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Encyclopedia > Wind shear
Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.

Airplane pilots generally regard significant windshear to be a change in airspeed of 15 knots (7.7 m/s) and/or a change in azimuth of 30 degrees or greater per thousand feet (300 m) of altitude change. Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. It is also a key factor in severe thunderstorms. An additional hazard is turbulence often associated with wind shear. Some knots: 1. ... Azimuth is the horizontal component of a direction (compass direction), measured around the horizon, from the north toward the east (i. ... Altitude is the elevation of an object from a known level or datum. ... A rolling thundercloud over Enschede, The Netherlands. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ...

Sound is significantly distorted by wind shear, becoming significantly bent downward where temperature inversions exist within the atmosphere. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into living long life cycles and producing severe weather. The meteorological concept of thermal wind deals with how differences in wind with height are dependent on horizontal temperature differences. Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ... NOAA scientists observe severe weather using a mobile doppler radar and a helicopter (in the distance) Severe weather phenomena are weather conditions that are hazardous. ... The thermal wind is not actually a wind, but a wind difference between two pressure levels and , with . ...

Where and when it is strongly observed GA_googleFillSlot("encyclopedia_square");

Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.

Weather situations where shear is observed include: Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... A photograph of the surface curl soon after an intense microburst impacted the surface A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage. ...

• 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. 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, turbulent eddies known as rotors associated with lee waves may form, which are dangerous to ascending and descending aircraft.[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 spreading 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.

A knot is a unit of speed abbreviated kt or kn. ... The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. ... Windward is the direction from which the wind is blowing at the time in question. ... Categories: Aeronautics | Meteorology | Stub ... This article or section does not cite its references or sources. ... For other uses, see Friction (disambiguation). ... The curl phase soon after an intense microburst impacted the surface Downburst damages in a straight line. ...

Planetary boundary layer

See also: Ekman layer, Ekman spiral, Planetary boundary layer, and Surface layer

The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars, when compared to the winds in frictionless flow well above the Earth's surface.[2] This layer where friction slows and changes the wind is known as the planetary boundary layer, and is thickest during the day and thinnest at night. Daytime heating thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft due to insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer and thereby increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night. It has been suggested that this article or section be merged with Ekman spiral. ...  Ekman spiral effect. ... The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. ... The surface layer is the top layer in a body of water, such as an ocean. ... This article is about Earth as a planet. ... The word isobar derives from the two ancient Greek words, Î¹ÏƒÎ¿Ï‚ (isos), meaning equal, and Î²Î±ÏÎ¿Ï‚ (baros), meaning weight. In meteorology, thermodynamics, and similar science (and engineering), an isobar is a contour line of equal or constant pressure on a graph, plot, or map. ... Not to be confused with insulation. ...

Effects on flight

Gliding

Glider ground launch due to wind shear.

When landing, wind shear is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it.[4]

Wind shear is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long wingspan, which exposes them to a greater wind speed difference for a given bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident.[4][5] The distance AB is the wing span of this Aer Lingus Airbus A320. ... Flight dynamics is the study of orientation of air and space vehicles and how to control the critical flight parameters, typically named pitch, roll and yaw. ...

Soaring

The albatross is an expert in dynamic soaring using wind shear.

Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds including albatrosses. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.[6] By then turning downwind, and diving through the wind gradient, they can also gain energy.[7] Original photo by Uwe Kils larger image on http://www. ... Original photo by Uwe Kils larger image on http://www. ... Genera Diomedea Thalassarche Phoebastria Phoebetria Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes (the tubenoses). ... Dynamic soaring is a flying technique used to gain kinetic energy without effort by repeatedly crossing the boundary between air masses of significantly different horizontal velocity. ... Dynamic soaring is a flying technique used to gain kinetic energy without effort by repeatedly crossing the boundary between air masses of significantly different horizontal velocity. ... This is a list of types of soaring birds, which are birds that can maintain flight without wing flapping, using rising air currents. ... Genera Diomedea Thalassarche Phoebastria Phoebetria Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes (the tubenoses). ...

Impact on passenger aircraft

In the United States, a string of fatal accidents near thunderstorms downed passenger airliners during final descent and initial ascent, including Eastern Air Lines Flight 66 in New York (1975), Pan Am Flight 759 in New Orleans (1982), and Delta Air Lines Flight 191 at Dallas-Fort Worth (1985). The common cause in these air disasters was low level windshear. Eastern Air Lines Flight 66, a Boeing 727-225 with registration number N8845E, was operating New Orleans-Moisant-New York Kennedy on the afternoon of June 24, 1975. ... Pan Am Flight 759 soon after crashing in Kenner, Louisiana On July 9, 1982, Pan American World Airways, Inc. ... This article is about a crash in 1985. ...

Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. 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 onboard.[8] Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed, and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed, descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact.[9] For the current aircraft, see Boeing VC-25. ... For other uses, see Washington, D.C. (disambiguation). ... â€œReaganâ€ redirects here. ...

Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.

Architecture

Main article: Wind Engineering.
The design of buildings must account for wind loads, and these are affected by wind shear. For engineering purposes, a power law wind speed profile may be defined as follows:[10][11] Wind Engineering is a field of engineering devoted to the analysis of wind effects on the natural and built environment. ...

$v_z = v_g cdot left( frac {z} {z_g} right)^ frac {1} {alpha}, 0 < z < z_g$

where:

$v_z$ = speed of the wind at height $z$
= gradient wind at gradient height $z_g$
$alpha$ = exponential coefficient

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. The effect of wind shear can be factored into the selection of twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.[12] For either of the songs named Sailing, see Sailing (song). ... Traditional wooden cutter beating. ... mizzen mast, mainmast and foremast Grand Turk The mast of a sailing ship is a tall vertical pole which supports the sails. ... The USS Monongahela (1862), an exemplar of the 19th century sailmakers craft A sailmaker is a person who makes and repairs sails for sailboats, typically working on shore in a sail loft. ... Sail twist is a phenomenon in sailing where the head of the sail is at a different point of sail from the foot of the sail. ... The lift force, lifting force or simply lift is a mechanical force generated by a solid object moving through a fluid. ... A sailor is a member of the crew of a ship or boat. ... A boom vang is an item of rigging in a sail-powered vessel (usually small ones, but it is sometimes found on larger ones as well). ...

Sound propagation

Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. Shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. The result of these differing sound levels is key in (noise pollution) considerations, for example from roadway noise and aircraft noise, and must be considered in the design of noise barriers.[13] This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways as well as noise barriers.[14] Sound is a vibration that travels through an elastic medium as a wave. ... Thunder is the sound made by lightning. ... A gunshot is the discharge of a firearm, and the sound effect thereof; the term can also refer to a wound caused by such a discharge. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... Noise pollution (or environmental noise in technical venues) is displeasing human or machine created sound that disrupts the environment. ... Roadway noise is the most prevalent form of environmental noise. ... Aircraft noise is defined as sound produced by any aircraft on run-up, taxiing, take off, over flying or landing. ... The sound tube in Melbourne, Australia, designed to reduce roadway noise without detracting from the areas aesthetics. ... Noise pollution (or environmental noise in technical venues) is displeasing human or machine created sound that disrupts the environment. ... The sound tube in Melbourne, Australia, designed to reduce roadway noise without detracting from the areas aesthetics. ...

Hodograph plot of wind vectors at various heights in the troposphere. Meteorologists can use this plot to evaluate vertical wind shear in weather forecasting. (Source: NOAA)

For sound propagation, the exponential variation of wind speed with height can be defined as follows:[19]

$U(h) = U(0) h ^ zeta$
$frac {dU} {dH} = zeta frac {U(h)} {h}$

where:

$U(h)$ = speed of the wind at height $h$, and $U(0)$ is a constant
$zeta$ = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52
$frac {dU} {dH}$ = expected wind gradient at height h

In the 1862 American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle,[20] because they could not hear the sounds of battle only six miles downwind.[21] Combatants United States of America (Union) Confederate States of America (Confederacy) Commanders Abraham Lincoln, Ulysses S. Grant Jefferson Davis, Robert E. Lee Strength 2,200,000 1,064,000 Casualties 110,000 killed in action, 360,000 total dead, 275,200 wounded 93,000 killed in action, 258,000 total... The Battle of Iuka was a United States Civil War battle fought from October 3 - September 19, 1862 in Iuka, Mississippi. ...

Wind turbines

Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this in turn affects the turbine operation.[22] The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.[23] The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas.[24] Horizontal-axis wind turbine, the Enercon model E-66 wind energy converter, in Germany. ...

For wind turbine engineering, an exponential variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as:[22]

$v_w(h) = v_{10} cdot left( frac {h} {h_{10}} right)^ a$

where:

$v_w(h)$ = velocity of the wind at height, h [m/s]
$v_{10}$ = velocity of the wind at height, h10 = 10 meters [m/s]
$a$ = Hellman exponent

Effects on tropical cyclones

Main article: Tropical cyclogenesis

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.[25] Global Tropical Cyclone Tracks Cyclogenesis is the technical term describing the development or strengthening of a surface low pressure system, or cyclone, in the atmosphere. ... Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004 Hurricane and Typhoon redirect here. ... This article is about the meteorological phenomenon. ...

Strong wind shear in the high troposphere forms the anvil-shaped top characteristic of the mature cumulonimbus cloud. The anvil may stretch several kilometers downwind in the direction of the shear.[26]

Effects on thunderstorms and severe weather

Main article: Severe thunderstorm

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. 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.[27] A severe thunderstorm is a thunderstorm with winds 58 mph or greater, 3/4 inch or larger hail, or tornadoes. ... A tornado in central Oklahoma. ... A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. ...

Thermal wind

Main article: Thermal wind

The thermal wind equation is

$f mathbf{v}_T = mathbf{k} times nabla ( phi_1 - phi_0 )$,

where the φx are geopotential height fields with φ1 > φ0, f is the Coriolis parameter, and $mathbf{k}$ is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero there, the equation reduces to stating that $nabla ( phi_1 - phi_0 )$ is small. Geopotential height is a vertical coordinate referenced to Earths mean sea level â€” an adjustment to geometric height (elevation above mean sea level) using the variation of gravity with latitude and elevation. ... The Coriolis frequency, f, is equal to twice the rotation rate of the Earth multiplied by the sine of the latitude. ... In mathematics, a unit vector in a normed vector space is a vector (often a spatial vector) whose length, (or magnitude) is 1. ... In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i. ... A noontime scene from the Philippines on a day when the Sun is almost directly overhead. ...

References

1. ^ National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.
2. ^ Glossary of Meteorology. E. Retrieved on 2007-06-03.
3. ^ (2003) Glider Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration, p. 7-16. FAA-8083-13_GFH.
4. ^ a b Piggott, Derek (1997). Gliding: a Handbook on Soaring Flight. Knauff & Grove, pp. 85-86, 130-132. ISBN 9780960567645.
5. ^ Knauff, Thomas (1984). Glider Basics from First Flight to Solo. Thomas Knauff. ISBN 0960567631.
6. ^ Alexander, R. (2002). Principles of Animal Locomotion. Princeton: Princeton University Press, p. 206. ISBN 0691086788.
7. ^ Alerstam, Thomas (1990). Bird Migration. Cambridge: Cambridge University Press, 275. ISBN 0521448220.
8. ^ National Weather Service Forecast Office, Riverton, Wyoming. http://www.crh.noaa.gov/riw/severe/microburst.php Downburst.] Retrieved on 2006-10-22.
9. ^ NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.
10. ^ Crawley, Stanley (1993). Steel Buildings. New York: Wiley, p. 272. ISBN 0471842982.
11. ^ Gupta, Ajaya (1993). Guidelines for Design of Low-Rise Buildings Subjected to Lateral Forces. Boca Raton: CRC Press, p. 49. ISBN 0849389690.
12. ^ Garrett, Ross (1996). The Symmetry of Sailing. Dobbs Ferry: Sheridan House, pp. 97-99. ISBN 1574090003.
13. ^ Foss, Rene N. (June 1978). "Ground Plane Wind Shear Interaction on Acoustic Transmission". WA-RD 033.1. Washington State Department of Transportation.. Retrieved on 2007-05-30.
14. ^ C. Michael Hogan, Analysis of highway noise, Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands ISSN 0049-6979
15. ^ Everest, F. (2001). The Master Handbook of Acoustics. New York: McGraw-Hill, pp. 262-263. ISBN 0071360972.
16. ^ Uman, Martin (1984). Lightning. New York: Dover Publications. ISBN 0486645754.
17. ^ Volland, Hans (1995). Handbook of Atmospheric Electrodynamics. Boca Raton: CRC Press, p. 22. ISBN 0849386470.
18. ^ Singal, S. (2005). Noise Pollution and Control Strategy. Alpha Science International, Ltd, p. 7. ISBN 1842652370. “It may be seen that refraction effects occur only because there is a wind gradient and it is not due to the result of sound being convected along by the wind.”
19. ^ Bies, David (2003). Engineering Noise Control; Theory and Practice. London: Spon Press, p. 235. ISBN 0415267137. “As wind speed generally increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels.”
20. ^ Cornwall, Sir (1996). Grant as Military Commander. Barnes & Noble Inc. ISBN 1566199131 pages = p. 92.
21. ^ Cozzens, Peter (2006). The Darkest Days of the War: the Battles of Iuka and Corinth. Chapel Hill: The University of North Carolina Press. ISBN 0807857831.
22. ^ a b Heier, Siegfried (2005). Grid Integration of Wind Energy Conversion Systems. Chichester: John Wiley & Sons, p. 45. ISBN 0470868996.
23. ^ Harrison, Robert (2001). Large Wind Turbines. Chichester: John Wiley & Sons, p. 30. ISBN 0471494569.
24. ^ Lubosny, Zbigniew (2003). Wind Turbine Operation in Electric Power Systems: Advanced Modeling. Berlin: Springer, p. 17. ISBN 354040340X.
25. ^ University of Illinois. Hurricanes. Retrieved 2006-10-21.
26. ^ Mcilveen, J. (1992). Fundamentals of Weather and Climate. London: Chapman & Hall, p. 339. ISBN 0412411601.
27. ^ University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.

Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 154th day of the year (155th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 150th day of the year (151st in leap years) in the Gregorian calendar. ...

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