Raindrop speeds defy expectations

WASHINGTON—It’s conventional wisdom in atmospheric science circles: large raindrops fall faster than smaller drops, because they’re bigger and heavier. And no raindrop can fall faster than its “terminal speed”—its speed when the downward force of gravity is exactly the same as the upward air resistance. Now, a team of U.S. and Mexican researchers has found that it ain’t necessarily so.

Some smaller raindrops can fall faster than bigger ones. In fact, they can fall faster than their terminal speed. In other words, they can fall faster than drops of that size and weight are supposed to be able to fall. And that could mean that the weatherman has been overestimating how much it rains, the scientists say.

“Existing rain models are based on the assumption that all drops fall at their terminal speed, but our data suggest that this is not the case,” explains Raymond Shaw, a physicist at Michigan Technological University in Houghton and a member of the research team. “If rainfall is measured based on that assumption, large raindrops that are not really there will be recorded.”

“If we want to forecast weather or rain, we need to understand the rain formation processes and be able to accurately measure the amount of rain,” he notes. The new results could alter scientists’ understanding of the physics of rain and improve the accuracy of weather measurement and prediction.

Shaw, Alexander Kostinski, also of Michigan Tech, and Guillermo Montero- Martínez and Fernando García-García of the Universidad Nacional Autónoma de México (National University of Mexico) in Mexico City, will publish their findings Saturday, June 13, in the American Geophysical Union’s journal, Geophysical Research Letters.

During natural rainfalls at the Mexico campus, the researchers gathered data on approximately 64,000 raindrops over three years. To study the raindrops, they used optical array spectrometer probes and a particle analysis and collecting system. They also modified an algorithm, or computational formula, to analyze raindrop sizes.

The scientists found clusters of raindrops falling faster than their terminal speed, and as the rainfall became heavier, they saw more and more of these unexpectedly speedy drops. Images revealed that the “super-terminal” drops come from the break-up of larger drops, which produces smaller fragments all moving at the same speed as their parent raindrop and faster than the terminal speed predicted by their size.

“In the past, people have seen indications of faster-than-terminal drops, but they always attributed it to splashing on the instruments,” Shaw says. He and his colleagues took special precautions to prevent such interference, including collecting data only during extremely calm conditions.

This research was supported in part by the National Science Foundation.