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PPL · EASA 050 · Wind
Buys Ballot's law: where is the low?
In the northern hemisphere you stand with your back to the wind. Where is the area of low pressure?
Explanation
Air starts to move from high towards low pressure, but the Coriolis effect turns it to the right in the northern hemisphere until it blows along the isobars. With your back to that wind, the high pressure is on your right and the low pressure on your left. This is Buys Ballot's law.
Step by step
- The pressure difference pushes the air from high towards low pressure.
- In the northern hemisphere the Coriolis effect turns it to the right, until it blows along the isobars.
- Back to the wind: high pressure on the right, low pressure on the left.
Why not the other answers
- On your right is the answer for the southern hemisphere, where the Coriolis effect turns the wind to the left.
- In front of you would be right only if the air flowed straight from high to low pressure. Above the friction layer the Coriolis effect turns it until it blows along the isobars.
- Behind you would mean the wind blows from low towards high pressure, against the force that drives it.
Tip. Near the ground friction turns the wind about 30° towards the low over land, so with your back to the surface wind the low is on your left and a little in front.
Original questions written by FlightDesk Aero and aligned with the EASA syllabus. They are not questions from the official EASA exam bank.