We characterize measurements of upper tropospheric (~300--150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on board the Upper Atmosphere Research Satellite (UARS) in terms of their spatial and temporal variability. We present the climatology of water vapor mixing ratio for October 1991--June 1997, which includes the seasonal means, the root-mean-square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upper tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the Walker circulation. We quantify the frequency of dry tropical observations in seasonal maps. The most prominent dry regions are located in the subtropics. On the 316 hPa surface there is some occurrence of low values of humidity throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern Hemisphere winter. The seasonal cycle of MLS water vapor is compared with Stratospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agreement is found between these two data sets for measurements near the extratropical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or tropospheric air. Frequency distributions display distinct characteristics and seasonal dependence for these two regions. ¿ 2000 American Geophysical Union