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106
features such as oceans , lakes , mountains , and deserts . Mesos cale winds are thos e which act
on a local scale, such as gust fronts. At the smallest scale are the winds which blow on a
scale of only tens to hundreds of metres and are essentially unpredictable, such as dust
devils and microbursts. Finally, there are special-case winds that come about as a
consequence of local geography.
Wind can also shape landforms, via a variety of eolian processes.
Global 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.
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 Old English 'trade',
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
equatorial 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
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