A multi-year dataset of lidar measurements of polar stratospheric clouds (PSCs) from two sites in the Arctic (Sodankyl¿, 67¿N, 27¿E. and Ny-¿lesund, 79¿N, 12¿E) has been analyzed with respect to the existence temperature of different PSC types. Due to the location of the measurement sites relative to the polar vortex, their observations are most of the time representative of vortex edge and vortex center conditions, respectively. The comparison of the aerosol lidar measurements with local and trajectory temperatures reveals that PSC occurrence is favored at the edge of the Arctic polar vortex. One possibility discussed here is a differential distribution of water vapor that could be responsible for the non-uniform threshold temperatures and thus differential PSC occurrence within the polar vortex. ¿ 2001 American Geophysical Union