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That fabled tear-shaped raindrop may make for a good color-by-numbers, but the truth is that raindrops are a lot more complicated and far less pointy.

Hosted by: Michael Aranda

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Sources:
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https://aip.scitation.org/doi/abs/10.1063/1.1656936
https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.49709640807
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https://www.nature.com/articles/nphys1340
https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/met.1724

Image Sources:
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https://www.gettyimages.com/detail/video/rain-drops-on-the-window-glass-in-4k-slow-motion-60fps-stock-footage/1328307651
https://www.gettyimages.com/detail/video/faucet-in-the-tub-dripping-water-stock-footage/1316112806
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https://www.gettyimages.com/detail/video/instructor-teaching-young-woman-skydiving-extreme-stock-footage/873604368
https://www.gettyimages.com/detail/video/rain-falling-on-pavement-in-slow-motion-stock-footage/950875276
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https://www.gettyimages.com/detail/video/torrential-rain-in-the-rainy-season-a-tropical-stock-footage/1160301686
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Michael: This SciShow video describes the closest thing in nature to a perfect sphere, as far as we know. To find the area of a sphere, you need pi - 3.14, etc. And that's only one of the many reasons we need pi, so to show our appreciation for this incredible numerical value, we are launching a very exciting new pi-themed calendar. You can get it for a limited time at ComplexlyCalendars.com

(SciShow theme)

On a rainy day, when you're lounging inside with a cup of hot cocoa, you'll see raindrops with all sorts of shapes hitting your window - long streaks, big round blobs, tiny spheres. Your window pane turns into an abstract painting pretty fast. But what you won't see falling outside is the well-known pointy-tipped raindrop shape you see in drawings, or even what you see dripping out of your faucet, because that's not what raindrops look like as they're falling.

The real cast of characters is way more interesting, with wobbling blobs, droplets that look like jellyfish and dinosaurs, and more. Welcome to the hidden world of raindrops.

Some of the first experiments that rigorously studied the shapes raindrops form as they fall were done at UCLA in the 1960s, and this wasn't just, like, a bunch of scientists sitting around being curious and bored. They figured that understanding what shapes raindrops make and why would also help with weather forecasting. And they were right, for the record. Their results did end up informing weather models.

In any case, though, they used a wind tunnel in their experiments to suspend droplets of water mid-fall by blowing air up at them. They basically created one of those indoor skydiving experiences, just for raindrops, and that allowed them to take detailed, in-flight photos of them. Their main finding was that raindrops looked pretty spherical; there were no pointy tops.

This is actually about what they expected to see based on previous research, and the reason why was surface tension. Water is a surprisingly clingy molecule. The pull of water molecules toward each other is often stronger than the pull of gravity dragging them down. This allows droplets of water to cling together, to walls, or to your skin. This is also why raindrops don't look like they do in cartoons. As surface tension pulls the water molecules together, any pointiness in the raindrop gets smoothed out.

But this team in the '60s also found that, when different forces get involved, the droplets don't stop at spheres. Like, although smaller raindrops tend to stay spherical, when droplets got to around 4 mm across, they had more flattened out bottoms, like hamburger buns. This shape forms because, as the droplet falls, it pushes against the air below it, and because bigger droplets get buffeted by the air more, they get flattened more.

And the factors that affect droplet shape and size don't stop there. Electric charge and even aerosols like soot from cars can also impact things, meaning human activity can also affect droplet shapes. In fact, so much goes into this process that different storms will generally have different-looking raindrops depending on the conditions.

Like, heavier rainstorms tend to have bigger raindrops, because there's just more water around. But when those raindrops get too big, they can often inflate and burst like the world's tiniest water balloons as they get buffeted by the air. Plus, if the air is moving in an especially erratic way during heavy rain, that can cause turbulence, which can push around the droplets in unpredictable ways and make them wobble.

And the next thing you know, those droplets are twisting and turning and moving through all kinds of distorted shapes. For instance, if you've ever seen rain at night back-lit by a streetlamp, you might have noticed a streaking effect. That's caused by the raindrops wobbling.

But burger buns, streaks, that's pretty tame compared to what happens when raindrops collide, combine, and break up. That's where the real fun starts. In the 1970s, researchers from the University of Toronto set up an experiment to photograph droplets colliding, and things got weird. They found that the shapes they noticed during collisions fell into four categories, which they called "sheet," "neck," "disk," and "bag."

The "sheet" is the most common type of collision, and it's when the smaller droplet tears a chunk off the larger one, leaving it looking like an upside down jellyfish. Meanwhile, the "neck" shape, also known as a "filament," is when the smaller droplet just glances off the larger one. Instead of savagely ripping a piece off, it just drags a stream of water along with it as the drops separate. It kind of looks like a long-neck dinosaur.

Next up, the "disk" shape is when the smaller drop hits near the center of the larger one in a nice, satisfying splat. The collision makes the droplets coalesce for a bit before moving apart. And lastly, there's the elusive "bag" shape. If you're playing raindrop bingo, this is going to be an especially hard box to check, because it's a rare variant of the "disk" caused by a dead-on collision. It results in a big lump, followed by a larger shower of droplets when everything breaks up.

Although these researchers in the '70s stopped at four categories, later teams expanded the list by describing other shapes they saw. For instance, they identified another variant on "disk" called "crown," which was caused by slower collision speeds. Like you might guess, it looks like a crown.

They also found the aftermath of the collision is different in each case, too, resulting in different numbers, sizes, and even more shapes of droplets after the droplets separate. But whatever the scenario, there's not a pointy-topped raindrop in sight. So the next time you're staring wistfully out the window on a rainy day, know that what you see passing you doesn't look like the tear-shaped form in cartoons, but the truth is way more interesting. It turns out there's lots to learn when studying the shape of water.

So raindrops are way cooler than that teardrop shape. They go through so many dynamic stages that it doesn't even begin to capture, and to learn about their qualities in the tiny spherical stage, we need pi. In fact, we need pi for so many reasons. We need it to make sense of this universe, and because it's so important to us, we've designed a calendar in homage to pi.

Each month represents one digit of pi, starting with the three hearts in an octopus, and if you didn't know that an octopus has three hearts, there's a very SciShow-y blurb in each month to tell you more about it. The calendar has all sorts of amazing holidays highlighted in it, like February 11th, which is International Day of Women and Girls in Science, and November 18th, which is LGBTQ+ STEM Day.

To get yours before they run out, you can head over to ComplexlyCalendars.com, or click the link in the description. Thanks for watching this video, and thanks for supporting SciShow.