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September 18, 2007 > TechKnow Talk

TechKnow Talk

The Coriolis Effect: A Swirling Controversy

Toilets flush clockwise in the northern hemisphere, counterclockwise in the southern hemisphere, right? Or is it the other way around? The TechKnow Guy doesn't usually debunk urban myths, but this one leads to an interesting discussion of natural science and how weather systems form.

The story goes something like this: because of a natural phenomenon called the Coriolis Effect, toilets and basins always swirl in one direction in the northern hemisphere and in the other in the southern. Let's dispense with the toilets first. During flushing, water is injected into the bowl through holes under the rim. These holes are set at an angle, which creates the swirling motion. The toilet flushes clockwise (CW) or counterclockwise (CCW) depending on the direction the holes are pointing.

"Very well," say the promoters of the myth, "that's due to the design of the toilet. But fill a circular basin with water, open the drain and it will swirl CCW in the northern hemisphere and CW south of the equator." It's easy enough to test this theory. Find a public restroom with several sinks, fill each, and open the drains. It's quite likely some will swirl in one direction and some in the other. That should be all the proof required to put this myth to bed.

But like most such stories, there is some truth to this one. The Coriolis Effect, or more accurately the Coriolis force, is very real. Given the right conditions, it would indeed cause the water in a basin to swirl CCW in the northern hemisphere and CW in the southern. But it is a force created by the rotation of the earth, and takes a long time to manifest itself. The sink drains in a matter of seconds; the direction of swirl is determined by small currents in the water left over from filling it, the shape of the sink, imperfections on its sides, or some combination of these factors.

A reasonable question at this point is why the water swirls at all. Why doesn't it simply flow downward directly through the drain? While this can sometimes happen, those leftover currents, currents formed as the draining water moves across the sides of the container, and turbulence near the drain are likely to induce some circular movement. Once this occurs, a physical law known as conservation of angular momentum comes into play. Consider the analogy of the spinning ice skater. As the skater's arms are brought closer to the body, the rate of rotation increases. Similarly, as the diameter of the basin decreases near the bottom, the water moves more quickly, accentuating the circular motion, and creating the swirling action.

Now let's return to the Coriolis force. The earth rotates CCW as viewed from above the north pole. Due to this rotation, objects traveling above the earth appear to take a curved path to the right of the direction of motion in the northern hemisphere and to the left in the southern hemisphere, as viewed from the perspective of the rotating earth. As mentioned above, this effect is very small over short time periods, but becomes significant over periods of hours or days, long enough for the earth's rotation to become a significant factor.

The Coriolis force is a difficult concept, one we don't encounter in everyday experience. One simplistic way to visualize it is to imagine sitting on a rotating carousel. If a ball is rolled from the edge of the merry-go-round toward the center, it will follow an apparently curved path. If the rotation is CCW, for example, the ball will veer away from the center to the right.

This deflection is a result of the rotation of the carousel, not due to any actual force exerted on the ball. For this reason, there is some debate as to whether the Coriolis force should be defined as a force at all. Whether it is perceived as a force actually depends on one's perspective, or frame of reference, either as part of the rotating system or external to it.

Probably the most important manifestation of the Coriolis force is in weather. Imagine sitting on a satellite orbiting the earth, and watching a hurricane in the northern hemisphere. Air rushes from surrounding areas of higher pressure toward the low-pressure center at its eye. But due to the earth's rotation beneath it, as the air travels towards the weather system, its path begins to curve to the right, just as it would on the carousel. So instead of rushing directly in to fill the low-pressure area, the incoming air is deflected to the right of the eye. In fact, the faster the air is moving the stronger the Corilis force pushing it to the right.

But the low pressure is like a magnet, pulling the surrounding air back toward it. At a point determined by the strength of the low pressure and the speed of the air, a balance is reached between the Coriolis force, pushing the air to the right, away from the center, and the pressure differential, or gradient, pulling the air back to its left, in towards the center. Thus, the atmosphere swirls in a CCW direction around the low-pressure center, unable to either break free or rush in to fill it. We are all familiar with satellite pictures of weather systems showing exactly this behavior. The low-pressure system is very much like a draining "basin in the sky."

Just the opposite situation applies in the southern hemisphere, where the air swirls in a CW direction around a low-pressure system. It is actually the same phenomenon, operating in exactly the same way. This is simply because the earth is rotating in what appears to be a CCW direction viewed from "above" and in a CW direction viewed from below. If this is difficult to visualize, have a friend spin a weighted string in a CW direction. Look at it over your friend's shoulder and note its CW spin. Then move around and face your friend, and see that is it now rotating in a CCW direction.

Without the Coriolis force, i.e., if the earth did not rotate, there would be no hurricanes, cyclones, or similar storms. The Coriolis force is strongest near the poles, and zero at the equator. Strong storms form only far enough north or south of the equator for the Coriolis force to be significant. Further, the Coriolis force causes the storms themselves, once formed, to veer away from the equator over time. In fact, hurricanes do not exist in equatorial regions because of the weak Coriolis force there.

Apart from weather, most of us have no experience with the Coriolis force. It does become a factor for calculating the trajectories of very long-range artillery shells, intercontinental missiles, and airplane flight paths, since these objects cover great distances and must be very accurate. And in addition to creating our weather, the Coriolis force is instrumental in the formation of the jet stream in the atmosphere as well as some of the large ocean currents.

If conditions were exactly perfect, the Coriolis force could in fact be demonstrated in a draining basin. But because it is such a tiny influence compared to other factors, such an experiment is very difficult to perform. The basin must be near-perfectly circular, smooth-sided, and symmetrical. A flat-bottomed or pan-shaped tub is best. It must be large and the drain hole very small, so it requires many hours to drain, giving the Coriolis force time to manifest itself. The drain hole must be precisely in the center, and the plug must be removed from below, so as not to introduce any motion into the water.

Given this "perfect" basin, it should be placed where no vibration, breeze, or other disturbance can affect it, and then filled with water. It must then sit for a long time, at least several days, for all the residual currents to completely dissipate. After the plug is carefully removed, it will eventually develop a CCW rotation (in the northern hemisphere) as it drains, due to the Coriolis force. The force of gravity pulling the water downward takes the place of the pressure gradient in the weather system.

Finally, credit should be given to French scientist Gaspard-Gustave Coriolis. Though he did not discover the force that bears his name, he completed a thorough mathematical description of it in 1835. Monsieur Coriolis could not have anticipated his legacy would be forever associated with the toilet.

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