Histograms of precipitable water from radiosondes and zenith neutral delay estimated by GPS are found at many locations to follow a lognormal distribution. This observation is consistent with a theoretical expression for precipitable water based on moisture flux. Two unimodal cases are identified: the traditional lognormal distribution is commonly found in subtropical and temperate climates while tropical oceanic environments tend to exhibit a reversed lognormal form. Bimodal cases, formed by combinations of either or both of these unimodal distributions, are found where seasonal (e.g., monsoons) or climatic (e.g., El Niño) variations generate distinct precipitable water modes with rapid transitions between them. This connection with the lognormal distribution suggests a basis for the parameterization both of precipitable water in climate models as well as of the delay due to water vapor in atmospheric models used for space geodesy.