Okay, let’s get right down to it. The one thing you really want to know about our living down here in the Southern Hemisphere. Not all these pithy little blog posts— plants, birds, elections, health care, famous Kiwis or our pathetic little adventures (thank you for your forbearance). Nope, no more waiting. What you really want to know and have been waiting for. Here it is:
DOES WATER GOING DOWN A DRAIN IN A BATH TUB OR TOILET HERE IN THE SOUTHERN HEMISPHERE ROTATE CLOCKWISE AS OPPOSED TO COUNTERCLOCKWISE IN THE NORTHERN HEMISPHERE?
Well, let’s find out.
What you’d be referring to is the Coriolis (co-re-o-lis) effect, named after the French scientist, Gaspard-Gustave Coriolis, who published a paper on this subject, rotating frames of reference, in 1835. He showed mathematically that if something is put in motion above the surface of a rotating object, the object’s path will curve in relation to the surface.
Here’s an example of this. Imagine you and your friend are sitting in the middle of one of those rotating merry-go-rounds in a playground—you know, the ones that make you sick after a certain age—and you throw a ball back and forth as you spin around. No problem. You easily throw it back and forth. But now you both move to opposite edges of the merry-go-round, sit down and face each other. The edges of the merry-go-round are moving faster than the center. Now you are really spinning around, and if you don’t hang on, you could fall off. You throw the ball to your friend across from you, but each time the ball curves off and misses him or her. This is the Coriolis effect.
On a larger scale, the Earth is a giant spinning merry-go-round rotating from west to east as seen from above the North Pole. Just like our park merry-go-round, things spin faster the farther they are away from the axis or center of the spinning object. The equator, which is farther away from the axis, moves faster. In fact, the speed of the Earth is 1018 mph at the equator while, for example, it is around 820 mph at 37 degrees south latitude here in Tauranga.
Do you still have your ball from the merry-go-round? Good. So now, let’s say we stand in Texas and throw the ball—give it a good hefty throw— directly northward. We’d think the ball would land in Nebraska. But no, the ball curves off and lands to the right, maybe in Delaware. And if we stood south of the equator in, let’s say in India, and threw our ball, it would curve, in the direction of the rotation of the earth, and land not in the Indian Ocean, but maybe in Australia. This is the Coriolis effect.
Stuff that is not attached to the Earth’s surface like water or air is also affected by the Coriolis effect. When you see that big storm on the weather map spinning toward toward, and you are collecting batteries, flashlights and bottles of water, the direction of the swirling of the storm is due to the Coriolis effect.
In the atmosphere, high pressure areas move toward low pressure areas. In a hurricane, cyclone or typhoon (all words meaning the same thing), high pressure air moves toward a central low pressure area. If the Earth were not rotating, the high pressure air would move directly toward the low pressure center with no swirling effect being created. But because of the Coriolis effect, in the Northern Hemisphere, the surrounding air keeps getting deflected to the right causing a counterclockwise swirling pattern as it tries to return to the low pressure center. In the Southern hemisphere, the opposite is true, producing a clockwise swirling pattern. As an interesting aside, there are no cyclones near the equator because there is essentially no Coriolis effect.
So what about the whole water swirling down the drain situation? Does it swirl in different directions in different hemispheres due to the Coriolis effect?
Unfortunately—for some reason I wanted this to be true— there is NO difference in the direction of water swirling down a drain in a sink, tub or toilet in the Northern Hemisphere in comparison to the Southern Hemisphere due to the Coriolis effect. This is because the Coriolis effect is simply too weak to affect a tiny body of water to any measurable amount, particularly when compared to other forces.
Rather, the direction water swirls down a drain depends on the direction in which the water is introduced into the basin and the basin’s geometric shape and abnormalities in the basin’s surface. Even seemingly small biases in any of these can predispose water to begin rotating down a drain in one direction or the other.
So there it is, another myth . . . going down the drain.
Nonetheless, the idea of water going down the drain in different directions on either side of the equator is perpetuated. There are a number of videos on YouTube (I have included one at the bottom of this post) showing scam artists deceiving tourists near the equator on the rotation of water. In the demonstrations, the scammers while allegedly standing directly over the equator allow a filled basin to empty from a central drain in the basin. The water drains directly downward with no swirling.
Then they move several meters (!) away to the north and repeat the demonstration showing how the water now rotates counterclockwise going down the drain. Then they move a few meters toward the south and demonstrate a clockwise draining of the water from the basin.
What’s going on here? Is your trusty blog poster (me) wrong? No. Depending on the video you watch, if you look closely, the charlatan will either pour the water into the basin from one side or the other, which gives it a rotational bias when it drains out, or when the basin is set down, the charlatan will either turn the basin slightly when they set it down or turn themselves to either the right or the left depending on which way they want the water to go down the drain. Or if the basin isn’t level when it is set down, more water on one side will affect which direction the water will flow out of the basin. Any small seemingly insignificant action can produce a slight rotational force which will influence the direction water flows out the drain.
So I had to see if I could reproduce the scam myself. The second video is one I made demonstrating the so-called Coriolis effect on either side of an imaginary equator that runs across my dining room table. After leaving water in a plastic basin to sit for a few seconds, when I pulled out the plug the water drained directly downward without any spiraling. Then I moved about a foot north of my equator, and by turning the basin slightly as I set it down and pulling the plug, the water rotated counterclockwise. I repeated the experiment on the opposite side of my equator and again was able to make the water swirl down the drain in the opposite direction, clockwise. Now all I need is some gullible tourists!
YouTube demonstration of water rotating in opposite directions.
My YouTube attempt to reproduce the above effect.