Wind shear is a change in wind speed and/or direction over a short distance. It can occur either horizontally or vertically and is most often associated with strong temperature inversions, microbursts, macrobursts or convective activity. Wind shear can occur at high or low altitudes and is a very serious threat to aviation safety. Four common sources of low-level wind shear aredescribed below
Wind shear is a change in wind speed and/or direction over a short distance. It can occur either horizontally or vertically.
Frontal Activity
Not all fronts have associated wind shear. In fact, shear is normally a problem only in those fronts with steep gradients. As with so many things associated with weather, there is no absolute rule, but a couple of clues tell you that wind shear may occur:
The temperature difference across the front at the surface is 5°C or more.
The front is moving at a speed of at least 30 knots.
Thunderstorms
Gusty winds are associated with mature thunderstorms and are the result of large downdrafts striking the ground and spreading out horizontally. These winds can change direction by as much as 180 degrees and reach velocities of 100 knots as far as 10 miles ahead of the storm. The gust wind speed may increase by as much as 50% between the surface and 1,500 feet, with most of the increase occurring in the first 150 feet. The implications for a shear during approach in such a case are obvious.
The other hazard associated with the thunderstorm is the downburst which is an extremely intense, localized downward motion of air. The power of the downburst can actually exceed aircraft climb capabilities, not only those of light aircraft but high-performance jets as well. It would be best to avoid such areas.
Temperature Inversions
Overnight cooling creates a temperature inversion a few hundred feet above the ground. When coupled with high winds from what is known as the low-level jet stream, this inversion can produce significant wind shear close to the ground.
Surface Obstructions
Wind shear from surface obstruction is generally associated with hangars or other buildings near the runway. The sudden change in wind velocity can seriously affect a landing. Another type of surface obstruction – mountains – can also affect wind shear. Some airfields are close to mountain ranges, and mountain passes are close to the final approach paths. Strong surface winds blowing through these passes can cause serious localized wind shear during the approach. The real problem with such shear is that it is almost totally unpredictable in terms of magnitude or severity. A pilot can expect such shear whenever strong surface winds are present.
Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts. Hover over the numbers below to understand how a downburst, and the associated wind shear is a significant threat to aviation safety.
Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts. Click on the numbers below to understand how a downburst, and the associated wind shear are a significant threat to aviation safety.
Wind shear can be classified according to its effect on aircraft performance.
Increasing Performance Windshear: Occurs when the shear causes an increase in indicated airspeed.
Decreasing Performance Windshear: Occurs when the shear causes a decrease in indicated airspeed