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Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind wind speed and/or wind direction over a relatively short distance in the 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.
Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near and caused by , fronts, areas of locally higher low-level winds referred to as low-level jets, near , 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.
The thermal wind equation is
where the are geopotential height fields with , is the Coriolis parameter, and is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since is small or zero, such as near the equator, the equation reduces to stating that is small.
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.
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.
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.
Wind shear can also produce lee 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 coming up from below, it will produce significant shear waves that can be used for soaring.
Windshear can be detected using Doppler radar. Airports can be fitted with low-level windshear alert systems or Terminal Doppler Weather Radar, and aircraft can be fitted with airborne 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.
The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refraction upward, away from listeners on the ground, producing an acoustic shadow at some distance from the source. 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, because they could not hear the sounds of battle only six miles downwind.
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. This low-level wind shear can cause a large bending moment in the shaft of a two-bladed turbine when the blades are vertical. The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.
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. 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 , 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. By then turning downwind, and diving through the wind gradient, they can also gain energy. It has also been used by gliding on rare occasions.
Impact on passenger aircraft
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.
Sailing
Wind shear affects sailboats in motion by presenting a different wind speed and direction at different heights along the 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.
Sound propagation
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 , 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 . 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 .
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.Professor John Twidell. Wind Engineering. Retrieved on 2007-11-25.
Zbigniew Lubosny (2025). 354040340X, Springer. 354040340X
See also
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