A Popular Weather Myth Down the Drain

Does water spin down the drain one way in the Northern Hemisphere and the opposite way in the Southern Hemisphere? Despite the popularity of this myth, it isn't true. The volume of water in a sink or toilet is too small to be significantly influenced by the Coriolis effect

OK, let's stretch the imagination a little and take a field trip. Picture yourself shrunk to the size of an air molecule, blowing on the wind. You start out in Ohio on a day with clear, sunny weather and high atmospheric pressure. Low pressure exists to the east, just off the coast of Maine. Since wind wants to go from high to low, you are carried east toward the center of the low pressure system. That's caused by something we call "pressure gradient force."

Hold on! You can't just go in a straight line. Suddenly the Coriolis effect kicks in. That bends your motion to the right. Also, if you get close the ground, you start bouncing off of trees and buildings; you're slowed down by friction, and that weakens the Coriolis effect slightly. (The Coriolis effect increases as speed increases.) Suddenly, pressure gradient force has a slight edge in this tug of war. You lurch back toward the low. But the Coriolis effect hasn't given up -- it tugs you back to the right.

Picture it as a series of small steps: First you take a step toward the low (due to pressure gradient force), then a step to the right (Coriolis effect), then another step toward the low, and so forth. After a while, you'll see that you're dancing in a counter-clockwise spiral around the center of the low.

In the Northern Hemisphere, wind blows counter-clockwise around low pressure and clockwise around high pressure. In the Southern Hemisphere, the directions are reversed.

In the next lesson we'll look at fronts and the jet stream.