Monday, April 22, 2024

Toilet water turned upside down in the southern hemisphere? False

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Maria Gill
Maria Gill
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This article is part of a column rumor detectorClick here for other texts.

The origin of the rumor

Already in the sixties, an article was published in the magazine temper nature He stated that this contested hypothesis has re-emerged from time to time. The theory also had two minutes of fame on an episode of Simpsons It was published in 1995 with the title BART vs Australia.

The premise sometimes refers to the bathroom and sink.

Coriolis effect

According to the defenders of this idea, it is the rotation of the Earth that will be called into question, especiallyCoriolis effect that can be produced.

It must first be remembered that because the circumference of the Earth is greater at the equator than near the poles, anything in it must travel a greater distance to complete one revolution in 24 hours. This means that if you stand at the equator, you are “moving” faster than if you are near the poles – even if you don’t feel like it.

The result is that the winds and ocean currents circulating in this system throughout the planet cannot travel in a straight line indefinitely: they follow curves, described in the nineteenth century by the French mathematician Gaspard Gustave de Coriolis.

For example, air masses that move northward in the northern hemisphere are being drawn “to the right” (eastward) as they fly over the portion of the globe that is “moving” more slowly. Conversely, in the Southern Hemisphere, air masses moving towards the south, gravitate towards the “left” (always eastward). The Coriolis Effect, and thus affects the winds, also affects ocean currents.

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This is especially amazing for hurricanes. As shown in National Oceanic and Atmospheric AdministrationA cyclone is a large system that directs air toward the center (the eye of a cyclone). The reason is that the pressure difference between the center (low pressure) and perimeter (high pressure) of the system makes the center act like a sink drain. Moreover, since a hurricane can be hundreds of kilometers in diameter, this is enough for the Coriolis effect to also enter the scene: the winds are deflected by the rotation of the Earth. The result, a hurricane Don’t turn in the same direction In the northern hemisphere than in the southern hemisphere: clockwise in the south and vice versa in the north.

very weak effect

Does the same principle apply to toilets? Those who make this analogy miss two things. On the one hand, a hurricane is much larger. On the other hand, the Coriolis force is less than it appears: the wind acceleration attributed to it is barely one-millionth of that due to gravity, when we are at mid-latitudes. This was explained by Thomas Humphrey in 2001, a scientist cited by the magazine Scientific American.

There are mathematical methods for calculating whether the effect of the Earth’s rotation is a determining factor in the movement of a mass of air or a fluid. This takes into account the latitude as well as the rotation speed and size of the system.

For example, in a normal low pressure system that spans half of the United States, wind speeds are, after all, moderate. By virtue of the so-called Rossby’s name, this system will be affected by the Earth’s rotation, and we will observe the Coriolis effect.

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On the contrary, a hurricane with fast-rotating winds and a relatively small size, will not be affected by the Earth’s rotation. In fact, meteorologists have described hurricanes that somehow rotate, regardless of the hemisphere.

What about toilets?

Regarding whether Coriolis force is at play in baths, experiments have been conducted in the sixties. They aimed to determine if the Earth’s rotation affected the movement of water in the discharge basin. This was indeed the case, but only if all other factors were controlled in the laboratory (sink is completely symmetrical, drains well in the center, there is no air movement on the surface, etc.).

American meteorologist Tom de Liberto Enjoy calculating the Rossby number for a two-meter-diameter children’s pool. He concluded that in order to see the effect of the earth’s rotation when emptying the pond with the help of a valve, the velocity of water circulation must be less than 0.0001 m/s. So the water should be almost completely still.

If the Coriolis force is too low to have an effect in the pool, it can be assumed that this is more true for the toilet bowl. In addition, when the toilet is flushed, the water that is pushed into it begins a very rapid rotational movement that greatly exceeds the effect of the earth’s rotation. In fact, factors such as the shape of the bowl or the position of the drain are more important in determining the direction of rotation.


Although the Coriolis effect has a real influence on many meteorological phenomena, it is not noticeable when flushing the toilet.

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Photo: Jarlhelm / Wikipedia Commons

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