A periodicity was observed to drive the 1992 late winter formation of stratospheric clouds over McMurdo Station, Antarctica. Lidar and meteorological data show that intense stratospheric coolings, accompanied by generation of clouds, resulted from the transit of air parcels proceeding from latitudes near the edge of the polar vortex. Lidar depolarization measurements show that large cloud particles could survive for several days in undersaturated air. In the occurrence of further coolings, these particles would act as preferential growth nuclei. Depolarization measurements also indicate that most of the late winter inner vortex sulfuric acid aerosol was frozen. Periodically observed transit of vortex airmasses outside the terminator could be the cause of the wintertime ozone losses, occasionally observed at the very center of the continent. ¿ American Geophysical Union 1993