The spectra of underwater noise generated by rain, hail, and snow have been measured in a lake at a depth of 35 m, for a variety of atmospheric conditions. Rain noise spectra, for light winds (<1.2 m s−1), have a sharp peak at 13.5 kHz with a steep falloff (∼60 dB/oct) on the low-frequency side and a more gradual falloff (9 dB/oct) on the high-frequency side. A quasi-flat spectral regime exists in the frequency interval 2–10 kHz. Wind, for speeds increasing above 1.2 m s−1, progressively rounds the peak. The spectral level at 15 kHz (i.e., near the peak) shows a linear dependence on the log of the rain rate with wind speed as a parameter. Correlation of the rain noise spectra with raindrop-size distributions suggests that low frequencies are generated by the larger drops, although this aspect of the problem needs further work. Hail noise spectra have rounded maxima appearing between 2 and 5 kHz with an approximately 10-dB falloff on each side. The spectrum of underwater sound generated by gently falling snow shows a linear increase in level, averaging 5 dB/oct, when plotted against the log of frequency.

Measurements of underwater noise produced by rain over the open ocean were made at a site some 1 km off Ucluelet on the west coast of Vancouver Island, British Columbia, Canada. A hydrophone was placed near the bottom in 55 m of water in the path of the prevailing eastward moving Pacific storm systems. The spectral noise-level dependence on rain rate between zero and 20 mm/h was determined for various frequencies and sea states. A straight line, least-squares fit to the sample values of spectral level plotted against the log of rain rate was found to best represent the relationship. Representative rain noise spectra for various rain rates and sea states were derived.

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