The gradient dift instability for F region plasma clouds is investigated retaining ion inertia effects. With the assumption that the ion gyrofrequency &OHgr; is greater than ion inertial time changes (∂/∂t+Vt⋅∇) and the ion-neutral collision frequency vin, a quadratic dispersion equation results. In the limit vin≫4cE0/BL (E0 and B are the ambient electric and magnetic fields, and L is the plasma cloud density gradient scale length), the growth rate &ggr; reduces to the usual result cE0/BL. In the opposite limit &ngr;in≪4cE0/BL,&ggr; = (cE0&ngr;in/BL)1/2. This result can be explained why high-altitude (~450-km) barium releases, where &ngr;in is less than unity and a factor of 10-100 smaller than it is at lower altitudes (~200km), striate on time scales similar to the lower altitude releases.