Friday Sept. 22, 2006

Quiz #1 was returned in class today.  The average was -49 (76%), the class homepage shows you how to calculate your percentage grade.  Be sure to check you quiz carefully to be sure that it was graded correctly and that the points missed were added up correctly.

The Experiment 2 materials were distributed in class today.  It won't take more than about 30 minutes to collect the experiment data.  Why not do the experiment this weekend, return the materials early next week, and pick up the supplementary information sheet (before you get busy with a new optional assignment, new 1S1P reports, and studying for the next quiz).

There is a new reading assignment.

Today we will:
1. Finish up the material on upper level charts
2. See a short demonstration that has been making the rounds through the other NATS 101 classes
3. Finish up with the Piccards by watching a short video segment on Bertrand Piccard's first successful trip around the globe nonstop in a balloon.


This is where we left off on Monday. 

A surface LOW positioned below the X on the upper level chart (to the east of the upper level trough) is in a perfect position to intensify.  Why?  Because you'll often find upper level divergence at that location.  Upper level divergence, if it is strong enough, can cause the surface LOW pressure to get even lower which will cause the storm to intensify.

We will see what upper level divergence looks like and the effect that it can have in the next few figures.  The following figures were on a handout distributed in class and should replace the figure on p. 42 in the photocopied class notes.



In the figure above we see a surface LOW (with 960 mb pressure).  Winds are spinning counterclockwise and spiraling in toward (converging) the center of the low.  These surface winds are moving air into the column of air and (as explained on the figure) should cause the pressure in the center of the LOW to increase.  Imagine that each arrow brings in enough air to increase the pressure at the center of the LOW by 10 mb.  You would expect the pressure at the center of the LOW to increase from 960 mb to 1000 mb as shown in the next figure.


Because the pressure at the center of the LOW has increased to 1000 mb (which we will pretend is about the same as the pressure in the surrounding atmosphere, there  is not longer a pressure difference and the converging winds are gone.

Now what if instead of increasing from 960 mb to 1000 mb, the pressure decreased, from 960 mb to 950 mb say?  How would you explain that?


This is just like a bank account.  You have $960 in the bank and make four $10 dollar deposits.  You would expect your bank account balance to increase from $960 to $1000.  What if your account balanced dropped to $950?  How would you explain that? 



The figure above shows what is going on.  We're back to the starting point again, we have a surface low with a central pressure of 960 mb.  The converging winds are each carrying in 10 mb worth of air into the column. 

What's new is the addition and removal of air at upper levels.  Imagine that 50 mb worth of air are added to the column and 50+50=100 mb worth of air are removed.  That's a net removal (net divergence) of 50 mb.  So now we have 40 mb worth of air being added at the ground (surface convergence) and 50 mb worth being removed at upper levels (upper level divergence).  The grand total is 10 mb of removal.  The surface pressure will decrease slightly.  That's why the 960 mb LOW drops to 950 mb.

You can apply the numbers in the right hand picture to the bank account problem.  You have $960 in the bank and make 4 $10 deposits.  However you also deposit $50 dollars and make 2 $50 withdrawals (the top of the picture).  That's a total of $90 being deposited and $100 being withdrawn.  Your bank account goes down $10.


Note with the lower central pressure, the converging winds will strengthen and carry more air into the column.  In this figure we have assumed the strengthening winds now each carry 12.5 mb worth of air into the column.  The converging surface winds are now carrying in as much air as is being lost at upper levels.  The surface LOW pressure center won't change, everything is in balance.

In a case like this where upper level divergence > surface convergence, the surface LOW pressure will get even lower (the low will "deepen") and the storm will strengthen.

The other possibility is that the upper level divergence < surface convergence.  In this case the LOW pressure will increase (the low will "fill") and the storm will weaken.

Click here for some additional examples.  By working through some additional examples you might increase your understanding of this material and build up your confidence (of course there's always a chance that more examples will just make this topic more confusing - the choice is yours)
We can apply these concepts to hurricanes and understand why hurricanes can intensify and get so strong.



You find surface low pressure in the center of a hurricane (a general name for a hurricane is tropical cyclone- the word cyclone refers to the winds that spin around low pressure).  Record-setting low sea level pressure values have all been observed in hurricanes.

In this first picture we consider the converging surface winds only.  These winds will add air to the cylinder of atmosphere that is the hurricane.  This will increase the surface pressure and the storm will fill (surface pressure will increase) and the storm will weaken.  The storm would eventually dissipate.

Now we know that sometimes hurricanes strengthen despite the converging winds that blow at the surface.


We will find, later in the semester, that high pressure is found at the tops of hurricanes.  This means the upper level winds will diverge.  If the upper level divergence exceeds the surface convergence, air will be removed from the colume, the surface pressure will get lower, and the hurricane will strengthen.  This is illustrated at the bottom of the figure above.  The surface LOW in the center of the hurricane gets lower and lower.  As the pressure drops the winds increase in speed and bring more air into the column. Eventually the convergence at the bottom and the divergence at the top of the hurricane balance and the storm doesn't strengthen any further.
This figure wasn't shown or discussed in class.

Now that you have some idea of what upper level divergence looks like you are in a position to understand another one of the relationships between the surface and upper level winds. 

One of the things we have learned about surface LOW pressure is that the converging surface winds create rising air motions.  The figure above gives you an idea of what can happen to this rising air (it has to go somewhere).  Note the upper level divergence in the figure: two arrows of air coming into the point "DIV" and three arrows of air leaving (more air going out than coming in is what makes this divergence).  The rising air can, in effect, supply the extra arrow's worth of air.

Three arrows of air come into the point marked "CONV" on the upper level chart and two leave (more air coming in than going out).  What happens to the extra arrow?  It sinks, it is the source of the sinking air found above surface high pressure.
Now back to something we have already covered.  People will float or sink in a pool of water depending on whether they are slightly less dense or slightly more dense than water. 




  Everyone floats in the Great Salt Lake because the density of the salty water is higher, around 1.1 g/cm3


If you place a can of regular Pepsi and a can of Diet Pepsi in water, you will find that the can of diet cola floats while the regular cola sinks.


The can of diet cola is mostly water will a little bubble of air inside the top of the can.  The density of this combination of cola, air, and aluminum (in the can) is less than the density of water.

The regular cola is similar except that instead of water there is a mixture of water and corn syrup (sweetener).  The corn syrup makes this can more dense than water.

We finished class looking at a short video segment of the first non-stop trip around the globe in a balloon.  Bertrand Piccard was a member of the 2-man team aboard the Breitling Orbiter 3.  They completed their trip in March, 1999.