[Encyclios]

Regional atmospheric division

Because the principal physical and chemical characteristics of the atmosphere are a function of height above the Earth’s surface, it was determined to divide the atmosphere into concentric regions, each defined by uniform properties and characteristics. A first division, made on the basis of chemical constitution, includes two regions: the homosphere and the heterosphere. The homosphere extends to an altitude of about 90-100 km, maintaining a fairly constant chemical composition. The heterosphere, above 100 km, has a composition that varies with altitude, with the final predominance of helium and hydrogen, while carbon dioxide, ozone and water vapor are missing. The two zones are separated by a transition layer called homopause.

The most widely used division and adopted by the International Union of Geodesy and Geophysics in 1951 is that based on the trend of temperature with altitude. In this scheme the following atmospheric regions are recognized: troposphere, tropopause, stratosphere, stratopause, mesosphere, mesopause, thermosphere, thermopause, exosphere.