The accuracy required for a correct interpretation of differential reflectivity (ZDR) is typically estimated to be between 0.1 and 0.2 dB. This is achieved through calibration, defined as the identification of the constant or time-varying offset to be subtracted from the measurements in order to isolate the meteorological signals. We propose two innovative steps: the automated selection of sufficiently homogeneous sections of Plan Position Indicator (PPI) scans at 90∘ elevation, performed in both rain and snow, and the ordinary kriging interpolation of the median ZDR value of the chosen radar volumes. This technique has been successfully applied to five field campaigns in various climatic regions. The availability of overlapping scans from two nearby radars allowed us to evaluate the calibration approach, and demonstrated the benefits of defining a time-varying offset. Even though the method has been designed to work with both solid and liquid precipitation, it particularly benefits radar systems with limited access to rain measurements due to the deployment in mountainous or polar regions or to issues affecting the lowest range gates.