The results of measurements of water vapor vertical profiles in the upper troposphere and lower stratosphere on board a meteorological sounding balloon with the use of a FLASH-B optical fluorescent hygrometer are presented. These results were obtained during the LAUTLOS-WAVVAP international experiment on the comparison of balloon hygrometers, which was carried out in January - February 2004 in northern Finland. Humidity values are determined inside, outside and at the boundary of the circumpolar stratospheric vortex. It is shown that at a height of 20 km, the difference between the mixing ratios of water vapor outside and inside the vortex can attain 1 ppmv and increase with height. It is inferred that the laminae of the vertical humidity profile at the stratospheric vortex boundary is in good agreement with the layering of the potential vorticity profile, which points to a dynamic nature of these layers. It is shown that decreased values of the water vapor content observed in the polar stratosphere are not always controlled by dehydration processes associated with the formation and sedimentation of particles of polar stratospheric clouds.