Wind shear
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Wind shear is a difference in wind speed and/or direction between two points in the atmosphere. Depending whether the two points are at different altitudes or at geographically different locations, shear can be either vertical or horizontal shear.
Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. An explanation in regard to vertical wind shear can be seen here: (wind gradient). It is also a key factor governing the severity of thunderstorms. An additional hazard is turbulence often associated with wind shear.
Weather situations when shear is observed are:
- Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 °C or more, and the front moves at 15 kt or faster. Remember that fronts are three-dimensional creature; thus frontal shear can be observed at any altitude between surface and tropopause.
- Obstacles to flow. When wind above a mountain blows from the direction of mountain, vertical shear is obseved at lee side.
- Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the inversion top. Change in wind can be 90 degrees in direction and 40 kt in speed. Even a nocturnal low level jet can sometimes be observed. Density difference causes addiotional problems to aviation.
See also
Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Of these accidents, 15 occurred during take-off, 3 during flight, and 8 during landing. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately 1 each 10 years due to onboard detection, and Doppler radar on the ground.
External links
- National Science Digital Library - Wind shear (http://www.nsdl.arm.gov/Library/glossary.shtml#wind_shear)fi:Tuulishear