It seems obvious that head and tail winds are equally likely. That is, assuming the direction of the wind and your flight are both random, head and tail winds should be equally likely. But it’s wrong.

Of course, even if head and tail winds were equally likely, you would spend more time flying in headwinds, simply because they slow you down. But that’s not the reason I’m talking about here.

The reason is simple. When the wind is 90* to your direction of flight, you have to turn toward it slightly to maintain your desired direction of flight, so it slows you down. Visualize the entire 360* circle that the angle of the wind can have relative to your direction of flight. If wind at exactly 90* slows you down, then more than half of the range of the circle slows you down. A wind from the side must be slightly behind you in order for the loss of speed turning toward it, to be countered by the gain in speed it adds pushing you along. In other words, when the wind is from the side, it must be slightly behind you to break even.

Of course, the same applies to boats. But not cars, because you don’t need to steer into a crosswind when driving (well you do, but it requires so much less correction as to be insignificant).