Using high-speed videography, a pair of researchers may have cracked the mystery of how raindrops take shape between the clouds and the ground.

The conventional view held that raindrops take shape as a result of the forces resulting from numerous collisions and mergers with other drops as well as from the atmospheric drag that fragments the drops; these interactions cause raindrops to adopt a variety of sizes by the time they reach the ground. But according to Emmanuel Villermaux, a physicist at the University of Provence in France, collisions and mergers turn out not to be an important part of the equation after all—the critical factor in shaping raindrops is the drag-induced breakup of single drops, not the interactions between them.

Villermaux is the lead author on the paper presenting these findings, published online today by Nature Physics. (Scientific American is part of the Nature Publishing Group.)

"The common belief was that once very small droplets have aggregated inside the cloud," they form lumps big enough to fall from the cloud, Villermaux explains. "These lumps, while they fall to reach the ground, interact with their neighbors in a kind of aggregation session to build up the distribution of the sizes in the rain as it falls."

The problem, he says, is that those interactions are unlikely to happen in the relatively empty airspace through which rain falls. "Once the drops have left the cloud they are so dilute in the air that they are not likely to interact so much," he says. "Their frequency of collision is not enough for many interactions to occur from the base of the cloud to the ground."

Villermaux and then–graduate student Benjamin Bossa tracked falling drops of water with a high-speed video camera [see video below], breaking down the evolution from drops to droplets frame-by-frame. To accelerate a process that ordinarily develops over a lengthy free fall, Villermaux and Bossa released their simulated rain into an upward countercurrent of air.

The drops flattened out as a result of the drag force exerted by the air, sometimes forming an extending ligament that subsequently shredded into smaller drops and sometimes inflating into a downturned bag that would then burst into numerous fragments.

The deformation and ensuing breakup of single drops produced a menagerie of smaller droplets with the same distribution of sizes that is found in natural rainfall at ground level, pointing to the key role of the disintegration of single drops in determining the droplets' final state.

"As soon as you've understood what occurs at the scale of a single, isolated drop, irrespective of any interaction with its neighbors, you realize that quantitatively the fragment products of a breaking drop represent the statistical content you have in rain," Villermaux says. "So the interaction mechanism is not necessary to account for the statistical content of rain. That's the key observation."

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