Hadley cells. Image credit: NASA.
Hadley circulation is an average thermal circulation in a planet's atmosphere due to warm air rising at lower latitudes, moving to higher latitudes, then descending and moving back to lower latitudes nearer the surface. On Earth, such circulations, known as Hadley cells, exist between the equator and 30° N/S (the most prominent and permanent cells), 30° N/S and 60° N/S, and 60° N/S and the poles.
On Venus, a single cell extends from the equator to the poles. On Mars, Hadley circulation is strongest in the northern winter, especially when the atmosphere is dusty, and weakest at the equinoxes, while during solstice seasons, it consists of a strong cross-equatorial surface flow with rising motion in the summer hemisphere, return flow at high altitude, and descent in mid-latitudes of the winter hemisphere.
The effect was first discussed in 1735 by the English meteorologist George Hadley (1685–1768).