"The second atmosphere layer is the stratosphere extending from +/- 20 km to around 50 km. It has high concentrations of ozone and low concentrations of water".
(Air transport, global warming and the environment, Editorial, Transportation Research Part D, Elsevier, 2009)
It is "the atmospheric layer above the tropopause which is very stable and characterized by low moisture content and absence of clouds".
(http://virtualskies.arc.nasa.gov/glossary/S.html)
=> In brief page