Description

The impact on aviation of increasing water vapour concentrations in the stratosphere, linked to climate change, have yet to be fully understood. This article explains the phenomenon of convective overshooting of the tropical tropopause layer (TTL) and associated hydration of the lower stratosphere, the terminology, and links to other SKYbrary relevant content.

Tropical Tropopause Layer (TTL)

The TTL is a transition layer in the tropics between the troposphere, where temperature decreases with height, and the stratosphere, where temperature increases with height. The TTL is important as a gateway to the wider stratosphere. 

Cold point tropopause (CPT)

The cold point tropopause is the point in the atmosphere where the temperature reaches its lowest value as air ascends from the troposphere into the stratosphere. The CPT is important because it is where air from the tropopause undergoes final dehydration before entering the stratosphere. The very low temperatures at the tropical CPT reduce water vapour concentration to a few parts per million.

Factors like deep convection, cirrus clouds, and wave activity in the TTL can influence the temperature and height of the CPT.

The region above the CPT is referred to as the upper TTL.

Brewer Dobson circulation (BDC)

Brewer Dobson circulation carries air from the upper TTL into the tropical and then extratropical stratosphere on timescales of a few years, significantly influencing stratospheric composition. See the separate article on Brewer Dobson circulation for more information.

Level of zero radiative heating (LZRH)

The atmosphere gains heat through the absorption of solar radiation (shortwave) and the absorption of infrared radiation (longwave) emitted by the Earth's surface and atmosphere. It loses heat through the emission of infrared radiation to space.

At the LZRH, the net effect of these radiative processes is zero; the atmosphere is neither gaining nor losing heat due to radiation.

The CPT is above the LZRH; the majority of the air that penetrates above the LZRH is likely to ascend into the stratosphere and the same applies to air passing through the CPT, meaning that temperatures at the CPT exert significant control on stratospheric water vapour concentration.

Tropical lower stratosphere (TLS)

The region immediately above the TTL is often referred to as the tropical lower stratosphere (TLS), characterised by low temperatures and a relatively stable air mass where ozone-rich stratospheric air mixes with humid tropospheric air.

Convective overshooting

Convective overshooting, when particularly strong thunderstorm complexes penetrate deep into the TTL, provides a secondary pathway into and through the TTL and, more rarely, directly into the TLS. This can result in increases in concentrations of water vapour in the stratosphere. It has yet to be determined whether this hydration of the TLS is significant.

Overshooting top of a cumulonimbus viewed from an aircraft crossing the Democratic Republic of Congo [Source: wikicommons. Author: Max.kit, 20 December 2016]

Overshooting storms also carry pollutants into the stratosphere, altering the stratosphere's composition and chemistry, potentially impacting ozone and leading to climate change. 

Further reading

• "Moisture transport by convective overshoots in the tropical tropopause layer", C W Powell et al. DAMTP, Centre for Mathematical Sciences, University of Cambridge. Article in the June 2025 RMetS journal "Weather".