Above each of Earthˇs hemispheres are three loops of atmospheric circulation: the polar cell, the Ferrel cell, and the Hadley cell. These loops are composed of ascending and descending movements, caused by the pressure gradient, and horizontal movements, caused by the Coriolis force. In each circuit, the warm air rises, moves at high altitude, descends again as it cools, then is warmed again as it moves on the surface in a constant direction.