| Section A | A index | 391-399 of 917 terms |
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alidade—A stationary instrument, mounted on a stand, that measures the angle subtended at the stand by the horizon and an object in space. The clinometer is a portable form of alidade frequently used with a ceiling projector to determine the height of clouds. An alidade usually measures the elevation angle only; a theodolite measures azimuth as well.
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alimentation—Generally, the process of providing nourishment or sustenance; thus, in glaciology, the combined processes that serve to increase the mass of a glacier or snowfield; the opposite of ablation. The deposition of snow is the major form of glacial alimentation, but other forms of precipitation along with sublimation, refreezing of meltwater, etc. also contribute. The additional mass produced by alimentation is termed accumulation.
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Alisov's classification of climate—A genetic scheme, based on physical causes (i.e., circulation types, airmass types, fronts, etc.), for classifying climate proposed by Alisov in the 1950s as distinct from an empirical method based almost entirely on observations.
Alisov, B. P., 1954: Die Klimate der Erde, Deutscher Verlag der Wissenschaften, Berlin, 277 pp.
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alkalinity—The relative acidity of any solution expressed in a pH range of numbers. The pH value is the negative common logarithm of the hydrogen-ion concentration in a solution, expressed in moles per liter of solution. A neutral solution, that is, one that is neither acidic nor alkaline, such as pure water, has a concentration of 10 moles per liter; its pH is thus 7. Acidic solutions have pH values ranging with decreasing acidity from 0 to nearly 7; alkaline or basic solutions have a pH ranging with increasing alkalinity from just beyond 7 to 14. In seawater, the alkalinity is a measure of the excess of hydroxyl ions over hydrogen ions, generally expressed as milliequivalents per liter.
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alkanes—Hydrocarbon species (also known as paraffins) with general formula CnH2n + 2 where n is an integer. Alkanes contain only saturated bonds and are relatively unreactive in the atmosphere, where they react by hydrogen abstraction to form alkyl radicals. Major sources include natural gas emissions, biomass burning, evaporative emissions, and fuel combustion.
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alkenes—(Also called olefins.) Monoalkenes are hydrocarbon species with general formula CnH2n, where n is an integer, containing one unsaturated carbon–carbon bond. Alkenes are emitted in large quantities by automobiles and by vegetation. They react moderately to very rapidly with hydroxyl radicals and with ozone, which makes them major contributors to atmospheric reactivity, but limits their concentrations in the atmosphere. More complex dialkenes, with more than one double bond, can also be formed.
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alkylperoxy radicals—Organic free radicals formed from the addition of oxygen to alkyl radicals. Their major atmospheric reaction, with NO, is an integral component of ozone generation in the troposphere.
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alkynes—Highly unsaturated organic compounds containing a carbon–carbon triple bond, with general formula CnH2n − 2, of which acetylene, or ethyne, C2H2 is the simplest member. Alkynes are emitted to the atmosphere mainly as a result of incomplete combustion.
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