Abstract
The rate of change of surface pressure is defined by the horizontal divergence of momentum integrated vertically through the entire atmosphere. As geostrophic winds are rigorously nonβdivergent, surface pressure cannot change when the motion is everywhere geostrophic (Jeffreys, 1919). Pressure changes are therefore determined by the field of geostrophic departure. Ignoring friction, the geostrophic departure is proportional to and directed at right angles to the horizontal acceleration whence pressure changes are defined solely by the field of acceleration, that is by dynamical considerations. Popular conceptions of pressure changes due to simple translation or advection of existing fields of pressure, or to thermal effects caused by direct heating or by advection of air of a different temperature, are invalid without a discussion of the dynamical processes involved.
The approach to the theory of the movement and development of depressions and anticyclones is through a study of the departure of the velocity from the geostrophic value, or what is the same thing, a study of the field of acceleration. By expanding the acceleration into partial derivatives to time and space four terms, one of which is the well known isallobaric term, are obtained. Some justification for considering the terms as physically distinct is given.
It is shown that isallobaric effects alone cannot account for the integrated divergence associated with pressure developments and it is suggested that all four terms may be significant. The effect of the vertical space derivative, proportional to the vertical velocity and to the horizontal gradient of temperature, previously enunciated as a formal proposition by Durst and Sutcliffe (1938), is stressed as a novel contribution which may be of fundamental importance.