The Coriolis effect causes global wind currents to blow in a diagonal direction due to the rotation of the Earth. If the Earth didn't rotate, wind currents would blow straight north and south from the poles to the equator due to the pressure differences between the two regions.
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.
