Very large raindrops splashing on the ocean surface.
Very large raindrops splashing on the ocean surface during an experiment at Kwajalein Atoll in the central Pacific Ocean in 1998. Photo by Jeffrey A. Nystuen, Applied Physics Laboratory, University of Washington.

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Large raindrops in wind offshore of Westport, WA.
Sound courtesy of Jeffrey A. Nystuen, Applied Physics Laboratory, University of Washington.
Click either choice below to hear the light rain:
 
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Light rain recorded just below the surface. The sound is similar to a wind chime.
Sound courtesy of Sonatech, Inc., http://www.sonatech.com/ Released under Creative Commons License, non-commercial attribution.

Animation of large raindrop striking the water surface.
Animation of a large raindrop. For large (2.0-3.5 mm diameter) and very large (greater than 3.5 mm) raindrops, the splash becomes energetic enough that a wide range of bubble sizes are trapped underwater during the splash. For very large raindrops, the splat of the impact is also very loud with the sound spread over a wide frequency range (1-50 kHz).
Courtesy NASA Earth Observatory. Animation by Susan Gonnelli, NASA Television
Description
Rain falling on the ocean makes a loud and distinctive sound. Individual raindrops make sound under water in two ways. The impact of the raindrop hitting the ocean surface makes the first sound. Following the initial impact, sound can radiate from bubbles trapped under water during the splash. For most raindrops, the sound produced by the bubbles is louder. Raindrops of different sizes produce different sounds. Small raindrops (0.8-1.2 mm) are remarkably loud because they generate bubbles with every splash. They produce sounds between 13–25 kHz. Medium raindrops (1.2-2.0 mm) do not generate bubbles and are therefore surprisingly quiet. Large (2.0-3.5 mm) and very large (>3.5 mm) raindrops trap larger bubbles, which produce sound frequencies as low as 1 kHz.

The unique characteristics of the sounds produced by different kinds of rainfall allow scientists to identify and measure raindrop size, rainfall rate, and other features of rainfall over the ocean. Rain is one of the most important components of weather and climate. Improved understanding of the global pattern of rainfall is needed to improve weather and climate forecasting. Information on rainfall over the ocean helps meteorologists, oceanographers, and climatologists.

Rain can be a source of underwater noise. Even common occurrences such as heavy rain can increase noise levels by up to 35 underwater dB across a broad range of frequencies (from roughly 1000 Hz to greater than 50,000 Hz) (see: What are common underwater sounds?). Extreme rain events produce very loud signals, sometimes as much as 50 underwater dB above the background noise level.
 
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References

  • Jeff Nystuen, Applied Physics Laboratory, University of Washington, "Listening to Raindrops." (Link)
  • Ma, B.B., Nystuen, J.A., and Lien, R.-C. 2005, "Prediction of underwater sound levels from rain and wind." Journal of the Acoustical Society of America 117(6): 3555-3565. 
  • Nystuen, J.A. 2001, "Listening to raindrops from underwater: An acoustic disdrometer." Journal of Atmospheric and Oceanic Technology 18(10): 1640-1657. 
  • Nystuen, J.A., Amitai, E., Anagnostou, E.N., and Anagnostou, M.N. 2008, "Spatial averaging of oceanic rainfall variability using underwater sound: Ionian Sea rainfall experiment 2004." Journal of the Acoustical Society of America 123(4): 1952-1962. 
  • Wenz, G. M. 1962, "Acoustic ambient noise in the ocean: Spectra and sources." Journal of the Acoustical Society of America. 34, 1936-1956. 
Additional Resources

  • Nystuen, J.A., "Applied Physics Laboratory, University of Washington: Air-Sea Exchange." (Link)
  • 2000, "Climate Change Gets Wet." Space Daily News, March 2, 2000. (Link)
  • "NASA Earth Observatory: Listening to Raindrops." (Link)