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
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