Definition

The tropopause is the upper limit of the troposphere and therefore constitutes the boundary between it and the Stratosphere.

According to the World Meteorological Organisation, the "first tropopause" is conventionally defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided also that the average lapse rate between this level and all higher levels within 2 km does not exceed 2°C/km. If the average lapse rate above this "first tropopause" between any level and all higher levels within 1 km exceeds 3°C/km, then a "second tropopause" is defined by the same criterion as the first. This second tropopause may be either within or above the 1 km layer.

Near the mid-latitudes there may be two layers of tropopauses: polar and tropical. For aviation purposes, however, significant weather charts generally show one tropopause, using the average heights of the two tropopauses to denote its height in flight level.