A weather vane is used to show the direction from which the wind blows. Traditionally, weather vanes are mounted on taller structures including houses and barns. The reason weather vanes are posted in high locations is to prevent interference and to catch the purest breezes.
The key piece of a weather vane is the central pivoting arrow or pointer. The pointer is usually tapered at one end to provide balance and to catch even light winds. The larger end of the pointer acts as a sort of scoop that catches the wind. Once the pointer turns, the larger end will find a balance and line up with the source of the winds.