A change in wind direction or speed, either vertically or horizontally, within a short distance in the atmosphere.
Wind shear is a gradient, which means it exhibits a specific change of wind velocity over a given distance. Wind shear can be horizontal, vertical, or both. If the wind direction changes as well as the wind speed, the actual wind shear will be greater than the wind speed shear alone. The most critical wind shears for pilots occur over horizontal distances of 1 mile or less and over vertical distances of 1,000 feet or less.
Wind shear that occurs below 2,000 feet above ground level, or along the aircraft’s final approach or takeoff and climb path is called low-level wind shear (LLWS). Low-level wind shear is most hazardous when the wind shifts from a headwind to a tailwind. This shear will cause a decrease in airspeed equal to the decrease in wind velocity and can adversely affect the performance of the aircraft. These shears are considered significant when they cause airspeed changes of 15 knots or more. In recent years, an effective low-level wind shear alert system has been developed. These detectors are being installed at airports to warn pilots of the possibility of low-level wind shear in the vicinity. Three of the most common types of low-level wind shears are airmass wind shears, thunderstorm-associated wind shears, and topographical wind shears.
Airmass wind shears commonly develop during calm, fair nights, during which the ground may become cooler than the overlying airmass, creating a surface temperature inversion. This nocturnal inversion is very stable and impedes mixing of the airmasses above and below the inversion. As a result, the surface air remains calm, while the winds aloft increase, because they are not slowed by surface friction. Vertical wind shear can be remarkably strong through the inversion. Wind shears should be expected whenever wind speeds at 2,000 to 4,000 feet above the surface are 25 knots or greater. After daybreak, the sunlight heats the ground, and the inversion and the wind shear dissipate. In cold-winter climates, however, when the ground is covered with snow and ice, inversions, with their accompanying wind shears, can persist throughout the day and night.
Thunderstorms produce strong updrafts and downdrafts throughout their lifecycles. The cumulus stage of thunderstorms is characterized by updrafts. Upon reaching the mature stage, the storm has both updrafts and downdrafts. Dissipating thunderstorms have mostly downdrafts. These updrafts and downdrafts can cause severe and even extreme turbulence that can cause an aircraft to experience structural damage.
The most severe form of wind shear produced by a thunderstorm is a microburst. A microburst is a core of cool, dense air that descends rapidly from the thunderstorm. When the column of air reaches the ground, it spreads out in all directions and rolls back over itself, forming a vortex ring. Microbursts usually do not last longer than fifteen minutes, although some can linger as long as thirty minutes. There is usually heavy rain in the microburst, although in drier climates, the rain can evaporate before reaching the ground, creating a dry microburst.
Microbursts pose a danger to aircraft because of their strong downdrafts and because the wind direction shifts 180 degrees across the center of the microburst. In a microburst with wind speeds of 45 knots, the shear across the microburst will be 90 knots. The wind shear created by the microburst can exceed the operating capabilities of even heavy transport jets. On February 8, 1985, a Delta Air Lines Lockheed L-1011 Tristar encountered a severe microburst while attempting to land in a thunderstorm at Dallas-Fort Worth, Texas. The aircraft crashed 6,300 feet short of the runway, broke up, and burst into flames, killing 134 of the 163 passengers and crew on board.
Air moving across hills, mountains, and valleys can create wind shears. As air moves across the ridges of hills or mountains, the airstream is compressed and deflected upward. The compressed air speeds up. The change in direction and speed of the air creates wind shear on the tops of the hills and mountains. Higher mountains and faster winds produce stronger wind shears.
Air moving across a valley with gently sloping sides will produce downdrafts on one side and updrafts on the other side. These drafts can produce significant shears if the wind is strong. In canyons with steeply sloped sides, there may be sharp downdrafts on the leeward side of the canyon. In addition, wind may travel through the canyon at right angles to the airflow above it. Several aircraft have crashed in canyons due to the unpredictable wind shears they encountered.
Wind shear can be associated with jet streams, high-level frontal activity, and the tropopause, the region at the top of the earth’s troposphere, the lowest, densest part of the atmosphere. When high-level stable layers of air are displaced vertically, they produce atmospheric gravity waves. These waves can develop crests, much like ocean waves. The air is usually cloudless at these altitudes, so there are no visual clues to alert pilots to the presence of these shears, although if clouds are present, they will often reflect the wave pattern of the air. These wind shears are a common source of clear-air turbulence.
Ahrens, C. D. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 5th ed. St. Paul, Minn.: West, 1994. A classic, profusely illustrated text on meteorology that includes discussions of the effects of weather on aircraft. Federal Aviation Authority. Aviation Weather. Washington, D.C.: U.S. Department of Transportation, Federal Aviation Administration and U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, 1975. The definitive manual on aviation weather, clearly and tersely written. _______. Pilot Wind Shear Guide. Washington, D.C.: U.S. Department of Transportation, Federal Aviation Administration, 1988. A detailed discussion of the various types and causes of wind shear and its hazards to pilots. Lester, Peter F. Aviation Weather. Englewood, Colo.: Jeppesen Sanderson, 1997. A basic meteorology text written especially for pilots.