The parcel method of investigating the susceptibility of the atmosphere to particular disturbances involves postulating parcel displacements in an undisturbed environment and deducing the likelihood of such a displacement by estimating the kinetic energy change that would result. On the convective scale, such a method has provided realistic and practical criteria for the existence and intensity of such motions. On the larger scale, theoreticians tend to reject such arguments as heuristic at best.
This paper is an attempt to reconsider the role of parcel theory on different length scales and, in particular, in the application of symmetric instability ideas to frontal rainbands. It is concluded that parcel theory has to be applied with care but that in some situations it can provide a useful—though partial—insight into atmospheric disturbances.