Vertical wind shear is a change in wind speed or direction with a change in altitude. It describes how the wind shifts as you move up or down through the atmosphere.
How It Works#
Wind rarely behaves the same at every altitude. Vertical wind shear occurs when air at one altitude moves differently from air just above or below it. The difference can be in speed, direction, or both.
Shear is measured as the rate of wind change per unit of altitude. Strong shear means the wind changes rapidly over a short vertical distance. Weak shear means the change is gradual.
Several sources produce vertical wind shear:
- Jet streams passing overhead
- Temperature inversions, where a layer of warm air sits above cooler air
- Low-level wind shear near thunderstorms
- Terrain effects, such as airflow disrupted by mountains or ridgelines
Low-level wind shear (LLWS) is especially hazardous. It occurs below 2,000 feet AGL (above ground level) and can catch pilots off guard during approach and departure, the two most critical phases of flight.
Example in Aviation#
A turboprop commuter aircraft is on final approach in instrument meteorological conditions (IMC). At 1,500 feet AGL, the aircraft experiences a sudden airspeed increase of 20 knots, followed immediately by a sharp airspeed loss. The crew has flown into a shear layer: wind at the lower altitude is opposing the aircraft's heading, then shifts to a tailwind within seconds.
The crew applies full go-around power and escapes without incident. Their aircraft's onboard wind shear alerting system detected the energy change and triggered a warning just before the speed loss peaked.
Why It Matters#
Vertical wind shear can overpower an aircraft's ability to maintain a stable flight path. A sudden airspeed loss in the shear zone reduces lift exactly when the aircraft is close to the ground with little room to recover. This is a known factor in several fatal approach and departure accidents.
Pilots train to recognize shear conditions, monitor pilot reports (PIREPs) and terminal forecasts, and respond correctly when an aircraft's energy state degrades. Understanding shear is not optional knowledge; it is a survival skill in low-altitude operations.
Key Takeaways#
- Vertical wind shear is a change in wind speed or direction with altitude.
- It is most dangerous below 2,000 feet AGL during approach and departure.
- Sources include jet streams, temperature inversions, and thunderstorms.
- Onboard alerting systems and PIREPs help pilots detect and anticipate shear.
- The correct response to a wind shear warning is immediate go-around power.