I was thinking the other day about something my father said to me before I left for Chile, namely for me to “look out for the different direction that water flows down the plughole in the southern hemisphere”.  Once I started to think more and more, I felt somewhat dubious with respect to the veracity of this “fact”.  Why should water flow in the opposite direction in the southern hemisphere to the northern?

Let’s take a look at the facts for a bit.  Ok, we can safely assume that everywhere on the Earth has equal gravitation strength (yes there are minor anomalies, but they’re negligible in this context).  Additionally, the physical properties of water are pretty well known and fixed.

From these pieces of information, taking the assumption that all plugholes are of equal size, then we may conclude that all water flows out of the sink at the same rate.  From here we can consider two different options.

First, if we assume that the rotation rate of the water varies according to latitude, then at the equator, in order to flow differently at southern latitudes, it must flow straight down (so that south of the equator, it would flow the opposite way).  Not convinced that at the equator, every plughole has no rotation? No, me neither.  What factor would cause this anyway?

Our second possibility is even more ridiculous.  If we assume that the rotation rate of the water is also fixed, with only the direction varying, then the following should happen.  In the northern hemisphere, water would (for the sake of argument) rotate at 10 rpm in a clockwise fashion, while in the southern hemisphere, it would be 10 rpm anticlockwise (but both sinks would lose an equal volume of water per unit time).  If we consider the limit as latitude tends to 0 degrees, then at 1 metre north of the equator, the water would rotate differently to that found at 1 metre south of the equator, yet be exactly the same as the water found well within the arctic circle, several thousand miles away.

The most bizarre example would be the case where the equator bisects your plughole.  Assuming that water in the northern and southern hemispheres flows in different directions, the water in this case would rotate in two different directions at once.

Feel free to prove me wrong, but don’t quote the “Coriolis effect”.  For that to be the cause you need a plug the size of a hurricane (which somehow I doubt you possess), or to have extremely sensitive and tightly controlled laboratory conditions.

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