[Christ]

I think pulling in rain and ice/snow/crapola would affect the benefit by displacing the actual volume of air that comprises the boundary layer as well.

I.E. if there is a boundary layer 1 inch thick, and you take a sample of the boundary layer that is 1x1inch, you should have 1 cubic inch of sampled air... air. Nevermind the obvious pressure gradient in measuring the exact sample. But if rain/snow/ice/other get sucked in by the vacuum which holds the boundary layer to the concave shape, you're displacing part of that sampled air, and since the boundary layer of air is creating our energy, you're displacing some of that energy.

This discussion is "water at my nostrils" in terms of what I know and can apply to it, so please, if this is wrong, correct it at will, I'd appreciate a minor education in it.