Does a bath drain
the other way down south?
"Drains swirl the opposite way in the Northern and Southern Hemispheres." You've probably heard it. Short answer: it isn't true. But it isn't total nonsense either. The force is real — it's just wildly, hopelessly too small. Let's count exactly how small.
Fill a basin with water and pull the plug. A swirl forms. Note which way it spins.
Do it again. This time it might spin the other way. Same house, same basin, 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 override that.
If Earth's spin were what decided the swirl's direction, none of this could happen. So what's actually going on with that claim?
Because Earth is spinning, anything moving gets nudged slightly off its straight-line path. This is exactly what decides which way a typhoon's swirl turns. It's not made up.
In a basin, its size is only about one millionth of gravity. It gets completely buried under a slight tilt in the plug hole or leftover swirl from filling the water.
In other words, this myth mixes up "does it exist" with "does it matter". Let's go through it step by step.
The force is real, no question
Earth turns once a day. At the equator that's a speed of over 1,600 km/h. We're all riding on it.
On something spinning, even if you move in a straight line, the scenery around you drifts off course. Throw a ball on a spinning playground roundabout: seen from outside it goes straight, but to the rider it curves. Same idea (Figure 1).
This "looks-curved" effect pushes to the right of travel in the Northern Hemisphere and to the left in the Southern Hemisphere. So typhoons really do swirl opposite ways in each hemisphere. One glance at a satellite photo confirms it.
None of this is myth. Weather forecasting works precisely because this effect is built into the calculations.
So why doesn't it work in the bath?
The problem is scale. How strong this effect is depends on how large an area, and how long a time, something moves over.
A typhoon is hundreds of kilometres across and spins for days. Earth's tiny nudge has plenty of time to build up.
A basin is 30 cm across, and the water drains in tens of seconds. There's no time for anything to build up.
Do the sums, and this force on basin water comes out to only about one millionth of gravity. Meanwhile, the swirl's actual direction is set by things like:
- A slight leftover spin from filling the basin — the biggest factor by far. Water from a tap is always turning one way or the other
- An imperfectly symmetrical drain — screw holes, hair catchers, tiny scratches
- A slightly tilted basin
- The motion of your hand pulling the plug
- Slow convection from temperature differences in the water
Every one of these is orders of magnitude bigger than Earth's spin effect. So the swirl's direction comes down to pure chance on the day.
It's "is it big compared to everything else".
What if you strip away every other factor?
Here's 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 method was rigorous:
- A shallow, round tank nearly 2 metres across
- A drain built to be as close to perfectly symmetrical as possible
- After filling, leaving it undisturbed for over 18 hours so any leftover swirl died away completely
- Covering it to block air currents, holding room temperature steady
- Pulling the plug remotely and gently, without disturbing the water
The result: a counter-clockwise swirl in the Northern Hemisphere runs, a clockwise swirl in the Southern Hemisphere runs, reproduced reliably. Exactly as theory predicts.
So here's the picture: the effect is real. But for it to show up, everything else has to be erased first. An everyday basin never meets that bar.
At tourist spots along the equator, you'll sometimes see a demonstration where moving a bucket of water just a few metres across the equator line supposedly flips the direction of its swirl.
This has no scientific basis. Moving a few metres can't flip this effect, and in any case the effect is exactly zero right on the equator (it's the weakest possible spot for it).
In the demonstration, the swirl is quietly set up by how the bucket is placed, the direction water is poured, or where a leaf is dropped in. Once you know the trick, it's a fun show in its own right. See if you can spot how it's done.
Where this force really does matter
- Typhoons, lows and highs — their swirl direction flips between hemispheres. It's also why wind follows the isobars on a weather map: this effect balances out the pressure difference
- Trade winds and westerlies — they set the direction of Earth's large-scale wind bands
- 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 big to ignore
- Foucault's pendulum — a device that demonstrates Earth's spin by slowly rotating its swing plane
What they share is a large scale, a long time, or both.
Something you can check at home
- Fill a basin with water. Leave it for 2–3 minutes until the surface settles
- Pull the plug and note which way the swirl goes
- Fill it again. This time stir it gently clockwise by hand before pulling the plug
- Do it once more, this time stirring counter-clockwise before pulling the plug
- Compare the three results
It should keep spinning whichever way you stirred it. Your hand is overwhelmingly stronger than Earth's spin. The first case (waiting quietly) may go a different way on different days — that too is decided by tiny differences in room air or water temperature. Try it a few times and confirm there's no consistent pattern.
Summary
The claim that a bath drains the opposite way in each hemisphere is not true. But the force itself is real, and it does decide which way typhoons swirl. Existing and mattering turned out to be two different things.
In science, what matters isn't "does it exist"
but "how much".
The force that really matters around your bath or shower is something much closer to home. Why a shower curtain clings inward toward you is explained in Why does the shower curtain cling to you while you're showering?
Want more? — terms, formulas, and where this fits in the textbooksLabels show which level each part belongs to, from middle school through active research
- Middle schoolcovered in middle-school science
- High schoolcovered in high-school "Physics Basics"/"Physics"
- High school+high-school "Physics", or advanced/column material in textbooks
- Universitynot covered in high school — university-level geophysical fluid dynamics
- Researchnot settled even at university level — what researchers are still working out
Middle schoolTerms: words around rotation
- Coriolis force: the name for the effect discussed above — things moving on a rotating system appear to curve. Named after the 19th-century French scientist Coriolis.
- Apparent force (inertial force): a force that only appears from the point of view of something rotating or accelerating. The feeling of being pulled forward when a train brakes hard is the same family.
- Rotation (Earth's spin): Earth turning once a day. As an angular speed, that's about 0.0000729 radians per second.
- Foucault's pendulum: a device where a long pendulum, left swinging, slowly rotates its plane of swing — a visible demonstration of Earth's spin.
High schoolHigh school+Working it out with formulas: how many orders of magnitude apart are a bath's swirl and the Coriolis effect?
There's a claim that "in the Northern Hemisphere, bath swirls turn clockwise". Whether it's true doesn't need arguing — a calculation settles it. Work out the size, and compare it with the 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 turns | one rotation per day [rad/s] |
| v speed of the moving thing | units: m/s |
| sinφ factor 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 it can be compared directly with 9.8. No need to argue over "does it matter or not" — just divide.
First, work out Ω. A day is 86,400 seconds, and one rotation is 2π, so:
| Calculate Ω | (2 × 3.14) ÷ 86400 ≒ 0.0000727 |
| At latitude 35° | sinφ ≒ 0.57 |
| Speed of the bath water | v ＝ 0.1 m/s (assumed) |
| Substitute | 2 × 0.0000727 × 0.1 × 0.57 ≒ 0.0000083 |
| Gravity is | 9.8 m/s² |
| Fraction of gravity | 0.0000083 ÷ 9.8 ≒ 0.00000085 |
About one millionth. That's the answer.
One millionth is a size so small that a 0.001 mm tilt in the tub alone would beat it. The drain's shape, a little leftover swirl before you pull the plug, the flow from filling the tub, 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 in a lab under extreme conditions (water left standing for days, a symmetrical container), it has reportedly been confirmed. It's not that the effect doesn't work — it gets buried.
Look at a typhoon with the same formula, and the picture changes. There's one number that lets us compare: the ratio of "momentum" to "Coriolis".
ratio ＝ v ÷ (f × L) (f ＝ 2 × Ω × sinφ ≒ 0.000083, L is the scale)
| Bath v ＝ 0.1 m/s, L ＝ 0.5 m | 0.000083 × 0.5 ＝ 0.0000415 |
| Ratio | 0.1 ÷ 0.0000415 ≒ 2410 |
| Typhoon v ＝ 30 m/s, L ＝ 500000 m | 0.000083 × 500000 ＝ 41.5 |
| Ratio | 30 ÷ 41.5 ≒ 0.72 |
When this ratio is well above 1, Coriolis can be ignored; around 1 or below, it starts to matter. Bath: 2410. Typhoon: 0.72. A gap of over 3,000 times.
Look at the denominator and it's clear: "size" and "time" are what decide whether it matters. A typhoon spans 500 km and spins for days. A bath is 50 cm and lasts tens of seconds. Coriolis is weak, but given something big and slow, it has time to add up.
"Earth's spin doesn't affect a bath but does affect a typhoon" is hard to accept on gut feeling alone. But plug in the numbers twice, compare them, and a factor of 3,000 falls straight out. That's the value a formula gives you.
High schoolHigh school+Working out the actual size
The size of the Coriolis force is given by this formula.
F = 2 m Ω v sin φ
m is mass, Ω is Earth's angular speed of rotation (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" quoted in the main text comes from this calculation. It shifts a bit 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 demonstration can't be genuine. It gets stronger the closer you get to the poles.
UniversityThe number that decides whether it matters
There's a convenient quantity for comparing scales: 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, Coriolis dominates. Much greater than 1, and it's negligible.
| 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 a lot |
| Basin (L ≒ 0.3 m, U ≒ 0.1 m/s) | Ro ≒ 4,000 → completely negligible |
A basin's Rossby number is about 10,000 times a typhoon's. One single number shows that the same-looking phenomenon is governed by an entirely different force. In geophysical fluid dynamics, this number decides which equations are valid to use.
Incidentally, to see the Coriolis effect in a lab, you need to shrink this Ro. That means either making U extremely small (slowing the flow) or making L large (using a big tank). The 1960s experiments' "2-metre tank" and "18-hour standing time" were chosen to push both of these at once.
ResearchWhat's still not fully understood
- The theory of drain vortices themselves remains surprisingly hard. Whether an air core forms at the vortex centre, when it stays stable, and when the vortex first spins up, all depend intricately on water depth, viscosity, drain shape and initial disturbances. In practice, people rely on numerical simulation — no tidy predictive formula exists.
- Which direction it turns can't, in principle, be predicted. An infinitesimally small difference in starting conditions decides the outcome, so it's effectively the same as a coin flip. Unpredictability is itself a feature of this phenomenon.
- The 1960s experiments aren't easy to reproduce. The conditions are extremely strict, and a slight air current or temperature difference throws the result off. Attempts to reproduce them for teaching purposes have been made in various countries, but success rates are low. It's an experiment that got the "right" result, yet is oddly hard to casually verify.
- Continental-scale effects are still debated. The claim that "rivers in the Northern Hemisphere erode their right bank more" (proposed in the 19th century) is hard to separate from terrain and geology, and how far it generalises remains unsettled.
Rather than stopping at "the myth is wrong", sorting out exactly where it's right and where it isn't is the most interesting part of this topic.
How this maps onto textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: Earth's rotation / how forces work / weather | The spinning-disc example, typhoon swirl direction |
| High school | Physics: circular motion and inertial force | Its role as an apparent force |
| High school+ | Physics: non-inertial frames (advanced material in many textbooks) | F = 2mΩv sinφ, why it's zero at the equator |
| University | Geophysical fluid dynamics, meteorology, physical oceanography | Rossby number, geostrophic wind, designing lab conditions |
| 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 (Northern Hemisphere experiment).
- Trefethen, L. M. et al., The Bath-Tub Vortex in the Southern Hemisphere, Nature 207, 1084–1085, 1965 (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 Baer, K. E. von, and the ongoing debate over verifying it (unresolved).
※ 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 to aid understanding and vary with conditions. When observing at home, don't leave the water running unattended.