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Coriolis Effect on Water Flow Myths

1) The assumption that water spins counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere cannot be made. While the Coriolis effect causes hurricanes to spin in these directions, it is too weak to impact small volumes of water like those in a toilet. 2) Another factor is the Rossby number, which compares inertial and Coriolis forces. For toilet water, the Coriolis effect would only be noticeable if the water's flow rate was less than 2x10^-6 m/s. 3) In summary, the Coriolis effect is too weak relative to gravity and inertial forces to impact the direction of spin of water during a typical toilet flush.

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David Xiaohan Chen
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43 views2 pages

Coriolis Effect on Water Flow Myths

1) The assumption that water spins counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere cannot be made. While the Coriolis effect causes hurricanes to spin in these directions, it is too weak to impact small volumes of water like those in a toilet. 2) Another factor is the Rossby number, which compares inertial and Coriolis forces. For toilet water, the Coriolis effect would only be noticeable if the water's flow rate was less than 2x10^-6 m/s. 3) In summary, the Coriolis effect is too weak relative to gravity and inertial forces to impact the direction of spin of water during a typical toilet flush.

Uploaded by

David Xiaohan Chen
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© © All Rights Reserved
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Can we assume that water spins counter-clockwise in the northern

hemisphere and clockwise in the southern clockwise?


We cannot make the above assumption. I estimate that the average
toilet flush volume is one gallon and that the water flushes in no more than
five seconds. Under these conditions, inertial forces and gravity will be the
dominant forces. If we can indeed make our first assumptions, it would be
because of the Coriolis effect. Coriolis effect is caused by the rotation of the
earth. Points on the equator must rotate much faster than points closer to
the two poles (1). As a result, hurricanes spin counter-clockwise in the
northern hemisphere and clockwise in the southern hemisphere. Coriolis
effect is too weak to have an effect at the equator. As a result, hurricanes do
not occur at the equator (1). However, the Coriolis effect is 30 million times
less than gravity. Relative to the spin of bathroom water, the earth rotates
too slowly for the Coriolis effect (3). The water volume is also too small and
the water doesnt spin long enough for the Coriolis effect to show an effect
ton the water. Thus it cannot be assumed that water will flow
counterclockwise in the northern hemisphere and clockwise in the southern
hemisphere (2).
Another important factor in disproving this assumption is the Rossby
number. The Rossby number is a dimensionless number that is the ratio of
inertial to Coriolis forces for a certain flow of a fluid that is rotating (4). A
small Rossby number indicates a system in which Coriolis forces dominate.
For a baseball player throwing a ball, the number is 32,000, so it does not

matter which hemisphere he pitches the ball (4). However, unguided


howitzer shells stay in the air long enough for Coriolis effect to matter (4).
The water in your toilet would need to a flow rate of less than 2x10^-6 m/s
for Coriolis forces to have an effect (5).
1. https://www.loc.gov/rr/scitech/mysteries/coriolis.html
2. http://www.huffingtonpost.com/2012/10/25/water-drainequator_n_2010379.html
3. https://www.washingtonpost.com/news/speaking-ofscience/wp/2015/06/03/videos-synced-up-across-the-globe-prove-onceand-for-all-which-way-water-swirls/
4.

Lakshmi H. Kantha; Carol Anne Clayson (2000). Numerical Models of Oceans and Oceanic
Processes. Academic Press. p. 103. ISBN 0-12-434068-7.

5. http://math.ucr.edu/home/baez/physics/General/bathtub.html

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