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.

Due to the changes in tropopause heights, especially in mid-latitude polar frontal systems, in certain cases stratospheric air may be brought into the warm air troposphere as a result of the lower tropopause height over the cold air and the jetstream associated with polar frontal system. The airflow effect may “draw” stratospheric air from above and enter the warm air troposphere by way of the jet stream.

Description

The tropopause occurs at approximately 20,000 feet over the poles and at approximately 60,000 feet above the equator. The International Standard Atmosphere (ISA) assumes that the average height of the tropopause is 36,000 feet.

Due to the tropospheric effect of temperature decrease with height, in general the temperature at the Tropopause is lower equatorward and higher poleward.

The location of the tropopause is of interest to flight crew because it indicates the altitude at which temperature becomes constant with increasing altitude, which is an import factor in performance and fuel calculations. It also gives an indication of the location of jet streams and the high winds and turbulence associated with them. In general, clouds and weather occur below the tropopause in the troposphere; however, deep tropical convective systems can break through the tropopause, especially over land. Overshooting cumulonimbus tops are examples of such situations.

The turbulence felt as a result of jet streams and other weather phenomena doesn't necessarily end at the tropopause. Turbulence can continue well above the tropopause, however the general rule to climb or descend to avoid turbulence most effectively (as opposed to horizontally by directional changes) still applies.

Troposphere

Jet Streams

Jet Streams, associated with frontal weather, where two different air masses meet, occur at or below the tropopause.