Wind shear (WS)

Wind shear is a marked change in wind velocity over a short distance — vertically, horizontally, or both. Close to the ground it is a serious hazard, because an aircraft on approach or departure can lose or gain airspeed faster than it can be corrected for.

In a METAR it is reported against the runways affected. WS RWY24 reports shear on runway 24; WS ALL RWY reports it on all of them. The phase may be narrowed with WS TKOF RWY24 or WS LDG RWY24 where the hazard applies to departure or arrival specifically.

A TAF can forecast non-convective low-level wind shear in a different form: WS followed by the height in hundreds of feet, then the wind at that height. WS020/25045KT forecasts 250° at 45 kt at 2 000 ft — typically a low-level jet or a strong inversion, shearing against a much lighter surface wind. Convective shear is not forecast this way; it is implied by the thunderstorm groups, which is why TS and wind shear are read together.