Coriolus Effect and Drains
Name: Roger W.
I know that the vortex of water going down a drain spins clockwise in the
northern hemisphere and counterclockwise in the southern hemisphere. What happens at
the equator? Is this effect greater closer or further away from the equator?
The the vortex of water going down a drain spins clockwise in the northern hemisphere
and counterclockwise in the southern hemisphere is myth, not fact. You can demonstrate
this by starting the water spinning in one direction or the other and it will continue
to move in the initial direction. The myth is based on Coriolis forces that are real,
and do operate on large masses such as cyclonic winds but the effect is far too weak to
affect the flow of a small mass of water in a sink. Engineers have studied the draining
of tanks from several perspectives and they find that the draining is to some extent
"chaotic" that is large effects can depend upon minor changes in the
"initial conditions". For example, water in a large tank has a "long memory". If the
tank is stirred in one direction and the "plug" is pulled hours later the direction of
the vortex tends to "remember" the initial direction of the stirring. In addition the
shape of the tank, and the detailed construction of the drain bias the direction of
You do raise an interesting question however, "What happens to a cyclonic storm that
form well away from the equator and travels across the equator?" I do not know but
a meteorologist should know.
Check out the web site
The effect is greater away from the equator. At the equator, there would be no
preference. You would not be able to predict which way the vortex spins. Very
close to the equator, the vortex spins clockwise most of the time.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012