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.