ICAO 9817 PDF

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.