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Wind, in the most general sense, is the movement of air. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the planet, lasting eons. The two major influences on the atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet ( Coriolis effect). 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 an 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.
Given a difference in barometric pressure between two air masses, a wind will arise between the two which tends to flow from the area of high pressure to the area of low pressure until the two air masses are at the same pressure, although this will be strongly modified by the Coriolis effect.
While all winds are the movement of air more or less parallel to the Earth's surface, they come in a variety of forms. There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds, such as the jet streams. There are synoptic winds that result from pressure differences in surface airmasses at the middle latitudes, and there are winds that come about as a consequence of geographic features such as oceans, lakes, mountains, and deserts. MesoscaleSpatial scale provides a "shorthand" form for discussing relative lengths, areas, distances and sizes. A microclimate, for instance, is one which might occur in a mountain valley or near a lakeshore, whereas a megatrend is one which involves the whole pla winds are those 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 eolianEolian (or aeolian processes pertain to the activity of the winds. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments. Although water is much more po 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 windsA katabatic wind is a wind blowing down a gradient caused by buoyancy forces ie the air is cool. Over the major Ice sheets of Antarctica and Greenland katabatic winds blow with great constancy particularly during the polar night, when heat loss by radiati of the major Ice sheetAn ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 square kilometers (12 million acres). The only current ice sheets are Antarctic and Greenland; during the last ice age the Laurentide ice sheet covered much os of AntarcticaAntarctica (from Greek nu;ταρκτικ&sigmaf opposed to the arctic) is a continent surrounding the Earth's South Pole. It is the coldest place on earth and is almost entirely covered by ice. It is not to be confused with the and GreenlandThis article is about Greenland, the island dependency of Denmark. For information about the town of Greenland, see Greenland, New Hampshire. Greenland ( Greenlandic: Kalaallit Nunaat "The Land of the Greenlanders (Kalaallit)"; Danish: Gronland , an Arcti. 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 cellThe major driving force of atmospheric circulation in the tropical regions is solar heating. Because of the Earth’s 23. 5 degree axial tilt, the sun is never more than a few tens of degrees from directly overhead at noon in the tropics; as a consequence,, 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 circular 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 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.
Records of tornadoes in northerly latitudes are spotty and incomplete because of the vast amount of uninhabited terrain and lack of monitoring, and it is certain that tornadoes have gone unseen and unreported. The deadly Edmonton tornado of 1987, which ranked as an F4 on the Fujita scale and killed 27 people, is evidence that powerful tornadoes can occur north of the 50th parallel.
The Edmonton, Canada tornado of 1987 is evidence that powerful tornadoes can develop at high latitudes.
The jet streams are rapidly moving upper-level currents. Travelling generally eastward in the tropopause, the polar jets reside at the juncture of the Ferrel cell and the Polar cell and mark the location of the polar cold front. During winter, a second jet stream forms at about the 30th parallel, at the interface of the Hadley and Ferrel cells, as a result of the contrast in temperature between tropical air and continental polar air.
The jet streams are not continuous, and fade in and out along their paths as they speed up and slow down. Though they move generally eastward, they may range significantly north and south. The polar jet stream also marks the presence of Rossby waves, long-scale (4000 - 6000 km in wavelength) harmonic waves which perpetuate around the globe.