INTRODUCTION

Low-level wind shear, in the broadest sense, encompasses a family of air motions in the lower levels of the atmosphere, ranging from small-scale eddies and gustiness that may affect aircraft as turbulence, to the large-scale flow of one air mass layer past an adjacent layer. Included among the wide variety of phenomena that produce such air motions are thunderstorms, land/sea breezes, low-level jet streams, mountain waves and frontal systems. In order to understand, in this context, the common denominator linking such varied phenomena, it is necessary to explain the meaning of the term "wind shear". The most generalized explanation of wind shear is "a change in wind speed and/or direction in space, including updrafts and downdrafts". From this explanation it follows that any atmospheric phenomenon or any physical obstacle to the prevailing wind flow that produces a change in wind speed and/or direction, in effect, causes wind shear.

Wind shear is always present in the atmosphere and its presence is often visible to an observer. Examples are cloud layers at different levels moving in different directions; smoke plumes sheared and moving in different directions at different heights; rotating suspended debris and/or water droplets in the relatively innocuous dust devils and in the extremely dangerous water spouts and tornadoes; the "wall-like" leading edge of dust/sandstorms; and trees bending in all directions in response to sudden gusts from a squall line. All these visual effects testify to the universal presence of wind shear and wind shear-causing phenomena in the atmosphere.

The significance of wind shear to aviation lies in its effect on aircraft performance and hence its potentially adverse effects on flight safety. Although wind shear may be present at all levels of the atmosphere, its occurrence in the lowest level — 500 m (1 600 ft) — is of particular importance to aircraft landing and taking off. During the climb-out and approach phases of flight, aircraft airspeed and height are near critical values, therefore rendering the aircraft especially susceptible to the adverse effects of wind shear. As will become clear in subsequent chapters, the response of aircraft to wind shear is extremely complex and depends on many factors including the type of aircraft, the phase of flight, the scale on which the wind shear operates relative to the size of the aircraft and the intensity and duration of the wind shear encountered.

Having drawn attention to the prevalence of wind shear in the atmosphere and its potential danger to aircraft, in order to keep things in perspective, it should be pointed out that, considering the high number of aircraft landings and take-offs which take place around the world, only a very small number of aircraft encounter difficulties which result in accidents and, of these accidents, in only a fraction is wind shear a factor. Nevertheless, the fact that wind shear has contributed to aircraft accidents in the past is sufficient reason for everyone engaged in aviation operations to understand the dramatic effect that wind shear can have on aircraft performance, particularly during the landing and take-off phases.