$z$ is altitude and $q_{\rm s}$ is the vapor mole fraction corresponding to the saturation vapor pressure at that altitude. The first and second cases on the right hand side correspond to cloud-free and cloudy conditions, respectively. Note that under all conditions $q_{\rm t}(z) = q_{\rm v}(z-\Delta z)$ in the Lewis model, reflecting the assumption that only vapor is transported upwards.

Beyond this simple model, Lewis (1969) considered the partitioning of chemical species in some detail, and also calculated pseudo-adiabatic lapse rates. Here we simply assume that each condensate results from the saturation of a single condensable, and fix the lapse rate as input from observations or an external model.

For an example, we calculate an ammonia cloud profile from the Lewis model (Figure 1) using the Jovian temperature profile from Voyager (Lindal et al. 1981), the relation for vapor pressure given in Appendix \[pvap\], and a sub-cloud mole fraction of 3 $\times$ $10^{-5}$ (a wide range of abundances below the expected base of the Jovian ammonia cloud have been reported; we adopt the value at 0.6 bars retrieved by Kunde et al. 1982 for the Northern Equatorial Belt, which also agrees with the best-fit values of Carlson, Lacis, & 