From AMS Glossary
A device to measure air temperature based on the principle that the speed of sound varies as the square root of temperature.
The most common application is in so-called sonic anemometers, where the time-of-flight of sound pulses between a pair of acoustic transducers is used to determine the speed of sound. This measurement is converted to temperature via the formula
where cs is the speed of sound, T the air temperature (K), e the partial pressure of atmospheric water vapor (mb), and P the atmospheric static pressure (mb).