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 π,
And that’s only one of the many reasons we need π.
So, to show our appreciation for this incredible numerical value,
we are launching a very exciting new π themed calendar.
You can get it for a limited time at ComplexlyCalendars.com
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 smears,
有长条状的 大圆点状的 小点状的
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 in forming 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 its spheres.
Like although smaller raindrops tended to stay spherical,
when droplets got to around four millimeters 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 backlit by a street lamp,
you might have noticed a streaking effect,
that’s caused by the raindrops wobbling.
But burger buns, streaks, that’s pretty tame,
但跟雨滴碰撞 融合 破碎后形成的形状相比
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”, “disc” and “bag”.
分别定义为“丝状” “条状” “盘状”以及“包状”
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 “disc” 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 “disc”
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 of “disc” called “crown”,
例如 他们定义了另一种“盘状”的变形 “王冠状”
which was caused by slower collision speeds like,
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 π.
In fact, we need π 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 π.
Each month represents one digit of π,
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 you 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.