Tropopause followed by stratosphere
Definition
Rising from the planetary surface of the Earth, the tropopause is the atmospheric level where the air ceases to become cool with increased altitude and becomes dry, devoid of water vapor. The tropopause is the boundary that demarcates the troposphere below from the stratosphere above, and is part of the atmosphere where there occurs an abrupt change in the environmental lapse rate (ELR) of temperature, from a positive rate (of decrease) in the troposphere to a negative rate in the stratosphere. The tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided that the average lapse-rate, between that level and all other higher levels within 2.0 km does not exceed 2°C/km. The tropopause is a first-order discontinuity surface, in which temperature as a function of height varies continuously through the atmosphere, while the temperature gradient has a discontinuity.Tropical Tropopause Layer Cold Trap
In 1949 Alan West Brewer proposed that tropospheric air passes through the tropopause into the stratosphere near the equator, then travels through the stratosphere to temperate and polar regions, where it sinks into the troposphere.This is now known as Brewer-Dobson circulation.
Because gases primarily enter the stratosphere by passing through the tropopause in the tropics where the tropopause is coldest, water vapor is condensed out of the air that is entering the stratosphere. This ″tropical tropopause layer cold trap″ theory has become widely accepted.This cold trap limits stratospheric water vapor to 3 to 4 parts per million.