Any departure of the state of a system from its equilibrium or the deviation of a variable from its mean.

In the first definition, stable systems (such as Earth’s climate) return to their equilibrium from perturbations (such as volcanic eruptions), whereas unstable systems (such as baroclinic or convective storms) seek a different arrangement of the state variables.

In the second definition, perturbation refers to the deviation of a variable from its mean. It is central to Reynolds decomposition:

u

u ¯ +

u

′ . {\displaystyle u={\overline {u}}+u^{\prime }.}

Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, 666 pp., https://doi.org/10.1007/978-94-009-3027-8.

Term edited 10 June 2025.