Wind Shear
Wind shear describes situations in which wind changes speed and/or direction along the horizontal and/or vertical axis. Changes along the vertical axis are known as vertical wind shear, while changes along the horizontal axis are known as horizontal wind shear.
Wind shear is a major hazard, especially for flights operating at low levels (below 500 m or approximately 1600 ft). Wind shear can affect flight performance in various ways. A headwind gust will increase aircraft speed and force an aircraft to accelerate and/or fly above its intended path.

Wind shear due to updrafts and downdrafts (the most dangerous kind) can displace an aircraft upward or downward. A tailwind gust will decrease aircraft speed and force it to fly below its intended path and/or decelerate.

Finally, crosswind shear can make an aircraft drift laterally away from its intended flight path.

The WMO criteria for forecasts of hazardous low-level windshear/turbulence require that one or more of the following be met:
- Mean surface wind of 20 kt or greater
- The magnitude of the vector difference between the mean surface wind and gradient (2000 ft) wind is 40 kt or greater
- Thunderstorms or heavy showers exist within 10 km
- Significant wind shear already has been reported by aircraft in the vicinity