Ask A Scientist Physics Archive

name         Roger W.
status       other
age          40s

Question -   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?
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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 
flow.
    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.
	
Vince Calder
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Check out the web site
http://earthsci.org/geopro/ocean/ocean.html#Circulation%20in%20the%20Atmosp here
---Unterman
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Roger,
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
Physics Professor
Illinois Central College
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