Its weird that one it depends if there is a hole right through to the back or not so the wind can get through.
A pressure bubble will form up and act like there is a window there, you can see the label off the belts flapping about a bit.
To be honest I have always been amazed how the yanks keep those hats on. We used to watch them play golf in Aberdeen on the links and they always had them on and didn't struggle. Pair of DC clamping them on as well probably helps?