Does bathwater really swirl
the other way south of the equator?
"The plughole vortex spins one way in the Northern Hemisphere and the other way in the Southern Hemisphere." You've probably heard this one. The short answer: it isn't true. But it isn't pure nonsense either. The force behind it is real — it's just wildly too small to matter here. Let's find out exactly how small.
Fill a sink with water and pull the plug. A vortex forms. Note which way it spins.
Do it again. This time it may well spin the other way — same house, same sink, same hemisphere.
Try it in the bath too. Stir the water by hand before pulling the plug, and it keeps spinning the way you stirred it. Being in the Northern Hemisphere doesn't flip the direction on its own.
If Earth's rotation were what decided the direction, none of this should happen. So what's actually going on with that claim?
Because Earth is spinning, anything in motion drifts slightly off its straight-line path. This exact force is what sets the direction of a typhoon's spin. It isn't a made-up effect.
In a sink, its strength is only about one millionth of gravity. It's completely swamped by a slight tilt in the plug's shape or leftover swirl from filling the water.
So the mix-up here isn't whether the force exists — it's whether it actually matters. Let's go through it step by step.
The force is real, no question
Earth turns once a day. At the equator that means moving at over 1,600 km/h. We're all riding along with it.
On something rotating, even if you move in a straight line, the scenery around you drifts instead. Throw a ball on a spinning merry-go-round: seen from outside it flies straight, but to the rider it looks like it curves. Same idea here (Figure 1).
This "apparent curve" effect pushes to the right of the direction of travel in the Northern Hemisphere, and to the left in the Southern Hemisphere. That's why typhoons really do spin in opposite directions north and south of the equator — one glance at a satellite photo confirms it.
None of this is a rumour. Weather forecasting works precisely because this effect is built into the calculations.
So why doesn't it matter in the bath?
The issue is scale. How strong this effect is depends on how large an area, and how long a time, the motion covers.
A typhoon is hundreds of kilometres across and spins for days. Earth's tiny nudge has plenty of time to pile up.
A sink is 30 cm across, and the water drains in tens of seconds. There's no time for anything to accumulate.
Run the numbers and this force on sink water comes out to only about one millionth of gravity. Meanwhile, the direction of the vortex is actually decided by things like:
- Leftover swirl from filling the water — by far the biggest factor. Water poured from a tap is always spinning one way or another
- The drain isn't perfectly symmetric — screw holes, hair catchers, tiny scratches
- The basin is slightly tilted
- Hand movements while pulling the plug
- Slow convection from temperature differences
Every one of these is vastly bigger than the effect of Earth's rotation. So the direction ends up down to chance, each time.
It's "is it big compared to everything else?"
What if you set up the conditions perfectly?
This is the interesting part. People have actually tested it.
In the 1960s, experiments were run in the United States, and a few years later in Australia. The methods were rigorous:
- A round, shallow tank nearly 2 metres across
- A drain built as close to perfectly symmetric as possible
- After filling, leaving it to sit for over 18 hours to let any residual swirl die out completely
- Covering it to block air currents, and keeping room temperature constant
- Pulling the plug remotely and gently, so as not to disturb the water
The result: the Northern Hemisphere experiment reliably produced a counter-clockwise vortex, and the Southern Hemisphere one a clockwise vortex. Exactly as theory predicts.
In other words: the effect is real. But for it to show itself, everything else has to be wiped out first. In an everyday sink, those conditions are never even close to being met.
At tourist spots along the equator, you sometimes see a demonstration where moving a bucket just a few metres across the equator line supposedly flips the direction of the water's vortex.
This has no scientific basis. Moving a few metres can't switch this effect on or off, and in any case, the effect is exactly zero right on the equator — it's the weakest possible spot for it.
In these demonstrations, the spin is quietly built in — through how the bucket is set down, the direction water is poured, or where a leaf is dropped. Once you know the trick, it's a fun bit of showmanship in its own right. See if you can spot it.
Where this force really does matter
- Typhoons, lows and highs — vortices spin opposite ways in each hemisphere. Wind following the isobars on a weather map is this effect balancing out the pressure difference
- Trade winds and westerlies — these set the direction of planet-scale winds
- Ocean currents — involved in the direction of ocean gyres and coastal upwelling
- Long-range shells and rockets — over tens of kilometres, the drift becomes too large to ignore
- Foucault's pendulum — a device that visibly shows the plane of a swinging pendulum slowly rotating, proving Earth's spin
What they all have in common: a large scale, a long duration, or both.
Something you can check at home
- Fill a sink with water. Wait 2-3 minutes for the surface to go still
- Pull the plug and note the direction of the vortex
- Fill it again. This time stir it gently clockwise by hand before pulling the plug
- Do it once more, this time stirring counter-clockwise
- Compare the three results
It should keep spinning whichever way you stirred it. Your hand is overwhelmingly stronger than Earth's rotation. The first case (waiting quietly) can flip direction from day to day — that's decided by tiny differences in room air and water temperature too. Try it a few times and see there's no pattern to it.
Summary
The claim that bathwater vortices reverse by hemisphere is not true. But the force behind it is real, and it genuinely does decide the spin direction of typhoons. Existing and actually mattering turned out to be two different things.
What matters in science isn't "does it exist?"
It's "how much of it is there?"
The force that really does matter around your bath or shower is something much closer to home. Why a shower curtain gets pulled in toward you is explained in Why does the shower curtain cling to you while you shower?.
Want to go deeper? Terms, equations, and how this connects to the classroomLabels below show whether it's middle-school level or an open research question
- MSCovered in middle-school science
- HSCovered in high-school physics
- HS+Advanced high-school physics, or a textbook sidebar/enrichment topic
- Univ.Not covered in high school — university-level content (geophysical fluid dynamics)
- ResearchNot settled even at university level — something researchers are actively studying
MSTerms: words about rotation
- Coriolis force: the name for the effect described above — "on a rotating system, moving objects appear to curve." Named after the 19th-century French scientist Coriolis.
- Apparent force (inertial force): a force that only shows up when viewed from something rotating or accelerating. The feeling of being pulled forward when a train brakes hard is the same family of effect.
- Rotation (of Earth): Earth turning once per day. In angular velocity, that's about 0.0000729 radians per second.
- Foucault's pendulum: a device where a long pendulum, left swinging, has its plane of swing slowly rotate. It makes Earth's rotation visible.
HSHS+Working it out: how many orders of magnitude apart are the bath vortex and the Coriolis effect?
There's a claim that "in the Northern Hemisphere, bath vortices spin clockwise." Whether that's true doesn't need debate — it's settled by calculation. Just work out the size and compare it to other forces.
a = 2 × Ω × v × sinφ
| a strength of the sideways push | units: m/s² (same units as gravity's 9.8) |
| Ω how fast Earth spins | one rotation per day [rad/s] |
| v speed of the moving object | units: m/s |
| sinφ coefficient set by latitude | 0 at the equator, 1 at the poles |
The key point: this a comes out in the same units as gravity. So we can compare it directly against 9.8. No need to argue over whether it "matters" — just divide.
First let's work out Ω. A day is 86400 seconds, and one rotation is 2π, so:
| Calculating Ω | (2 × 3.14) ÷ 86400 ≒ 0.0000727 |
| At latitude 35° | sinφ ≒ 0.57 |
| Speed of bathwater flow | v = 0.1 m/s |
| Substituting | 2 × 0.0000727 × 0.1 × 0.57 ≒ 0.0000083 |
| Gravity is | 9.8 m/s² |
| Fraction of gravity | 0.0000083 ÷ 9.8 ≒ 0.00000085 |
Roughly one millionth. That's the answer.
One millionth is small enough that a 0.001 mm tilt in the tub alone would beat it. The shape of the plug, slight leftover rotation before pulling it, the flow from filling with water, warps in the floor — every one of these is orders of magnitude bigger than the Coriolis effect.
"Clockwise because it's the Northern Hemisphere" doesn't hold up. But under extremely controlled lab conditions (water left standing for days, a symmetric container), it has reportedly been confirmed. It's not that it doesn't act — it's that it gets buried.
Run the same equation for a typhoon and the picture changes. There's one number useful for comparison: the ratio of "momentum" to "Coriolis."
ratio = v ÷ (f × L) (f = 2 × Ω × sinφ ≒ 0.000083, L is the scale)
| Sink v = 0.1 m/s, L = 0.5 m | 0.000083 × 0.5 = 0.0000415 |
| Taking the ratio | 0.1 ÷ 0.0000415 ≒ 2410 |
| Typhoon v = 30 m/s, L = 500000 m | 0.000083 × 500000 = 41.5 |
| Taking the ratio | 30 ÷ 41.5 ≒ 0.72 |
When this ratio is much bigger than 1, Coriolis can be ignored; around 1 or below, it starts to matter. Sink: 2410. Typhoon: 0.72. A difference of over 3,000-fold.
Look at the denominator and it's clear. Scale and duration decide whether it matters. A typhoon spans 500 km and spins for days. A sink spans 50 cm for tens of seconds. Coriolis is weak, but given something large and slow, it gets time to add up.
"Earth's rotation doesn't affect the bath but does affect a typhoon" is a hard thing to accept intuitively. But plug in two sets of numbers and compare, and it comes out clearly as a factor of 3,000. That, I think, is where equations earn their keep.
HSHS+Working out the actual magnitude
The strength of the Coriolis force is given by:
F = 2 m Ω v sin φ
Here m is mass, Ω is Earth's angular velocity (about 7.29 × 10⁻⁵ /s), v is speed, and φ is latitude.
| Coriolis force | 2 × 1 × 7.29×10⁻⁵ × 0.1 × sin35° ≒ 8.4 × 10⁻⁶ N |
| Gravity on the same water | 1 × 9.8 = 9.8 N |
| Ratio | about 1 in 1.17 million |
※ The "one millionth" figure in the main text comes from this calculation. It shifts somewhat with latitude and flow speed.
The key detail is the sin φ term. At the equator (latitude 0°), sin 0° = 0, so the Coriolis force is exactly zero. That's why the equator demonstrations don't hold up. It gets stronger closer to the poles.
Univ.The number that decides "does it matter"
There's a convenient quantity for comparing magnitudes: the Rossby number.
Ro = U / (f L) (U is speed, L is the scale of the phenomenon, f = 2Ω sin φ is the Coriolis parameter)
When this number is much less than 1, the Coriolis force dominates. When it's much greater than 1, it can be ignored.
| Typhoon (L ≒ 500 km, U ≒ 30 m/s) | Ro ≒ 0.7 → matters |
| Low-pressure system (L ≒ 1,000 km, U ≒ 10 m/s) | Ro ≒ 0.1 → matters strongly |
| Sink (L ≒ 0.3 m, U ≒ 0.1 m/s) | Ro ≒ 4,000 → completely negligible |
The Rossby number for a sink is roughly 10,000 times that of a typhoon. This single number shows that, despite describing "the same" kind of phenomenon, an utterly different force is in control. In geophysical fluid dynamics, this number decides which equations are even valid to use.
Incidentally, to see the Coriolis effect in a lab, you need to make this Ro small — either by making U extremely small (slowing the flow) or making L large (using a big tank). The 1960s experiments used "a 2-metre tank" and "18 hours of standing" precisely because that targets both at once.
ResearchWhat's still not well understood
- The theory of the drain vortex itself remains surprisingly hard. Whether an air core forms at the centre of the vortex, under what conditions it's stable, and when the vortex spins up at all — these depend intricately on water depth, viscosity, drain shape, and initial disturbances. In practice this is handled by numerical simulation, and no simple predictive formula exists.
- Which way it spins is, in principle, unpredictable. Vanishingly small differences in initial conditions decide the outcome, making it effectively a coin toss. That unpredictability is itself a feature of the phenomenon.
- The 1960s experiments aren't easy to replicate. The conditions are extremely demanding, and a bit of air movement or temperature difference can throw off the result. Educational attempts to reproduce it have been made in various countries, without much success. It's "an experiment that produced correct results" yet remains stubbornly hard to casually verify — an awkward subject to teach with.
- The effect at continental scale is still debated. The claim that "riverbanks erode more on the right bank in the Northern Hemisphere" (first proposed in the 19th century) is hard to separate from the effects of terrain and geology, and how far it generalises remains unsettled.
Rather than stopping at "the myth is wrong," teasing apart exactly where it's right and where it breaks down is what makes this topic genuinely interesting.
How this connects to the classroom (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: Earth's rotation / forces / weather | The spinning-platform example, typhoon spin direction |
| HS | Physics: circular motion and inertial forces | Its role as an apparent force |
| HS+ | Physics: non-inertial frames (enrichment topic in many textbooks) | F = 2mΩv sinφ, why it's zero at the equator |
| Univ. | Geophysical fluid dynamics, meteorology, physical oceanography | Rossby number, geostrophic wind, experimental design |
| Research | Fluid dynamics (unresolved) | Theory of drain vortices, unpredictability of direction, difficulty of replication |
- Shapiro, A. H., Bath-Tub Vortex, Nature 196, 1080–1081, 1962 (the Northern Hemisphere experiment).
- Trefethen, L. M. et al., The Bath-Tub Vortex in the Southern Hemisphere, Nature 207, 1084–1085, 1965 (the Southern Hemisphere experiment).
- Vallis, G. K., Atmospheric and Oceanic Fluid Dynamics (a standard textbook on the Rossby number and geostrophic flow).
- Japan Meteorological Agency (気象庁), explanatory material on the structure of typhoons and low-pressure systems.
- The 19th-century claim by Karl Ernst von Baer (Baer, K. E. von), and the ongoing debate over how it has since been tested (the conclusion remains unsettled).
※ The figures used in these calculations are representative estimates. They vary with latitude, flow speed, and container size.
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding and can vary with conditions. When observing this at home, please don't leave running water unattended.