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{{short description|Difference in wind speed or direction over a short distance}}
{{other uses}}

[[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]]

'''Wind shear''' ({{IPAc-en|ʃ|ɪər}}; also written '''windshear'''), sometimes referred to as [[wind gradient]], is a difference in [[wind]] [[wind speed|speed]] and/or [[wind direction|direction]] over a relatively short distance in the [[Earth's atmosphere|atmosphere]]. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.<ref name="Shear">{{cite web|url=http://www.hurricanescience.org/glossary/?letter=V#glossaryword373|title=Vertical wind shear. Retrieved on 2015-10-24}}</ref>

Wind shear 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 significant effects on the control of an aircraft, and it has been the only or a contributing cause of many aircraft accidents.

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. 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|access-date=2007-11-25|publisher=Integrated Publishing}}</ref>
[[File:Downdraft Wind shear-2.jpg|thumb|right|Down draft winds with associated [[virga]] allow these [[cloud]]s in the eastern sky at civil [[twilight]] to mimic [[aurora borealis]] in the [[Mojave Desert]].]]

==Definition==
Wind shear refers to the variation of wind [[velocity]] over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of {{convert|30|knot|m/s|lk=in}} for light aircraft, and near {{convert|45|knot|m/s}} for airliners at flight altitude.<ref>[[FAA]] [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/b3fb7dd636fb870b862569ba0068920b/$FILE/AC00-54.pdf FAA Advisory Circular Pilot Wind Shear Guide.] {{Webarchive|url=https://web.archive.org/web/20061014025906/http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/b3fb7dd636fb870b862569ba0068920b/$FILE/AC00-54.pdf |date=2006-10-14 }} Retrieved on 2007-12-15.</ref> Vertical speed changes greater than {{convert|4.9|knot|m/s}} also qualify as significant wind shear for aircraft. Low-level wind shear can affect aircraft airspeed during takeoff and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of {{convert|30|knot|m/s|disp=sqbr}}).<ref>{{cite web|title=Wind Shear |url=http://oea.larc.nasa.gov/PAIS/Concept2Reality/wind_shear.html |publisher=NASA |access-date=2007-10-09 |url-status=dead |archive-url=https://web.archive.org/web/20071009144924/http://oea.larc.nasa.gov/PAIS/Concept2Reality/wind_shear.html |archive-date=2007-10-09 }}</ref> The rationale for this additional caution includes:{{cn|date=February 2024}}
*microburst intensity can double in a minute or less,
*the winds can shift to excessive crosswinds,
*{{convert|40|-|50|knot|m/s}} is the threshold for survivability at some stages of low-altitude operations, and
*several of the historical wind shear accidents involved {{convert|35|-|45|knot|m/s}} microbursts.
Wind shear is also a key factor in the formation of severe thunderstorms. The additional hazard of [[turbulence]] is often associated with wind shear.{{cn|date=February 2024}}

==Occurrence==
{{See also|Jet stream}}
[[Image:Microburstnasa.JPG|thumb|right|250px|[[Microburst]] schematic from NASA. The direction of travel is downward until the air current hits ground level, at which point it spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.{{cn|date=February 2024}}]]
Weather situations where shear is observed include:
*[[Weather fronts]]. Significant shear is observed when the temperature difference across the front is {{cvt|5|C-change|F-change|0}} or more, and the front moves at {{convert|30|knot|m/s}} or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and [[tropopause]], and can 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=Jet Streams in the UK |url=http://www.bbc.co.uk/weather/features/understanding/jetstreams_uk.shtml|publisher=BBC|access-date=2008-05-08 |url-status=dead |archive-url=https://web.archive.org/web/20080118235955/http://www.bbc.co.uk/weather/features/understanding/jetstreams_uk.shtml |archive-date=January 18, 2008 }}</ref> The CAT is strongest on the anticyclonic shear side of the jet,<ref>{{Cite journal |doi = 10.1175/1520-0493(1997)125<1251:PMOCAT>2.0.CO;2|issn = 1520-0493|year = 1997|volume = 125|issue = 6|pages = 1251–1259|title = Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows|journal = Monthly Weather Review|last1 = Knox|first1 = John A.|bibcode = 1997MWRv..125.1251K|doi-access = free}}</ref> usually next to or just below the axis of the jet.<ref>{{Cite journal |last1=Clark |first1=Terry L. |last2=Hall |first2=William D. |last3=Kerr |first3=Robert M. |last4=Middleton |first4=Don |last5=Radke |first5=Larry |last6=Ralph |first6=F. Martin |last7=Neiman |first7=Paul J. |last8=Levinson |first8=David |date=2000-04-01 |title=Origins of Aircraft-Damaging Clear-Air Turbulence during the 9 December 1992 Colorado Downslope Windstorm: Numerical Simulations and Comparison with Observations |journal=Journal of the Atmospheric Sciences |language=EN |volume=57 |issue=8 |pages=1105–1131 |doi=10.1175/1520-0469(2000)057<1105:OOADCA>2.0.CO;2 |issn=0022-4928|doi-access=free |bibcode=2000JAtS...57.1105C }}</ref>
*Low-level jet streams. When a nocturnal low-level jet forms overnight above 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 non-convective wind shear as it is not due to nearby thunderstorms.<ref name="IP"/>
*Mountains.<ref>National Center for Atmospheric Research. [http://www.ucar.edu/communications/quarterly/spring06/trex.jsp T-REX: Catching the Sierra’s waves and rotors] {{webarchive|url=https://web.archive.org/web/20061121051137/http://www.ucar.edu/communications/quarterly/spring06/trex.jsp |date=2006-11-21 }} 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 degrees in direction and {{convert|40|knot|m/s}} 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>{{Cite journal |last=Fujita |first=Tetsuya Theodore |date=1985 |title="The Downburst, Microburst, and Macroburst," by Fujita, T. Theodore, SMRP Research Paper Number 210, 1985. |url=https://swco-ir.tdl.org/handle/10605/262010 |access-date=2023-10-30 |website=swco-ir.tdl.org |language=en-US}}</ref>

==Horizontal component==

===Weather fronts===
{{main article|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 that 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 the 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===
[[File:20150226-08h40 to 09h45 - JBay - Wind shear.ogv|thumb|Wind shear along the coast with low-level clouds moving towards the east and higher-level clouds moving towards the south-west]]
The magnitude of winds offshore is nearly double the wind speed observed onshore. This is attributed to the differences in friction between landmasses 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==

===Thermal wind===
{{main article|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}} and {{math|''p''0}}, with {{math|''p''1 < ''p''0}}; 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

:<math>f \mathbf{v}_T = \mathbf{k} \times \nabla \left( \boldsymbol{\varphi}_1 - \boldsymbol{\varphi}_0 \right) \,,</math>

where the {{math|'''φ'''}} are [[geopotential height]] fields with {{math|'''φ'''1 > '''φ'''0}}, {{math|''f''}} is the [[Coriolis parameter]], and {{math|'''k'''}} 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''}} is small or zero, such as near the equator, the equation reduces to stating that {{math|∇('''φ'''1 − '''φ'''0)}} is small.<ref name="Holton"/>

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.{{cn|date=February 2024}}

=== Effects on tropical cyclones ===
[[File:Chaparral Supercell 2.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 = [https://archive.org/details/fundamentalsofwe0000mcil/page/339 339] | url = https://archive.org/details/fundamentalsofwe0000mcil/page/339 }}</ref>]]
{{See also|Tropical cyclogenesis#Weak vertical wind shear}}
[[Tropical cyclone]]s are, in essence, [[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 intensification. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low-level center.{{cn|date=February 2024}}

===Effects on thunderstorms and severe weather===
{{main article|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. 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 causes the thunderstorm to dissipate.<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 forces surface winds to slow and back counterclockwise near the surface of [[Earth]] blowing inward across isobars (lines of equal pressure) when compared to the winds in frictionless flow well above Earth's surface.<ref>{{cite web | title=AMS Glossary of Meteorology, Ekman layer | publisher = [[American Meteorological Association]] | url=http://glossary.ametsoc.org/wiki/Ekman_layer | access-date=2015-02-15}}</ref>{{failed verification|date=November 2020|reason=Cited source does not mention winds aloft.}} 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 are most emphasized at night.{{cn|date=February 2024}}

====Effects on flight====
{{main article|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 the 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.{{cn|date=February 2024}}

=====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 produce [[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 produce 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 = 978-3-9808838-2-5
}}</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.]]
[[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.]]

Windshear can be extremely dangerous for aircraft, especially during takeoff and landing. Sudden changes in wind velocity can cause rapid decreases in [[airspeed]], leading to the aircraft being unable to maintain altitude. Windshear has been responsible for several deadly accidents, including [[Eastern Air Lines Flight 66]], [[Pan Am Flight 759]], [[Delta Air Lines Flight 191]], and [[USAir Flight 1016]].{{cn|date=February 2024}}

Windshear can be detected using [[Doppler radar]].<ref name="NASA conference">{{cite book |title=Airborne Wind Shear Detection and Warning Systems |date=July 1990 |publisher=NASA Conference Publication 10050, Part 1 |page=214 |url=https://books.google.com/books?id=AdIhAQAAIAAJ |access-date=20 November 2022 |language=en}}</ref><ref name="Windshear Substantiating Data">{{cite book |title=Windshear Training Aid: 4.0 - Windshear Substantiating Data |date=February 1987 |publisher=Federal Aviation Administration |page=4.2–95 |url=https://books.google.com/books?id=p_w5AQAAMAAJ |access-date=20 November 2022 |language=en}}</ref> Airports can be fitted with [[low-level windshear alert system]]s or [[Terminal Doppler Weather Radar]], and aircraft can be fitted with [[airborne wind shear detection and alert system]]s. 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 systems by 1993. The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by windshear 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 |access-date=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 = [https://archive.org/details/symmetryofsailin00garr/page/97 97–99] | isbn = 1-57409-000-3 | url-access = registration | url = https://archive.org/details/symmetryofsailin00garr/page/97 }}</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
| access-date = 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|issn=0049-6979|doi= 10.1007/BF00159677 |bibcode= 1973WASP....2..387H |title= Analysis of highway noise |year= 1973 |last1= Hogan |first1= C. Michael |journal= Water, Air, and Soil Pollution |volume= 2 |issue= 3 |pages= 387–392 |s2cid= 109914430 }}</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, producing 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 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 several 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.] {{Webarchive|url=https://web.archive.org/web/20071025044627/http://www.multi-science.co.uk/windeng.htm |date=2007-10-25 }} Retrieved on 2007-11-25.</ref>

[[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 cause 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==
*[[Aviation safety]]
*[[Low-level windshear alert system]]
*[[Sailing]]
* [[Cumulonimbus and aviation]]

==References==
{{reflist|2}}

==External links==
* [https://web.archive.org/web/20060924180030/http://education.arm.gov/nsdl/Library/glossary.shtml#Wind_shear National Science Digital Library – Wind shear]

{{DEFAULTSORT:Wind Shear}}
[[Category:Wind]]
[[Category:Weather hazards to aircraft]]
[[Category:Microscale meteorology]]
[[Category:Articles containing video clips]]
[[Category:Aerodynamics]]
[[Category:Aeronautics]]

← Older revision		Revision as of 15:54, 17 April 2025
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	~~{{short description\|Difference in wind speed or direction over a short distance}}~~
	~~{{other uses}}~~

	[[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]]		[[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]]

Wind shear - Revision history

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wikipedia>Randy Kryn: uppercase per direct link (Mojave Desert)

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