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Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction at different altitudes. Horizontal wind shear is a change in wind speed between fixed points on the ground at any given altitude. Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft due to its effects on control of the aircraft, and it has been a sole or contributing cause of many aircraft accidents. Wind shear is sometimes experienced by pedestrians at ground level when walking across a plaza towards a tower block and suddenly encountering a strong wind stream that is flowing around the base of the tower. This phenomenon is a concern for architects. Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather. The thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences, and explains the existence of the jet stream.〔(【引用サイトリンク】title=LOW-LEVEL WIND SHEAR.] Retrieved on 2007-11-25 )〕 ==Definition== Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (22 m/s) for airliners at flight altitude.〔FAA (FAA Advisory Circular Pilot Wind Shear Guide. ) Retrieved on 2007-12-15.〕 Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of 30 knots).〔(【引用サイトリンク】url=http://oea.larc.nasa.gov/PAIS/Concept2Reality/wind_shear.html Wind Shear. )〕 The rationale for this additional caution includes: (1) microburst intensity can double in a minute or less, (2) the winds can shift to excessive cross wind, (3) 40-50 knots is the threshold for survivability at some stages of low-altitude operations, and (4) several of the historical wind shear accidents involved 35-45 knot microbursts. Wind shear is also a key factor in the creation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wind shear」の詳細全文を読む スポンサード リンク
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