Wind shear is one of the most significant meteorological hazards to pilots. Here's what you need to know about where wind shear commonly occurs at low altitudes.
A Quick Overview Of Wind Shear
Wind shear is a dramatic change in wind speed and/or direction over a short distance. It can occur either horizontally or vertically and will often lead to large airspeed, altitude, and course deviations. Wind shear can occur at both high and low altitudes, but we'll focus on low altitudes today.
1) Frontal Wind Shear
According to the FAA, "Not all fronts have associated wind shear. In fact, shear is normally a problem only in those fronts with steep wind 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 10F (5C) or more.
- The front is moving at a speed of at least 30 knots.
You can get clues about the presence of wind shear during the weather briefing by checking these two factors. If these factors are present, be prepared for the possibility of shear on approach (FAA).
2) Wind Shear From Thunderstorms
Thunderstorms produce the most severe weather you can find in aviation. And you don't need to be inside a thunderstorm to find serious wind shear. Strong downdrafts and microbursts can form underneath them.
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 percent between the surface and 1,500 feet, with most of the increase occurring in the first 150 feet. The implications for wind shear like this on final approach are obvious (FAA).
3) Wind Shear From Temperature Inversions
Pilots who have flown in the Southwest, Southern California, or Colorado are familiar with this weather pattern. Overnight cooling creates a temperature inversion a few hundred feet above the ground. When coupled with high winds from above, this inversion can produce significant wind shear close to the ground.
As the inversion dissipates, the area of wind shear and gusty winds move closer to the ground. In some areas of the Southwest, a 90-degree change in direction and 20-30 knot increases in surface winds in a few minutes are not uncommon (FAA).
4) Wind Shear From Surface Obstructions
When the air near the surface of the Earth flows over obstructions, such as bluffs, hills, trees, or buildings, the normal horizontal wind flow is disturbed and transformed into a complicated pattern of eddies and other irregular air movements.
Where else can you find wind shear close to the ground? Tell us in the comments below.
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