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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The frequency dependence of acoustic volume backscattering strength has been measured at depths of 100, 200, 300, and 400 ft in Saanich Inlet, British Columbia, Canada, over a 24-h period. The measurements were made for comparison with open-ocean (Pacific) results, since the biology of the inlet has been extensively studied, and it serves to some extent as a reference water volume. In the course of carrying out the above measurements, the backscattering spectral characteristics and strength of a school of young hake, of apparent mean size of 11 in., was obtained. The time series measurements revealed unchanging scattering characteristics at the 300- and 400-ft depths, with broad peaks near 1.2 and 4.8 kHz superimposed on otherwise flat spectra. Greater variability was observed at the 200-ft depth, while the greatest variability was observed near the surface, i.e., at the 100-ft observation depth. The fish school, which was observed to be closely packed (less than a fish length spacing near the surface) and randomly oriented, was shown to have a flat backscattering spectrum between 1 and 9 kHz and an estimated backscattering strength of −47±3 dB.

The characteristics of underwater noise in the ocean generated by precipitation are important to weather forecasters and oceanographers since they permit the detection and measurement of rain over the ocean by remote (i.e., buoyed or bottom-mounted) acoustic sensors. We have recently observed the character of the underwater noise generated by rain, hail, and snow. The spectrum of rain noise, for wind speeds below 1.5 m/s, shows a peak of 13.5 kHz with a sharp cutoff on the low-frequency side and a gradual falloff (7 dB per octave) on the high-frequency side. Stronger winds smear the peak. Hail spectra show a peak at 3.0 kHz with a gradual (roughly 11 dB per octave) falloff on both sides. The spectrum of snow noise is unique. Our instrumentation permitted the measurement of the drop (or stone) size distributions in the precipitation. These findings will enhance the art of remote acoustic sensing in oceanograpy. [Work supported by: Supply & Services Canada; Institute of Ocean Sciences, Sidney, BC, Canada; Atmospheric Environment Service, Toronto, Canada; and Department of National Defence, Canada.]