Trade inversion

From Glossary of Meteorology

trade inversion

A characteristic temperature inversion usually present in the trade-wind streams over the eastern portions of the tropical oceans.

It is found in large-scale subsiding flows constituting the descent branches of the Hadley cell and Walker circulation. The subsidence warming in the inversion layer is balanced by radiative cooling and evaporation from the tops of trade cumuli. The height of the base of this inversion varies from about 500 m at the eastern extremities of the subtropical highs to about 2000 m at the western and equatorial extremities. In the equatorial trough zone and over the western portions of the trade-wind belt, the inversion does not exist as a mean condition, although it appears in certain weather patterns. The strength of the inversion varies enormously, occasionally being more than 10°C over 1 km, but sometimes being absent altogether, especially in the Northern Hemisphere. The inversion is generally strongest when the height of its base is lowest, and vice versa. The thickness of the inversion layer varies from only a few meters to more than 1000 m. On the average its thickness is about 400 m. The airflow below the inversion is very moist and filled with cumulus clouds (trade cumuli). Above it, the air is warm and exceedingly dry; this structure is so characteristic of the trade current that tropical analysts think of the tropical troposphere as consisting of a lower moist and an upper dry layer.

Riehl, H. 1954. Tropical Meteorology. ch. II.