What is a sonic anemometer and on what principle does it work

A sonic anemometer uses sound to measure wind speed. It sends a sound signal from a fixed transmitter to a fixed receiver, and by measuring the time it takes for the sound to arrive, can compute the speed of sound. Wind speed will increase or decrease the speed of sound depending on whether it is a tail wind or a head wind. By measuring the speed of sound in both directions the wind speed along that axis can be calculated from the difference of the two measurements. A two axis or three axis instrument can measure total wind speed.