The tropopause is the upper boundary of the troposphere, a layer of the earth's atmosphere near the ground. In the troposphere, the temperature generally decreases with increasing altitude, with restricted exceptions called inversions. However, at a height of about 6 mi (10 km) at the poles 9 mi (15 km) at the equator, the temperature abruptly becomes constant with increasing altitude. This isothermal region is called the stratosphere, and the interface between it and the troposphere is called the tropopause. Mixing of air across the tropopause is slow, occurring on a time scale of weeks on the average, while tropospheric mixing is more rapid.

The existence of the stratosphere is largely caused by the absorption of solar energy, mostly ultraviolet light, by oxygen to form ozone. The balance of the heating is caused by absorption of other parts of the solar spectrum by other trace gases.

See Also

Ultraviolet Radiation