1. Method of processing a radar image obtained from a radar, comprising the steps of: an automatic evaluation of the altitude of the isotherm at zero degrees Celsius, called zero isotherm, using a processing of the reflectivity information conveyed by pixels forming all or part of the radar image, the processing comprising a generation of at least two intermediate values of the altitude of the zero isotherm, from variations in the reflectivity information within the radar image, the value of the altitude of the zero isotherm being a function of the intermediate values, each radar image having N vertical slices, the processing of the information being performed on each vertical slice, one after the other.