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How a Raindrop Is Like an Exploding Parachute
ScienceNOW Daily News ^ | 20 July 2009 | Karen C. Fox

Posted on 07/27/2009 1:21:16 AM PDT by neverdem

Enlarge ImagePicture of raindrop

Boom! A drop of water flies apart in midair.

Credit: Emmanuel Villermaux

Here's a question for a rainy day: How do clouds create such a wide variety of raindrop sizes? The answer, according to stunning new high-speed movies, is much simpler than physicists thought.

The idea has 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. But trying to unravel how the drops crash and break up led to a tough set of equations.

The new movies, however, show a much more straightforward process. Researchers snapped 1000 pictures a second of an isolated water drop as it fell through an ascending air stream. The drop first flattens into a pancake shape, which then balloons like a parachute. The bottommost rim of this chute has a thick, irregularly corrugated rim. Pressure from the air drag eventually breaks the chute apart into numerous smaller droplets--their wide range of sizes is due to the wide range of sizes of the bumps in the rim.

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Credit: Emmanuel Villermaux

Falling water. Watch a water drop break apart in midair.

Overall, the process is sufficient to account for a wide variety of raindrop sizes without needing to resort to drops colliding in midair, says lead author and physicist Emmanuel Villermaux of Aix-Marseille Universite in Marseille, France. More importantly, he says, the equations needed to describe the exploding drops are far less complex than those that would be needed to describe many drops colliding with each other, breaking up and coalescing repeatedly over time.

If a single raindrop breaks up in a statistically predictable way, then determining the wide range of sizes in an entire rain shower varies only with the intensity of the rainfall: Heavier rain leads to larger initial drops and a broader size distribution versus fine mists with homogenous, small drops. Villermaux and Aix-Marseille colleague Benjamin Bossa publish their findings online today in Nature Physics.

Physicist Jens Eggers who studies the dynamics of water drops at the University of Bristol in the United Kingdom is breathing a sigh of relief. "I was expecting things to get complicated, with lots of empirical relationships thrown together," he writes in an e-mail to Science. "Instead, based on a few physical ideas, the authors manage to explain a beautiful empirical relationship ... in a simple and universal way."

Atmospheric scientists, who have long believed that raindrop size is determined inside a cloud and from complex interactions as they fall, may take more convincing. "Mainstream cloud physicists will reject this thesis," e-mails Ramesh Srivastava, an atmospheric scientist who studies cloud dynamics at the University of Chicago in Illinois. Srivastava says that rain size distribution in practice does not seem to correlate to the paper's predictions; whereas Villermaux says evidence shows correlation at 100 meters below the cloud after the drops have had a chance to break up.

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."


TOPICS: Culture/Society
KEYWORDS: parachute; photography; physics; raindrop
There's a short video at the source. The last image reminds me of someone dumping the proverbial "can o' peas." Airborne!
1 posted on 07/27/2009 1:21:16 AM PDT by neverdem
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To: neverdem

Thanks! I am sure Sassy ( my 9r old) will love reading about this. Pandy


2 posted on 07/27/2009 1:42:05 AM PDT by pandoraou812 (elected officials should be required to pass drug, alcohol & dementia testing)
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To: neverdem

Nice article. Can’t use it this info any time soon, or ever... but is interesting.


3 posted on 07/27/2009 2:29:56 AM PDT by lmr (God punishes Conservatives by making them argue with fools.)
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To: lmr
Can’t use it this info any time soon, or ever... but is interesting.

I don't know what it is, but I sense an application.

4 posted on 07/27/2009 2:37:22 AM PDT by neverdem (Xin loi minh oi)
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To: neverdem

I suspect that the critics of this hypothesis are correct in that it does not explain how the big raindrops come about. The idea that raindrops grow by coalescence (assumed in the article) was largely abandoned many decades ago. It is estimated that about 90% of rain forms by the Bergeron Effect. (Look it up!).

It has also long been known that raindrops do funny things as they fall, including tumbling. The parachute effect probably happens only with drops of a certain size.

Very interesting, though, and the film is fun to watch. How does one get 1000 exposures per second? My Sony camera can’t do that.


5 posted on 07/27/2009 4:02:22 AM PDT by docbnj
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To: neverdem

i wonder how representative this is of what happens in the real world. i see different sized drops falling from the sky. also, looking closely at the story, there’s this tidbit... “...as it fell through an ascending air stream”. there are no “ascending air streams” in the real world. looking at the pics/video, i could see how an ascending air stream could affect a drop in the way depicted...but normally (sans ascending air stream) drops appear to fall intact.....


6 posted on 07/27/2009 4:54:35 AM PDT by mreerm
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To: neverdem
Sometimes the limits on our knowledge is surprising. One would think that by now, we'd have the computational power to simulate this stuff and be able to make predictions. (Testable predictions are what makes real science IMO.) It's fascinating that we had to resort to high-speed photography to identify what would appear to be a fairly simple process.
7 posted on 07/27/2009 8:02:00 AM PDT by zeugma (Will it be nukes or aliens? Time will tell.)
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To: neverdem
Sometimes the limits on our knowledge is surprising. One would think that by now, we'd have the computational power to simulate this stuff and be able to make predictions. (Testable predictions are what makes real science IMO.) It's fascinating that we had to resort to high-speed photography to identify what would appear to be a fairly simple process.
8 posted on 07/27/2009 8:02:40 AM PDT by zeugma (Will it be nukes or aliens? Time will tell.)
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To: mreerm

I suspect the ascending air stream was used to simulate the distance the drop would fall in the real world. They probably weren’t able to get the budget for a 2 mile long observation chamber.


9 posted on 07/27/2009 8:05:52 AM PDT by zeugma (Will it be nukes or aliens? Time will tell.)
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To: mreerm
there are no “ascending air streams” in the real world.

FALSE.

Completely and utterly false.

10 posted on 07/27/2009 8:06:40 AM PDT by UCANSEE2 (Where's this tagline thing everyone keeps talking about?)
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To: neverdem
Don't need a video, don't need a study. Stand in a hotel room, one of the old ones that allowed you to actually open the windows, make sure you are at least 4 stories about ground level, no higher than about 8 stories because that makes it hard to see. Get a glass of water, wait until people(this is optional)are walking along the sidewalk below, pour the water out of glass while leaning out the window. You will see the water break up into a circular pattern of small drops, thus putting the lie to the earlier theory of cohesion etc.

Be prepared to answer questions from the police and/or the hotel management about water falling on people below your window, claim innocence and say it must have come from the story above you, or below you(in case you are wondering, this scientific experiment was done in Louisville Kentucky in Sept. 1959 by several new army recruits just posted to Fort Knox Kentucky for training on the M48A2 tanks).

11 posted on 07/27/2009 8:15:02 AM PDT by calex59 (I, me, myself, am actually Jim Thompson)
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To: neverdem

Fourier analysis


12 posted on 08/19/2009 9:06:09 AM PDT by onedoug
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