Wind shear is a change in wind speed and/or direction over a short distance, either vertically (with altitude) or horizontally. It is a fundamental atmospheric phenomenon that influences cloud development, aviation safety, severe storm formation, and turbulence. Wind shear is expressed in units of speed per distance (e.g., knots per 1,000 feet, or m/s per km).
Vertical wind shear is the most meteorologically significant form. Low-level shear (0–2 km) is essential for tornado formation in supercells: it creates horizontal rotating tubes of air that the storm's updraft tilts into the vertical, producing the mesocyclone. Deep-layer shear (0–6 km) determines whether thunderstorms become organised, long-lived supercells or short-lived ordinary cells. In the tropics, low shear favours hurricane development, while high shear tears developing tropical cyclones apart.
For aviation, wind shear — especially near the ground during takeoff and landing — is a critical hazard. Microbursts produce the most dangerous form, with wind speed changes exceeding 15 m/s over just a few hundred metres. Frontal passages, low-level jets, sea breezes, and mountain-induced turbulence also create significant shear. Modern aircraft carry onboard predictive wind shear systems using forward-looking radar, and airports in at-risk areas deploy ground-based detection systems. In weather forecasting, wind shear parameters (such as the 0–6 km bulk shear) are key inputs for severe weather outlooks issued by meteorological services.