Differential reflectivity (ZDR) and circular depolarization ratio (CDR) radar signals are directly related to each other and to the median drop size diameter of rainfall when the drop sizes are exponentially distributed. These polarization parameters are examined for both static and oscillating raindrop shapes along with their relationships with rainfall rate and liquid water content estimates derived from ZDR or CDR and reflectivity factor (Z) measurements. Propagation effects at S band wavelengths are also considered and shown to be particularly important in CDR measurements.