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'Subsidence' in the Earth's atmosphere is most commonly caused by cold temperatures: as air cools, it becomes more dense and moves towards the ground, just as warm air becomes less dense and moves upwards. Subsidence generally causes high barometric pressure as more air moves into the same space: the polar highs are areas of almost constant subsidence, as are the horse latitudes, and these areas of subsidence are the sources of much of the world's prevailing wind. Subsidence also causes many smaller-scale weather phenomena, such as morning fog. An extreme form of subsidence is a downburst, which can result in damage similar to that produced by a tornado. A milder form of subsidence is referred to as downdraft.