The occurrence of equatorial spread F on the bottomside of the F layer is likely the result of a process often referred to as the collisional interchange instability or generalized Rayleigh-Taylor instability. The traditional approach to the analysis of this instability with sheared zonal flow has been to calculate the eigenvalues of the linearized system. It is well documented that the introduction of shear has a stabilizing effect on the eigenvalues and significantly increases the wavelength corresponding to the fastest growing eigenmode. In this paper it is argued that the well-accepted conclusions drawn from eigenvalue analyses are not correct for cases of geophysical interest. A calculation of the &egr; pseudospectra demonstrates that the system is highly nonnormal and that large-amplitude transients may exist even when all of the eigenmodes are decaying with time. Transient effects are shown to be of fundamental importance to the evolution of the system over a wide range of horizontal wavelengths. This viewpoint is consistent with some aspects of VHF radar observations of F region gravitational interchange dynamics and may explain the presence of irregularities which appear to be confined to the bottomside of the F layer. The techniques discussed are of general interest to the analysis of nonnormal linear systems. ¿ 1999 American Geophysical Union