The ozone hole that forms over the Antarctic each year between August and early October has reappeared.

Nasa animation of the ozone hole developing over Antarctica

Satellite observations made with the Total Ozone Mapping Satellite (TOMS) show that it is near its greatest ever extent, about 26 million square kilometres (10 million square miles).

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.

Complex condition

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.