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What Do Raindrops Really Look Like?

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06:54   |   Sep 25, 2018

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What Do Raindrops Really Look Like?
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  • [OPEN]
  • I just got done skydiving, indoors!
  • So I could feel what it’s like to be a raindrop.
  • Because when you picture a raindrop, you’re probably picturing it wrong.
  • But before we get into that, I’m going back in there, because that was awesome…
  • Ok, so if I ask you to draw a raindrop, most of you would probably draw this.That’s how
  • we’ve been doing it since we were kids, right?
  • But this is wrong.
  • A falling raindrop doesn’t look like this… it’s physically impossible.
  • Now, when water drips  – say, from a faucet – each drop does kiiiind of take a teardrop
  • shape, as its tail drags behind it… but only for a split second.
  • Pretty quickly, the drops become blob-shaped.
  • And that’s because surface tension takes over.
  • Surface tension happens because water molecules are more attracted to each other than the
  • air around them.
  • So once they split from the faucet or whatever they fall from, they form the shape with the
  • smallest surface area for its volume, which is a sphere.
  • So raindrops can never form that old teardrop shape.
  • But they also aren’t perfect spheres.
  • Because raindrops are falling.
  • Fast.
  • And this means they’re subject to air resistance.
  • Now, air is a fluid.
  • It’s obviously not wet, the way we typically think of a “fluid”.
  • But in physics, a fluid is just a substance that deforms, or flows, around an object when
  • that object is pushing on another one.
  • Now, if you’ve ever held your hand outside the car, you’ve felt the air deform, or
  • flow, around your hand.
  • But the air also exerts a force against your hand, and to keep your hand still, your muscles
  • have to exert an equal force in the opposite direction.
  • This is what happens when water’s falling through the atmosphere!
  • Several forces are acting on it at once.
  • Gravity is pulling it down.
  • Collisions with air molecules provide a force in the other direction.
  • And there’s attractive forces between the water molecules holding the drop together.
  • All of these combined, flatten out the spherical drop into a sort of hamburger-kind of shape.
  • Of course, how do we know for sure that’s what they look like?
  • We can’t exactly go up in the sky with a magnifying glass and fall along with raindrops
  • to examine their shape… but we can do that down here on Earth.
  • With a really big fan.
  • These droplets are suspended in a vertical wind tunnel.
  • The same kind used for indoor skydiving.
  • Droplets suspended in a vertical wind tunnel are experiencing the same net forces as falling
  • raindrops.
  • Only instead of the droplets falling and hitting the air on their way down, the air is rising
  • and hitting the drops on its way up.
  • As an object begins to fall, due to gravity, it accelerates.
  • Its velocity increases.
  • Until the force of collisions with air molecules is equal to the force of gravity pulling it
  • down.
  • At this point, it stops accelerating.
  • The velocity levels off.
  • This is terminal velocity.
  • Different objects have different terminal velocities, depending on their surface area,
  • their mass, things like that.
  • Now, my body wants to get to the ground because of gravity.
  • And the tunnel is blasting air up, for me, at about 95 or 100 mph… (label with 95-100
  • mph underneath 43 - 45 m/s)
  • For a professional stunt flyer, the wind speed could be as high as 150 mph.
  • I’m clearly not a professional.
  • The point is, I can float in a wind tunnel because of those opposing forces: gravity
  • in one direction and the collisions of air molecules in the other.
  • An object floating in a wind tunnel is experiencing the same net forces as an object falling at
  • terminal velocity.
  • So when we suspend a droplet of water in the wind tunnel, we’re seeing exactly what we’d
  • see if we were falling through the air, next to a raindrop, at terminal velocity!
  • And what we see is definitely not the old shape we drew when we were kids.
  • Real raindrops actually come in four rough shapes.
  • We’ll call them: spheres, burger buns, pancakes, and parachutes.
  • Around 1900, a farmer-turned-amateur scientist named Wilson Bentley started putting out pans
  • of flour, to collect raindrops and measure their size.
  • He measured 70 different rainstorms this way!
  • And Bentley realized no matter what the conditions, most raindrops that hit the ground are small.
  • And around the same time, German physicist Philipp Lenard figured out a way to look at
  • raindrops as they were falling -  he built a vertical wind tunnel!
  • What he saw was that the biggest drops didn’t stay big for long.
  • Remember how I said spherical, blobby shapes minimize surface area thanks to surface tension?
  • The smallest cloud droplets start out as these spheres.
  • But on the way down, small drops bump into each other and combine into bigger ones.
  • Those larger raindrops have more surface area for air to push on, so they flatten out even
  • more.
  • Once a drop reaches five-to-six millimeters – that’s about the size of a housefly
  • – it’ll go from bun shaped, to parachute shaped.
  • And as it gets bigger, it rips itself apart: The force from air becomes more than the attraction
  • between water molecules, and it scatters into a bunch of smaller, rounder drops.
  • So how big can a raindrop be?
  • That’s hard to say - but in tests they rarely ever hit 7mm across before they break apart.
  • So.
  • Physics tells us rain is more pancakes and hamburgers than teardrops.
  • Hopefully I didn’t ruin your childhood.
  • Stay curious.

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What do raindrops look like? Exactly how we drew them as kids, right? Wrong! Teardrop-shaped rain is physically impossible. This week I went inside a vertical wind tunnel to bring you the true shape of rain.

Special thanks to iFly Indoor Skydiving in Austin, TX! Learn more about their STEM field trips: https://www.iflyworld.com/programs/stem-field-trips/

REFERENCES:

Dr. Scot Martin, Laboratory of Environmental Chemistry at Harvard University https://www.seas.harvard.edu/directory/smartin

Terminal Velocity of Raindrops Aloft
https://journals.ametsoc.org/doi/10.1175/1520-0450%281969%29008%3C0249%3ATVORA%3E2.0.CO%3B2

The Terminal Velocity of Fall for Water Droplets in Stagnant Air
https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281949%29006%3C0243%3ATTVOFF%3E2.0.CO%3B2

Single-Drop Fragmentation Determines Size Distribution of Raindrops
https://www.nature.com/articles/nphys1340

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