Prevailing winds — the general circulation of the atmosphere Prevailing winds are winds which come about as a consequence of global circulation patterns. These include the Trade Winds, the Westerlies, the Polar Easterlies, and the jet streams. Because of differential heating and the fact that warm air rises and cool air falls, there arise circulations that (on a non-rotating planet) would lead to an equator-to-pole flow in the upper atmosphere and a pole-to-equator flow at lower levels. Because of the Earth’s rotation, this simple situation is vastly modified in the real atmosphere. In almost all circumstances the horizontal component of the wind is much larger than the vertical — the exception being violent convection. The Trade Winds are the most familiar consistent and reliable winds on the planet, exceeded in constancy only by the katabatic winds of the major ice sheets of Antarctica and Greenland. It was these winds that early mariners relied upon to propel their ships from Europe to North and South America. Their name derives from the Middle High German trade, akin to Old English trod meaning “path” or “track”, and thus the phrase “the wind blows trade”, that is to say, on track. The Trades form under the Hadley circulation cell, and are part of the return flow for this cell. The Hadley carries air aloft at the equator and transports it poleward north and south. At about 30°N/S latitude, the air cools and descends. It then begins its journey back to the equator, but with a noticeably westward shift as a result of the action of the Coriolis force. Along the east coast of North America, friction twists the flow of the Trades even further clockwise. The result is that the Trades feed into the Westerlies, and thus provide a continuous zone of wind for ships travelling between Europe and the Americas. The Westerlies, which can be found at the mid-latitudes beneath the Ferrel circulation cell, likewise arise from the tendency of winds to move in a curved path on a rotating planet. Together with the airflow in the Ferrel cell, poleward at ground level and tending to equatorward aloft (though not clearly defined, particularly in the winter), this predisposes the formation of eddy currents which maintain a more-or-less continuous flow of westerly air. The upper-level polar jet stream assists by providing a path of least resistance under which low pressure areas may travel. The Polar Easterlies result from the outflow of the Polar high, a permanent body of descending cold air which makes up the poleward end of the Polar circulation cell. These winds, though persistent, are not deep. However, they are cool and strong, and can combine with warm, moist Gulf Stream air transported northward by weather systems to produce violent thunderstorms and tornadoes as far as 60°N on the North American continent.