The AFU and Urban Legend Archive Science Coriolis Force coriolis force tyson debunking Select a topic - - - - - - - - Home Searches AFU FAQ - - - - - - - - AFU Animals Books Celebrities Classic Collegiate Death Disney Drugs Food GIF Language Legal Medical Misc Movies Politics Products Religion Science Sex Songs TV - - - - - - - - Other sites

I am often asked by students whether their toilet bowls will flush clockwise or counterclockwise in the Southern Hemisphere. This would, of course, be important information if you were ever kidnapped and blindfolded and dropped off in a strange land. If we assume a commode of conventional size, then this "toilet bowl test" will fail because the answer lies in the manufacturer's design. But if your toilet bowl were a few hundred miles in diameter, then the Coriolis force of the rotating Earth would easily overcome the random water currents and force the bowl to empty its contents in a counterclockwise swirl. If you had Southern Hemisphere friends with an equally large toilet, then theirs would indeed empty in the opposite (clockwise) direction.

The circulation within oversized flush toilets is a natural consequence of of motion on the surface of an object that rotates. We owe our detailed understanding of the effect to the French engineer and mathematician Gaspard Gustave de Coriolis, who, in 1835, described the laws of mechanics in a rotating reference frame.

Earth's surface is an excellent place to demonstrate why the origin of the Coriolis force is relatively simple. Our planet rotates on its axis approximately once every twenty-four hours. Over that period, objects on the equator travel a circle with a circumference of nearly 25,000 miles, which corresponds to a speed of more than 1,000 miles per hour. By forty-one degrees north, the latitude of New York City and the American Museum of Natural History, the circumference traveled is only about 19,000 miles, and the west-to-east speed is approximately 800 miles per hour. As you continue to increase in Earth latitude (north or south of the equator), your west-to-east speed decreases until it hits exactly zero miles per hour at the poles. (For this reason, most satellites are launched as close to the equator as possible, enabling them to get a good "running start" in their eastward orbits.)

Imagine a puffy cloud in the Northern Hemisphere and a meteorological low-pressure system directly to its north. The cloud will tend to move toward the low. But during the journey, its greater eastward speed will enable the cloud to overtake the low, which is itself in motion, and end up east of its destination. Another puffy cloud that is north of the low will also tend to move toward the low, but will naturally lag behind and end up west of the system. To an unsuspecting person on Earth's surface, these curved north-south paths would appear to be the effects of a mysterious force (the Coriolis force), yet no true force was ever at work.

When puffy clouds approach a low-pressure system from all directions, you get a merry-go-round of counterclockwise motion, better known as a cyclone. In extreme cases, you get a monstrous hurricane with wind speeds upward of a hundred miles per hour. In the Southern Hemisphere, the same conditions will create a cyclone that spirals clockwise. Those in the military who target missiles and artillery shells know all about the Coriolis force and normally calculate the appropriate corrections needed for accuracy. In an embarrassing military moment of World War I, English battle cruisers engaged two German warships at a range of nearly ten miles near the Falkland Islands in the Southern Hemisphere -- but they forgot to reverse their Coriolis correction. Despite this and other gunnery problems, the English eventually won the battle with about sixty direct hits but not before more than a thousand shells had fallen in the ocean.

In high school I knew all about the Coriolis force, but I never had the opportunity to test in on something as large as a swimming pool until the summer after my junior year when I worked as a lifeguard. At the midsummer cleaning, I carefully opened the drain valve to the pool and observed the circulation. The water funneled in the "wrong" direction -- clockwise. The last time I checked, I was life-guarding in Earth's Northern Hemisphere, so I was tempted to declare the Coriolis force a hoax. But a fast back-of-the-envelope calculation verified that the difference in Coriolis velocity across the pool was a mere half inch per minute. This is slow. The water currents from somebody just climbing out of the pool or even a gentle breeze across the water's surface would easily swamp the effect, and I would end up clockwise half the time and counterclockwise the other half. To demonstrate the insignificance of the Coriolis force on this scale would have required emptying and refilling the pool dozens of times. But each try would dump 15,000 cubic feet of water and diminish my job security. So I didn't.

The air circulation near high-pressure systems, which are inelegantly known as anti-cyclones, is a reverse picture of our cyclone. On Earth, these high-pressure systems are the astronomer's best friend because they typically repel clouds. The surrounding air still circulates, but it does so without the benefit of clouds as tracers. The circulation around low- and high-pressure systems, known as geostrophic winds, presents us with the paradox that the Coriolis force tends to move air along lines of constant pressure (isobars), rather than across them.

Now imagine, if you will, a place that is 1,400 times larger than Earth, has an equatorial speed that is about twenty-five times as fast, and has a deep, thick, colorful atmosphere. That place is the planet Jupiter, where a day lasts just nine hours and fifty-six minutes. Jupiter is a cosmic garden of atmospheric dynamics where all rotationally induced cloud and weather patterns are correspondingly enhanced. In the most striking display of the Coriolis force in the entire solar system, Jupiter lays claim to the largest, most energetic, and longest-lived storm ever observed. It is an anticyclone that looks like a great red spot in Jupiter's upper atmosphere; we call it Jupiter's Great Red Spot. Discovered in the mid-1660s by the English physicist Robert Hooke and, separately, by the Italian astronomer Giovanni Cassini, the feature has persisted for more than 300 years. But it was not until the twentieth century that the modern interpretation of the Spot as a raging storm was supplied by the Dutch-born American astronomer Gerard Kuiper.

The Great Red Spot is bigger than Earth, but its size and shape have varied over the years. It lives in Jupiter's southern hemisphere and rotates counterclockwise, which immediately tells us we have a high-pressure system. The coloration, from orange red to a barely visible pale cream, is generally attributed to various concentrations of phosphorus and sulfur compounds. Close-up images from the Voyager flyby missions of the late 1970s revealed a maelstrom of colorful curlicues at the interface of the Great Red Spot and the surrounding atmosphere. There were also strikingly resolved horizontal "belts" and "zones," interlaced with countless smaller cyclones and anticyclones, which give Jupiter the appearance of an archaeological cross section of a Big Mac hamburger, bun included. Above all else, however, the Voyager data posed renewed theoretical challenges. They resolved Jovian features down to twenty miles in diameter -- astonishingly small when one remembers Jupiter's size relative to Earth's. Models of cosmic phenomena are often clean and tidy until they are tested outside of the limits in which they were formulated. When higher image resolution comes along, for example, many models are discarded, others are modified, and some are freshly invented. But jumps in resolution have always been followed by a deeper understanding of the universe.

Whatever else a model of Jupiter's atmosphere is designed to explain, it should as a minimum account for basic properties of the Great Red Spot, such as its longevity, its distinguished size, and that it is an anticyclone. An ideal model would be able to account for all atmospheric motion on Jupiter. The tools available to the theorist are Newton's laws of motion as adapted to the properties of gases and liquids -- otherwise known as fluid mechanics.

Although little is known about the structure of Jupiter's underlayers, contemporary models do capture the basic feature of the Great Red Spot. Jupiter radiates more heat than it receives from the sun and has enormous interior thermal reservoirs that can drive its atmospheric flow patterns. One source is the radioactive decay of trace elements, while another is the leftover heat from Jupiter's initial contraction from a proto-planetary cloud to a planet in the early solar system. The sustained source of energy for the spot could also (or instead) be tapped from other sources. On Earth, hurricanes are partly driven by the latent heat released to the atmosphere when raindrops condense out of the air. A similar mechanism may dominate in Jupiter's atmosphere as its gases condense toward its liquid interior. The Spot has also been observed (and successfully modeled) to dine upon smaller, turbulent eddies in its vicinity. This cannibalistic behavior is yet another source of energy. Clues to the deeper cloud layers will be gained when the spacecraft Galileo passes Jupiter (in December 1995) and parachutes a miniprobe that will measure temperature, density, composition, wind speeds, and electrical storms as it descends through the outer atmosphere.

For now, there is no reasonable hope of describing every one of Jupiter's surface features in detail. A more realistic approach is to construct an atmospheric model that provides a statistically equivalent picture of Jupiter's surface features. In other words, a model of a Big Mac can approximate all Big Macs even though it may not look like any one in particular.

One nagging problem with models that always produce a single, sustained anticyclone is the blunt reality that Jupiter's northern hemisphere is devoid of a twin Great Red Spot. Clearly, if models show that big spots are inevitable, then the north ought to have one, too. Elsewhere in the solar system, the Coriolis force has given rise to what is called Neptune's Great Dark Spot. Like Jupiter's Great Red Spot, it is a southern hemisphere anticyclone of epic proportions with a twin in the north. This is a problem that may require an as-yet-unexplored north-south asymmetry in both Jupiter's and Neptune's internal structure. One way to create such an asymmetry would be a cosmic collision. The July 1994 encounter between Jupiter and the dozens of crumbled comet parts from Shoemaker-Levy 9 left visible and sustained scars on Jupiter's outer gaseous surface. The long-term effects of this impulse of deposited energy remains to be seen. Will the scars form stable new structures among the cloud tops? Or will the scars dissipate completely into the atmosphere? For the moment, feel free to consider the new blemishes to be extra ingredients in your hamburger.