From AMS Glossary
(Or bidirectional wind vane.) A sensitive wind vane used in turbulence studies to obtain a record of the horizontal and vertical inclinations of the wind vector.
The instrument consists of two lightweight airfoil sections mounted orthogonally on the end of a counterbalanced rod that is free to rotate in the horizontal and vertical planes. The angular positions of the vane are commonly detected by potentiometers. Some bivanes have a propeller on the end opposite the airfoils to enable measurement of the total wind speed.