The stratified variations in temperature were deduced from the behavior of sound waves transmitted through the atmosphere, which travel faster in warm air than in cold air. Weather balloons carrying electronic equipment are launched to ascertain conditions in the stratosphere; information on this atmospheric layer is also acquired from earth-orbiting satellites.
Within the stratosphere at altitudes of 12 to 30 mi (19–48 km) is the ozone layer. Its capacity to intercept most of the sun's ultraviolet rays is fundamental to the maintenance of life on the earth. Without this filtering effect, the sun's full radiation would destroy animal tissue, but sufficient ultraviolet radiation reaches the earth to support the activation of vitamin D in humans. Elevated temperatures found in the ozone layer result from its absorption of radiant energy.
Measurements of Antarctica's ozone layer have registered a consistent seasonal
hole, or thinning, in the layer above the South Pole since 1985, and since then similar thinnings have been found over other areas of the world. There is evidence that the ozone is being broken down by chlorine atoms that are released when sunlight breaks up substances such as chlorofluorocarbons (CFCs). Montreal Protocol and its amendments now ban these substances and have set time limits on the production of others that may also affect the ozone layer.
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