Some Tiny Raindrops Land Moving Faster Than Their Terminal Velocity

By Allison Bond | June 22, 2009 3:45 pm

rainWho knew this spring’s soggy weather fell under the umbrella of physics research? Scientists found that when raindrops fall faster than physics predicts, the drops have actually broken into smaller droplets, according to a study in the journal Geophysical Research Letters. And because weather services gauge rainfall based on the velocity at which droplets fall–conventional wisdom holds that large drops should hit the ground at a higher speed than do smaller droplets–these results could improve the way we predict weather.

All falling objects have a so-called terminal velocity, a speed they can’t surpass due to air resistance. Therefore, larger drops generally should fall faster because their heftier size helps them power through air resistance more easily than little drops. (In the extreme case, think of fog: water droplets so small they don’t fall at all.) But data showing small drops sometimes impact the ground at the same speed as larger ones showed this conventional wisdom was wrong, and has puzzled scientists for years. To solve the mystery, the researchers collected a shower of data using optical equipment over a period of several years. The team clocked about 64,000 raindrops falling in Mexico City. The researchers measured their sizes and velocities only in extremely calm conditions, so the wind that often accompanies rain could not skew the data. They found that some drops plummeted faster than the so-called terminal velocity for their size [ScienceNOW Daily News].

To explain the phenomenon of fast-flying droplets, the researchers found the expeditious outliers … came in clusters. These superfast droplets probably come from the breakup of fast-moving large drops [Scientific American]. When a fat drop is speeding toward the ground, any droplets that break off of it also are falling at the big drop’s velocity, which could be higher than the smaller drop’s terminal velocity. Lead researcher Alexander Kostinski used the analogy of someone throwing a bottle from a train to explain the process. If the train is standing still, the bottle flies a lot slower than if the train is speeding along at 60 miles an hour [Los Angeles Times].

These results could have a sizable impact on the economy, a third of which is impacted by weather forecasting. That’s because meteorologists build weather models with the assumption that the largest raindrops fall at the greatest speeds and hit the ground with the most force … because the speed ultimately reached by a raindrop (or any falling object) is related to its mass [Los Angeles Times]. Who knows what improvements in weather prediction this study could precipitate.

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Image: flickr / laffy4k

CATEGORIZED UNDER: Environment, Physics & Math
MORE ABOUT: earth science, rain
  • http://N/A Anonymous

    Wow, what a useless and uninteresting scientific study. Maybe research groups should spend their money on something useful for a change.

  • leo

    Perhaps anonymous needs to think outside the box a bit. Studies like these can help answer questions related to engineering, medicine, sports, and the environment, just to name a few.

  • Twit Ter

    Leo, to which box are you refering? The article did not mention a box.

  • Ben R

    No pun intended… right?

  • Karl

    Is it possible that the smaller, high speed drops had been a larger drop that just split up before hitting the ground before they slowed down? The article does not say the the drops were tracked for long periods, probably only just as they hit the ground. This might mean that current rainfall estimations are not that far off and do not need adjusting. Interesting finding that may need more research.

  • Tesla

    Obviously, science needs to redefine “terminal velocity” and the world of physics will be back on its feet again. Whew, that was a close one!

    Maybe the faster ones are ones that have inverted and are experiencing less wind resistance because they are now pointy side down.

  • MyReferenceFrame

    I agree with number 1: what a useless discovery at least on the face of it. The author is stretching in that last sentence, grasping for relevance with all his/her might. Interesting but how this info will help predict weather is beyond me. But I guess that’s the interesting part about discoveries like these: the applications of this knowledge, or simply the principle behind it, may help solve problems we don’t even know about yet.


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