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[[Image:Cirrus clouds2.jpg|thumb|right|[[Cirrus clouds|Cirrus]] uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.]]
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'''Wind shear''', sometimes referred to as '''windshear''' or [[wind gradient]], is a difference in [[wind]] [[wind speed|speed]] and [[wind direction|direction]] over a relatively short distance in the [[Earth's atmosphere|atmosphere]]. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across [[Weather front|fronts]] and near the coast, and vertical shear typically near the surface, though also at higher levels in the atmosphere near upper level jets and frontal zones aloft.
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Wind shear itself is a [[microscale meteorology|microscale meteorological]] phenomenon occurring over a very small distance, but it can be associated with [[mesoscale meteorology|mesoscale]] or [[synoptic scale]] weather features such as squall lines and cold fronts. It is commonly observed near [[microburst]]s and [[downburst]]s caused by [[thunderstorm]]s, fronts, areas of locally higher low level winds referred to as low level jets, near [[mountain]]s, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft due to its effects on control of the aircraft, and it has been a sole or contributing cause of many aircraft accidents.
'''MathML'''
:<math forcemathmode="mathml">E=mc^2</math>


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]].<ref name="IP">{{cite web|url=http://www.tpub.com/weather3/6-15.htm|title=LOW-LEVEL WIND SHEAR.] Retrieved on 2007-11-25|last=Publishing|first=Integrated}}</ref>
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==Definition==
'''source'''
Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 [[knot (unit)|knots]] (15&nbsp;m/s) for light aircraft, and near 45 [[knot (unit)|knots]] (22&nbsp;m/s) for airliners at flight altitude.<ref>[[FAA]] [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/b3fb7dd636fb870b862569ba0068920b/$FILE/AC00-54.pdf FAA Advisory Circular Pilot Wind Shear Guide.] Retrieved on 2007-12-15.</ref> Vertical speed changes greater than 4.9&nbsp;knots (2.5&nbsp;m/s) also qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of 30 knots, Ibid.).<ref>{{cite web|title=NASA|url=http://oea.larc.nasa.gov/PAIS/Concept2Reality/wind_shear.html Wind Shear.|accessdate=2007-10-09}}</ref> The rationale for this additional caution includes (1) microburst intensity can double in a minute or less, (2) the winds can shift to excessive cross wind, (3) 40-50 knots is the threshold for survivability at some stages of low-altitude operations (Ibid.), and (4) several of the historical wind shear accidents involved 35-45 knot microbursts. Wind shear is also a key factor in the creation of severe thunderstorms. The additional hazard of [[turbulence]] is often associated with wind shear.
:<math forcemathmode="source">E=mc^2</math> -->


==Where and when it is strongly observed==
<span style="color: red">Follow this [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering link] to change your Math rendering settings.</span> You can also add a [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering-skin Custom CSS] to force the MathML/SVG rendering or select different font families. See [https://www.mediawiki.org/wiki/Extension:Math#CSS_for_the_MathML_with_SVG_fallback_mode these examples].
{{See also|Jet stream}}
[[Image:Microburstnasa.JPG|thumb|right|250px|[[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:
*[[Weather fronts]]. Significant shear is observed when the temperature difference across the front is 5&nbsp;°C (9&nbsp;°F) or more, and the front moves at 30 [[Knot (unit)|knots]] 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. Vertical wind shear above warm fronts is more of an aviation concern than near and behind cold fronts due to their greater duration.<ref name="IP"/>
*Upper-level jet streams. Associated with upper level jet streams is a phenomenon known as [[clear air turbulence]] (CAT), caused by vertical and horizontal wind shear connected to the wind gradient at the edge of the jet streams.<ref>{{cite web|title=BBC|url=http://www.bbc.co.uk/weather/features/understanding/jetstreams_uk.shtml Jet Streams in the UK.|accessdate=2008-05-08}} {{Dead link|date=April 2012|bot=H3llBot}}</ref> The CAT is strongest on the cold air side of the jet,<ref>M. P. de Villiers and J. van Heerden. [http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=73339 Clear air turbulence over South Africa.] Retrieved on 2008-05-08.</ref> usually next to or just below the axis of the jet.<ref>CLARK T. L., HALL W. D., KERR R. M., MIDDLETON D., RADKE L., RALPH F. M., NEIMAN P. J., LEVINSON D. [http://cat.inist.fr/?aModele=afficheN&cpsidt=1345004 Origins of aircraft-damaging clear-air turbulence during the 9 December 1992 Colorado downslope windstorm : Numerical simulations and comparison with observations.] Retrieved on 2008-05-08.</ref>
*Low-level jet streams. When a nocturnal low-level jet forms overnight above the Earth's surface 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 since it is not due to nearby thunderstorms.<ref name="IP"/>
*Mountains. When winds blow over a mountain, vertical shear is observed on the [[Windward and leeward|lee]] side. If the flow is strong enough, turbulent [[Eddy (fluid dynamics)|eddies]] known as "rotors" associated with [[lee waves]] may form, which are dangerous to ascending and descending aircraft.<ref>National Center for Atmospheric Research. [http://www.ucar.edu/communications/quarterly/spring06/trex.jsp T-REX: Catching the Sierra’s waves and rotors] Retrieved on 2006-10-21.</ref>
*[[Inversion (meteorology)|Inversions]]. When on a clear and calm night, a radiation inversion is formed near the ground, the [[friction]] does not affect wind above the top of the inversion layer. The change in wind can be 90&nbsp;degrees in direction and 40&nbsp;kt in speed. Even a nocturnal (overnight) low level jet can sometimes be observed. It tends to be strongest towards sunrise. Density differences cause additional problems to aviation.<ref name="IP"/>
*[[Downburst]]s. When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three dimensional boundary. The stronger the [[outflow boundary]] is, the stronger the resultant vertical wind shear will become.<ref>[[Ted Fujita|Fujita, T.T.]] (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.</ref>


==Horizontal component==
==Demos==


===Weather fronts===
Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]:
{{main|Weather fronts}}
Weather fronts are boundaries between two masses of air of different [[density|densities]], or different temperature and moisture properties, which normally are [[convergence zone]]s in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in [[temperature]] and may also differ in [[humidity]]. Wind shear in the horizontal occurs near these boundaries.
[[Cold front]]s feature narrow bands of [[thunderstorm]]s and [[severe weather]], and may be preceded by [[squall line]]s and [[dry line]]s. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomes [[stationary front|stationary]], it can degenerate into a line which separates regions of differing wind speed, known as a [[shear line (meteorology)|shear line]], though the wind direction across the front normally remains constant. In the [[tropics]], [[tropical wave]]s move from east to west across the [[Atlantic basin|Atlantic]] and eastern [[Pacific basin]]s. Directional and speed shear can occur across the axis of stronger tropical waves, as northerly winds precede the wave axis and southeast winds are seen behind the wave axis. Horizontal wind shear can also occur along local land breeze and [[sea breeze]] boundaries.<ref name="DR">David M. Roth. Hydrometeorological Prediction Center. [http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf Unified Surface Analysis Manual.] Retrieved on 2006-10-22.</ref>


===Near coastlines===
The magnitude of winds offshore are nearly double the wind speed observed onshore. This is attributed to the differences in friction between land masses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes change the wind on shore during daylight hours.<ref>Franklin B. Schwing and Jackson O. Blanton. [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0485(1984)014%3C0193%3ATUOLAS%3E2.0.CO%3B2&ct=1 The Use of Land and Sea Based Wind Data in a Simple Circulation Model.] Retrieved on 2007-10-03.</ref>


==Vertical component==
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** Orca: There is ongoing work, but no support at all at the moment [[File:Orca-mathml-example-1.wav|thumb|Orca-mathml-example-1]], [[File:Orca-mathml-example-2.wav|thumb|Orca-mathml-example-2]], [[File:Orca-mathml-example-3.wav|thumb|Orca-mathml-example-3]], [[File:Orca-mathml-example-4.wav|thumb|Orca-mathml-example-4]], [[File:Orca-mathml-example-5.wav|thumb|Orca-mathml-example-5]], [[File:Orca-mathml-example-6.wav|thumb|Orca-mathml-example-6]], [[File:Orca-mathml-example-7.wav|thumb|Orca-mathml-example-7]].
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===Thermal wind===
==Test pages ==
{{main|Thermal wind}}
Thermal wind is a meteorological term not referring to an actual [[wind]], but a ''difference'' in the [[geostrophic wind]] between two [[pressure level]]s <math>p_1</math> and <math>p_0</math>, with <math>p_1< p_0</math>; in essence, wind shear. It is only present in an atmosphere with horizontal changes in [[temperature]] (or in an ocean with horizontal gradients of [[density]]), i.e. [[baroclinicity]]. In a [[barotropic]] atmosphere, where temperature is uniform, the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low (and high) temperature in the same manner as the [[geostrophic wind]] flows around areas of [[Low pressure area|low]] (and [[High pressure area|high]]) [[pressure]].<ref name="Holton">James R. Holton (2004). ''An Introduction to Dynamic Meteorology.'' ISBN 0-12-354015-1</ref>


The ''thermal wind equation'' is
To test the '''MathML''', '''PNG''', and '''source''' rendering modes, please go to one of the following test pages:
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:<math>f \mathbf{v}_T = \mathbf{k} \times \nabla ( \phi_1 - \phi_0 )</math>,
*[[Inputtypes|Inputtypes (private Wikis only)]]
 
*[[Url2Image|Url2Image (private Wikis only)]]
where the <math>\phi_x</math> are [[geopotential height]] fields with <math>\phi_1 > \phi_0</math>, <math>f</math> is the [[Coriolis parameter]], and <math>\mathbf{k}</math> is the upward-pointing [[unit vector]] in the [[vertical direction]]. The thermal wind equation does not determine the wind in the [[tropics]]. Since <math>f</math> is small or zero, such as near the equator, the equation reduces to stating that <math>\nabla ( \phi_1 - \phi_0 )</math> is small.<ref name="Holton"/>
==Bug reporting==
 
If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de .
This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to the [[tropopause]] which is (even though other factors are also important) the result of the temperature contrast between equator and pole.
 
===Effects on tropical cyclones===
[[Image:Thunderhead.anvil.jpg|thumb|right|Strong wind shear in the high [[troposphere]] forms the anvil-shaped top of this mature [[cumulonimbus]] cloud, or thunderstorm.<ref>{{cite book | last = Mcilveen | first = J. | title = Fundamentals of Weather and Climate | publisher = Chapman & Hall | location = London | year = 1992 | isbn = 0-412-41160-1 | pages = 339}}</ref>]]
{{main|Tropical cyclogenesis}}
[[Tropical cyclone]]s are basically [[heat engine]]s that are fueled by the [[temperature gradient]] between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting strengthening. Vertical wind shear tears up the "machinery" of the heat engine causing it to break down. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low level center.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/grow/home.rxml Hurricanes.] Retrieved 2006-10-21.</ref>
 
===Effects on thunderstorms and severe weather===
{{main|Severe thunderstorm}}
Severe thunderstorms, which can spawn [[tornado]]es and hailstorms, require wind shear to organize the storm in such a way as to maintain the [[thunderstorm]] for a longer period of time. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and kills the thunderstorm.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/wind/home.rxml Vertical Wind Shear] Retrieved on 2006-10-21.</ref>
 
{{clear}}
 
===Planetary boundary layer===
[[File:PBLimage.jpg|thumb|right|Depiction of where the planetary boundary layer lies on a sunny day]]
{{see also|Ekman layer|Ekman spiral|Planetary boundary layer|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 (lines of equal pressure), when compared to the winds in frictionless flow well above the Earth's surface. This layer where friction slows and changes the wind is known as the planetary boundary layer, sometimes the Ekman layer, and it 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 by calming the surface wind which increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/browse?s=e&p=14 E.] Retrieved on 2007-06-03.</ref>
 
====Effects on flight====
{{main|Aeronautics|Gliding}}
 
=====Gliding=====
[[Image:FAA-8083-13 Fig 7-20.PNG|thumb|right|Glider ground launch affected by wind shear.]]
In gliding, wind gradients just above the surface affect the takeoff and landing phases of flight of a [[Glider (sailplane)|glider]].
Wind gradient can have a noticeable effect on [[ground launch]]es, also known as winch launches or wire launches. If the wind gradient is significant or sudden,
or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding
the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the
gradient.<ref>{{cite book
| title = Glider Flying Handbook
| year = 2003
| publisher = U.S. Federal Aviation Administration
| location = U.S. Government Printing Office, Washington D.C.
| id = FAA-8083-13_GFH
| pages = 7–16
| url=http://www.faa.gov/library/manuals/aircraft/glider_handbook/
}}</ref>
 
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.<ref name=Piggott>{{cite book | last = Piggott | first = Derek | title = Gliding: a Handbook on Soaring Flight | publisher = Knauff & Grove | year = 1997 | isbn = 978-0-9605676-4-5 | pages = 85–86, 130–132}}</ref>
 
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 [[Roll (flight)|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.<ref name=Piggott/><ref>{{cite book | last = Knauff | first = Thomas | title = Glider Basics from First Flight to Solo | publisher = Thomas Knauff | year = 1984 | isbn = 0-9605676-3-1 }}</ref>
 
=====Parachuting=====
Wind shear or wind gradients are a threat to parachutists, particularly to [[BASE jumping]] and [[wingsuit flying]]. Skydivers have been pushed off of their course by sudden shifts in wind direction and speed, and have collided with bridges, cliffsides, trees, other skydivers, the ground, and other obstacles.{{Citation needed|date=February 2012}} Skydivers routinely make adjustments to the position of their open canopies to compensate for changes in direction while making landings to prevent accidents such as canopy collisions and canopy inversion.
 
=====Soaring=====
Soaring related to wind shear, also called [[dynamic soaring]], is a technique used by [[soaring birds]] like [[albatross]]es, who can maintain flight without wing flapping. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.<ref>{{cite book | last = Alexander | first = R. | title = Principles of Animal Locomotion | publisher = Princeton University Press | location = Princeton | year = 2002 | pages = 206 | isbn = 0-691-08678-8 }}</ref> By then turning downwind, and diving through the wind gradient, they can also gain energy.<ref>{{cite book | last = Alerstam | first = Thomas | title = Bird Migration | publisher = Cambridge University Press | location = Cambridge | year = 1990 | pages = 275 | isbn = 0-521-44822-0 }}</ref> It has also been used by [[gliding|glider pilots]] on rare occasions.
 
Wind shear can also create [[lee wave|wave]]. This occurs when an [[atmospheric inversion]] separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused by [[thermal]]s coming up from below, it will create significant shear waves that can be used for soaring.<ref>{{cite book
|last = Eckey
|first = Bernard
|title = Advanced Soaring Made Easy
|publisher = Eqip Verbung & Verlag GmbH
|year = 2007
|ISBN = 3-9808838-2-5
}}</ref>
 
=====Impact on passenger aircraft=====
[[Image:Windshearaircraftnasa.gif|thumb|right|200 px|Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.]]
Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. 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 lift generated by the wings, and 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.
[[File:Delta 191 wreckage.jpg|thumb|left|Wreckage of [[Delta Air Lines Flight 191]] tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene.]]
As the result of the accidents in the 1970s and 1980s, most notably following the 1985 crash of [[Delta Air Lines Flight 191]], in 1988 the U.S. [[Federal Aviation Administration]] mandated that all commercial aircraft have [[Airborne wind shear detection and alert system|on-board wind shear detection systems]] by 1993. 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.<ref>{{cite web |author=National Aeronautics and Space Administration, [[Langley Research Center]] |url=http://web.archive.org/web/20100329221032/http://oea.larc.nasa.gov/PAIS/Windshear.html|title=Making the Skies Safer From Windshear|date=June 1992|accessdate=2012-11-16}}</ref> Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler [[weather radar]] units on the ground ([[NEXRAD]]).{{fact|date=February 2014}} The installation of high-resolution [[Terminal Doppler Weather Radar]] stations at many U.S. airports that are commonly affected by wind shear has further aided the ability of pilots and ground controllers to avoid wind shear conditions.<ref name=tdwr-nws>{{cite web |url=http://www.erh.noaa.gov/gsp/tdwr/info/specs.html
|title=Terminal Doppler Weather Radar Information |accessdate=4 August 2009 |publisher=National Weather Service }}</ref>
 
====Sailing====
Wind shear affects [[sailboats]] in motion by presenting a different wind speed and direction at different heights along the [[mast (sailing)|mast]]. The effect of low level wind shear can be factored into the selection of [[sail 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 low level wind shear, for example using a [[boom vang]].<ref>{{cite book | last = Garrett | first = Ross | title = The Symmetry of Sailing | publisher = Sheridan House | location = Dobbs Ferry | year = 1996 | pages = 97–99 | isbn = 1-57409-000-3 }}</ref>
 
====Sound propagation====
{{see also|Speed of sound}}
Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by [[refraction]] phenomenon. The audibility of sounds from distant sources, such as [[thunder]] or [[gunshot]]s, is very dependent on the amount of shear. 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 barrier]]s.<ref>{{cite journal
| publisher = Washington State Department of Transportation.
| url = http://www.wsdot.wa.gov/Research/Reports/000/033.1.htm
| title = Ground Plane Wind Shear Interaction on Acoustic Transmission
| accessdate = 2007-05-30
| version = WA-RD 033.1
| author = Foss, Rene N.
| date = June 1978
}}</ref> 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 barrier]]s.<ref>{{cite journal|url=http://www.springerlink.com/content/x1707075n815g604/|title= 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}}</ref>
 
[[Image:Hodographe NOAA.PNG|thumb|[[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]])]]
 
The [[speed of sound]] varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is [[refraction|refracted]] upward, away from listeners on the ground, creating an [[acoustic shadow]] at some distance from the source.<ref>{{cite book | last = Everest | first = F. | title = The Master Handbook of Acoustics | publisher = McGraw-Hill | location = New York | year = 2001 | isbn = 0-07-136097-2 | pages = 262–263 }}</ref> In the 1862, during the [[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,<ref>{{cite book | last = Cornwall | first = Sir | title = Grant as Military Commander | publisher = Barnes & Noble Inc | year = 1996 | isbn = 1-56619-913-1 | page = 92}}</ref> because they could not hear the sounds of battle only six miles downwind.<ref>{{cite book | last = Cozzens | first = Peter | title = The Darkest Days of the War: the Battles of Iuka and Corinth | publisher = The University of North Carolina Press | location = Chapel Hill | year = 2006 | isbn = 0-8078-5783-1 }}</ref>
 
====Effects on architecture====
[[Wind engineering]] is a field of [[engineering]] devoted to the analysis of [[wind]] effects on the natural and [[built environment]]. It includes strong winds which may cause discomfort as well as extreme winds such as [[tornadoes]], [[hurricanes]] and storms which may cause widespread destruction. Wind engineering draws upon [[meteorology]], [[aerodynamics]] and a number of specialist [[engineering]] disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering.<ref>Professor John Twidell. [http://www.multi-science.co.uk/windeng.htm Wind Engineering.] Retrieved on 2007-11-25.</ref>
 
<!-- Deleted image removed: [[Image:EU Windmill.jpg|thumb|right|[[Wind turbine]]s in [[Douglas]], [[Central Scotland]]]] -->
[[Wind turbine]]s 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.<ref name=Heier>{{cite book | last = Heier | first = Siegfried | title = Grid Integration of Wind Energy Conversion Systems | publisher = John Wiley & Sons | location = Chichester | year = 2005 | isbn = 0-470-86899-6 | pages = 45}}</ref> This low level wind shear can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.<ref>{{cite book | last = Harrison | first = Robert | title = Large Wind Turbines | publisher = John Wiley & Sons | location = Chichester | year = 2001 | isbn = 0-471-49456-9 | pages = 30}}</ref> The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.<ref name=Lubosny>{{cite book | last = Lubosny | first = Zbigniew | title = Wind Turbine Operation in Electric Power Systems: Advanced Modeling | publisher = Springer | location = Berlin | year = 2003 | isbn = 3-540-40340-X | pages = 17}}</ref>
 
==See also==
*[[Air safety]]
*[[Low level windshear alert system]]
*[[Sailing]]
 
==References==
{{reflist|2}}
 
==External links==
*[http://education.arm.gov/nsdl/Library/glossary.shtml#Wind_shear National Science Digital Library - Wind shear]
 
{{good article}}
 
{{DEFAULTSORT:Wind Shear}}
[[Category:Wind]]
[[Category:Weather hazards to aircraft]]
[[Category:Microscale meteorology]]

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