CORRECT!

1). What is an important characteristic of wind shear?

a). It is primarily associated with the lateral vortices generated by thunderstorms.
b). It usually exists only in the vicinity of thunderstorms, but may be found near a strong temperature inversion.
c). It may be associated with either a wind shift or a windspeed gradient at any level in the atmosphere.
The answer is:
c). It may be associated with either a wind shift or a windspeed gradient at any level in the atmosphere.


(from Wikipedia):

Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant windshear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (22 m/s) for airliners.[2] Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways.[3] It is also a key factor in the creation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.

Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences. Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact. As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground. (NEXRAD)[21]

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