From Glossary of Meteorology
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Any departure introduced into an assumed steady state of a system.

The magnitude is often assumed to be small so that product terms in the dependent variables may be neglected; the term “perturbation” is therefore sometimes used as synonymous with “small perturbation.” The perturbation may be concentrated at a point or in a finite volume of space; it may be a wave (sine or cosine function); in the case of a rotating system, it may be symmetric about the axis of rotation; or it may be a displacement by the parcel method. The mathematical work in an instability problem may be facilitated by the perturbation technique, whether or not the equations are linearized. In synoptic meteorology, this term is used for any departure from zonal flow within the major zonal currents of the atmosphere. It is especially applied to the wavelike disturbances within the tropical easterlies.
See easterly wave;
compare disturbance.