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On Mon, 3 Jan 2000 you wrote:
> Am I the victim of an old-wives' tale, or does bathwater spiral clockwise in
> the northern hemisphere and anti-clockwise in the southern (or vice-versa).
> Supposedly something to do with the Carolis effect (sorry if mis-spelt).
Hello! This is an excellent question, as it is one of the most common
misconceptions about the Earth that exists. Most astronomers would even
get it wrong!
If you had a perfectly smooth bathtub in a perfectly round shape, and you
filled it full of water, let the water sit for a few days (to let every
last bit of turbulence from the filling of the bathtub die out), and then
let the water drain though a single hole in the middle of the bathtub,
then the water would begin to circulate (counter-clockwise in the northern
hemisphere, and clockwise in the southern hemisphere). This is exactly
what happens with weather systems. A low-pressure system is just like
this ideal bathtub - air is rushing in to the middle of the low pressure
system to try and fill the low pressure area, and water is rushing in
toward the drain to try and fill it.
However, bathtubs and toilets do not have these perfect shapes, and the
direction that the water circulates is determines by the shape of the
bathtub or by any turbulence that may be present from filling the bathtub.
The Coriolis forces are swamped by any minor imperfections.
It is possible to estimate the Coriolis forces in a bathtub. If we have a
bathtub at the North Pole (where Coriolis forces would be greatest, so
assume it is a hot tub so it doesn't freeze), and the bathtub is one meter
in radius, and the water drains from the edge of the bathtub to the drain
at the center in one second, then the deflection of the water will be less
than approximately 0.05 millimeters. This would be measurable, but is
rather tiny, and definitely nowhere near the circulation seen in most
bathtubs. A larger bathtub or a faster draining rate would amplify the
deflection.
In short, any "experiment" purporting to show the Coriolis effect in a
bathtub, toilet, or pan of water is a fraud, although not necessarily
intentional. But a glance at a satellite picture of clouds shows true
Coriolis effects without the need for a hottub at the North Pole!
I hope this clears things up, and thanks for writing!
Sincerely,
Kurtis Williams
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