What causes the water going down a drain to swirl clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere?
Category: Earth Science Published: December 18, 2012
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Water swirling down a drain does not always go a certain direction. You can get the same sink to swirl water clockwise one minute and counter-clockwise the next. This misunderstanding has its basis in a very real effect: the Coriolis force. The Coriolis force is in the same family as the centrifugal force. It is an inertial force caused by the rotation of an object. It is not imaginary or fictional, but is very real in the rotating reference frame. The Coriolis force tends to make things on the surface of the object to spiral a certain direction. As the earth rotates, this motion causes everything on the surface to experience the Coriolis force, including the water in your sink. But, the Coriolis force is so weak that it doesn't really do anything until it acts on a lot of material. Your sink simply does not have enough water to be affected by the Coriolis force. On the other hand, hurricanes usually spin counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere because they have enough material to be affected by the Coriolis force, as detailed in the textbook "An Introduction of Dynamic Meteorology" by James R. Holtone. Tornadoes are too small to be affected by the Coriolis force and spin in any direction. The spinning direction of drain water is random, determined mostly by how the water is bumped, sloshed, or distributed when it starts to drain.