The effects of clear air entrainment and mixing on the cloud optical thickness are investigated. Three different approximate expressions of the cloud optical thickness are derived according to the type of mixing and the presence or absence of secondary droplet activation. They depend upon the liquid water path as well as upon the optical thickness and liquid water path of corresponding adiabatic cloud. The influence of the cloud droplet concentration on the cloud optical thickness values is shown. These expressions are used to deduce a parameterization of the optical thickness, that is further validated with experimental data. ¿ American Geophysical Union 1993