What is the principle of relativity?
When one frame of reference moves in a straight line and at a constant velocity relative to another,
no physical process can distinguish which one is moving and which one is at rest.

Well, here you can feel the wind moving at a constant velocity but you cannot see it,
until it has picked up some of the moving water
and using the wharf as a frame of reference,
we can now see the angles the wind is taking.

What a load of twaddle I am talking about!!!