Instability Criteria and Growth of Baroclinic Disturbances

The instability criterion and the rate of growth of long baroclinic disturbances in horizontally uniform zonal flow is derived for a sloping streamsurface. Both quantities are expressed by one explicit and simple function. Unstable disturbances are characterized by upward motion toward the cold air and downward motion toward the warm air. The slope of the streamsurface is eliminated, and the growth equation is compared with the results of other studies. The maximum rate of growth is small enough to indicate that numerical prediction of the development of baroclinic disturbances is practicable. Other results are that long waves may occur in a baroclinic atmosphere above a flat earth as well as above a spherical earth and that on a spherical earth unstable disturbances occur only north of a critical latitude.