From Glossary of Meteorology
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In atmospheric electricity, the process by which neutral atmospheric molecules (small ions) or other suspended particles (mainly large ions) are rendered electrically charged chiefly by collisions with high-energy particles.

Cosmic rays and radioactive decay are the main sources of atmospheric ionization. In the lower atmosphere, decay electrons of mu-mesons plus alpha particles from radioactive gases, as well as beta particles and gamma rays, serve to ionize air molecules. The rate at which these agents ionize the air is expressed in units of one ion pair per cubic centimeter per second, symbolized by I. Cosmic rays at sea level yield about 2I, both over land and at sea. Radioactive gases contribute about 5I over land areas at sea level, while radioactive materials in the soil and rocks themselves yield about 4I. At heights above about 5 km, only cosmic rays provide significant ionization, and this contribution finally reaches a maximum at about 13 km, above which the rate decreases, due to decreasing air density and consequent lack of target molecules for the cosmic rays.
See photoionization.