The Coriolis force associated with the rotation of the Planet generates
'Planetary Vorticity'...this is the force that generates opposite rotation
in the different hemispheres. In a fluid, there is also 'Relative
Vorticity', which is the sum of the angular momentum of the local motion,
independent of the planet's motion (like if you stirred your bathtub with
your hand). So the total vorticity if a fluid body is the sum of the
Planetary and Relative vorticity. In the large scale ocean, dimensional
analysis of the Navier-Stokes equations shows that the Planetary Term
dominates the Relative Term, so large scale motions like tropical storms
follow the Northern-Southern Hemisphere rotation rules. In your bathtub, a
similar dimensional analysis shows that the Relative Vorticity terms are
many orders of magnitude larger than the Planetary Vorticity terms. Unless
you can design an experiment where you suppress EVERY local fluid motion in
your bathtub (very difficult, but theoretically possible), the planetary
effect can have no effect.

Also, this business about watching the swirl in the bathtub change as you
cross the Equator is totally bogus. The Coriolis effect goes to zero at the
equator (it is a maximum at the pole), and near the equator is so small that
even the large scale effect of the planet's rotation is barely seen.

Rick Romea
Seattle, WA
rromea at stioptronics.com
206-523-5831 (Home)
425-827-0460 X 316 (Work)

-----Original Message-----
From: Stan Ray <stanray at wolfenet.com>
To: tweeters at u.washington.edu <tweeters at u.washington.edu>
Date: Sunday, June 14, 1998 7:47 PM
Subject: Vortices


>This is not a bird related topic, but it does have to do with a natural
>phenomenon. I have tried a Yahoo search under several different topics,
>but have not gotten anywhere. Here is the question:
>
>We all have heard that bath tub drain vortices rotate in opposite
>directions depending on which hemisphere (north or south) they are located
>in. Does anyone have a reliable reference that documents this idea? If
>so, you can answer my querie directly to me and not clutter Tweeters with
>non-bird information.
>Thanks
>
>Stan Ray
>