HADLEY CELL

simple model of the Earth's atmospheric circulation that was proposed by George Hadley (1735). It consists of a single wind system in each hemisphere, with westward and Equatorward flow near the surface and eastward and poleward flow at higher altitudes. The tropical regions receive more heat from solar radiation than they radiate back into space, and the polar regions radiate more than they receive; because both areas have nearly constant temperatures, Hadley theorized that warm air must therefore rise near the Equator, flow poleward at high altitudes, and lose heat to the cold air present near the poles. This cooler and denser air then descends and flows Equatorward at low levels until it nears the Equator, where it is warmed and rises again. Hadley devised this model in an attempt to explain the westward- and Equatorward-flowing trade winds, but he ignored the Coriolis effect of the Earth's rotation, which deflects moving objects (including air) sideways and precludes a simple north-south circulation. The Ferrel cell, a model with a circulation opposite to that of the Hadley cell, was subsequently proposed to account for the midlatitude westerly winds. The tricellular theory (q.v.) includes two Hadley cells and one Ferrel cell.