| Summary: | Several observational studies have shown a tight relationship between tropical precipitation and column-integrated water vapor. We show that the observed relationship in the tropics between column-integrated water vapor, precipitation, and its variance can be qualitatively reproduced by a simple and physically motivated two-layer model. It has previously been argued that features of this relationship could be explained by analogy with the theory of continuous phase transitions. Instead, our model explicitly assumes that the onset of precipitation is governed by a stability threshold involving boundary-layer water vapor. This allows us to explain the precipitation-humidity relationship over a broader range of water vapor values, and may explain the observed temperature dependence of the relationship.
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