If a south wind is tailwind, a north wind will be headwind, and east wind and west wind will be side winds. What about the rest of the angles? When does a tailwind becomes a side wind? My intuition says that WSW and ESE wind would be neutral and the 120 degrees inside those 2 points will be different degrees of tailwind.

I guess there is a absolutely rational answer based in physics rather than what I experience when I ride.