Maximum ozone hole area over the southern hemisphere, historically (9 September 2000) and currently (11 September 2017)

False-colour view of total ozone over the North (Arctic) and South (Antarctic) poles. The purple and blue colours indicate the lowest ozone presence, while yellow and red indicate higher ozone presence. Ozone concentration is commonly measured in Dobson Units. One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimetres thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere.