Absolute humidity See humidity

absorbed dose The amount of energy absorbed by a tissue or other substance from incident radiation. It is measured in grays (Gy): one gray = the transfer of one joule of energy to one kilogram of material.

absorption 1. The retention of radiant energy by an object, e.g. by the pigments of a photosynthetic organism.

2. The uptake of small nutrient molecules into the body of an organism. In animals this takes place after digestion of larger food molecules. In plants it includes the uptake of water and solutes.

3. See adsorption.

abundance A measure of the number of individual organisms in an area. An assessment of frequency (the relative abundance of each species present in an area), gives a quick subjective estimate. cover and abundance may be combined for a subjective community description (see Braun-Blan-quet scale). Quantitative measures of abundance include density (the number of individuals in a given area), cover, or the frequency of occurrence of a species in randomly placed quadrats.

abyssal zone The ocean-floor environment between 4000 and 6000 meters in depth. It is characterized by extremely high water pressure, low temperatures and nutrient levels, and an almost total absence of light. See also benthic zone.

acclimation See acclimatization.

acclimatization A reversible change in the physiology or morphology of an organism in response to changes in its environment. The term is generally used in the context of the natural environment. The term acclimation tends to be used in a laboratory context.

acid A compound that acts as a proton donor in aqueous solution, i.e. it releases hydrogen ions or protons, giving a pH less than 7. Acids turn litmus paper from blue to red, and react with alkalis (bases) to yield neutral salts. See pH. Compare alkali.

acid rain Rain with a very low pH (below pH 5.6 and often below pH 4), due to pollution from oxides of nitrogen and sulfur released by the burning of fossil fuels or other industrial emissions. These pollutants combine with water in the atmosphere, forming nitric and sulfuric acids, a process that may be catalyzed by other pollutants such as ammonia, hydrogen perox

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