Rain calms the crash of ocean waves.

Rain Calms the crash of ocean waves

Crashing waves form walls of white water that can make a sailor beg for mercy. But a driving rain, according to new laboratory experiments, can actually reduce the number of large, breaking waves, its pelting force calming the sea like a sedative. This somewhat counterintuitive finding may change the way scientists interpret radar measurements of wind speed at the ocean surface.

British oceanographer Stephen A. Thorpe of the University of Southampton says he came across his first hint of the calming effect of raindrops a decade ago while recording underwater sound reflected from the ocean's surface. Thorpe found that raindrops, which produce a characteristic, high-frequency hiss when they strike the ocean (SN: 1/4/86, p.4), seemed to reduce the number of breaking waves.

To verify that observation and to learn how raindrops could stop large waves from breaking, he and graduate student Michael Tsimplis recently conducted a simplified experiment. By generating waves in a shallow, 43-foot-long water tank and setting up 3,200 dripping hypodermic needles to simulate steady rainfall, they found that rain exerts its influence through an indirect route.

The researchers report in the Dec. 21/28 NATURE that water droplets in their experiment suppressed the formation of mid-sized water waves -- those with about 4 inches between crests. Because wind relies on mid-sized waves to transfer energy to larger waves, eventually causing these giants to crash, rain can help slow or stop the process before it builds force, Thorpe says. Though rain creates its own small ripples, he adds, its ability to dissipate mid-sized waves -- probably through the turbulence it creates at the ocean's top layer -- has an overall quieting effect.

An understanding of precipitation's tranquilizing properties may offer insights to several phenemona, including the placid, ripple-free appearance of a lake after a rain, Thorpe says. And the relative absence of large, crashing waves means that radar should bounce off the ocean surface with a more crystal-like scattering pattern during a rainstorm. Scientists unaware of raininduced alterations in the radar signal might incorrectly estimate wind speed, he suggests.

Thorpe calls his water tank experiment "the first step in the investigation" and says he plans next to simulate the effects of wind. But the current evidence, he says, strongly indicates that "rain can be a friend to a sailor."