A rooftop wind-direction indicator is a device known as a VANE, which is a four-letter word. A VANE is a flat plate or structure that is mounted on a roof and rotates freely to indicate the direction of the wind. It is usually used to measure wind speed and direction in order to adjust the orientation of wind turbines or other wind-powered devices. Vanes are also used to determine the direction of air flow in ventilation systems.