Bimodality in upper-tropospheric water vapor distributions is shown to be a common feature in the tropics, implying sharp gradients between dry and moist regions in space and time. A method of testing for and quantifying bimodality is introduced. Using this method, the bimodality of water vapor is surveyed in both satellite and in situ observations, as well as a hierarchy of model simulations.

Bimodality implies that the homogenization time for moisture, whether physical (mixing) or mathematical (averaging), is long compared to a radiative-subsidence drying time after an injection of moisture by convection. This radiative drying time varies with altitude, and is shortest (about a day) near 11 km. It is shown that the local bimodality found in point measurements and cloud-resolving model simulations disappears with very modest averaging (~1 day, ~250 km). However, bimodality reappears on the global scale, where climatically dry and moist regions are separated so widely that synoptic and large-scale mixing and smearing times exceed the drying time.