Imagine that you are standing on the North Pole, with your arms outstretched. You are spinning at a rate of one full circle every twenty-four hours, because you are standing on a spinning surface. However, you are not moving in space (except as the Earth orbits the Sun); you are spinning like a very slow top on the North Pole.
Now imagine that you are standing on the equator. You are not spinning; you are orbiting the Earth's center, but your body is not being spun around itself.
Now imagine that you are standing on the South Pole. Again, you are spinning, but in the opposite direction: here you are spinning clockwise, instead of counterclockwise.
This is the Coriolis Effect: the Earth's rotation causes anything on the Earth that isn't perfectly on the equator to spin, and the effect gets strong enough as you get closer to the poles. Now, you don't feel this spinning, because you're too small: even with your arms outstretched and standing on the pole, you still only make one complete circle every twenty-four hours, a speed that is not measurable to all but the most precise equipment. However, a weather system a thousand miles across, will find that the actual speed and direction of the air movement (i.e. wind) at one side of it will be quite different from the air movement at the other side. Because the wind carrying the air and the weather is spinning, the weather systems themselves will begin to spin, and because the Coriolis Effect spins in the opposite direction in one hemisphere versus the other, the direction of the spin will be opposite in the two hemispheres: storms swirl counterclockwise north of the equator, and clockwise south of the equator.
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