This is an appropriate point to look at a common misconception involving the Coriolis force.  You might have heard that the Coriolis force causes water to spin in different directions when it drains from sinks or toilet bowls in the northern and southern hemisphere.it does in the northern hemisphere.  We will find that this is not really the case.  Draining water can spin in either direction in either hemisphere.

The Coriolis force does cause winds to spin in opposite directions around 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).


Air starts to move inward towards the centers of low pressure.  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 identify the northern hemisphere and the southern hemisphere picture above.




The same kind of idea applies to high pressure except that the air starts moving outward.  The Coriolis force then turns it to the right or left.

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


The wind can spin in either direction in either hemisphere.


 For high pressure the PGF points outward.  Without the CF winds can't spin around High pressure because there is nothing to provide the needed inward force.


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.

In the classroom version of this course we watch a short video that seems to indicate otherwise.  The video was filmed in Kenya right on the Equator.  A young man drains a bucket a few tens of feet to the north of the Equator and then an equal distance to the south of the line.  The video shows the water spinning in opposite directions.  When the bucket is placed right at the Equator the water doesn't seem to spin at all while it drains. 

The gentleman in the video, who apparently makes a living demonstrating the Coriolis effect, was just very good at getting the draining water to spin one direction or another as he moved on opposite sides of the equator.  Probably the most difficult part would be to get the water draining without spinning, which is what he was able to do when standing right on the equator.

Also, the CF is zero at the Equator.  You would have to move a 100 miles or more to one side or the other before the CF becomes significant.  Nonetheless I give the students in the classroom version of the course the following optional assignment (it is probably my favorite assignment of the semester).
You must have access to a toilet with water
that spins when the toilet is flushed. 
Flush the toilet and make a note of whether
the water spins clockwise or counterclockwise. 
Then either email me the result or write down
the result on a piece of paper and turn it.
No photographs please.

Many of the toilets on campus won't work. 
The water doesn't spin, there is just a
sudden whoosh as the water is forced down the drain.

Here are 3 of the responses I got in a previous class.


We'll devote the rest of this lecture to another situation where the Coriolis force can be neglected because it is much weaker than the pressure gradient force.

Differences in temperature (such as you might find between a coast and the ocean or between a city and the surrounding country side) can create horizontal pressure differences. The horizontal pressure gradient can then produce a wind flow pattern known as a thermal circulation. 

Because these are generally relatively small scale circulations, the pressure gradient is much stronger than the Coriolis force. 

By applying some of the concepts we learned earlier in the semester we can really understand pretty well how thermal circulations develop.



We'll start here along a sea coast.  In this picture the air temperatures and pressures on both sides of the picture are the same.


A beach will often become much warmer than the nearby ocean during the day (the sand gets hot enough that it is painful to walk across in barefeet).  The warm air over the land will expand upward.  Note how the 900 mb level has moved upward in the picture.  We've left the temperature of the water the same as it was in the earlier picture.  The 900 mb level is found at the same altitude above the ocean.The cooler air over the ocean will shrink and move downward.  The 900 mb level drops below the level of the green line.  So on the left side of the figure we find 910 mb on the left side and 900 mb on right (at a constant altitude above the ground).

Another way of figuring out the upper level pressure pattern is to remember that pressure decreases relatively slowly in warm low density air.  There is only a 90 mb drop between the ground the green line on the left side of the picture.  Pressure decreases more rapidly with altitude (a 100 mb drop) in the cooler higher density air on the right side.  We end up with the same upper level pressure pattern.



The temperature differences have created an upper level pressure gradient (pressure difference), higher pressure (910 mb) on the left and lower pressure (900 mb) on the right.  The resulting PGF causes air to start to blow from left to right.


Once the air aloft begins to move it will change the surface pressure pattern.  The air aloft leaving the left side of the picture will lower the surface pressure (from 1000 mb to 990 mb).  Adding air at upper levels to the right side of the picture will increase the surface pressure (from 1000 mb to 1010 mb).  Surface winds will begin to blow from right to left.


You can complete the circulation loop by adding rising air above the surface low pressure at left and sinking air above the surface high at right.  The surface winds which blow from the ocean onto land are called a sea breeze (meteorologists try to specify where the wind is coming from).  Since this air is likely to be moist, cloud formation is likely when the air rises over the warm ground.  Rising air expands and cools.  If you cool moist air to its dew point, clouds form.

It is pretty easy to figure the directions of the winds in a thermal circulation without going through a long-winded development like this.  Just remember that warm air rises.  Draw in a rising air arrow above the warm part of the picture, then complete the loop.

At night the ground cools more quickly than the ocean and becomes colder than the water.  Rising air is found over the warmer ocean water (sea below).  The thermal circulation pattern reverses direction.  Surface winds blow from the land out over the ocean.  This is referred to as a land breeze.




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



Cities are often warmer than the surrounding countryside, especially at night.  This is referred to as the urban heat island effect.  This difference in temperature can create a "country breeze."  This will sometimes carry pollutants from a factory outside the city back into the city or odors from a sewer treatment plant outside of town back into town.

The Asian monsoon (the term monsoon refers to a seasonal change in the direction of the prevailing winds) is a large scale circulation pattern and is much more complex than a simple thermal circulation.  However you can use the thermal circulation concept to get a general understanding of what to expect at different times of the year.




In the summer India and SE Asia become warmer than the oceans nearby.  Surface low pressure forms over the land, moist winds blow from the ocean onshore, and very large amounts of rain can follow.  The view above at left is from above, the view at right is from the side.





The winds change directions in the winter when the land becomes colder than the ocean.

You can also use the thermal circulation to understand some of the basic features of the El Nino phenomenon (you find a discussion of the El Nino on pps 135-139 in the photocopied Classnotes).

First here is what conditions look like in the tropical Pacific Ocean in non-El Nino years (top and side views again)




Cold ocean currents along the west coasts of N. America and S. American normally converge at the equator and begin to flow westward (see top view above).  As the water travels westward it warms.  Some of the warmest sea surface waters on the earth are normally found the western Tropical Pacific.  A temperature gradient becomes established between the W. and E. ends of the tropical Pacific. The crossectional view above shows the normal temperature and circulation pattern found in the equatorial Pacific Ocean.   You would find surface high pressure in the east and low pressure in the west.  Note that the wind circulation pattern is the same as the simple thermal circulation we studied above.

Every few years El Nino conditions occur and the cold currents don't make it to the Equator.  Warm water is carried from the western Pacific to the eastern Pacific





Now surface high pressure is found in the west and surface low pressure and rising air is found in the E. Pacific (the reversal in the surface pressure pattern is referred to as the southern oscillation).  Indonesia and Australia often experience drought conditions during El Nino events.  In the desert SW we expect slightly wetter than normal conditions (perhaps 20% wetter than normal).  Wetter conditions are also found in California and in the SE US.