Parameterization of effective radius (re) as proportional to the cube root of the ratio of cloud liquid water content (L) to droplet concentration (N), i.e., re=&agr;(L/N)1/3, is becoming widely accepted. The principal distinction between different parameterization schemes lies in the specification of the prefactor &agr;. This work focuses on the dependence of &agr; on the spectral dispersion of the cloud droplet size distribution. Relationships by Pontikis and Hicks [1992> and by Liu and Hallet [1997> that account for the dependence of &agr; on the spectral dispersion are compared to each other and to cloud microphysical data collected during two recent field studies. The expression of Liu and Hallet describes the spectral dependence of &agr; (or re) more accurately than the Pontikis and Hicks relation over the observed range of spectral dispersions. The comparison shows that the different treatments of &agr; as a function of spectral dispersion alone can result in substantial differences in re estimated from different parameterization schemes, suggesting that accurately representing re in climate models requires predicting &agr; in addition to L and N. ¿ 2000 American Geophysical Union