This paper is devoted to the improvement of semi-physical fire spread models. In order to improve them, a theoretical approach based on the multiphase concept was carried out. The multiphase approach which considers the finest physical phenomena involved in fire behaviour was reduced by making several assumptions. This work led us to a simplified set of equations. Among these, a single equation for the thermal balance was obtained by using the thermal equilibrium hypothesis. This approach has been applied to the improvement of our semi-physical model in order to take into account increasing wind influence. The predictions of the improved model were then compared to experimental data obtained for fire spread conducted across pine needle fuel beds. To this end, different slope values and varying wind velocities were considered. The experimental tendency for the variation of the rate of spread was predicted. Indeed, it increases with increasing wind velocity for a given slope as well as for a given wind with increasing slope.