The Coriolis Effect

The Coriolis effect is the apparent curving of moving objects, such as air or water, caused by Earth's rotation. It is not a true force -- it is what an observer on the rotating Earth perceives, since the ground beneath the object is moving too. In the Northern Hemisphere, this effect bends the path of moving air to the right; in the Southern Hemisphere, it bends to the left. The effect is strongest near the poles and weakens to almost nothing at the equator, since the speed of Earth's rotation relative to the surface varies by latitude. The Coriolis effect plays a major role in weather. It is the reason hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, since the storm's inward winds bend as they spiral toward its center. Ocean currents are shaped by the same effect, steering large circular systems called gyres. A common myth claims the Coriolis effect determines which way water spins when it drains from a sink or bathtub. In reality, the effect is far too weak at that small scale to matter -- the direction water drains depends mainly on the basin's shape and any leftover motion in the water, not on which hemisphere the sink is in.