Wednesday, September 22, 1999 Published at 17:13 GMT 18:13 UK
Ozone hole opens again
The ozone hole on 11 September
By BBC News Online Science Editor Dr David Whitehouse
The ozone hole that forms over the Antarctic each year between August and early October has reappeared.
Ozone is a molecule that is composed of three oxygen atoms. It is responsible for filtering out harmful ultra-violet radiation (less than 290 nanometres) from the Sun.
Ozone is constantly being made and destroyed in the stratosphere, about 30 km above the Earth. In an unpolluted atmosphere, this cycle of production and decomposition is in balance.
But the emission of chlorofluorocarbons (CFCs) in recent decades has upset this equilibrium.
The man-made chemicals, once popular refrigerants, rise into the stratosphere where they are broken down by the Sun's rays. Free chlorine atoms then act as catalysts to decompose ozone.
The whole process requires complex meteorological conditions that are peculiar to the polar stratosphere during the long, dark winter months. It is only when these conditions have been established and the sunlight returns in the Antarctic spring that the infamous ozone hole starts to appear.
Scientists are also worried about ozone depletion in the Northern Hemisphere.
They predict that we will see the ozone hole for many decades to come, until global efforts to reduce CFC emission begin to take effect.