From Glossary of Meteorology


Measure of the effective concentration of a chemical in the gas or liquid phase.

The activity is usually less than the mass concentration, from which it differs due to the nonideal nature of gases and solutions. In the limit of very dilute mixtures, the activity is the
same as the concentration for liquid solutions, or the partial pressure for gases. The activity coefficient, usually denoted by γ or f, is the ratio of the activity to the actual physical concentration.

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.