The Missing Coriolis Effect

Uncle Thanky

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Senior Member.
The scale is too small in all 3 cases.

The time, space and velocity scales are important in determining the importance of the Coriolis force. Whether rotation is important in a system can be determined by its Rossby number, which is the ratio of the velocity, U, of a system to the product of the Coriolis parameter,upload_2017-4-22_16-48-21.png, and the length scale, L, of the motion:

The Rossby number is the ratio of inertial to Coriolis forces. A small Rossby number indicates a system is strongly affected by Coriolis forces, and a large Rossby number indicates a system in which inertial forces dominate. For example, in tornadoes, the Rossby number is large, in low-pressure systems it is low, and in oceanic systems it is around 1. As a result, in tornadoes the Coriolis force is negligible, and balance is between pressure and centrifugal forces. In low-pressure systems, centrifugal force is negligible and balance is between Coriolis and pressure forces. In the oceans all three forces are comparable.[25]

An atmospheric system moving at U = 10 m/s (22 mph) occupying a spatial distance of L = 1,000 km (621 mi), has a Rossby number of approximately 0.1.

A baseball pitcher may throw the ball at U = 45 m/s (100 mph) for a distance of L = 18.3 m (60 ft). The Rossby number in this case would be 32,000.

Baseball players don't care about which hemisphere they're playing in. However, an unguided missile obeys exactly the same physics as a baseball, but can travel far enough and be in the air long enough to experience the effect of Coriolis force. Long-range shells in the Northern Hemisphere landed close to, but to the right of, where they were aimed until this was noted. (Those fired in the Southern Hemisphere landed to the left.) In fact, it was this effect that first got the attention of Coriolis himself.
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estimating the Rossby numbers here with orders of magnitude U ~ 1 m/s and L ~10 m resp. 1 m
the Rossby numbers here are about 1000 to 10000. Which means that the coriolis force has a small to negligible effect here
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Senior Member.
can you explain that to me in non-nerd speak? how come we see a swirl in the kitchen sink at times? vs. his experiments.

In basic terms, it means that the forces that control the vortex (swirl) in a drain are somewhat random, and because the drains are so small when compared to a storm system (Hurricanes) there's not enough size to always cause a vortex. Most vortexes seen in water are the result of water swirling around submerged rocks or where currents moving in different directions merge.

In Math terms:


This effect is too small to affect your bathtub, but it's still observable under the right conditions.According to Wikipedia, Otto Tumlirz conducted several experiments in the early 20th century that demonstrated the effects of the Coriolis forces on a draining tub of water. The tub was allowed to settle for 24 hours in a controlled environment before the experiment began. This was enough to damp out the residual angular momentum left over from filling the tub up to the point where Coriolis effects were dominant.
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Senior Member.
It is a bit like the effect a frictional force will have on an object. For instance when you pull a table cloth from under a fully covered table rather slowly you will end up with a great mess. But if you do it very quickly everything will stay more or less in its place and you will get applause. Another factor is whether the tablecloth is rough or smooth. So the frictional force between the plates etc and the cloth won't be able to accelarate the plates depending on "cloth speed" and "smoothness". You might call the combination of the two the ratio of inertia to the friction force.
Likewise because the angular velocity of the Earth is comparatively small (once about every 24 hours) the question whether the coriolis force is able to "drag" things around depends more or less on the time it takes to cross the sink or the lake, which in turn is determined by the typical speed (say 1 m/s) and the length scale (say 1 m). The coriolis parameter f is 2(2pi/86164 seconds) x sin (latitude) and has on mid latitudes the order of magnitude of 1/10000
So swirls in bathtubs etc originate from initial movements in the water etc. that are strenghtened by preservation of angular momentum when the water moves inward (like an art skater making a pirouette) independent from the coriolis force.
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Mick West

Staff member
So swirls in bathtubs etc originate from initial movements in the water etc. that are strenghtened by preservation of angular momentum when the water moves inward (like an art skater making a pirouette) independent from the coriolis force.

The misconception about the Coriolis effect is used at tourist traps on the the equator to "demonstrate" they are moving across the equator. The huckster has a basin of water that they drain, showing a clockwise spiral. They then walk "over the equator" (which is generally not the exact equator anyway) and show that the basin drains the other way. It's just a trick where they force the direction of the spiral either in the initial filling of the basin, or by giving it a bit of a wobble.


Senior Member.
Another example that bears out Mick's point:

“Is it possible to detect the Earth’s rotation in a draining sink? Yes, but it is very difficult. Because the Coriolis force is so small, one must go to extraordinary lengths to detect it. But, it has been done. You cannot use an ordinary sink for it lacks the requisite circular symmetry: its oval shape and off-center drain render any results suspect. Those who have succeeded used a smooth pan of about one meter in diameter with a very small hole in the center. A stopper (which could be removed from below so as to not introduce any spurious motion) blocked the hole while the pan was being filled with water. The water was then allowed to sit undisturbed for perhaps a week to let all of the motion die out which was introduced during filling. Then, the stopper was removed (from below). Because the hole was very small, the pan drained slowly indeed. This was necessary, because it takes hours before the tiny Coriolis force could develop sufficient deviation in the draining water for it to produce a circular flow. With these procedures, it was found that the rotation was always cyclonic.”
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From accessed 15.02/2015

Flat Earth supporters often cite the "missing" bathtub coriolis, while strangely remaining silent about the evidence of large scale effects seen in weather systems, storms and hurricanes, which do reliably rotate in opposite directions in the northern and southern hemispheres.

They also keep quiet about the manuals for artillery directors and snipers that describe the need to take coriolis into account:

I've removed one example because the post has now been made private.

14. The next important factor in drift is Coriolis acceleration. Unlike the gyroscopic effect, this particular acceleration does not depend on the resistance of the air. It would be present if the projectile were fired in a vacuum. It does, however, depend on the latitude and gearing of the gun and on its range. Its effect is opposite north and south of the equator. For long range guns it is sometimes almost as great as the gyroscopic effect.

15. The trajectory of a projectile is always measured using the firing range or some other part of the earth's topography as a frame of reference. The frame of reference is considered fixed with the projectile doing all the moving. This is a false assumption, since the earth is not fixed but is rotating on its axis once a day. In reality, therefore, the frame of reference is uniformly accelerated, which for normal latitudes means it is being swung in a giant curve.

16. Figure 3-4-7 (A) and (B) illustrates what happens to the projectile and the frame of
reference in the northern hemisphere. To an observer on the moon (A) the projectile goes in a straight line from G to T while the gun, because of the earth's rotation, goes in a curve line from G to P. To an observer on earth (B) the gun does not appear to move so that the observer considers G and P to be the same. The projectile, however, appears to curve mysteriously over to the right. Actually it is not the projectile swinging to the right but the earth (frame of reference) swinging to the left...

17. The difference in the length of the trajectory is due to the velocity imparted to the
projectile by the rotation of the earth, which the moon observer adds in but the earth observer leaves out. Drift from Coriolis acceleration is maximum at the poles and negligible at the equator. The acceleration also affects range, but in the opposite sense, being maximum when fired along the equator and negligible at the poles.
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Active Member
The Exposing PseudoAstronomy Podcast covered this subject in their latest episode.

...the Coriolis Force is rather weak as fictitious and real forces go. While it's always there, or the effect is there, it simply doesn't apply much in our daily lives because it's drowned out by everything else.

For example, if I fill a bowl with water, the residual motions of all the water molecules within it from me filling it is going to be huge relative to any Coriolis Force. Same thing goes if I fill a bathtub, or a pool.

If I were to drain any container of water out the bottom, the shape of that hole and all its imperfections - either designed or not - is going to dominate by many powers of 10 over the amount of any rotational effects that would be caused by the Coriolis Force.

In a toilet bowl in particular, there are usually jets of water that are aimed at an angle - to the left or to the right - along the top of the bowl such that when you flush, it has a larger chance of dislodging material and moving it down the drain. Those jets' directions are what will cause the water in the toilet bowl to spin either clockwise or counter-clockwise, or, if you're special, those jets are straight down and there may be no rotation at all.
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