The layer above the tropopause is called the stratosphere—because Léon-Philippe Teisserenc de Bort (1855-1913), the French meteorologist who discovered and named it, thought that at this height the atmospheric gases separated into layers, or strata, according to their masses. This was incorrect, but the name stuck.
Temperature remains constant with height through the lower stratosphere, but it begins to increase with height above about 12 miles (19 km) and the rate of increase accelerates above 20 miles (32 km). The temperature at the upper boundary of the stratosphere, known as the stratopause, is sometimes higher than 32°F (0°C). The warming is due to the absorption of ultraviolet (UV) radiation by oxygen and ozone.
The stratopause is at about 34 miles (55 km) over the equator and the poles and about 31 miles (50 km) over middle latitudes. It is higher in winter than in summer. The air pressure at this height is about 0.015 lb. in.-2 (0.100 pascal, Pa; 1 millibar, mb).
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