Wind flow is certainly one of the most important factors affecting the behaviour of wildfires. However its effect upon the propagation of the fire front is not yet understood. The relationship between the rate of spread (ROS) and the wind speed velocity appears in the literature through a power law relation with an exponent varying between 0.5 and 1.5. Some experimental results related also some contra intuitive phenomena, such as a decrease of the ROS as the wind speed exceeds a certain value. We have explored this problem using numerical simulations of surface fires through homogeneous vegetation layer. The numerical results have been analysed in terms of rate of spread, fire line intensity, Byram's convection number. Two regimes of fire propagation (plume dominated and wind driven) have been clearly identified. We also proposed a detailed analysis of the unsteady behaviour of fire intensity signals (average value, standard deviation, power spectrum analysis) and the consequences in terms of predictability of fire behaviour (see Figure 1). A set of numerical simulations was also carried out in adding a sinusoidal variation of the inlet wind velocity.