Boom! A drop of water flies apart in midair.

Emmanuel Villermaux

How a Raindrop Is Like an Exploding Parachute

Here's a question for a rainy day: How do clouds create such a wide variety of raindrop sizes? The answer, according to stunning new high-speed movies, is much simpler than physicists thought.

The idea has been that raindrops grow as they gently bump into each other and coalesce. Meanwhile, more forceful collisions break other drops apart into a scattering of smaller droplets. All this action would explain the wide distribution of shapes and sizes. But trying to unravel how the drops crash and break up led to a tough set of equations.

The new movies, however, show a much more straightforward process. Researchers snapped 1000 pictures a second of an isolated water drop as it fell through an ascending air stream. The drop first flattens into a pancake shape, which then balloons like a parachute. The bottommost rim of this chute has a thick, irregularly corrugated rim. Pressure from the air drag eventually breaks the chute apart into numerous smaller droplets--their wide range of sizes is due to the wide range of sizes of the bumps in the rim.

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Falling water. Watch a water drop break apart in midair.
Credit: Emmanuel Villermaux

Overall, the process is sufficient to account for a wide variety of raindrop sizes without needing to resort to drops colliding in midair, says lead author and physicist Emmanuel Villermaux of Aix-Marseille Universite in Marseille, France. More importantly, he says, the equations needed to describe the exploding drops are far less complex than those that would be needed to describe many drops colliding with each other, breaking up and coalescing repeatedly over time.

If a single raindrop breaks up in a statistically predictable way, then determining the wide range of sizes in an entire rain shower varies only with the intensity of the rainfall: Heavier rain leads to larger initial drops and a broader size distribution versus fine mists with homogenous, small drops. Villermaux and Aix-Marseille colleague Benjamin Bossa publish their findings online today in Nature Physics.

Physicist Jens Eggers who studies the dynamics of water drops at the University of Bristol in the United Kingdom is breathing a sigh of relief. "I was expecting things to get complicated, with lots of empirical relationships thrown together," he writes in an e-mail to Science. "Instead, based on a few physical ideas, the authors manage to explain a beautiful empirical relationship ... in a simple and universal way."

Atmospheric scientists, who have long believed that raindrop size is determined inside a cloud and from complex interactions as they fall, may take more convincing. "Mainstream cloud physicists will reject this thesis," e-mails Ramesh Srivastava, an atmospheric scientist who studies cloud dynamics at the University of Chicago in Illinois. Srivastava says that rain size distribution in practice does not seem to correlate to the paper's predictions; whereas Villermaux says evidence shows correlation at 100 meters below the cloud after the drops have had a chance to break up.

Regardless of who's right, the work isn't likely to see application any time soon. Villermaux says the findings are unlikely to aid weather forecasting or climate modeling, for example. "It's just for the pleasure of understanding."