Hadley cell

Lore & Background

The Hadley cell is a large-scale tropical atmospheric circulation in which air rises near the equator, flows poleward at an altitude of 12–15 kilometers near the tropopause, cools and descends in the subtropics around 30 degrees latitude, and returns equatorward near the surface. This thermally direct circulation arises from differences in solar heating and insolation between the tropics and subtropics. On average, a circulation cell exists on each side of the equator, with the Southern Hemisphere cell slightly stronger and extending marginally into the Northern Hemisphere. During summer and winter, the circulation is dominated by a single cross-equatorial cell, with rising air in the summer hemisphere and sinking air in the winter hemisphere. The lower branches of the Hadley circulation manifest as the prevailing trade winds, which converge moisture and air in the tropics to form the Intertropical Convergence Zone (ITCZ), the location of Earth’s heaviest rainfall. Seasonal shifts in the ITCZ, linked to the Hadley cell’s variability, drive monsoons. The descending branches create subtropical ridges and suppress rainfall, placing many of the world’s deserts and arid regions in the subtropics. The circulation also transports heat, angular momentum, and moisture meridionally, contributing to the subtropical jet stream and maintaining global thermal equilibrium. The Hadley cell covers nearly half the Earth’s surface, from roughly the Tropic of Cancer to the Tropic of Capricorn, and occupies the full depth of the troposphere.

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.