The underwater sounds produced by impacting snowflakes

In 1985, Scrimger [Nature 318, 647 (1985)] reported measurements of noise l evels significantly above the ambient level for snow falling on a quiet fre shwater lake. He examined only the time-averaged sound levels and did not r eport measurements of individual snowflake impacts. Subsequently, the noise produced by individual and multiple snowflake impacts was examined for a n umber of different snowfalls. The radiated acoustic signals generated by th e impact of individual snowflakes upon a body of water have a remarkable si milarity to each other and differ principally in the frequency of the emitt ed sound wave. The acoustic signal of a snowflake impact thus generates a c haracteristic signature for snowfall that is clearly distinct from other fo rms of precipitation noise. Various aspects of this signature suggest that the radiated acoustic waveform from a snowflake impacting with water is due to the entrainment of a gas bubble into the liquid, and the subsequent osc illation of this bubble as it establishes its equilibrium state. Various sc enarios are presented for bubble entrainment and approximations to the ampl itude of the radiated signal and the acoustic waveform are obtained. (C) 19 99 Acoustical Society of America. [S0001-4966(99)02710-1].