash: Water returning to the ocean from waves washing onto a beach.
Barrier island: A long, narrow, wave-built island lying parallel to the mainland and separated from it by a lagoon or bay.
Barrier reef: A coral reef surrounding an island or lying parallel to the shore of a continent, separated from land by deep lagoon. Coral debris islands may form along the reef.
Base: A substance that combine with a hydrogen ion (H) in solution.
Bathyal zone: The ocean between about 200 and 4000 meters deep.
Beach: A zone of unconsolidated (loose) particles extending from below water level to the edge of the coastal zone.
Beaufort wind scale: A system for estimating wind velocity (named for its inventor).
Benthic zone: The zone of the ocean bottom (pelagic zone).
Benthos: Aquatic life living on, in or at the bottom of a water body like oceans, seas, lakes, rivers etc.
Biochemical Cycle: The various circuits of flowing elements and materials (carbon, Oxygen, phosphorus, water) that combine Earth’s biotic and abiotic systems; the cycling of materials is continuous and renewed through the biosphere and the life processes.
Biodegradable: Able to be broken by natural processes into simpler compounds.
Biodiversity: An abbreviation of ‘biological diversity’ and usually taken to mean the total number of species presently living on Earth.
Biodiversity hotspots: An small area of land that contains an exceptional number of endemic species and are at high risk from human activities.
Biogeography: The study of the distribution of plants and animals and related ecosystems, the geographical relationships with related environment over time.
Biological control: The control of pest population without using chemical pesticides, most commonly by stimulating parasites and predators of the pest.
Biological factors: A biologically generated aspect of the environment, such as predation or metabolic waste products, that affects living organisms. Biological factors usually operate in association with purely physical factors such as light and temperature.
Biological resource: A living animal or plant collected for human use. Also called a living resource.
Bioluminescence: Biologically produced light.
Biomass: The total mass of living organisms on Earth or per unit area of a landscape; also, the weight of the living organisms in an ecosystem.
Biome: A large terrestrial ecosystem characterized by specific plant communities and formations; usually named after the predominant vegetation in the region. In other words, it is a well-demarcated environment that contains a complex of organism defining the ecology of the region. Tundra and rain forest are examples.
Biosphere: That area where the atmosphere, lithosphere, and hydrosphere function together to form the context within which life exists; an intricate web that connects all organisms with their physical environment.
Biosynthesis: The initial formation of life on the Earth.
Biota: The total flora and fauna of a region.
Biotic succession: Biotic communities are not static but these keep on changing continuously with time, so much so that with time some communities get replaced by others. Such an orderly and progressive replacement of one community by another, until a relatively stable community called the ‘climax community’ occupies the area, is known as ecosystem development or biotic succession.
Black smoker: A hydrothermal vent on the ocean floor emitting fluid containing iron, manganese, and copper. These tend to be black and resemble smoke. Vent fluids containing zinc and arsenic are usually white, and known as ‘white smokers’.
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