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PPL · EASA 050 · Wind
The wind at 2 000 ft from the surface wind
Over flat land in the northern hemisphere the surface wind is 240° / 12 kt. What is the most likely wind at 2 000 ft, above the friction layer?
Explanation
Friction with the ground slows the wind near the surface, and a slower wind is turned less by the Coriolis effect, so it blows across the isobars towards the low pressure. Over land the surface wind is backed by about 30° and blows at about half the speed of the wind above the friction layer. Going up to 2 000 ft, the wind therefore veers by about 30° and doubles.
Step by step
- Direction: 240° + 30° = 270°, a veer (clockwise).
- Speed: 12 kt × 2 = 24 kt.
- Wind at 2 000 ft ≈ 270° / 24 kt.
Why not the other answers
- 210° turns the wind the wrong way. Climbing out of the friction layer in the northern hemisphere the wind veers, clockwise; it backs as you descend towards the surface.
- 250° / 18 kt are the values over the sea, where the surface is smoother: a turn of about 10° and a surface wind of about two thirds of the wind above.
- 12 kt keeps the surface speed. Friction also slows the wind: over land the surface wind is only about half of the wind above the friction layer.
Tip. The same rule works on the way down: on the approach the wind backs and weakens as you descend, so expect the drift to change in the last 2 000 ft.
Original questions written by FlightDesk Aero and aligned with the EASA syllabus. They are not questions from the official EASA exam bank.