and I wanted to definitively answer the

question: does water actually swirl the

opposite direction down the drain in the

other hemisphere? At the time, I was

living in Sydney, Australia, and Destin

was in Huntsville, Alabama. Those are

about the same latitude, just opposite

sides of the Equator, so we created two

videos that were meant to be watched

simultaneously, and if you want to

synchronize them yourself, you can click

the link in the description, but we also

recognize that that was a hassle for

some people, particularly watching on

mobile devices, and that's why I'm now

releasing this extra wide video of both

videos synchronized. The Simpsons did a

whole episode based on toilet flushing

the opposite direction in Australia. Plus,

other shows have supposedly demonstrated

this effect.

Destin: So I have seen documentaries

that seemed to indicate that which

hemisphere you're in determines which

way the water is going to swirl, but

there's this other group of people, and

they seem really confident that it

doesn't matter where you're at, the

water's going to swirl however it wants.

Derek: so is this a real effect or not?

Hank: the application of this principle to

draining water in Earth's two hemispheres

is just bunk.

Destin: But if you ever just look

for yourself and figured out which way your

toilets swirls,

Derek: if you try it yourself

you'll find inconsistent results.

Destin: Here in Alabama I've noticed that some turn

counterclockwise and some go clockwise

Derek: this sink sometimes drains one way and

sometimes the other way.

Destin: You see, most will it have little jets in it, so the

swirl direction is determined by the

design of the toilet and not which

hemisphere you're in.

Derek: In any container of

water there's always going to be some

rotation: the water is not perfectly

still, and it is this, rather than the

hemisphere, that determines which way the

water will swirl down the drain.

Derek: so it's a myth: crossing the equator does not

mean the toilet's going to change

directions. But what if we come into the

garage and do a more controlled

experiment?

Derek: But what if we could

eliminate all motion from the water?

Destin: This is a one and a half meter kiddie pool.

Derek: I have here a five foot wide kiddie pool.

Destin: Instead of filling the pool with no vorticity at

all, I'm going against the way it's

supposed to drain.

So I'm trying to fill

it with a flow going clockwise.

Derek: I actually fill the pool in the

anti-clockwise direction to be sure that

any clockwise motion we see is no

due to the way I filled the pool.

Destin: So let's let the water settle for a complete day

so that we know that it's perfectly still.

Derek: I've left this water sitting here for 24

hours. So it seems like I damped all of

the motions from the filling.

Destin: I'm not going to reach in and pull a plug out

because that would induce some vorticity.

I'm going to use this valve that I have

connected to the bottom of the pool.

Derek: I really hope this works. Destin, wish me luck.

Destin: Good luck Derek.

Derek: I'm about to pull

the plug.

Destin: okay opening the valve in

Both: three, two, one

Destin: Water should be flowing

Derek: And the pool is draining. But you can't see any

motion of the water just yet.

been flowing for a couple of minutes and I

haven't seen anything yet, so we're going to

put some dye in it.

Derek: To help us see where the water is blowing I'm going to put

some food coloring in on the four

cardinal directions around the pool.

Destin: Check it out, it's like a tornado like right off

the bat.

We got a counterclockwise rotation.

We filled it up clockwise, and now

it's going counterclockwise.

Derek: You can clearly see that the water is

flowing clockwise, in this direction, and

that makes sense because that's how it

should flow in the southern hemisphere

due to the Earth's rotation.

Destin: We have a kiddie pool in my garage, and the whole

Earth is rotating and the water's going

counterclockwise because I'm in the

northern hemisphere. It's real! This is real!

Derek: But you can see what a tiny little

effect it is and what extraordinary

lengths I had to go to to see this

effect. So really, you're not going to see

it in the bathtub and you're not going

to see it in a sink or in a toilet

because there are other sources of

angular Momentum that totally wash out this effect.

Destin: we just proved it because we

just eliminated variables.

Destin: The Coriolis Effect is real.

Derek: The Coriolis Effect, There you have it. It works!

To understand how it works, imagine a pool

with one edge touching the South Pole.

Destin: Think about a pool near the North Pole.

Derek: the pool is stationary relative to Earth,

but every day it's actually completing one full revolution.

Destin: the Earth is spinning on its axis, so the pool spins around the

pole once a day.

Derek: Now you can see the side

of the pool furthest from the pole

travels much farther every day than the

side right next to the pole.

Destin: the pool is moving but the part that's

closest to the Equator has more momentum,

and the part that's closest to the pole has less.

Derek: So the outer side of the pool is

moving fastest towards the east and as

you get closer to the pole the velocity

decreases down to zero.

Destin: Think about these velocities relative to the drain in the middle.

Derek: Now, imagine we drain the pool

Destin: When we pull the plug, all the water

starts moving towards the middle.

Derek: Water from the far side is moving too fast

relative to the drain, and so it gets out

ahead, whereas water from near the pole

is going too slow, and so it lags behind.

Destin: The side nearest the equator is going

faster, so that water outruns the drain,

but the water nearest the pole is going

slower, so it falls behind.

Both: So when the water approaches the drain it swirls

(counter)clockwise.

Destin: this is the reason

hurricanes swirl counterclockwise in

the northern hemisphere,

Derek: And this is the

reason cyclones swirl clockwise in the

southern hemisphere.

Destin: The center of the

hurricane has lower pressure, just like a

drain, so the hurricane swirls just like our pool.

Derek: The higher pressure air rushes into

the eye of the storm, and just like in

our pool, swirls in the direction

dictated by the hemisphere.

Both: And that's the truth about toilet swirls.

[End of Sync]

Derek: Hey, I hope

you enjoyed that, and thanks for watching.

If you haven't subscribed to Smarter

Every Day, Destin's channel, yet then

click here, or if you want to see his

latest video, click here, and if you're

not subscribed to Veritasium for some

reason, you can do that by clicking here,

or see my latest right here. [disappears]