Proposed by George Hadley in 1735, it is the tropical convection system that describes the atmospheric thermal circulation of air from the equator (0 degrees latitude) poleward to 30 degrees both north and south of the equator. When saturated air at the intertropical convergence zone (ITCZ), located approximately at the equator, is heated, convergence and convection causes it to rise into the upper atmosphere, creating an area of low pressure at the earth's surface. As the pocket of saturated air at the ITCZ rises, it cools, releasing large amounts of precipitation. Rather than remaining in the tropics, the air then flows horizontally towards the higher latitudes. At approximately 30 degrees both north and south of the equator, the pocket of air in the upper atmosphere has sufficiently mixed and cooled enough to return to the surface. To complete the circle, the area of low pressure at the equator pulls air in from the higher latitudes.