Weather Watch:  Storms and Draining Water

A common misconception is that the water drains counterclockwise in
the Northern Hemisphere and clockwise in the Southern Hemisphere due
to the Coriolis force, the force that results from the rotation of the
Earth. The Coriolis force is responsible for the counterclockwise
motion around low-pressure systems and the clockwise motion around
high-pressure systems in the Northern Hemisphere.  Because the
rotation rate of the water going down the drain (once every few
seconds) is much larger than the rotation rate of the earth (once a
day), only motions that persist for many hours or more (many weather
systems, for example) can be affected noticeably by it.  The shape of
the sink or any initial rotation in the water has more to say about
the direction water flows down drains than the Coriolis force.  For
more about the Coriolis force and sinks, see
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

-David Schultz, NOAA/National Severe Storms Laboratory