Wind shear (/ʃɪər/; also writtenwindshear), sometimes referred to aswind gradient, is a difference inwindspeed and/ordirection over a relatively short distance in theatmosphere. 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.[1]
Wind shear is amicroscale meteorological phenomenon occurring over a very small distance, but it can be associated withmesoscale orsynoptic scale weather features such as squall lines and cold fronts. It is commonly observed nearmicrobursts anddownbursts caused bythunderstorms, fronts, areas of locally higher low-level winds referred to as low-level jets, nearmountains, 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 thetroposphere also inhibitstropical cyclone development but helps to organize individual thunderstorms into longer life cycles which can then producesevere weather. Thethermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences and explains the existence of thejet stream.[2]
Wind shear refers to the variation of windvelocity over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30knots (15 m/s) for light aircraft, and near 45 knots (23 m/s) for airliners at flight altitude.[3] Vertical speed changes greater than 4.9 knots (2.5 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 30 knots [15 m/s]).[4] The rationale for this additional caution includes:[citation needed]
Wind shear is also a key factor in the formation of severe thunderstorms. The additional hazard ofturbulence is often associated with wind shear.[citation needed]
Weather situations where shear is observed include:
Weather fronts are boundaries between two masses of air of differentdensities, or different temperature and moisture properties, which normally areconvergence zones 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 intemperature and may also differ inhumidity. Wind shear in the horizontal occurs near these boundaries.Cold fronts feature narrow bands ofthunderstorms andsevere weather and may be preceded bysquall lines anddry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomesstationary, it can degenerate into a line that separates regions of differing wind speed, known as ashear line, though the wind direction across the front normally remains constant. In thetropics,tropical waves move from east to west across theAtlantic and easternPacific basins. 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 andsea breeze boundaries.[10]
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.[11]
Thermal wind is a meteorological term not referring to an actualwind, but adifference in thegeostrophic wind between twopressure levelsp1 andp0, withp1 <p0; in essence, wind shear. It is only present in an atmosphere with horizontal changes intemperature (or in an ocean with horizontal gradients ofdensity), i.e.,baroclinicity. In abarotropic 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 thegeostrophic wind flows around areas oflow (andhigh)pressure.[12]
Thethermal wind equation is
where theφ aregeopotential height fields withφ1 >φ0,f is theCoriolis parameter, andk is the upward-pointingunit vector in thevertical direction. The thermal wind equation does not determine the wind in thetropics. Sincef is small or zero, such as near the equator, the equation reduces to stating that∇(φ1 −φ0) is small.[12]
This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to thetropopause which is (even though other factors are also important) the result of the temperature contrast between equator and pole.[citation needed]
Tropical cyclones are, in essence,heat engines that are fueled by thetemperature 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.[citation needed]
Severe thunderstorms, which can spawntornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain thethunderstorm 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.[14]
The atmospheric effect of surface friction with winds aloft forces surface winds to slow and back counterclockwise near the surface ofEarth blowing inward across isobars (lines of equal pressure) when compared to the winds in frictionless flow well above Earth's surface.[15][failed verification] This layer where friction slows and changes the wind is known as theplanetary boundary layer, sometimes theEkman 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 toinsolation, 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.[citation needed]
In gliding, wind gradients just above the surface affect the takeoff and landing phases of the flight of aglider.Wind gradient can have a noticeable effect onground launches, 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.[16]
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.[17]
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 longwingspan, which exposes them to a greater wind speed difference for a givenbank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident.[17][18]
Wind shear or wind gradients are a threat to parachutists, particularly toBASE jumping andwingsuit 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] 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.[citation needed]
Soaring related to wind shear, also calleddynamic soaring, is a technique used bysoaring birds likealbatrosses, 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.[19] By then turning downwind, and diving through the wind gradient, they can also gain energy.[20] It has also been used byglider pilots on rare occasions.
Wind shear can also producewave. This occurs when anatmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused bythermals coming up from below, it will produce significant shear waves that can be used for soaring.[21]
Windshear can be extremely dangerous for aircraft, especially during takeoff and landing. Sudden changes in wind velocity can cause rapid decreases inairspeed, leading to the aircraft being unable to maintain altitude. Windshear has been responsible for several deadly accidents, includingEastern Air Lines Flight 66,Pan Am Flight 759,Delta Air Lines Flight 191, andUSAir Flight 1016.[citation needed]
Windshear can be detected usingDoppler radar.[22][23] Airports can be fitted withlow-level windshear alert systems orTerminal Doppler Weather Radar, and aircraft can be fitted withairborne wind shear detection and alert systems. 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.[24]
Wind shear affectssailboats in motion by presenting a different wind speed and direction at different heights along themast. The effect of low-level wind shear can be factored into the selection ofsail 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 aboom vang.[25]
Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" byrefraction phenomenon. The audibility of sounds from distant sources, such asthunder orgunshots, is very dependent on the amount of shear. The result of these differing sound levels is key innoise pollution considerations, for example fromroadway noise andaircraft noise, and must be considered in the design ofnoise barriers.[26] This phenomenon was first applied to the field ofnoise pollution study in the 1960s, contributing to the design of urban highways as well asnoise barriers.[27]
Thespeed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound isrefracted upward, away from listeners on the ground, producing anacoustic shadow at some distance from the source.[28] In 1862, during theAmerican Civil WarBattle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle,[29] because they could not hear the sounds of battle only six miles downwind.[30]
Wind engineering is a field ofengineering devoted to the analysis ofwind effects on the natural andbuilt environment. It includes strong winds which may cause discomfort as well as extreme winds such astornadoes,hurricanes, and storms which may cause widespread destruction. Wind engineering draws uponmeteorology,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.[31]
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.[32] This low-level wind shear can cause a large bending moment in the shaft of a two-bladed turbine when the blades are vertical.[33] The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.[34]
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