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.
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. 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.