The Earth's general atmospheric circulation in the tropics and subtropics is simply described. Intense heating by the sun at the equator causes air to rise and spread out polewards in the upper troposphere. As this air moves toward the poles it cools and begins to descend back to the Earth's surface at 200 -300 north and south of the equator. Upon reaching the Earth's surface this air either returns to the equator or moves polewards. Where air rises in this circulation, low air pressure forms and there is intense instability and condensation of moisture with subsequent heavy rainfall. Where air descends, high air pressure forms with intense evaporation, clear skies and stability. This tropical circulation of air is termed the Hadley cell and the high pressure that forms encircles the globe coincident with the great sub-tropical deserts. Australian climate is dominated by this circulation cell and the deserts that form as a consequence. The Hadley cells migrate annually with the apparent movement of the su n north and south of the equator.