More on Spirals

Several readers have called in with essentially the same question: Is the funnel one sometimes sees as water goes down the bathtub drain caused by the same processes that cause spirals in clouds and in the aurora? And is it true that the direction of the swirling water is opposite in the southern hemisphere?

Usually, when one pulls the plug in the bathroom sink the water tends to be moving either clockwise or counterclockwise. As the water moves toward the drain, the preexisting clockwise or counterclockwise motion is enhanced because of a law of physics that requires the circular motion to be maintained. It is maintained only if the speed of the rotating water increases as it nears the drain. The truth is that one can create in the draining water a spiraling funnel that is either clockwise or counterclockwise, depending upon how you stir the water just before opening the drain.

However, if the water is perfectly still and there is no disturbance as the drain is opened, the water should tend to develop a counterclockwise flow pattern as it drains away in the northern hemisphere, and a clockwise pattern if it drains in the southern hemisphere. This preference is caused by the earth's rotation, the so-called Coriolis force. The Coriolis force in the northern hemisphere is stronger the closer one is to the North Pole.

This fact allows Alaskans to claim bigger and better Coriolis forces than exist elsewhere in the Union, and especially in Texas. Within our state, Ketchikan sits at the bottom of the Coriolis totem pole and Barrow at the top.

There is shear flow in the water spiraling down the drain; however, the shear itself is not the cause of the rotating pattern. Therefore, the pattern in the water is not caused by the same mechanism that creates spirals in the aurora or that can sometimes be seen in clouds when one air mass moves past another.

For associated information, see article #215 .