The instability characteristics of three-dimensional Northern Hemisphere average winter tropospheric flow are examined in a two-layer spherical quasi-geostrophic model. All the growing modes from the fastest down to the first stationary but growing mode have been examined for three cases (1, 2a and 3) having increasingly larger static stability parameters. Comparisons with observations and with the results of the corresponding barotropic model are presented. It is found that the modes for cases 1, 2a and 3 can be divided into six approximately distinct classes. Class A modes are rapidly propagating monopole cyclogenesis disturbances with largest amplitudes in the geographical locations of the storm tracks in the Pacific and Atlantic Oceans. Class B and C modes are respectively Pacific and Pacific and Atlantic onset-of-blocking dipole modes with longer periods than class A modes and which like class A disturbances, have westward tilts with height. They also have largest amplitudes upstream of the observed regions of mature blocks in the Pacific and Atlantic Oceans. Class D modes are stationary but growing modes with structures similar to the Pacific-North American teleconnection or anomaly pattern while class E modes have second largest periods of just over 40 days and have structures similar to the North Atlantic oscillation. Both class D and E modes are essentially equivalent barotropic and are mainly confined to the key regions of largest amplitude in the lower layer, while the upper layer patterns are more extensive and complex. Class F modes have periods intermediate between class C and E modes and properties intermediate between class B and C and D and E disturbances. The structural changes that occur in the time evolution of observed anomalies such as blocks are compared with the instability solutions. It is found that the development of mature anomalies may be thought of as approximately consisting of two stages which, for the Pacific-North American pattern, are as follows. The formation of a rapidly growing and eastward propagating dipole disturbance of the class B type, which tilts westward with height and has largest amplitude upstream of the region of mature blocks, initiates the process. The mode then changes into a disturbance similar to class D modes, through the operation of nonlinear effects. Finally, the class D mode amplifies without phase propagation and through largely equivalent barotropic effects to form the mature anomaly. A similar process occurs for the North Atlantic pattern.