Hadley cell

Lore & Background

The Hadley circulation is named after George Hadley, who in 1735 postulated the existence of hemisphere-spanning circulation cells driven by differences in heating to explain the trade winds. Other scientists later developed similar arguments or critiqued Hadley's qualitative theory, providing more rigorous explanations and formalism. The existence of a broad meridional circulation of the type suggested by Hadley was confirmed in the mid-20th century once routine observations of the upper troposphere became available via radiosondes.

Reader's Guide

The Hadley cell is a fundamental component of Earth's atmospheric circulation, covering almost half the planet's surface area and occupying the entire depth of the troposphere. Its significance lies in its role as a thermally direct circulation that redistributes heat from the equator toward the subtropics, maintaining global thermal equilibrium. The lower branches of the Hadley circulation manifest as the prevailing trade winds, which converge air and moisture in the tropics to form the Intertropical Convergence Zone, where the Earth's heaviest rains occur. The sinking branches give rise to oceanic subtropical ridges and suppress rainfall, creating many of the world's deserts and arid regions in the subtropics. Observations and climate modelling indicate that the Hadley circulation has expanded poleward since at least the 1980s as a result of climate change, with an accompanying but less certain intensification; model projections suggest it will widen and weaken throughout the 21st century. Analogous circulations may occur on other planets such as Venus and Mars.

Did You Know?

• The Southern Hemisphere Hadley cell is slightly stronger on average than its northern counterpart, extending slightly beyond the equator into the Northern Hemisphere.
• During summer and winter, the Hadley circulation is dominated by a single, cross-equatorial cell with air rising in the summer hemisphere and sinking in the winter hemisphere.
• The Hadley circulation is named after George Hadley, who in 1735 postulated the existence of hemisphere-spanning circulation cells driven by differences in heating to explain the trade winds.
• The existence of the Hadley circulation was confirmed in the mid-20th century once routine observations of the upper troposphere became available via radiosondes.