ABSTRACT
Air moves relentlessly and atmospheric motion is the key to understanding Earth’s climates and so to understanding many Earth surface patterns and processes. On a planetary scale, the temperature gradient between the equator and the poles is the foremost driver of air movements. This thermal gradient drives a vast overturning of air, first suggested by Edmund Halley in 1686 and elaborated by George Hadley in 1735, called the Hadley circulation. On Venus, there is one grand Hadley cell in each hemisphere: a huge convective current of air rises at the Venusian equator, moves polewards, sinks over the poles, and then returns at ground level to its equatorial origin. On the Earth, the Hadley circulation breaks down into three component cells in each hemisphere (Figure 16). Heat released as water evaporated from the tropical oceans condenses, mainly in the inter-tropical convergence zone (ITCZ) or equatorial low-pressure trough, and largely powers the equatorial Hadley cell. At ground level, the air returning towards the ITCZ produces the trade winds. The middle or Ferrel cell flows in the reverse direction, that is, equatorwards aloft and polewards at the surface. The third cell, known as the polar cell, is rather weak.
