MadSci Network: Earth Sciences
Query:

Re: Where do drains change there flowing direction?

Date: Wed Oct 28 15:01:17 1998
Posted By: Suzanne Willis, professor,Northern Illinois University
Area of science: Earth Sciences
ID: 908492984.Es
Message:

Hello, Logan -

Well, actually, the flow of water down drains has nothing to do with
which hemisphere you are in; the Coriolis effect is much too small.
It will be determined by random motions in the water (or, in the
case of toilets, by the direction of the incoming water).
Experiments have been done which show that the water will indeed
drain in opposite directions in the northern and southern
hemispheres if you are careful enough to eliminate all motion in
the water first; this is very difficult to do, and will not happen
by itself (it also requires perfectly symmetric containers).

This is explained in the sci.physics FAQ, along with an equation that
lets you calculate how still the water must be (as a function of how
large the vortex is) in order for the Coriolis effect to be noticeable.
Here is the reference:
http://math.ucr.edu/home/baez/physics/bathtub.html

Another reference which discusses the same subject, and which includes a
description of the experiments which did show a preferred direction (so
you can get an idea of how small the effect really is), as well as
a description of an equitorial scam (watch out for this
if you are ever in Kenya!) is the Bad Meteorology page:
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

Now, that being said, there is a real Coriolis effect; it is what
determines the direction of the trade winds, and of the rotation of large
storms such as hurricanes. There is a wide band around the equator known as
the doldrums where the winds tend to be weak and variable in direction;
this is the dividing line between the northern and southern hemispheres,
but it is a wide line, not a thin one. Also, there would be general air
circulation in the atmosphere even if the earth did not rotate, as the
equator would still be hotter than the poles; warm air would rise at the
equator and travel towards the poles, where it would sink, and there would
still be a relatively windless zone around the equator.
   


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