1. A method of processing a radar image, said method comprising the steps of: transmitting data from a radar; acquiring the radar image from said data; automatically evaluating an altitude of a zero isotherm, the evaluation comprising processing reflectivity information conveyed by pixels forming all or part of the radar image, the processing of the reflectivity information comprising generating at least two intermediate values of the altitude of the zero isotherm, from variations in the reflectivity information within the radar image, a value of the altitude of the zero isotherm being a function of the intermediate values, wherein the radar image has N vertical slices, the processing of the reflectivity information is performed on each vertical slice, one after the other, the zero isotherm is an isotherm at zero degrees Celsius, and the processing of the reflectivity information further comprises: for each vertical slice of the radar image, applying at least two distinct methods of processing of the reflectivity information to obtain the at least two intermediate values of the altitude of the zero isotherm; and generating for each of at least two distinct methods and for all the vertical slices, at least one main intermediate value of the altitude of the zero isotherm, from a selected set of intermediate values of the altitude of the zero isotherm, wherein the value of the altitude of the zero isotherm is evaluated from the main intermediate values of the altitude of the zero isotherm.