Wednesday, Nov. 7, 2012

There's a common misconception involving the Coriolis force.  You might have heard that water spins in a different direction when it drains from a sink or a toilet bowl in the southern hemisphere than it does in the northern hemisphere.  You might also have heard that this is due to the Coriolis force or the Coriolis effect. 

The Coriolis force does cause winds to spin in opposite directions around large scale high and low pressure centers in the northern and southern hemisphere.  The PGF starts the air moving (in toward low, out and away from high pressure) then the Coriolis force bends the wind to the right (N. hemisphere) or to the left (S. hemisphere).

Here's what you end up with in the case of low pressure (you'll find these figures on p. 130 in the photocopied ClassNotes):


Air starts to move inward toward low pressure (the dots show this initial motion).  Then the Coriolis force causes it to turn to the right or left depending on which hemisphere you're in.  You should be able to say which of the pictures above is the northern hemisphere and which is the southern hemisphere picture.


The same kind of idea applies to high pressure except that the air starts moving outward  (the dots aren't included here).  The Coriolis force then turns it to the right or left.

There are situations where the PGF is much stronger than the CF and the CF can be ignored.  A tornado is an example.  The PGF is much much stronger than the CF and the CF can be ignored.  Winds can blow around Low pressure because the PGF points inward.


The wind can spin in either direction in either hemisphere.

Note that without the CF, winds can't spin around High pressure because there is nothing to provide the needed inward force.



OK, what about water draining from sinks, buckets, toilets etc.


There's just an inward pointing PGF, no CF.  Water can spin in either direction in either hemisphere.  What causes the inward pointing PGF?  The water at the end of the spinning water is a little deeper than in the middle.  Since pressure depends on weight, the pressure at the outer edge of the spinning water is higher than in the center.  This creates the inward pointing pressure gradient (pressure difference) force.

Water draining from a sink or toilet can spin in either direction.  It doesn't matter where you're located. 

But this something we should probably checkout for ourselves, so here is one of my favorite optional assignments of the semester.




















































































































Here are some additional examples of thermal circulations or large scale circulations that resemble thermal circulations.