Thanks Dennis,
My head is still hurting over this. To paraphrase you, if I have understood correctly, the inertia is relative to the ground, not the air flow caused by the wind. This explains the proposed phenomenon neatly if correct.
My problem is based on my (infantile) grasp of Newtonian laws of motion. I feel the inertia should be related to the air flow, and that the ground is simply not relevant being in a different inertial frame. Am I wrong?
Its a long time since I did A level physics. I could well be wrong.
Edit: you just posted above as I was writing this. Apologies if you have just explained.