[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.