Raindrops go it alone

Single drips shatter to produce a wide variety of sizes

2:04pm, July 20, 2009
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View a video of shattering raindrops at the bottom of this article.

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.

The results may simplify conventional wisdom about raindrop sizes, comments Yangang Liu, an aerosol expert at Brookhaven National Laboratory in Upton, N.Y.

Raindrops fall to earth once they’ve grown big enough to escape from the clouds. These drops sometimes grow as large as 10 centimeters across (larger than a baseball), says Emmanuel Villermaux, coauthor of the new study. But drips hitting the ground range in diameter from less than 1 millimeter to 6 millimeters. Scientists used to think that the assortment, first carefully quantified in the 1940s, was a result of complicated interactions between the drops as they fell — like bumping, which can cause raindrops to splinter or coalesce. But Villermaux, thought that collisions would be too rare to explain the distribution of raindrop sizes.

Seeking another 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 from a nozzle. Though the drop fell only a few meters, the researchers applied an upward air current to simulate the experience of a raindrop during its long fall from the sky.

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

Villermaux and Bossa measured the diameters of these resulting pieces. Raindrops with a starting diameter of 6 millimeters broke into pieces that ranged from around 1 millimeter to just under 4 millimeters in diameter.

After creating mathematical equations to describe this shattering, the researchers found that the breakup of individual drops alone was enough to explain the staggering variety of raindrops — no collisions necessary. “You don’t need this interaction ingredient to understand how drops fragment,” Villermaux says. “You just need a single drop.”

What’s more, the equations reveal that a drop 6 millimeters in diameter or larger is likely to shatter before reaching the ground. This diameter is right around the largest raindrop size observed, both in the current study and in studies from the 1940s.

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.”

Over the course of 60 milliseconds, a single drop of water shatters to produce a variety of smaller drops. This solitary explosion may account for the different sizes of raindrops that hit the ground.

Credit: Emmanuel Villermaux

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