Acoustic rain gauge
From AMS Glossary
acoustic rain gauge
An instrument designed to determine rainfall over lakes and oceans.
A hydrophone is used to sense the sound signatures for each drop size as rain strikes a water surface. Since each sound signature is unique, it is possible to invert the underwater sound field to estimate the drop-size distribution within the rain. Selected moments of the drop-size distribution yield rainfall rate, rainfall accumulation, and other rainfall properties.
Nystuen, J. A., et al. 1996. A comparison of automatic rain gauges. J. Atmos. Oceanic. Technol.. 13. 62–73.