THE CORIOLIS EFFECT: WHY A TORNADO ROTATES COUNTERCLOCKWISE IN THE NORTHERN HEMISPHERE

In the northern hemisphere, a projectile will seem to curve to the right due to the rotation of Earth. This is known as the Coriolis effect. For example, suppose your projectile is aimed north. It has an eastward component of velocity due to the rotation of Earth. When it gets to its destination, the ground there will have a lesser eastward velocity because going north gets you closer to the axis of rotation. Therefore the projectile moves to the east relative to Earth. Try a south-aimed projectile. This time Earth moves faster to the east at the destination, so the projectile ends up west relative to Earth. In both cases the projectile curved to the right.

Does this mean a left-handed pitcher has an advantage in the northern hemisphere? No, in that short distance the Coriolis effect is insignificant. But it is not insignificant for cannon shots. This is where the effect was first noticed.

Consider two air masses moving toward a low-pressure region halfway between them. They both curve to the right, and thus swirl counterclockwise (as viewed from above) around the low pressure region.

This is spin up. If you think your head is spinning now, wait til you read the rest of it, where you can find out about those other spins.

Water draining from a bathtub will tend to swirl, but it can go either way, depending on conditions. Make a circular hole in the center of a bowl with circular symmetry and start with water perfectly still, then when draining it will swirl counterclockwise in the northern hemisphere and clockwise in the southern. This has been done experimentally, and it works if there is circular symmetry to a high degree of precision.

The Foucault pendulum, pronounced foo ko (I mean Foucault is pronounced this way, not pendulum): Free to swing in any vertical plane, the plane changes with respect to Earth, making 1 complete revolution in 24 hours. Viewed from above, it goes clockwise in the northern hemisphere, counter or anticlockwise in southern hemisphere, and not at all at the equator.

River meandering is also explained by the Coriolis effect, believe it or not. Einstein first pointed that out. Now swirl back to the main physics page.

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