

Video: See an exploding water droplet Video: See an exploding water droplet

The short lifetime of a raindrop is a complicated and explosive affair, physicists have revealed. Using high-speed video footage they have solved a longstanding conundrum of what determines the size of raindrops.

A century ago, physicists put out sheets of absorbent paper in showers to record raindrop size, and discovered a surprising variety. Most are under a millimetre across, but others span 5 mm – and providing rainfall level is constant, drops exhibit the same variation in every shower.

But over the intervening years, no one has been able to explain why this should be. The favoured explanation has been that the large water droplets produced by clouds collide with one another to release watery shrapnel that produces the abundant smaller droplets seen at ground level.

Diesel clue

This explanation has its problems, though, Emmanuel Villermaux at Aix-Marseille University in France points out.


Even in a heavy shower there may be only a few thousand drops in every cubic metre, and at most a few tens of collisions between these droplets on their way to the ground from a cloud 1 kilometre up. “That’s not enough for a stable distribution [of droplet sizes], he says.

High-speed footage captured by Villermaux and colleague Benjamin Bossa at the University Institute of France in Paris suggests a more unexpected explanation. They think individual droplets inflate and then explode to create the smaller droplets so common at ground level.

The pair got the idea from the unusual but well known transformations of fuel droplets travelling at high speed in diesel engines. As they travel, drops flatten from a sphere into a pancake-like disc; this catches passing air and inflates like a liquid parachute that eventually explodes in a shower of smaller droplets.

Eye on the sky

“Nobody had made the connection with rain,” says Villermaux. The researchers used a high-speed camera to record the way a large water droplet falling into an upward jet of air goes through the same process.

When they measured the size distribution of the droplets resulting from the explosion, they found it matched the results expected from rainfall.

The next step is to demonstrate that the same phenomenon occurs with real rain drops falling from the clouds. “Surveys are on the way,” he says. “[Researchers] are adding high-speed cameras to airplanes flying through precipitation. The evidence will come.”

Journal reference: Nature Physics (DOI: 10.1038/nphys1340)