concentration in Earth atmosphere as a whole is about negligible. The ozone layer is mainly found in the lower portion of the stratosphere from approximately fifteen to thirty kilometres above the surface of Earth. The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson. It was deduced that the missing radiation was being absorbed by something in the atmosphere. Eventually the spectrum of the missing radiation was matched to only one known chemical which is ozone. Its properties were explored in detail by the British meteorologist Dobson who developed a simple spectrophotometer that could be used to measure stratospheric ozone from the ground. Dobson established a worldwide network of ozone monitoring stations which continue to operate to this day. The Dobson unit a convenient measure of the amount of ozone overhead is named in his honour. Atmospheric research revealed that the ozone layer was being depleted by chemicals mainly chlorofluorocarbons released by industry. Concerns that increased UV radiation due to ozone depletion threatened life on Earth including increased skin cancer in humans and other ecological problems led to bans on the chemicals and the latest evidence is that ozone depletion has slowed or stopped. The United Nations General Assembly has designated September sixteenth as the International Day for the Preservation of the Ozone Layer. Venus also has a thin ozone layer at an altitude of hundred kilometres above the planet surface. The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman. Ozone in the Earth stratosphere is created by ultraviolet light striking ordinary oxygen molecules containing two oxygen atoms and splitting them into individual oxygen atoms. The atomic oxygen then combines with unbroken oxygen to create ozone. The ozone molecule is unstable and when ultraviolet light hits ozone it splits into a molecule of oxygen and an individual atom of oxygen a continuing process called the ozone oxygen cycle. The thickness of the ozone layer varies worldwide and is generally thinner near the equator and thicker near the poles. Thickness refers to how much ozone is in a column over a given area and varies from season to season. The reasons for these variations are due to atmospheric circulation patterns and solar intensity. The majority of ozone is produced over the tropics and is transported towards the poles by stratospheric wind patterns. In the northern hemisphere these patterns known as the Brewer Dobson circulation that make the ozone layer thickest in the spring and thinnest in the fall. When ozone is produced by solar UV radiation in the tropics it is done so by circulation lifting ozone lacking air out of the troposphere and into the stratosphere where the sun photolyzes oxygen molecules and turns them into ozone. The ozone is carried to higher latitudes and drops into lower layers of the atmosphere.