<P> The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet (UV) radiation . It contains high concentrations of ozone (O) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere . The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million . The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 20 to 30 kilometres (12 to 19 mi) above Earth, although its thickness varies seasonally and geographically . </P> <P> The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson . Measurements of the sun showed that the radiation sent out from its surface and reaching the ground on Earth is usually consistent with the spectrum of a black body with a temperature in the range of 5,500--6,000 K (5,227 to 5,727 ° C), except that there was no radiation below a wavelength of about 310 nm at the ultraviolet end of the spectrum . 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, ozone . Its properties were explored in detail by the British meteorologist G.M.B. Dobson, who developed a simple spectrophotometer (the Dobsonmeter) that could be used to measure stratospheric ozone from the ground . Between 1928 and 1958, 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 honor . </P>

In earth's atmosphere where is the ozone layer