From AMS Glossary
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 = 200R1.6, 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 = aRb, where a and b are adjustable parameters.
Marshall, J. S., and W. McK. Palmer, 1948: The distribution of raindrops with size. J. Meteor., 5, 165–166.