Friday Apr. 18, 2014

Pavarotti & Sting "Panis Angelicus" on a Friday morning before Easter.  You can read more about the song itself here.

All of the various reports turned in earlier this week have been graded and were returned in class today.  The Expt. #3 revised report are due next Wednesday, April 23.  Everything else is due the Wednesday after than, April 30.

The 1S1P reports on the Koppen Climate Classification system have all been graded and were returned.  There was a large discrepancy between the average grades from Grader #2 (the easier grader) and Grader #3.  Because of that 3 points were added to all of the reports graded by Grader #3.

I'm hoping to have grade summaries ready to hand out in class next Wednesday.  They will include the 1S1P reports on Rainbows, Mirages, and the Green Flash but not the topics turned in this week on Regional Winds & Foucault's Pendulum.

And here's a scary thought.  The Final Exam scheduled for this section of the class is 3 weeks from today (3:30-5:30 pm).

The day was devoted to finishing up the material on the 3-cell model, forces and winds.  We'll start a new section on Thunderstorms, Tornadoes, and Lightning next Monday.  That will take us up to Quiz #4 most likely. 

We need to quickly reconstruct the map we were working on at the end of class on Wednesday.  We started with the blank map below and added features in a step by step manner.  Leave some room off to the left for names of all the features that need to be drawn in.



You need a starting point, I would suggest remembering that there's a belt of low pressure at the equator, the equatorial low.  Then remember the belts of pressure alternate.  Low pressure at the equator and then high pressure at 30 N and 30 S, these are the subtropical highs.  Winds move away from high pressure at 30 degrees toward low pressure at the equator and low pressure at 60 degrees (not shown in the figure).  The Coriolis force turns the wind to the right in the NH and to the left in the southern hemisphere.


The winds blowing between the equator and 30 degrees latitude are the trade winds.  They converge at the equator and form the intertropical convergence zone (ITCZ).  The convergence causes air to rise so this is a region of cloudy skies.  The low pressure belt at the equator is also referred to as the doldrums.  This is a region of fairly calm surface winds and sailing ships used to become stuck there (becalmed I think is the word).  Calm winds are also found at 30 degrees latitude, the horse latitudes.

Winds found between 30 and 60 latitude blow from the west and are called the prevailing westerlies.

Hurricanes and middle latitude storms have been drawn in.  Winds blow from the west between 30 and 60 degrees latitude where middle latitude storms are found.  Those storms move from west to east.  Hurricanes on the other hand are found in the tropics between the equator and 30 degrees latitude.  The trade winds move hurricanes from east to west.





3-cell model predictions:
belts of high and low pressure

 important real world features:
centers of high pressure &
a belt of low pressure at the equator


The 3-cell model assumes that the earth is of uniform composition and not tilted toward or away from the sun.  It predicts belts of high pressure at 30 N and 30 S latitude as shown above at left.  Because the real world has oceans and continents we find centers of high pressure, not belts, located near 30 latitude. 




The high pressure center off the East Coast of the US is called the Bermuda High.  The Pacific High is found off the west coast.  Don't worry about the names of the Highs off the east and west coasts of South America.

Winds blowing around these centers of high pressure create some of the world's major ocean currents. The California current is a cold southward flowing current found off the west coast of the US.  The Gulf Stream is the warm northward flowing current along the east coast.

Some information about the El Nino phenomenon was stuck into the online class notes from Wednesday even though we didn't cover it in class.  The normal temperature pattern in the tropical Pacific Ocean is shown above.    You can now better understand why this is true.  Two cold ocean currents, the California current north of the equator and its analog in the southern hemisphere meet at the equator in the eastern Pacific.  That is why the water there is so cold.  The water then warms as it moves westward.  Some of the warmest ocean water in the world is found in the western equatorial Pacific (that is also where hurricane formation is most likely).  During an El Nino event the two cold ocean currents stop short of the equator.  Warm water from the western Pacific moves eastward.  The ocean water temperature pattern basically reverses.  This has a profound effect on weather around the globe.



No tilt
Low pressure at the equator, high pressure at 30S & 30N
 summer tilt
L and H move north of their normal positions
 winter tilt
L and H move south of their normal positions

The 3-cell model assumes that the earth isn't tilted.  That of course isn't the case in the real world.  The North Pole tilts toward the sun in June.  The figures above shown the effect this has on the 3-cell model features.  In June the low pressure belt moves north of the equator (nominally to 23.5 degrees latitude).  The two subtropical highs also move north of their nominal positions at 30 degrees latitude.  In the winter all the features shift southward. 

This is perhaps seen more clearly on the two figures below.



Here's the summer picture (North Pole tilted toward the sun).  The ITCZ which is normally found at the equator has moved north of the equator.


 And the winter pictures, all the features have moved southward.

The movement of the Pacific High north and south of its nominal position near 30 degrees latitude is part of what causes our summer monsoon in Arizona.   The term monsoon is often used incorrectly in S. Arizona to refer to a thunderstorm. 

A monsoon is a seasonal change
in the direction of the prevailing winds.

Probably the best known monsoon is the Indian Monsoon.  This was also discussed briefly in the notes from Wednesday's class.






In the winter the Pacific High is found south of 30 N latitude (the bottom of the figure above).  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean is moist (though the coastal water is cold so this air isn't as moist as it would be if it came off warmer water).  Before reaching Arizona the air must travel over high mountains in California.  The air loses much of its moisture as it does this (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.

 During the summer, the Pacific High moves north of 30 N latitude.  Winds on the southern side of the subtropical high have an easterly component.   Moist air originating in Mexico and from over warm water in the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

Tucson gets about 12 inches of rain in a normal year.  About half of this comes during the "summer monsoon" season. 

Lastly we returned to the toilet flushing experiment and 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. 



Just like a rock will always start to roll downhill and never uphill, 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 Coriolis force then turns it to the right or left.

Note that in all four cases above the inward pointing force is stronger than the force pointing outward.  This is what provides the net inward force needed for something to move in a circular path.  The Coriolis force, because it is the one pointing inward, plays a key role in this case.  An inward force is needed to keep anything moving in a circular path. 

The figures above were actually on a handout distributed in class that looked like this.


Remember the term cyclone refers to winds spinning around a center of low pressure.  Anticyclone refers to high pressure.

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. 



You need an inward pointing force in order for wind to blow in a circular path.   It is possible for winds to spin around low pressure.  The inward pointing PGF provides the inward force that is needed.  The PGF points outward around centers of high pressure.  Winds can't spin around high pressure when there is just a pressure gradient force.  There's nothing to supply the needed inward force.

This is what happens when water drains from a sink or toilet.  The water can spin in either direction in either hemisphere.  What causes the inward pointing PGF?


The water at the edges 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.  The toilet flushing experiment should show about equal numbers of clockwise and counterclockwise spinning water.  What has the experiment actually shown so far?
16 students have reported counterclockwise spinning
18 students have reported clockwise spinning

You can't do too much better than that (the numbers had changed a little bit by the 2 pm class: 17 CCW versus 24 CW).

Class ended (a little early) with an opportunity to earn a Green Card.  The following figure was on the back of the class handout.


Which two of the figures above would be possible if there were just Coriolis Force and no pressure gradient force. 

This a good question.  It looks hard.  Don't just guess at the answer.  Ask you self what do I know about the Coriolis Force and circular motion.  If you weren't in class today you can still try to earn a Green Card .  Turn in a sheet of paper on Monday with your name and the answer to the question above.  Your answer should include some explanation about how you solved the problem.