SCIENTISTS have discovered that when snow falls on water, each snowflake makes a sound, inaudible to humans. Like the wail of an approaching fire engine, this sound reaches a climax and then fades away, all in about one ten-thousandth of a second.
A raindrop or hailstone penetrates the surface of the water, but the lighter snowflake rests on top. Soon, though, it melts, and the “scream,” referred to above, occurs. This was noticed about 15 years ago, but the matter wasn’t pursued. In more recent years, however, the noise has become a nuisance to biologists in Alaska who use sonar to track migrating salmon. When the background clatter of falling snow masks signals from the fish, the monitoring exercise has to be aborted. What is the cause of this phenomenon?
New Scientist magazine explains that as the flake floats on the water, there is little noise below. But as soon as the flake starts to melt, water is sucked up by capillary action. It may be that at this time air bubbles are released from the flake or are trapped by the rising water. Each bubble vibrates as it struggles to reach an equilibrium with its surroundings, and in doing so, it sends out sound waves, somewhat like a ringing bell—but at a much higher pitch.
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