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Each falling raindrop may go it alone: study suggests drips shatter to produce a variety of sizes.

Raindrops keep falling on your head in all different sizes, and now researchers know why. The shattering of single raindrops after they leave clouds is enough to explain the wide variety of drop sizes, a study appearing online July 20 in Nature Physics shows, overturning the notion that complicated interactions between falling drops are to blame.

Explaining the details of raindrop shattering "is a very nice result," says Howard Stone of Princeton University, since "apparently, no one had connected a theory with the measurements."

Scientists used to think that the assortment in raindrop sizes was a result of splitting and coalescing as the drips bumped into each other during their descent. But study coauthor Emmanuel Villermaux thought that collisions would be too rare to explain the size distribution.

Seeking an alternative explanation, Villermaux and Benjamin Bossa, both of Aix-Marseille University in France, conducted experiments on isolated water droplets. A high-speed camera captured each contortion of a solitary drop as it fell a few meters. An upward air current simulated the experience of a raindrop during its fall from the sky.

Over tens of milliseconds, each drop flattened like a pancake because of drag. Next, water in the pancake shifted to the outer rim, forming what's called aligament shape. For some drops, these shapes looked like upside-down bags. Eventually, the ligament shape dramatically shattered into many smaller globules.

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After creating mathematical equations to describe this shattering, the researchers found that the breakup of individual drops alone could explain the staggering variety of raindrops. "You don't need this interaction ingredient to understand how drops fragment," Villermaux says. "You just need a single drop."

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Title Annotation:Matter & Energy
Author:Sanders, Laura
Publication:Science News
Article Type:Brief article
Geographic Code:1USA
Date:Aug 15, 2009
Words:273
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