Does Water Swirl the Other Way in the Southern Hemisphere?

Derek: A couple of years ago my friend Destin 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 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.

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