Linear growth rates for the gravitational Rayleigh-Taylor instability are calculated using the ionospheric electron density distribution obtained by solving the ion continuity equations including dynamics for O+, NO+, O2+, and N+2 ions. The instability model includes the contributions from plasma away from the magnetic equatorial plane. Effects of the transequatorial component of the thermospheric wind and the E¿B drift are investigated. Thermospheric winds are based on theoretical calculations. The E¿B drift velocities are modeled after the observations at Jicamarca and Fortaleza for different seasons. The results have shown that both effects are equally important to controlling the occurrence of equatorial spread F. Observed winter-summer asymmetry of the occurrence probability at the Jicamarca longitude is explained by the characteristics of the E¿B drift velocity model. However, in the case of Fortaleza, the observations cannot be explained strictly in terms of the E¿B drift effect and the wind effect, we have obtained a qualitative agreement between the observations and calculations over all seasons for both Jicamarca and Fortaleza. ¿ American Geophysical Union 1988