Wind shear
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Wind shear is a difference in wind speed and/or direction over a fairly short distance in the atmosphere. Wind shear can be divided into two different types; horiztontal and vertical wind shear.
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[change] Where and when it is strongly observed
Weather situations where shear is observed include:
- 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. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically.
- Low Level Jets. When a nocturnal low-level jet forms above the boundary layer ahead of a cold front, significant low level vertical wind shear can develop near the lower portion of the low level jet.
- Mountains. When winds blow over a mountain, vertical shear is observed on the lee side.[1]
- 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 additional problems to aviation.
- Downbursts. When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air that spreads out at ground level, both speed and directional wind shear can result at the leading edge of the three dimensional boundary. The stronger the outflow boundary, the stronger the resultant vertical wind shear.
[change] Sailing
In sailing, wind shear affects sailboats by presenting a different wind speed and direction at different heights along the mast. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height.
[change] Effects on thunderstorms and severe weather
Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time by separating the storm's inflow from its rain-cooled outflow.
[change] See also
[change] References
- ↑ National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.
- ↑ Mcilveen, J. (1992). Fundamentals of Weather and Climate. London: Chapman & Hall, p. 339. ISBN 0412411601.
