Caught on Film: Raindrop Forms Parachute, Explodes Into Motley Smaller Drops

By Eliza Strickland | July 21, 2009 3:26 pm

raindropThe pitter-patter of raindrops on your umbrella is caused by raindrops of all different sizes, and now physicists have a new explanation for how those raindrops form. A pair of researchers used a high-speed camera (video below the jump) to watch a single drop of water fall and change shape over the course of six-hundredths of a second, and found that the shattering of single raindrops after they leave clouds is enough to explain the wide variety of drop sizes [Science News].

Previously, the leading theory to explain the diversity of raindrops had 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 [ScienceNOW Daily News]. But lead researcher Emmanuel Villermaux says he questioned that theory, with its supposition of frequent collisions. Real raindrops are so sparse, he said, that it is likely a drop would “fall on its own and never see its neighbours” [BBC News].

For the experiment, reported in a study published in Nature Physics, the researchers trained the camera on a single drop falling from a faucet. 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 [Science News]. The air resistance first flattened the drop like a pancake, and then caused it to balloon upwards like an inflating parachute. Soon the air resistance overcame the cohesive forces keeping the drop together, and the parachute exploded into many tiny fragments. The size distribution of those fragments matched the diversity of sizes found in a rain shower, the researchers found.

But atmospheric scientists say they’re not yet ready to abandon the old model, and say they’re unconvinced by the new study. 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” [ScienceNOW Daily News], he says.

Related Content:
80beats: Some Tiny Raindrops Land Moving Faster Than Their Terminal Velocity
DISCOVER: Is It Wednesday? Better Bring an Umbrella
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DISCOVER: The Strange Forests that Drink–and Eat–Fog

Image: Emmanuel Villermaux

CATEGORIZED UNDER: Environment, Physics & Math
  • Roberto Ruth

    I think everything should be filmed with the high speed once.

  • Topaz

    What id like to know is whether after the “parachute” explodes to tiny fragments ( rain drops) Do those aswell turn into smaller parachutes? and the cycles repeats till colission?

  • hat_eater

    @topaz: I don’t think so. Let’s assume the drop disintegrates into droplets three times smaller in diameter. Their surface (and subsequently, the aerodynamic drag) is nine times smaller, but their mass (and subsequently, the gravity force accelerating them towards earth) is 27 times smaller. It’s a very rough estimate (call it a guess) but after such disintegration as was shown on the film to overcome the viscosity and surface tension that keep the droplet together and in more or less spherical form you’d need force in order of magnitude smaller than with the big drop. But the force that acts on it is now TWO orders of magnitude smaller.
    The mystery for me is how the big huge drops that fall the fastest and reach the ground first during a storm manage to stay in one piece all the way to the earth. Or perhaps most of the drops in the cloud are that big and the ones that reach the ground are just the incredibly lucky survivors that did not “parachute”?

  • hat_eater

    One more thing: I just read an older post on the same subject:
    http://blogs.discovermagazine.com/80beats/2009/06/22/some-tiny-raindrops-land-moving-faster-than-their-terminal-velocity/
    So, it seems that the initial velocity of at least some of the droplets is close to the velocity of the big drop. It’s rather inconsistent with what we saw on the film, but anyway, I think they decelerate before they start to “parachute”.

  • http://yahoo TEDJAGO

    Surely in real life the smaller drops would experience the same effect over and over. Why then dont we eventualy only have a “fog” layer…

  • Ralph

    See post #3.. basically the force that holds the droplet together is the surface tension of the drop. The strength of the surface tension is relative to the exposed area of the droplet, but uniform over the area, so that a smaller droplet experiences greater surface tension per accelerating mass than a larger droplet. Imagine a huge swimming-pool size ‘droplet’ of water encased in .5 mil plastic sheeting, versus a pool-ball size ‘droplet’ of water encased by the same sheeting. This is effectively the situation we have. Obviously the larger ‘droplet’ is more prone to be broken apart as it falls through the sky.

  • http://yahoo TEDJAGO

    Watch me pull a rabbit out of my – rain drop?

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