The effects of clear air entrainment and mixing on the cloud droplet effective radius are investigated. Approximate expressions of the droplet effective radius corresponding to extreme scenarios of mixing and secondary droplet activation are derived. They depend upon the liquid water path as well as upon the cloud droplet effective radius and liquid water path of the corresponding adiabatic cloud. The validity of these expressions is verified by using field experiment data. A parameterization of the mean droplet effective radius for tropical warm convective clouds is deduced from the above expressions and is further validated with the same field experiment data. ¿ American Geophysical Union 1995