Did you know? #2

"Dear Dr Hypothesis, I am about to make a long distance trip, and, despite many worries that have recently been highlighted by the press, I am actually more concerned about the direction water will drain down my plughole when I cross the equator. Is it true that it flows in a different direction in the southern hemisphere, and if so, why? Plughole Paul

DR HYPOTHESIS SAYS: This is a widely held misconception, Paul, but it does have some basis in scientific theory. The Coriolis force, which is driven by the rotation of the Earth, can drive fluids to flow anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. However, this force is very weak and so, on the scale of a sink or a bath, is almost always overcome by more important factors such as the temperature of the water or previous currents in the water. It should, in principle, be possible by to make water flow clockwise, anticlockwise and even straight down the same plughole by simply altering other variables such as those mentioned above."

From BlueSci Magazine

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But wait?! I remember I was really convinced of this Coriolis force when I went to Kenya on the equator. Our guide had a bucket of water and a large funnel on top of it. We used a floating piece of plastic to show the direction of water flow. We started from the north hemisphere, then the equator, then the south hemisphere. The water did drain the opposite direction when we passed the equator to the other side of Earth. I also remember the water drained vertically when we were directly on the equator!

Here's some evidence I experienced this (October 2004):

1. Equator


2. Northern hemisphere


3. Southern hemisphere