The Z–R relationship developed by J. S. Marshall and W. M. Palmer (1948) consistent with an exponential drop-size distribution. The relationship is Z = 200R^{1. 6}, where Z (mm6 m−3) is the reflectivity factor and R (mm h⁻¹) is the rainfall rate. The relationship is sometimes generalized to the form Z = aR^b, where a and b are adjustable parameters.