Rossow 1993 and Brooke et al. 1998). The cloud base appears at 0.42 bars, where the temperature is 129 K. Although absent in the figures of Lewis (1969) (likely due to reduced vertical resolution), in our interpretation of that model the vapor is not entirely depleted in the lowest reaches of the cloud (where $q_{\rm c} < q_{\rm t}$), hence $q_{\rm c}$ increases above the cloud base. Such an increase is found in terrestrial clouds of moderate vertical extent, where $q_{\rm c} < q_{\rm t}$, and hence $q_{\rm c}$ increases with altitude throughout their depths. However, at greater altitudes in this deep ammonia cloud, the vapor is so effectively depleted by condensation at the low temperatures that $q_{\rm v} \ll q_{\rm t}$, leading to a cold degeneracy: $q_{\rm c}(z) \approx q_{\rm t}(z) = q_{\rm s}(z-\Delta z)$, in which decreasing temperatures result in $q_{\rm c}$ diminishing with altitude. Note that the condensate abundance drops off rapidly above $\sim$0.13 bars due to increasing temperatures. Hence, the temperature minimum quite reasonably produces a cold-trap in the Lewis model.

Condensate particle sizes, the other ingredient needed for predicting cloud opacity, are not considered by Lewis (1969) or Weidenschilling & Lewis (1973).

Carlson et al. 