As temperatures drop this fall, days of heavy winds are becoming more common. Flying in high winds presents its own challenges. Even on the windiest days, though, it's not unusual for winds at the surface to die down not long after sunset. Still, dangerous wind shear can occur just as you're approaching the runway on short final. To look at why this may happen, let's review a bit about the wind:
Wind blows as a result of high pressure air flowing into areas of lower pressure. On a weather chart, we show areas of the same pressure as being connected by lines called isobars. The closer these isobars are, the greater the force causing wind to flow across them. This is the pressure gradient force and is what causes wind to blow. The force moves perpendicular to the isobars.
If this were the end of the story, winds strength and direction would be easy to predict, but in the real world things are more complicated. The rotation of the earth causes the blowing wind to "appear" to curve to the side when observed from the surface. The reason is that as the Earth moves, areas close to the Equator move faster than areas near the Poles, so as winds flows it is either left behind or leads in front of the surface. This is called the Coriolis Effect.
Wind speed and direction are therefore a result of the pressure gradient force, which initiates the wind, and the Coriolis Effect, which deflects the wind. In the Northern Hemisphere, it deflects to the right.
Pressure gradient force moves air from areas of high pressure to low pressure. Coriolis turns it right. The result is wind that flows parallel to isobars, with high pressure to the right and low pressure to the left (in the Northern Hemisphere).
The story gets more complicated near the surface. Ground features like mountains, hills, trees, and buildings slow the wind down. The slower the wind, the less effect Coriolis has on it. Pressure gradient force is unaffected, so the result is a weaker wind that curves in toward a low pressure area and out away from a high pressure zone.
During the day, sunlight warms the surface of the Earth, warming the air above it and causing it to rise in the form of thermals. As these thermals rise, it causes a mixing of air close to the surface with air further aloft. The more these layers of air mix, the less the change in winds between the surface and areas further above.
At night however, the lack of sunlight causes the thermals to disappear. Without the mixing of air at different levels present in the daytime, a distinct boundary layer appears between stronger winds aloft, and surface winds weakened by friction. On calm, clear nights, after a day of strong winds, this boundary layer can be strong.
What this means is that as you're approaching the runway to land on final, you could initially be experiencing a strong crosswind requiring a correction, and then experience a strong wind shear on short final as the wind weakens and changes direction. This wind shear can destabilize your approach and requires a change to your control inputs close to the ground.
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