wind shear, a sudden, drastic change in wind direction or speed over a comparatively short distance. Most winds travel horizontally, as does most wind shear, but under certain conditions, including thunderstorms and strong frontal systems, wind shear will travel in a vertical direction. Microburst wind shear is an extremely violent downward blast of air that hits the earth and radiates outward. With its sharp shifts in wind direction and relative wind speed, it can cause an aircraft to lose lift and crash, especially during takeoff or landing, when the slower speeds and closeness to the ground make altitude correction more difficult. Since 1996 all U.S. airliners have been required to be equipped with instruments that provide the pilot with advance warning of wind shear. See also weather and wind.