The Coriolis Force, explained and debunked
Now that we're firmly into hurricane season, with two hurricanes already having made landfall in the United States, it's an appropriate time to discuss the Coriolis force, which drives the circulation of hurricanes and all other major weather systems.
The Coriolis force (named for G.G. Coriolis, who discovered the force in 1835), like centrifugal force, is something of a pseudo-force in that it has no source. Gravity and electricity, for example, have mass and charge sources. However, anybody who's weathered a hurricane can tell that this pseudo-force often packs a very non-pseudo punch.
The Coriolis force and centrifugal force are both caused by one simple phenomenon, a rotating reference frame. An observer is in a rotating reference frame if while trying to stay at rest he is rotating around an axis, much as we are on the earth.
Centrifugal force arises from the fact that as you rotate around your rotation axis you're actually changing the direction of your motion constantly. For instance, imagine you're sitting at the end of the second hand on a clock, facing outward. At the 12 o'clock position, your momentum is to the right, but by the time you get to the six's position, your momentum is completely toward the opposite, to the left. The force that actually changed your direction of motion was the friction between your pants and the top of the second hand, but all the while your momentum (inertia) was trying to keep you moving in a straight line. You felt this changing of your momentum as the centrifugal force outward.
To understand the Coriolis force, it's easiest to think of yourself as being on the interior ring of a merry-go-round (moving counter-clockwise, say) while trying to throw a ball to your friend on the outermost ring. If you throw the ball right at your friend, you'll see that it sails wide right, behind his back. It will look to you as if the ball curved by several feet. Why? Because the outer ring is moving much faster than the inner ring, so your ball, traveling on a straight line relative to the ground, finds your friend zipping by to the left much faster than you were moving (relative to the ground) on the inside of the ride.
We can use our merry-go-round example to see how Coriolis forces work on the earth. A low-pressure system such as a hurricane pulls air inward. So, it pulls air down from the north and up from the south. But in the north, the earth's rotation has a slower velocity because the distance from the rotation axis is smaller. Think about the North Pole, for instance. There, the velocity due to rotation is zero because the distance from the axis is zero. So, the air being sucked in from the north is much like the ball thrown from the inside
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