A trajectory-based analysis of the northern hemisphere lower stratospheres of 1988/89 and 1991/92, from 30¿N to the pole, has been carried out, allowing estimates of the area subject to chlorine activation by polar stratospheric clouds to be made. In both winters a significant area is covered by activated air (i.e., air subject to temperatures cold enough for polar stratospheric cloud formation up to 20 days previously). Activated air in the middle latitudes is a combination of the temporary displacement of vortical air equatorward and in situ activation of extra-vortical air. Relative to 1988/89, 1991/92 shows increased activation in the middle latitudes. This was due to a displacement of low temperatures towards lower values of potential vorticity by an extreme anticyclonic forcing event. ¿American Geophysical Union 1994