A hydrodynamic instability arising from the existence of a meridional temperature gradient (and hence vertical shear of the mean flow and a thermal wind) in an atmosphere in quasigeostrophic equilibrium and possessing static stability. For reasonable values of the atmospheric parameters, the wavelength of maximum instability corresponds to that of a synoptic-scale disturbance. Such a system may be interpreted as converting potential energy of the basic flow into kinetic energy of the unstable perturbation.