Wind speed ranges from 1-10. You can get an idea of how fast it is blowing by looking at the movement of the clouds on the map view. Objects (like terrain, mountains, trees, earthen and stone walls can partially (or completely) block the wind. The best places to set up wind mills are in treeless sections of the meadows biome or in the plains.

Moder's forsaken power will cause the wind to blow at maximum speed where you are (and exactly in the direction you are sailing if you are on a ship).

Windmills make a good defense against deathsquitos since even at speed 1 a windmill blade hitting a deathsquito will kill it.