Falling faster—researchers confirm super-terminal raindrops

February 16, 2015 by Allison Mills, Michigan Technological University
Falling faster—researchers confirm super-terminal raindrops
Michigan Tech Physicists Make a Splash with Rain Discovery

Five years ago, a research team at Michigan Technological University and Universidad Nacional Autanoma de Mexico (National University of Mexico) detected tiny, super-fast raindrops. The finding was unexpected—small drops fell much faster than expected—and now this unexpectedly fast-falling rain has been verified.

Not only do these small raindrops fall faster than expected, they fall faster than they should be able to alone. As an object falls, two forces clash: Gravity pulls it down while air resists. Where the force of gravity matches the force of air resistance, the object reaches its "terminal speed." While the name sounds final, it's not. These small raindrops move faster—they are "super-terminal" raindrops.

Study co-author Alex Kostinski, a professor of physics at Michigan Tech, says confirming the speed was exciting, but not the most surprising result.

"What was so surprising was just how many drops violated the speed limit, so to speak," he says.

Over five months, the research team found super-terminal raindrops in all six rain storms they studied at a site just outside Charleston, S.C. Of the 1.5 million raindrops measured, all drops 0.8 mm and larger fell at expected speeds—and for drops sized 0.3 to 0.8 mm, 30 to 60 percent of them fell at super-terminal speeds.

To detect and measure the falling rain, the team used 22 instruments that optically track each drop as it passes through a laser beam. The equipment was crucial for ruling out instrument error and splashing as the source of the speedy drops.

Lead author Michael Larsen, an assistant professor of physics and astronomy at the College of Charleston, designed the experiment set-up. A Michigan Tech alumnus and former doctoral student of Kostinski's, Larsen says working on this research has helped "recapture some of the magic" of his graduate studies.

"The fact that a substantial fraction of drizzle-sized drops are moving faster than their terminal velocities suggest that we are not just seeing an outlier effect here," Larsen says. "That was a bit surprising to me and helped me realize that there's more science to be done."

With instrumental error ruled out, figuring out how to plug super-terminal drops into existing calculations may change rainfall estimates and erosion impacts. To do so calls into question a base assumption of many models. Kostinski says he is amazed "the assumption that rain consists of single, isolated drops, falling at prescribed speeds, has lasted so long," explaining that this assumption is deeply embedded in atmospheric science.

The quantitative extent of super-terminal raindrops' impact is yet unknown. And before calling to rewrite a bunch of calculations, Kostinski and Larsen want to figure how the drops form in the first place. One possibility is that the super-terminal rain breaks off of larger drops.

"Collisions may be very important, and rain may be more agitated than we think," says Kostinski, explaining the shape of falling raindrops. Certainly more restless than the endearing tear-shaped icon, falling raindrops are actually distorted spheres—flat on the downside, wobbling like a bowl of jello above. Caught between gravity and , the may break up and Kostinski explains, "As larger break up, fragments move at unexpected speeds."

Unexpected but now measured, verified and ready to be explored.

Explore further: Maybe it's raining less than we thought

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4.8 / 5 (5) Feb 16, 2015
Has downward wind been ruled out as a factor? Since small drops have a higher surface to volume ratio, such an effect may affect them more. Very interesting study.
5 / 5 (2) Feb 16, 2015
I would also suggest that since larger raindrops have a higher terminal velocity all it would take would be a larger raindrop to come apart near the measuring device to cause what appears to be super-terminal raindrops. This corruption of larger raindrops can be caused by dust or pollution in the air. And a reverse case of Roto's suggestion - an updraft could cause a larger raindrop to become super-critical pulling it to pieces.

Whenever you look close enough at supposedly known science you are bound to find exceptions.
5 / 5 (2) Feb 16, 2015
Also - could it be that these smaller raindrops are "slip-streaming" larger ones? That is: riding in the reduced atmospheric density on the trailing edge of a rapidly falling drop?
not rated yet Feb 16, 2015
Could be a statistical thing. With large drops the number of air molecules hitting it is very large so the number hitting it from top and bottom equalize. But with small particles the total number of air molecule impacts is much smaller, allowing for more statistical variation. Thus some water droplets will have more air molecules hitting from the top than the bottom and will be sped up.
5 / 5 (2) Feb 16, 2015
No mention of the shape of the smaller drops. Could it be they're elongated along the path of travel and offer lower drag targets to air molecules?
3 / 5 (2) Feb 16, 2015
Another possibility is force from electric fields - it's well known that even modest rain storms can generate large electric fields. Right here in my office is an inkjet printer that accelerates small droplets (roughly 3 picoliters or ~100 microns in diameter) with amazing precision but has no effect on larger drops. Since electric fields in caused by rainfall are much greater than my inkjet perhaps this is "the inkjet effect" on a larger scale?
not rated yet Feb 16, 2015
It could be one of two things. One, the larger ones may be moving slower due to a larger drag from the irregular shape at the bottom. The smaller drops with a more aerodynamic shape just don't fall as slow. Two, We need to study what is happening in the clouds themselves. Perhaps some of the drops are propelled downward giving them a higher velocity to begin with. That would also explain that faster drops are smaller as they break up with the added velocity
5 / 5 (1) Feb 16, 2015
Perhaps the larger drops have a rotation and when a smaller drop is shed it spins off at a higher speed - think a tire falling off of a trailer or car - the rotational center changes as does the speed to match the new center.
5 / 5 (4) Feb 16, 2015
Interesting article, but disappointing in its lack of detail:

How fast were "normal" raindrops falling?
How much faster were "super-terminal" raindrops falling?
Is the difference in terminal velocity vs. super-terminal velocity significant or minimal?

Some data to chew on would have been so enlightening!
2.5 / 5 (2) Feb 16, 2015
Something is adding to the pull of gravity, could be an electrostatic effect.
2 / 5 (5) Feb 16, 2015
Maybe it's not statistical. maybe it's molecular.

Maybe the smaller drops are somehow deforming to "miss" the air molecules.
4 / 5 (1) Feb 16, 2015
Ok so if smaller drops break off from a larger drop, there must be some force strong enough to break the surface tension and allow that to happen.
Say another rain drop nearby.
Now let's say 2 miles up where this happens, the first rain drop is newly forming (and unstable) - while the second rain drop is fully formed and falling at normal speeds.
If a droplet broke off the first - the slower moving rain drop, then followed the disturbance created by the second rain drop - it would be a pattern similar to birds flying "V shape". The droplet would take time to gain speed. The second drop is already moving at terminal velocity. Long as the droplet stays in that disturbance/wake left by the second drop - it would move faster then terminal velocity.

You can do the same experiment in water with your hand and a long toy. Try to move the toy through the water and it moves slowly - with much resistance. Now take your arm and move it through the water with the toy right behind it.
not rated yet Feb 16, 2015
Small droplet following in the wake of a larger rain drop.

----\\\\ <10 feet distance> ----->>>
----//// ------------------------------//////

Ten foot distance between the droplet and raindrop. Long as the droplet never moves fast enough to bridge that ten feet. It will always be in the disturbance left by the larger drop. Gravity is a constant, but in this situation - wind resistance behind the larger drop is not constant. Allowing the droplet to move faster, until it catches up. If droplet doesn't ever catch up to the larger rain drop - then the droplet would be moving faster then terminal velocity when it hits the ground.
not rated yet Feb 17, 2015
I wonder if this could be simulated in a wind tunnel? Measured horizontally so no gravity acceleration. Test for an unexpectedly large number of the smaller drops traveling slower than expected.
1 / 5 (1) Feb 17, 2015
AS the D'Arcy Weisbach equation calculates fluid friction, and the related Froud, Reynolds numbers calculate fluidic viscosity, dynamic and kinematic effects on terminal velocity, changes should ensue. This rainfall should be taken as a model for future applications in other media, like spacecraft seeking to avoid Einstein's limit, which may also be terminal velocity related. just like partial differential equations can get provisionally evaluated in special cases by demanding one or more of the independent function variables be held constant, so to can Einstein's equations if one delves deep into the assymptotic nature of time vs ds/dt plots.
5 / 5 (1) Feb 18, 2015
The air humidity is reaching peak levels. Near 100%.

They are probably torus shaped in the internal flow characteristics, rolling like smoke rings, adding mass at the leading edges and shedding it at the back. Adding and shedding via the molecular levels of near 100% saturation.

They may be long lived or short lived, difficult to say at this point. But, always exchanging mass as a rolling torus.

These are known possible considerations that fit the descriptive of this drop phenomena.

Viscosity drops in flowing systems, so we are looking at reduced viscosity in the internal flow of the drop, so that's covered, as a potential argument against the model.

As well, a flowing mass, with it's lower internal viscosity... holds it's level of inertia (the internal torus motions).

Once initiated and moving, it keeps moving. Just like a smoke ring.

Additionally...look to the recent article on internal behavior of water having multiple phases.
5 / 5 (1) Feb 18, 2015
I would also suggest that since larger raindrops have a higher terminal velocity all it would take would be a larger raindrop to come apart near the measuring device

Well, it says right in the article:
One possibility is that the super-terminal rain breaks off of larger drops.
"As larger drops break up, fragments move at unexpected speeds."


No mention of the shape of the smaller drops. Could it be they're elongated along the path of travel and offer lower drag targets to air molecules?

The shape is determined by the forces involved. And the terminal velocities for (singular) drops of certain diameters have been measured

A 'wake' explanation is unlikely to be the main factor (though it can certainly contribute). But the experimental setup only looks at drops that are 'far' from other drops.

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