Many previous works have been dedicated to the modeling of forest fires (or bush fires) using cellular automata (CA). Usually the transition rules used by the CA are either set or obtained by identification from experimental results. The main drawback of CA model for forest fires is the lack of sound knowledge on these transition rules. This work presents a physical model containing the main physical processes involved in bush fire propagationconvection, radiative transfer (nonlocal), and pyrolysis degradationto define the neighborhood and the transition rule tables needed by the CA. These transitions can then be expressed as functions of the properties of vegetation and flames. On the basis on this model, a study is carried out to analyze the combined effects of humidity and occupation density of vegetation on the fire propagation. Several thresholds on these parameters have been defined to understand for which conditions the fire propagates indefinitely.