The Ozone Hole is.......

an annual thinning of the ozone layer over Antarctica, caused by stratospheric chlorine. Other more moderate thinning's have also been called "ozone holes", such as that over the North Pole during certain weather conditions.The discovery of the annual depletion of ozone above the Antarctic was first announced in a paper by Joe Farman, Brian Gardiner and Jonathan Shanklin which appeared in Nature in May 1985.

The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total column ozone, above a point on the earth's surface, which is normally expressed in Dobson units. Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS).In 2012, it has been reported that the ozone hole had decreased to the smallest size since 2002.

The cause of the ozone holes is generally agreed to be CFC (chlorofluorocarbon) compounds which break down due to ultraviolet light and become free radicals containing chlorine high in the Earth's atmosphere. These radicals then break down the ozone catalytically. Ozone destruction due to chlorine radicals from CFCs can take place in the gas phase, but occurs particularly rapidly on the surface of polar stratospheric clouds (PSC), which form over the poles (particularly the south pole) during winter.The photochemical processes involved are complex but well understood, with UV radiation being involved in both the natural production and destruction of ozone, as well as the breakdown of CFCs into free radicals and the destruction of ozone by chlorine radicals. The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring; during winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions.CFCs are a byproduct of some chemical processes, and were also used in air conditioning/cooling units. They were also used as aerosol propellants. What makes CFCs so effective in breaking down ozone is that one CFC radical acts as a catalyst and can break down many ozone molecules. Furthermore, these radicals stay in the atmosphere for a very long time.Scientists have increasingly been able to attribute the observed ozone depletion to the increase of anthropogenic halogen compounds from CFCs, by the use of complex chemical transport models and their validation against observational data . These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments, and are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone.