One summer four and a half years ago, meteorologist David Atlas was sailing in Buzzards Bay, off Cape Cod, when a storm suddenly came up.
"The waves were about one and a half feet high at the time, ... and we thought we should batten down the hatches and get ready for intense rain," he recalls. "But to my utter surprise, as soon as the rain started, the sea became glassy and perfectly calm, except for the little ripples generated by the drops themselves." Atlas had witnessed something described by sailors for centuries: namely, that rain can calm choppy seas. But Atlas, a visiting scientist at NASA's Goddard Space Flight Center in Maryland, has now found scientific confirmation of the phenomenon - as well as a possible explanation.
After the incident in Buzzards Bay, Atlas combed through radar images of Earth made by a European Space Agency satellite, the ERS-1. One image showed a storm off Cape Hatteras, North Carolina, in July 1992. Atlas combined this image with ground-based radar data showing the amount of rain that had occurred at that time and place. The ERS-1 doesn't send its radar beam straight down at the planet; the beam is directed off to one side of the satellite's path. The strongest echoes sea was rough under most of the Hatteras storm, and Atlas found that those radar-bright areas coincided with areas of little or no rain. The waves had been flung up by a downdraft fanning out from the center of the storm. But in the storm core itself, where the rain was heavy, Atlas saw a dark hole in the radar image - a three-mile-long, 1.5-mile-wide zone that hadn't sent an echo back to the satellite at all, because the radar beam had just skipped off the flat, glassy sea like a stone.
How does rain flatten a sea? Waves, Atlas explains, are an organized, coordinated motion of water. But when a raindrop enters the water, it first creates a splash, and then an eddy. "Now imagine a lot of raindrops doing this," Atlas says. "When you have a bunch of these eddies, that is disorganized turbulence, or random motion, and the turbulence interferes midi the organized wave motion. The rain is simply stirring the water underneath, and when you stir the water, you kill the waves."