From AMS Glossary
An anemometer that measures linear components of the wind vector by determining the effect of the wind on transit times of acoustic pulses transmitted in opposite directions across known paths.
It operates on the principle that the propagation velocity of a sound wave in a moving medium is equal to the velocity of sound with respect to the medium plus the velocity of the medium. The wind velocity vector is determined by measuring the wind speed along three independent linear paths, for example, three orthogonal paths. The sonic anemometer is an absolute instrument and has the advantages of a very short time constant and an absence of moving mechanical parts.