first is “dust-like” (using the terminology of Prinn & Olaguer 1981) in which cloud particles grow and efficiently sediment out, resulting in relatively thin clouds limited by the local vapor pressure, as in the model of Lewis (1969). These dust-like clouds are assumed to prevail in the radiative region (stratosphere), where the temperature profile is stable and convection is suppressed.

The second fundamental type in the Lunine et al. (1989) study is a tropospheric cloud, in which downward transport by sedimentation is opposed by upward transport due to turbulent mixing. For this cloud type, two variations are considered by Lunine et al. (1989). For the first variation, described as “frozen-in,” cloud particles are so small that sedimentation is overwhelmed by upward transport due to turbulent mixing. In this case, the atmosphere is well-mixed with respect to condensate, and hence $q_{\rm c}$ is independent of altitude above the cloud base. For the second variation, which is intermediate to the dust-like and frozen-in cases, particles grow large enough in “convective” clouds to develop appreciable sedimentation velocities, and their downward sedimentation is balanced by their turbulent transport upward. For their calculations of specific brown dwarf models, Lunine et al. (1989) consider only the 