| Section A | A index | 141-149 of 917 terms |
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actinometer—The general name for any instrument used to measure the intensity of radiant energy, particularly that of the sun. Actinometers may be classified, accordingly to the quantities that they measure, in the following manner: 1) pyrheliometer, which measures the intensity of direct solar radiation; 2) pyranometer, which measures global radiation (the combined intensity of direct solar radiation and diffuse sky radiation); and 3) pyrgeometer, which measures the effective terrestrial radiation. See actinometry, bolometer, dosimeter, photometer, radiometer; compare sunshine recorder.
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actinometry—A term for the measurement of radiation, originally more general, but now used mainly to describe photochemical techniques of measuring ultraviolet radiation by chemical actinometers.
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actinon—A radioactive gas, symbol An; an inert gaseous element that is an isotope of radon and thoron. Actinon is a member of the uranium–actinium family of radioactive elements. Its radioactive half-life is extremely short, only 3.92 s, and its parent atoms (actinium x) are comparatively rare in the earth's crust, so actinon is considerably less important as an agency of atmospheric ionization than are radon and thoron.
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activated complex theory—(Also called transition state theory.) Theory of chemical reaction that relates the rate of reaction to an equilibrium between the reactants and an activated complex or transition state, which is a maximum energy configuration of the reactants as they proceed along the reaction coordinate.
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active layer—(Also called frost zone, mollisol.) That part of the soil included with the suprapermafrost layer (i.e., existing above permafrost) that usually freezes in winter and thaws in summer. Its bottom surface is the frost table, beneath which may lie permafrost or talik. The depth of the active layer varies anywhere from a few inches to several feet. Compare active permafrost.
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