UARS data and results from a three-dimensional transport model have been used to compare and contrast the extent of the early stages of chemical processing by polar stratospheric clouds (PSCs) during the 1991--1993 northern and southern hemisphere winters of the Upper Atmosphere Research Satellite (UARS) mission. The modeled location and timing of regions of polar processing agree quite well with the UARS microwave limb sounder (MLS), cryogenic limb array etalon spectrometer (CLAES) and Halogen Occultation Experiment (HALOE) observations in that enhanced ClO concentrations are generally found in sunlit regions of polar-processed air and decreases of ClONO2 and HNO3 accompany low temperatures and high values of the aerosol extinctions inside the vortex. For these winters it is found that there is a period of about 2 weeks during the northern hemisphere early winter where sporadic polar processing takes place. After this a persistent processing region is seen inside the polar vortex, and the vortex fills with polar-processed air in a bit more than 2 weeks from the first persistent PSC occurrence. In the southern hemisphere, polar processing also fills the vortex in a bit over 2 weeks. Estimates of the spectral aerosol measure of the aerosol spectrum from CLAES observations illustrate that PSC particles are seen where cold temperatures occur inside the polar vortex and heterogeneous conversion of chlorine species on PSCs are expected.¿ 1997 American Geophysical Union