Wind Shear
Wind Shear, in meteorology, a difference in the speed or direction of the wind, or in both the wind's speed and direction, at two points a short distance apart. Wind shear occurs under a variety of circumstances, such as when masses of warm and cold air meet, when air flows over rough terrain, or when currents of rising or sinking air (up-drafts or downdrafts) are formed.
A number of major airline crashes since the mid-1970's have been attributed to wind shear associated with microbursts, relatively small-scale and short-lived downdrafts that can occur in severe thunderstorms. The microburst consists of a blast of cooled air moving downward; as the air reaches the ground it spreads out in all directions, typically with a speed of 30 miles per hour (48 kmh) or greater. When an airplane that is taking off or landing flies through the outflow from a microburst, it encounters strong head winds, quickly followed by a downdraft and strong tail winds. Because the airplane is near the ground, the pilot has little time to react. In the early 1990's, airlines in the United States equipped their aircraft with sensors that warn the pilot the instant they detect flight characteristics typically caused by a microburst.