During the Polar Stratospheric Aerosol Experiment, total water was measured using an airborne Lyman-&agr; fluorescence hygrometer. The inlet is designed for anisokinetic sampling, thus large ice particles are detected sensitively. During the flight on January 24, 1997, temperatures of 200 K occurred close to the tropopause. Within this cold area, the existence of large particles with radii≥4 &mgr;m containing large amounts of water is observed. At the edge of this cloud, total water mixing ratios of 5--6 ppmv and close to saturation were measured at and below the tropopause. This observation together with analysis of the correlation of the H2O mixing ratio with other long-lived tracers indicates that the air masses had been dried by sedimentation of large ice particles. ¿ 1999 American Geophysical Union