Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Although natural phenomena can cause temporary ozone losses, chlorine and bromine released from man-made synthetic compounds such as CFCs are now accepted as the main cause of this depletion.
A common misconception is that there is an ozone hole above us in the sky which is letting in harmful ultraviolet (UV) radiation from the Sun. Ozone depletion, in fact, is occurring all over the world due to man-made pollution, at levels within the stratosphere, 19 to 30 km above the Earth's surface. However, in certain parts of the world, ozone depletion is particularly severe, and it is in the regions where the term "ozone hole" strictly applies.
Every September and October during the Southern Hemisphere springtime, significant ozone destruction is observed in the stratosphere above Antarctica, with losses of up to 60%. Levels of ozone are measured in Dobson Units (D.U.). The average amount of stratospheric ozone throughout the world is about 300 D.U. Ozone concentrations over Antarctica during the period of greatest depletion usually fall well below 200 D.U. When ozone concentrations are plotted on a map, the presence of a large ozone hole over the continent is striking.
The formation of the ozone hole over Antarctica is a consequence of the special atmospheric conditions which occur there, in particularly the very low stratospheric temperatures (below -80°C), the isolated wind patterns and the presence of continuous sunlight after the September equinox. Every summer (December to January) the hole repairs itself when stratospheric temperatures rise and the air above Antarctica mixes with the rest of the world's atmosphere. This cycle of ozone hole formation and reparation is repeated every year. The ozone hole over Antarctica has been forming every year since the early 1970s. In recent years the hole has become both larger and deeper, in the sense that more and more ozone is being destroyed.
Every March to April during the Northern Hemisphere springtime similar, but less pronounced ozone hole forms above the Arctic. The natural circulation of wind - the polar vortex - which isolates Antarctica from the rest of the world during the Southern Hemisphere winter and early spring, contributing to the ozone loss there, is much less developed in the Northern Hemisphere above the Arctic. In addition, stratospheric temperatures there are not as low as in the Antarctic, and consequently the loss of ozone is not as severe. However, the formation of even a moderate ozone hole above the Arctic region can give cause for considerable concern due to the greater populations in the higher latitudes of the Northern Hemisphere.