From AMS Glossary
The boundary between the troposphere and stratosphere, usually characterized by an abrupt change of lapse rate.
The change is in the direction of increased atmospheric stability from regions below to regions above the tropopause. Its height varies from 15 to 20 km (9 to 12 miles) in the Tropics to about 10 km (6 miles) in polar regions. In polar regions in winter it is often difficult or impossible to determine just where the tropopause lies, since under some conditions there is no abrupt change in lapse rate at any height. It has become apparent that the tropopause consists of several discrete, overlapping "leaves," a multiple tropopause, rather than a single continuous surface. In general, the leaves descend, step-wise, from the equator to the poles.