concept

Wind shear

The change in wind with height, or across horizontal distance — and crucially a change in both speed and DIRECTION. Directional shear is the part people forget: the wind at 500 m and the wind at 8 km can blow opposite ways, so the atmosphere is not one moving block but a deck of layers sliding over one another. At the margins of a jet stream, where speed changes sharply over a short distance, shear generates clear-air turbulence — violent buffeting in cloudless sky with no visual warning. To a flier that can read those layers, the same structure is a source of energy.