In the last 3 years, several flights from polar regions (Spitzbergen, 78 ¿N) to the subtropics (Cap Verde Islands, 16 ¿N) have been performed with the Airborne Millimeter- and Submillimeter-Wave Observing System aboard a Learjet of the Swiss Air Force. From the observation of the H2O rotational transition at 183.31 GHz, altitude-latitude cross sections of the middle atmospheric water vapor abundance for August 1994, October 1995, and March 1996 have been derived from the subtropical to Arctic regions. The key findings in this new H2O data set of the latitudinal variation from the lower stratosphere to the mesosphere are (1) the unusually high H2O abundance of 6.8 ppm at a potential temperature of 700 K in the spring 1996 polar vortex indicating a strong downward descent of ~14 km from October to March associated with air mass isolation, (2) the seasonal dependent modulations of the dry subtropical stratospheric H2O profile, (3) flat volume mixing ratio (VMR) contours between 35¿ and 65 ¿N due to isentropic mixing, (4) a maximum H2O VMR of more than 7.5 ppm in the subtropical mesosphere suggesting a disturbed mesospheric hydrogen partitioning possibly due to an enhanced conversion of methane and molecular hydrogen to water vapor, and (5) a general increase of middle atmospheric water vapor VMR of ~1.5 ppm since 1986. ¿ 1998 American Geophysical Union