Arctic ozone hole. Coloured satellite image of the atmospheric ozone layer over the northern hemisphere on 14th March 2000. Ozone thicknesses are colour-coded from dark blue (lowest), through cyan, green and yellow to orange (highest). The blue regions have an ozone layer that is about 40% thinner (at 100-200 Dobson units) than normal (300-340 Dobson units). The ozone layer's thickness over the Arctic region fluctuates from season to season and from year to year, meaning that the size of the hole over the Arctic (dark blue, upper centre) changes frequently. The Antarctic has had a constant decline in ozone and its hole is greater than the Arctic hole, spanning a maximum of 29.2 million square kilometres in 2000. Data from the Total Ozone Mapping Spectrometer (TOMS).