World Meteorological Organization (WMO). This Bureau operates a network of ozone monitoring stations, where the total amount of ozone above the station (the total column ozone) is monitored each day. The total column ozone is measured with an instrument called a Dobson spectrophotometer, compares the amount of sunlight at two ultraviolet wavelengths. One wavelength is affected strongly by ozone, the other is not. The difference between the two is directly related to the total column ozone.
Ozone is measured in Dobson Units (DU). 100 DU is equivalent to a 1 millimetre thick layer of pure ozone at sea level temperature and pressure.
What is the ozone hole?
The commonly accepted definition of the ozone hole, is the region above Antarctica where the ozone level is below 220 DU.
When did the ozone hole first appear?
The British Antarctic Survey first revealed the obvious decreases in ozone in the late 1970s to early 1980s. The Japanese station at Syowa also reported decreases in ozone.
When does the ozone hole occur?
The ozone hole starts to appear in September and breaks up in December.