From AMS Glossary
A thermometer based on the principle that the velocity of a sound wave is a function of the virtual temperature of the medium through which it passes.
By transmitting acoustic pulses in opposite directions between two transducers, the travel time difference can be used to infer the virtual temperature, provided that the wind speed across the acoustic path is known. Three-axis sonic anemometers, in addition to measuring three-dimensional winds, are also used as sonic thermometers. Sonic thermometers possess very short time constants and eliminate radiation error.