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.

Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. An explanation in regard to vertical wind shear is discussed in wind gradient. It is also a key factor governing the severity of thunderstorms. An additional hazard is turbulence often associated with wind shear. Wind shear also generally inhibits tropical cyclone development.

1 Where and when it is observed
2 Effects on aircraft
3 Effects on sound propagation
4 Effects on tropical cyclones
5 Effects on thunderstorms and severe weather
6 See also
7 References
8 External links

[edit] Where and when it is observed

Image:Microburstnasa.JPG

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 wind shear can develop. This is also known as nonconvective wind shear.
Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, strong eddies known as rotors (vertically oriented circulations to the lee of mountains) can form, which are dangerous to ascending and descending aircraft.<ref>National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.</ref>

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 spreading out at ground level, both speed and directional wind shear can result. The stronger the outflow boundary, the stronger the resultant vertical wind shear.

[edit] Effects on aircraft

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In the United States, a string of fatal accidents near thunderstorms downed passenger airliners during final descent and initial ascent in New York (1975), New Orleans (1982), and Dallas-Fort Worth (1985). Air Force One landed five minutes before one of the strongest downbursts ever recorded in the Washington, D.C. area at Andrews Air Force Base, with President Ronald Reagan aboard.<ref>National Weather Service Forecast Office, Riverton, Wyoming. Downburst. Retrieved on 2006-10-22.</ref> Ultimately, the culprit in these disasters was deemed to be wind shear. Strong low-level outflow from thunderstorms causes rapid changes in the wind character just above ground level. Initially, this outflow causes a headwind which causes the plane's acceleration, and causes a pilot to reduce engine power if they are unaware of the wind shear. As the plane passes into the region of the downdraft, suddenly dropping the planes airspeed due to the now reduced engine power, and the previous headwind becomes a tailwind. This can cause a plane to crash if close enough to ground level when the headwind switches to a tailwind. A pilot can correct for this wind change by increasing engine power, if there is enough time to react.<ref>NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.</ref>

As the result of the accidents in the 1970's and 1980's, the Federal Aviation Agency mandated in 1988 that all commercial aircraft were to have onboard windshear detection systems by 1993. Three airlines, United Airlines, Continental Airlines, and Northwest Airlines received extentions until the end of 1995 to install wind shear detection systems in their aircraft. The results of these efforts was immediate. 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 the mandated onboard detection, as well as the addition of Doppler radar units on the ground.

[edit] Effects on sound propagation

Vertical wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, sound waves being "bent" by a refraction phenomenon. This effect is important in understanding roadway noise and aircraft noise propagation and can be significant in the design of noise barriers.<ref>Washington State Department of Transportation. GROUND PLANE WIND SHEAR INTERACTION ON ACOUSTIC TRANSMISSION. Retrieved on 2006-10-21.</ref>

[edit] Effects on tropical cyclones

Tropical cyclones require low values of vertical wind shear so that their warm core can remain stacked above their surface circulation center, and further development as a warm-core cyclone can continue. Strongly sheared tropical cyclones tend to either level in intensity or dissipate due to the breakdown of their internal heat engine. <ref>University of Illinois. Hurricanes. Retrieved 2006-10-21.</ref>

[edit] Effects on thunderstorms and severe weather

Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such as a way as to maintain the thunderstorm for a longer period of time by separating the storm's inflow from its rain-cooled outflow. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which quickly cuts off its inflow of relatively warm, moist air and subsequently kills the thunderstorm.<ref>University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.</ref>