Atmospheric general circulation is organized into three meridional overturning cells in each hemisphere: the Hadley cell (thermally direct, driven directly by rising warm air near the equator and sinking cooler air in the subtropics), the Polar cell (also thermally direct, driven by rising air at the subpolar boundary and sinking cold air at the pole), and the Ferrel cell (located between the Hadley and Polar cells in the mid-latitudes), which is thermally indirect because its circulation (rising air at higher latitudes, sinking air at lower latitudes within its cell) is driven mechanically by the adjacent Hadley and Polar cells and eddy momentum transport rather than by direct thermal buoyancy forcing.