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Outdoor billiards. Rainfall sensors show that large drops collide and scatter on their way down to the ground.

National University of Mexico

Warp-Speed Raindrops

It's a rain race out there. In the meteorological equivalent of breaking the light-speed barrier, new research shows that the smaller droplets in a rainstorm often surpass what appears to be the speed limit for rain. The findings should help scientists devise models that could lead to more accurate weather forecasts.

Common sense dictates that larger raindrops should fall to the ground faster than smaller ones because they weigh more and can better overcome wind resistance. But anecdotal meteorology data have shown that when drops land, smaller ones are sometimes going just as fast as the biggest ones. That irregularity had puzzled scientists for many years; they usually attributed it to instrument problems. But now a team of physicists from Michigan Technological University (MTU) in Houghton and the National University of Mexico in Mexico City has found evidence that the phenomenon is real.

Over several years, the team clocked about 64,000 raindrops falling in Mexico City. The researchers measured their sizes and velocities only in extremely calm conditions, so the wind that often accompanies rain could not skew the data. They found that some drops plummeted faster than the so-called terminal velocity for their size--the speed, based on a well-established scale, at which air resistance counteracts the accelerative force of gravity.

Like the speed of light, the terminal velocity should be an absolute limit. But in a paper in press at Geophysical Research Letters, the team reports many observations of so-called superterminal drops, which form when larger drops collide and break up into bunches of small drops. Those smaller drops can then travel for a time as fast as the larger drops. For example, drops with a diameter of 100 micrometers are supposed to be limited to a terminal velocity of about 30 centimeters per second. But the researchers observed such drops hitting the ground at 3 to 4 meters per second.

"What surprised us was not so much seeing the superterminal drops," says physicist and co-author Raymond Shaw of MTU, "but seeing the deeper, compelling patterns." He explains that as rain falls harder, the fraction of superterminal, or speeding, small drops increases. At the same time, the proportion of the bigger drops decreases. That result, Shaw says, is "consistent with the notion that large drops break up to produce a swarm of speeding satellite droplets."

Shaw says there's a practical side to the research. "Weather forecasting models depend on simplified theories of how raindrops grow, [so] the more we understand about the interactions between drops, ... the more we can improve our ability to predict whether it will rain on tomorrow's picnic."

Environmental physicist Ana Barros of Duke University in Durham, North Carolina, says the paper confirms what she and colleagues have discovered about raindrop velocities in lab experiments. "There is a very large uncertainty about rainfall measurements" from sensors, she says.