From AMS Glossary
An instrument used to indicate or measure wind direction.
It consists basically of an asymmetrical, elongated object mounted at its center of gravity about a vertical axis. The end that offers the greater resistance to the motion of air moves to the downwind position. The direction of the wind may be determined by visual reference to an attached oriented compass rose. The direction may also be displayed or recorded by use of a potentiometer or a digital optical encoder.