A study is made of the effect of a long Planetary wave on the regions of preferential development, eddy momentum and heat fluxes, and the growth rates and phase speeds of growing baroclinic disturbances. The model used is a linear, spherical, two-layer quasi-geostrophic model with a basic state consisting of a 30° jet and an upper layer long planetary wave which together provide an approximate representation of the observed average Northern Hemisphere winter flow. The results of the baroclinic instability theory are compared with the observed regions of most actively developing baroclinic disturbances and eddy momentum and heat fluxes. Considering the crude vertical structure of two-layer models, the agreement between the theoretical and observed results is noteworthy. Details such as the observed latitudinal gaps between the jet-stream maxima and the regions of most actively developing eddies are reproduced in the model. Both these gaps and much of the horizontal variations of the theoretical and observed eddy momentum and heat fluxes are related to Phillips’ criterion for incipient instability. Finally, closure hypotheses, needed for statistical dynamical models in which long waves as well as zonally averaged quantities are to be predicted, are proposed.