A weather vane is also called a wind vane. It is a tool for measuring wind direction. It spins on a rod and points in the direction from which the wind ...
http://sln.fi.edu/tfi/units/energy/vane.html
Weather vane - Wikipedia, the free encyclopedia
A weather vane, also called a wind vane, is a movable device attached to an elevated object such as a roof for showing the direction of the wind. ...
http://en.wikipedia.org/wiki/Weather_vane
Wind Vane
Read about the wind vane on the nacelle. The wind vane measures the wind direction so the wind turbine can yaw to the right position.