During the polar winter, when the necessary polar stratospheric clouds can form for ozone depletion to occur, the polar vortex also strengthens which tends to i

During the polar winter, when the necessary polar stratospheric clouds can form for ozone depletion to occur, the polar vortex also strengthens which tends to isolate the polar stratosphere. During the summer, the vortex weakens, mixing occurs and the ozone depletion over the pole is reduced (the "ozone hole" is seasonal). In fact, as this occurs in the southern spring, areas of ozone-depleted stratosphere drift over inhabited lands to the north. More CFCs also enter the polar stratosphere through this mixing, though, ready to start the cycle again next polar winter.