Did our science teachers tell us the truth about water spinning in the drain?

We've all seen water swirl around in the drain or in the toilet, creating a vortex, also known as a whirlpool. We’ve also noticed, if we live in the northern hemisphere that the water almost always swirls in a counter clockwise direction. Even if we stir the water and make it go clockwise, the last little bit will swirl the other way just before going down the drain. And most of us have heard the claim that the water swirls the opposite way in the southern hemisphere. We grew up being told this in school but we probably weren’t completely convinced. It makes little sense that the effect on a tub full of water would be the opposite on the other side of an imaginary line on the earth.

These Canadian tourists traveled to Ecuador for a three week adventure in the Amazon and the Galapagos Islands, but part of their journey involved a visit to Quito, a city at the “middle of the world”. They had the opportunity to visit a scientific center that straddled the line between the northern and southern hemispheres. This experiment took place exactly on the equator, shown by the red line that can be seen running directly under the gold tub in the first of three parts of this demonstration. The guide explained that the swirling of the water is referred to as the Coriolis Effect and that it is caused by the rotation of the earth and the effect that the rotation has on free floating matter such as water and air. Anything north of the equator is subjected to a small, but noticeable force that spins non fixed objects in a counter clockwise direction. South of the equator, the force spins objects in a clockwise direction. He explained that this effects not only the water in the sink or toilet, but also the currents of the oceans and the air surrounding the earth. This is why cyclones and tornadoes spin in different directions, depending on their position relative to the equator.

The guide poured water into a tub and pulled the plug to drain the water into the bucket below. Sprinkling a few leaves on the surface of the water, he showed that there was no vortex right on the equator. Moving only a few feet to the southern hemisphere, he showed that the water swirled clockwise. Repeating this in the northern hemisphere, the vortex spun counter clockwise. Surprisingly, even a short distance from the equator, the Coriolis Effect was obvious. This effect is most obvious at the earth’s poles, but only at the equator can you see all three demonstrations within a few minutes. It turns out those grade eight science teachers were telling us the truth after all!