A method of correcting the reflectivity measurements performed by a radar such as a weather radar, a reflectivity measurement being associated with a resolution volume includes analyzing the current resolution volume to determine whether the plane representing the 0° C. isotherm passes through it. When the plane representing the 0° C. isotherm passes through the current resolution volume, the volume is split into two parts lying on either side of said plane, the attenuation associated with the resolution volume is determined by taking into account the contribution of each of the parts to the measured reflectivity. The reflectivity associated with the current resolution volume is corrected using the attenuation thus determined. An onboard weather radar implements the method.