You may have heard that water draining from a sink or toilet spins in one direction in the Northern Hemisphere and in the opposite direction in the Southern Hemisphere.  Is that really true?

If you want to earn 1S1P pts
 If you want to earn extra credit points
You should answer the question above and back up your answer with some explanation or evidence.  You should be able to do that in half a page (double spaced) or less.
 You should answer the question above and also answer the question found at the bottom of this reading section.


Spinning motions in cases where the PGF is stronger than the Coriolis force
The situations we have been looking at so far are representative of large "country size" storm systems.  There are smaller scale situations, though, where the PGF is much stronger than the CF and the CF can be ignored.  A tornado is an example.  Spinning water draining from a sink or toilet is another.  The PGF is much much stronger than the CF and the CF can be ignored. 




Large scale upper level winds blowing around Low pressure.  You  must take into account both PGF and CF forces. Winds only spin in a CCW direction around L in the northern hemisphere and only spin CW around L in the southern hemisphere.
 Smaller scale motions.  The CF is negligible.  A net inward force is need to keep winds spinning in a circular path.  The inward pointing PGF provides the needed net inward force in this case and winds can spin in either direction around the L in either hemisphere.



Water draining from a sink or toilet - direction of spin
This is what happens when water drains from a sink or toilet.  The PGF is present, there is no CF.  The water can spin in either direction in either hemisphere.  It might not be obvious though what causes the inward pointing PGF in the case of spinning water.



If you look carefully at some spinning water you'll notice the surface has a "bowl" or "funnel" shape as sketched above.  The water at the edges is a little deeper.  That additional water has more weight and produces more pressure.  The water in the middle is shallower, doesn't weigh as much and the pressure is lower.  Thus there is a PGF pointing from the edges into the center of the vortex.


Here's a picture of the "Old Sow" whirlpool in the Bay of Fundy (located between the Canadian provinces of New Brunswick and Nova Scotia).  It is apparently the largest whirlpool in the Western Hemisphere (source).  The Bay of Fundy also has some of the highest tides in the world.


The Great Toilet Flushing Experiment
You've no doubt seen  water draining from a sink or flushed toilet.  The spinning water has the same bowl-shape as shown above.  That produces an inward pointing PGF.  The PGF is much stronger than the CF, the CF can be ignored.  In these situations clockwise or counterclockwise spin should be equally likely.  Over the course of several semesters a few years ago we conducted an experiment in ATMO 170 to see if this is indeed the case.  Students would go out, note the direction of spin after flushing a toilet, and report back to me.  Because the Coriolis force does not play a role, we should expect to see roughly equal numbers of reports of clockwise (CW) and counterclockwise (CCW) spin.  The results of the experiment are summarized below:
A French "squatter" toilet in a service station near Toulouse.
(source of this image)
 A free, completely automated self cleaning "Sanisette" toilet now found on the streets of Paris. (source of this image)


Experiment results

 CW spin
 CCW spin
Fall 2013
 35
 29
Spring 2014
 24
 17
Fall 2014
 12
 24
Spring 2015
 17
 26
Fall 2015
 41
 33
Totals
 129
 129

The Coriolis force does not play a role in determining the direction of spin in a draining toilet or sink.  On these small size scales, the pressure gradient force is much larger than the Coriolis force.  So we should expect to see roughly equal numbers of counterclockwise and clockwise spinning motions.  You can see for totals of all the data collected over a period of 5 semesters, that was true.  The experiment couldn't have turned out better.  Because of that the experiment was discontinued after the Fall 2015 campaign.

Is it possible to find spinning winds around high pressure when the CF is not present?






The CF plays an essential role here, it is the force that provides the net inward force needed to keep the winds blowing in a circular trajectory.
 With just the PGF there's nothing to provide a net inward force.  Circular winds around centers of high pressure is not possible when there is no CF.


What if just the Coriolis force were present?
Have a look at the following figures showing CW and CCW spinning motions in both the Northern and Southern Hemisphere.  Students hoping to earn extra credit points on this assignment should determine which two of the following would be possible if just the Coriolis Force (CF) is present (hints: what is needed anytime something is moving in a circular path?  Is the CF able to meet that need in any of the following cases?)