Recent data suggest that due to the release into the atmosphere of CFCs by human activities, the ozone hole can as large as North America during a given astral spring. The CFCs are broken apart by UV light--to form free radicals of chlorine--after they have diffused into the upper stratosphere from the troposphere. This long term movement from the troposphere to the stratosphere is possible for these chlorine containing chemicals because of there long atmospheric lifetimes; however, this type of movement is not important for more reactive species such as tropospheric ozone because of their reactivity and therefore short atmospheric lifetime. In the Antarctic stratosphere, the reaction that converts reservoir species of chlorine into an active form--which destroys ozone--takes place on the surface of particles in polar stratospheric clouds as the temperature drops below about 200K. This is possible because of the unique isolation of the south polar vortex during the austral winter. The surfaces of these (nitric acid/water) clouds act as catalysts for reactions that release molecular chlorine which quickly photolyzes to chlorine's (radical) active state. It is this radical which destroys ozone.