Pre 2014 Blogs

How a Raindrop Is Like an Exploding Parachute

By Karen C. Fox
ScienceNOW Daily News
20 July 2009

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.

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."

http://sciencenow.sciencemag.org/cgi/content/full/2009/720/2?rss=1