Water vapor in the lower stratosphere was measured in situ by two aluminum oxide hygrometers mounted on the nose of an RB57 aircraft. Data were taken nearly continuously from January to May 1974 from an altitude of approximately 11–19 km as the aircraft flew between 70°N and 50°S over the land areas in the western hemisphere. Pseudomeridional cross sections of water vapor and temperature were derived from the flight data and show mixing ratios predominantly between 2 and 4 μg/g with an extreme range of 1–8 μg/g. Measurement precision was estimated by comparing the simultaneously measured values from the two flight hygrometer systems. Accuracy was estimated to be about ±40% at 19 km. A height-averaged latitudinal cross section of water vapor indicates symmetry of wet and dry zones. This cross section is compared with other aircraft measurements and relates to meridional circulation models.