Unless you spend a lot of time flying airplanes, you probably haven’t given a great deal of thought to the subject of wind shear.
Wind shear is the phenomenon of radically differing wind velocity and direction at slightly different altitudes. The occurrence of an adjoining northerly twelve-knot wind, say, and a southerly twenty-knot wind would constitute wind shear.
It is a phenomenon of grave concern to pilots. Wind shear causes the aircraft to feel turbulence and lose speed; if the differences between the adjoining winds are great enough, the effect can be extremely detrimental to sustained flight.
Wind shear is especially dangerous when it occurs at a low altitude in the vicinity of an airport, where it can affect planes in the vulnerable takeoff and landing phases of flight. Wind shear can cause a low-flying, slow-moving plane to lose airspeed and lift, sometimes so suddenly that the pilot does not have enough time to adjust to the new conditions and the plane crashes.
Although wind shear can occur at any altitude, a plane flying high and fast will usually have plenty of time to compensate for its negative effects. For this reason, low-level wind shear, wind shear below roughly 800 to 1,200 feet, is of greatest concern to flyers.
Many airports in the United States are equipped with the low-level wind-shear alerting system (LLWAS), which detects weather conditions likely to produce wind shear.
The system consists of a network of anemometers, fancy weathervanes, arranged to monitor wind speed and direction at various points around the airport. Data received by each of five or six anemometers on the perimeter of the airport and one at its center are fed into a computer in the control tower. If the computer detects strong discrepancies, it sets off an alarm. Control tower officials are then able to send out a wind shear warning.
A variety of meteorological phenomena can cause wind shear. Any kind of frontal zone, the boundary between different air masses, can produce it. Fast-moving cold fronts create prime wind shear conditions as they overtake slower warm air masses.
Thunderstorms are another wind shear breeding ground. They contain strong downward currents of air, called downdrafts and microbursts, that strike the earth and fan out horizontally. An airplane flying into a downdraft produced by a thunderstorm will experience all sorts of unenviable events. First, as the craft enters the downdraft, it meets with headwinds created by the flow of air across the earth’s surface, and its lift and airspeed are increased.
Then, as the airplane continues across the downdraft, downward currents become more pronounced and the plane is pushed toward the ground, it loses lift. At the same time, a rapid change from headwind to tailwind conditions causes the plane to lose airspeed, a situation that further erodes lift. And all of this can transpire in a matter of seconds.
Since wind shear often accompanies specific types of weather, experienced pilots can usually be alerted to it by checking for fronts and thunderstorms along the flight route, even before taking off.