<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> The stratosphere (/ ˈstrætəˌsfɪər, - toʊ - /) is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere . About 20% of the atmosphere's mass is contained in the stratosphere . The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth . The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation by the ozone layer . This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude . The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins . Near the equator, the stratosphere starts at 18 km (59,000 ft; 11 mi); at mid latitudes, it starts at 10--13 km (33,000--43,000 ft; 6.2--8.1 mi) and ends at 50 km (160,000 ft; 31 mi); at the poles, it starts at about 8 km (26,000 ft; 5.0 mi). Temperatures vary within the stratosphere with the seasons, in particular with the polar night (winter). The greatest variation of temperature takes place over the poles in the lower stratosphere; variations at lower latitudes and higher altitudes are smaller . Winds in the stratosphere can sometimes exceed 300 MPH, relative to the ground . </P> <P> The mechanism describing the formation of the ozone layer was described by British mathematician Sydney Chapman in 1930 . Molecular oxygen absorbs high energy sunlight in the UV - C region, at wavelengths shorter than about 240 nm . The oxygen atoms produced combine with molecular oxygen to form ozone . Ozone in turn is photolysed much more rapidly than molecular oxygen as it has a stronger absorption that occurs at longer wavelengths, where the solar emission is more intense . Ozone (O) photolysis produces O and O . The oxygen atom product combines with atmospheric molecular oxygen to reform O, releasing heat . The rapid photolysis and reformation of ozone heats the stratosphere resulting in a temperature inversion . This increase of temperature with altitude is characteristic of the stratosphere; its resistance to vertical mixing means that it is stratified . Within the stratosphere temperatures increase with altitude (see temperature inversion); the top of the stratosphere has a temperature of about 270 K (− 3 ° C or 26.6 ° F). This vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere . However, exceptionally energetic convection processes, such as volcanic eruption columns and overshooting tops in severe supercell thunderstorms, may carry convection into the stratosphere on a very local and temporary basis . Overall the attenuation of solar UV at wavelengths that damage DNA by the ozone layer allows life to exist on the surface of the planet outside of the ocean . All air entering the stratosphere must pass through the tropopause, the temperature minimum that divides the troposphere and stratosphere . The rising air is literally freeze dried; the stratosphere is a very dry place . The top of the stratosphere is called the stratopause, above which the temperature decreases with height . </P> <P> Sydney Chapman gave a correct description of the source of stratospheric ozone and its ability to generate heat within the stratosphere; he also wrote that ozone may be destroyed by reacting with atomic oxygen, making two molecules of molecular oxygen . We now know that there are additional ozone loss mechanisms, and that these mechanisms are catalytic meaning that a small amount of the catalyst can destroy a great number of ozone molecules . The first is due to the reaction of hydroxyl radicals OH with ozone . OH is formed by the reaction of electronically excited oxygen atoms produced by ozone photolysis, with water vapor . While the stratosphere is dry, additional water vapor is produced in situ by the photochemical oxidation of methane (CH). The HO radical produced by the reaction of OH with O is recycled to OH by reaction with oxygen atoms or ozone . In addition, solar proton events can significantly effect ozone levels via Radiolysis with the subsequent formation of OH . Laughing gas or Nitrous oxide (N O) is produced by biological activity at the surface and is oxidised to NO in the stratosphere; the so - called NOx radical cycles also deplete stratospheric ozone . Finally, chlorofluorocarbon molecules are photolysed in the stratosphere releasing chlorine atoms that react with ozone giving ClO and O . The chlorine atoms are recycled when ClO reacts with O in the upper stratosphere, or when ClO reacts with itself in the chemistry of the Antarctic ozone hole . Paul J. Crutzen, Mario J. Molina and F. Sherwood Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work describing the formation and decomposition of stratospheric ozone . </P>

What gas in the stratosphere causes temperatures to increase with altitude