Rumor has it that toilets flush backwards in Australia.
Which sounds dumb.
So I decided to investigate.
The Toilet Swirl
We’ve all heard the myth about how toilets flush differently in the Northern and Southern Hemisphere.
Sadly, once you research it, you quickly discover that the toilet swirl depends mostly on the shape and design of the toilet itself, rather than your position on earth.
But there is indeed a correlation between location and water swirls and that is due to no other than the earth’s own rotation. Smart people called it the Coriolis Effect.
Let’s take a hypothetical case: draining still water from similar containers, one located in the Northern Hemisphere and another in the Southern.
The Coriolis Effect
If we’d look at the earth from space, we’d see that it rotates counter-clockwise around its axis (at approx. 1000mph). This means that any point we consider within the earth’s surface has a tangential (sideways) velocity induced whit the earth’s rotation. Right now, we’re rotating at 1000mph. We just don’t notice it because we’re within the same reference frame as the earth.
Let’s consider random points on different spots of the earth’s surface. The points closer to the earth’s axis will have a lower sideways speed than the points near the equator, because these points are far more distant from the center of rotation, thus moving faster:
Considering now our case study: you just filled 2 pools with water on both hemispheres:
After waiting long enough for the water to come to absolute rest, you decide to open the drain to let the water out.
Let’s look at some random water particles and break down the dynamics:
- On the NH, the water particles on the upper side of the pool have lower tangential velocity than the particles on the southern side of the pool, because the south side is closer to the equator.
- On the SH, the upper side of the pool gathers points with higher sideways velocity, since it is closer to the equator, while the lower particles, closer to the earth’s axis, suffer less tangential velocity.
What happens once you open the drain valve?
Water will start moving toward the drain hole. In both pools, the particles with higher sideways speed, V, will move faster to the right than the ones with lower V.
The mass of faster points will move in the resulting direction and push the masses of slower points out of the way, thus the slower particles move in the opposite direction of the earth’s rotation.
The end results:
The Northern Hemisphere pool drain will induce a counter-clockwise rotation in the water, while the Southern Hemisphere pool water will rotate clockwise.
This is called the Coriolis Effect. It’s not a meme fellas, it’s science.
Not all toilets might entertain you with contrary swirls on opposite hemispheres, but this relationship between the earth’s rotational speed and fluids is real and finds applications in more than just baby pools.
It’s important to note that this effect is only manifested as described here if the water is absolutely still and has no external agents/forces in action. The distance between the most extreme water particles within a pool is too minimal to dominantly manifest the Coriolis effect, therefore any flow disturbances or singularities in the water and the pool’s surfaces could affect the direction of the swirl.
There are scenarios more dominantly determined by the Coriolis Effect, namely the rotation direction of Cyclones. This is also the reason why there are rarely any Tornados near the equator — it’s the weakest spot on Earth for the Coriolis forces to be manifested.
Here’s a spectacular tornado pic to make up for the sh*t drawings.
