The subject of this article concerns the unsteady effects (fire intensity, wind) upon the propagation and, more generally, the behavior of surface fires in open fields. The study focused on two sources of unsteadiness: the first one resulting from the regime of propagation (wind driven or plume dominated), which can affect greatly the behavior of the flame front and consequently the fire intensity, the second one resulting from the wind gusts associated with the conditions of flow of wind in real conditions. The study was based on numerical simulations, using a multiphase formulation, and on spectral analysis of the time evolution of the fire line intensity. The calculations were performed in 2D for a homogeneous vegetation layer (grassland) and for a large interval of wind conditions (10 m open wind velocity U10 ranged between 1 m/s and 25 m/s). The results have highlighted the link between the unsteady character of flame front behavior and the regime of propagation (plume dominated, wind driven). A particular interest was focused on the role played by two potential sources of instabilities, namely the Kelvin-Helmholtz instability (wind effects) and the thermo-convective instability (plume effects), upon the behavior of fires. A second set of simulations has been carried out using unsteady wind conditions, reproduced using sinusoidal boundary conditions for the streamwise velocity, with a frequency ranging between 0.5 Hz and 3 Hz.