Recent work has shown that water vapor levels near the boundary between the tropical troposphere and stratosphere depart from thermodynamic equilibrium (saturation with respect to ice) in a manner that appears to vary systematically with the microphysical nature of cumulonimbus cloud tops. This implies a control, independent of thermodynamics, on water near the tropopause, in contrast to the situation in most of the troposphere where observations and models have generally supported thermodynamic control of climatic water vapor. Here we investigate the relationship between the mean and seasonally varying state of the tropical upper troposphere and lower stratosphere, and convective transports of water including its stable isotopes, using a simple convective climate model. We argue that a gradual reduction with height in convective influence on water vapor and other constituents is consistent with observations, while an abrupt transition from a highly-mixed troposphere to an unmixed (vertically) stratosphere is not. We also present evidence that at heights where convective influence is waning, lofted ice tends to relax the atmospheric vapor toward equilibrium, but at a sufficiently slow rate so that the mean vapor levels reached will be expected to depend on process details as well as temperature. The altitudes where this is so are of great importance since they determine the water vapor content of the entire stratosphere.