| Abstract from DBPedia | Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/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 with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a 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 significant effects on the control of an aircraft, and it has been the sole or a contributing cause of many aircraft accidents. 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.ウインドシア (英: windshear)には、航空と気象の分野の定義があり、航空では飛行機が単位時間に受ける風ベクトルの変化量(m s−2)をいい、気象では2点間の風ベクトルの差を両地点の距離で除したもの(s−1)で、航空のウインドシアは飛行機の進行方向に沿う気象のウインドシアに飛行機の速度を乗じたものである。風の急変する場所にあるとき、重量のあるジェット輸送機は、揚力を急変させるが、この現象は離着陸時で揚力と高度が低レベルにある状態、及び音速に近い高速で飛行している時に問題となり、ウインドシアが注目される。飛行機はウインドシアによる対気速度減少分を補うために推力を増そうとするが、この際ジェット輸送機は、その重量のため加速が制限され、加速のタイミング遅れも手伝って事故になることもあり、またウインドシアによる対気速度の減少量が飛行機の加速性能に勝れば無事に飛行を継続できない。 (Source: http://dbpedia.org/resource/Wind_shear) |
