A hydrophone sits beneath the water surface and records the sounds of a summer thunderstorm passing overhead. The hissing of small raindrops hitting the water mingles with the gurgles and plops of larger drops. Occasionally, we hear the bass tones of thunder. These underwater sounds of a rain storm are being carefully recorded by hydrophones (underwater microphones) in remote stretches of ocean and analyzed by scientists to supplement global rainfall measurements.

Different sizes of raindrops produce dramatically different sounds as they hit water, primarily because some sizes of drops generate bubbles and others do not. Because the sound of rain underwater is loud and distinctive, we can use it to detect and measure raindrop sizes and amounts of rainfall over the ocean. Data from remote ocean areas is currently sparse, and this new recording technique will add new data and contribute to a global picture of rainfall. Scientists need these measurements to support climate studies of the distribution and intensity of global rainfall patterns.
 

large drop recording

hard rain recording

drizzle recording

Rain is one of the most important components of climate. Knowledge of its distribution and intensity is important not only to farmers and flood control planners, but also to meteorologists, oceanographers, and scientists who study climate (climatologists). Of particular interest to climatologists is the release of latent heat each time a raindrop forms. It takes energy to evaporate liquid water and this energy is stored in water vapor as “latent heat.” The formation of raindrops releases latent heat, which is one of the primary sources of energy that drives atmospheric circulation. Thus, scientists need to improve their understanding of the global distribution and intensity of rainfall to improve weather and climate forecasts. Furthermore, layers of relatively fresh water due to rain at the ocean surface are now thought to significantly affect oceanic circulation (Anderson et al. 1996), another important component of global climate. Unfortunately, rainfall is very difficult to measure, especially over the ocean where few people live and where rain gauges commonly used on land don't work. But we know that rain falling onto a tin roof makes a lot of noise, and so does rain falling onto water. In fact, rain falling onto water is one of the loudest sources of underwater sound, therefore we can measure oceanic rain by listening to it from below the ocean surface.

 How do raindrops make sound underwater?

The data used in this study are available in one or more of NASA's Earth Science Data Centers.