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The Coriolis Effect - Windows to the Universe

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If a cannonball were fired from Arizona to Montana it would curve toward the right because of the Coriolis effect. If it started out going due north it would end up northeast of where it started (blue arrow). If you were trying to hit a target due north of you then you would have to point your cannon toward the northwest (yellow line).
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Map courtesy of the Perry-Castaneda Library at the University of Texas

The Coriolis Effect

Let's pretend that you're on the top of a mountain in Arizona and you have a cannon. Let's make it a game; the object of the game is to to fire a cannonball and hit a flag pole that's on the top of another mountain straight north of you in Montana, almost a thousand miles (1600 km) away. Which way should we point our cannon?

You might think this is easy - if the flag pole is north we should point our cannon north, right? This is a good guess but it won't really work. Why? Because the Earth is spinning. The Earth's rotation would make it look like your cannonball was curving to the right so you'd miss the flag pole! This is called the Coriolis effect.

So what should you do? If you point your cannon a little bit toward the northwest, then when it curves to the right it will head right for the flag pole!

The Coriolis effect makes things look like they're curving toward the right in the northern hemisphere and toward the left in the southern hemisphere. This doesn't just work with cannonballs, it also works on winds and ocean currents. The Coriolis effect is what makes hurricanes spin around. It's also important on other planets, in stars, and in space.

Last modified January 8, 2010 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF