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Professor Hanington's Speaking of Science: What the heck is the Coriolis Effect?
Professor Hanington’s Speaking of Science

Professor Hanington's Speaking of Science: What the heck is the Coriolis Effect?

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F5 Tornado

F5 tornado, Manitoba 

Most people say when you flush your toilet the water spins counterclockwise as it drains due to the rotation of the Earth. They “prove” this by saying in Australia everyone knows the spinning is in the opposite direction because they are below the equator.

This I debunked myself many years ago at the Sydney airport after getting off a 15-hour flight from LAX. It still spun the same way as back in America, confirming the plumbing bonds between our two countries.

My son Peter, an enthusiastic plinkster, asked me to take a look at the way a rifle bullet’s path is affected by the spinning Earth, and does it matter what direction he shoots? In doing so I uncovered several interesting science facts about the Coriolis Effect and present them in this column.

Most people know that when viewed from a position where North is up, the Earth rotates on its axis west to east, or, as they say, counterclockwise. All planets except Venus and Uranus do as well, pointing to the idea that we evolved from some sort of spinning dust cloud eons ago.

Where the vast amount of energy came from to get our world spinning nobody knows but it can be shown to be enough to power our cities for thousands of years if we could only harness it somehow. A new source of Green Energy politicians have not thought of yet.

Because our Earth spins at a constant angular velocity, different latitudes have different linear speeds. At the poles your velocity is zero and that’s an easy one to see. But if you sitting in a cafe in Quito, Peru, on the equator, you are actually moving at the high clip of 1,000 miles per hour, moving from west to east. You don’t notice the speed however because everything else in the restaurant, the tables, the building, the waiter, is moving at the same rate and in the same direction. All is fine until you travel north and south where there are differences in these speed values. Sometimes, as in the case of hurricanes or tornados, these variations can lead to devastating results.

To picture the Coriolis Effect we have to start out with something not firmly attached to the ground. Consider a very large cannon such as the German Paris guns of WWI that could shoot 75 miles, on the equator firing a projectile north. As the bullet leaves the cannon it has both a north and eastward velocity. The north speed is the muzzle velocity and the eastward speed is the same as other objects at the equator, 1,000 mph. As the object leaves the equator and travels far enough north it will no longer be going east at the same speed the ground beneath it is.

The result is that an object travelling away from the equator will be heading east faster than the ground and will seem to be forced eastward by some mysterious force. Placing the cannon at the North Pole and firing it towards the equator results in a projectile with zero sideways velocity such that ground continues on beneath it and will seem to be forced west. In reality there is no actual force doing this, it’s just a consequence of the ground simply moving at different speeds below the path of the bullet.

Because our planet’s air is not connected to the ground like a railroad track or highway, you can see why such a mechanism would influence the spin direction of the Earth’s atmospheric and oceanographic circulation systems, including the Gulf Stream, the jet stream, trade winds, and hurricanes.

Tornadoes in the northern hemisphere start with large amounts of moving air. When low pressure hot air moves upwards from the tropics and is met by colder polar air heading south we get a spin situation analogous to our cannon-ball projectile movement mentioned above. Because the Coriolis Effect causes air rotation to occur, 95% of all tornadoes formed rotate in a counterclockwise direction. Check this out next time you watch the Wizard of Oz.

For those who like science history, the discovery of the Coriolis Effect had its beginnings in the observation of errant artillery shells by Italian scientist Riccioli who in 1651 said that the spinning Earth was to blame when shot missed their targets. He showed that cannonballs fired to the north are deflected to the east and adjustments must be made to compensate.

The acceleration equation for this was derived by Euler in 1749 and the effect was described in the tidal equations of Laplace thirty years later. In 1835, Gaspard-Gustave Coriolis published a paper on the energy yield of water wheels referring to this invisible force as a “compound centrifugal force.” The effect was known in the early 20th century as the “acceleration of Coriolis,” and by 1920 simply as the “Coriolis Force.”

Can one notice this when target practicing? The answer is yes if you are into long range shooting.

Experts say it will affect your aim at targets 1,000 yards or greater. When you target shoot either north or south in the United States the effect causes a miss to the right for both directions. If you are shooting west, the bullet will hit lower and if you are facing east, the target is going to be dropping and slightly moving away during the bullet transit time. This causes the bullet to hit higher.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at or


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