Monday, Apr. 11, 2016

Marcus Roberts "Bolivar Blues" (3:15), "I Got Rhythm" (6:08), if you have some extra time you really should watch this 60 Minutes segment on Marcus Roberts, Hans Otahal "Bumble Boogie" (4:36)

The Scientific Paper, revised Expt. #2, and a couple of Book Reports have all been graded and were returned in class today.  You can revise your report if you want to (no need to revise your report if you're happy with the grade you received).  The revised reports are due in two weeks, by Mon., Apr. 25.  Though if you could get them in before that would be helpful.  Please return your original report with your revised report.  This is especially important with the Scientific Paper and Book Reports.

Step #5 - Upper level winds, low pressure, northern hemisphere

Next we'll be looking at the upper level winds that develop around circular centers of high and low pressure.






We start with some stationary air at the bottom of the picture.  Because the air is stationary, there is no Coriolis force.  There is a PGF force, however.  The PGF at Point 1 starts stationary air moving toward the center of low pressure (just like a rock would start to roll downhill).  The dots show the initial motion

A rock would roll right into the center of the picture.  Once air starts to move, the CF causes it to turn to the right (because this is a northern hemisphere chart).    As the wind speeds up the CF strengthens.  The wind eventually ends up blowing parallel to the contour lines and spinning in a counterclockwise direction.  Note that the inward PGF is stronger than the outward CF.  This results in a net inward force, something that is needed anytime wind blows in a circular path.


Upper level winds spin counterclockwise around low pressure in the northern hemisphere.



Step #6 - Upper level winds, low pressure, southern hemisphere



We start again with some stationary air at Point 1 in this figure.  The situation is very similar.  Air starts to move toward the center of the picture but then takes a left hand turn (the CF is to the left of the wind in the southern hemisphere).  The winds end up spinning in a clockwise direction around low in the southern hemisphere.  The directions of the PGF, CF, and the net inward force are all shown in the picture.


Upper level winds spin clockwise around low pressure in the southern hemisphere.



Step #7 - Upper level winds, high pressure, northern hemisphere


Here initially stationary air near the center of the picture begins to move outward in response to an outward pointing pressure gradient force (PGF is pointing toward low pressure which is on the edges of the picture).  Once the air starts to move, the Coriolis force (CF) will cause the wind to turn to the right.  The dots show the initial outward motion and the turn to the right.  The wind ends up blowing in a clockwise direction around the high.  The inward pointing CF is stronger than the PGF so there is a net inward force here just as there was with the two previous examples involving low pressure.  An inward force is need with high pressure centers as well as with centers of low pressure.  An inward force is needed anytime something moves in a circular path.


Step #8 - Upper level winds, high pressure, southern hemisphere


This is a southern hemisphere upper level center of high pressure. The air starts to move outward again but this time takes a left hand turn and ends up spinning counterclockwise.  The net force is inward again.




Upper level winds review
Here's a quick review of much of what we covered in class on Tuesday.  Many of the figures below were on a handout distributed in class today.

Winds spin counterclockwise around L pressure in the northern hemisphere then switch direction and spin clockwise around L pressure in the southern hemisphere.   I think by just remembering a couple of things you can figure this out rather than just trying to memorize it.

The pressure gradient will start stationary air moving toward low pressure (just like a rock placed on a slope will start to move downhill)


The dots in the figure above show this initial motion and its in toward the center of the picture.  These must both be centers of Low pressure.  Then the wind will turn to the right or left depending on the hemisphere.  This is the effect of the Coriolis force, the CF turns wind to the right in the northern hemisphere and to the left in the southern hemisphere.   



The northern hemisphere winds are shown at left in the figure above, the southern hemisphere winds are shown at right.  The inward pointing force is always stronger than the outward force so that there is a net inward pointing force.

This initial motion is outward away from the center in the two figures below. 

Low pressure is on the outside edges of the picture.  High pressure must be found in the center of both pictures.



The outward moving air takes a right turn in the left figure above, a left turn in the right figure (you may need to rotate the picture so that you are looking downstream, in the direction the wind is blowing to clearly see the left hand turn).

Friction and surface winds

Next we'll try to understand why friction causes surface winds to blow across the contour lines (always toward low pressure).

With surface winds we need to take into account the PGF, the CF, and the frictional force (F).  That means we'll need some rules for the direction and strength of the frictional force.  Friction arises with surface winds because the air is blowing across (rubbing against) the earth's surface.





You're probably somewhat familiar with the effects of friction.  If you stop pedaling your bicycle on a flat road you will slow down and eventually come to a stop due to air friction and friction between the tires and road surface.  Friction always acts to slow a moving object it must point in a direction opposite the motion.

The strength of the frictional force depends on wind speed.  The faster you try to go the harder it becomes because of increased wind resistance.  It's harder to ride on a rough road than on a smooth road surface.  In the case of air there is less friction when wind blows over the ocean than when the air blows over land.  If the wind isn't blowing there isn't any friction at all.




The top figure (p. 128 in the ClassNotes) shows upper level winds blowing parallel to straight contours.  The PGF and CF point in opposite directions and have the same strength (the fact that there are only two forces present tells you these are upper level winds).  Note the CF is to the right of the wind, this is a northern hemisphere case.  The total force, the net force, is zero.  The winds would blow in a straight line at constant speed. 

We add friction in the second picture.  It points in a direction opposite the wind and acts to slow the wind down. 

Slowing the wind weakens the CF and it can no longer balance the PGF (3rd figure).  The stronger PGF causes the wind to turn and start to blow across the contours toward Low.  This is shown in the 4th figure. 




Step #10 - Surface winds blowing around H & L pressure in the N. & S. hemispheres.
I think you'll be surprised at how easy it is to determine whether each of the figures below (p. 129 in the ClassNotes) is a surface center of H or L pressure, found in the N or S hemisphere, and whether rising or sinking air motions/clear or cloudy skies would be associated with each figure.

Key point to remember: surface winds blow across the contours always toward low pressure.



It should be very easy to figure out which two of the figures above are surface centers of low and high pressure.




Next to determine whether each figure is in the northern or southern hemisphere we will imagine approaching the upper left figure in an automobile.  We'll imagine it's a traffic circle and the arrows represent cars instead of wind.



You're approaching the traffic circle, what direction would you need to turn in order to merge with the other cars.  In this case it's left.  That left turn is the Coriolis force at work and tells you this is a southern hemisphere map.

The remaining examples are shown below



Converging winds cause air to rise.  Rising air expands and cools and can cause clouds to form.  Clouds and stormy weather are associated with surface low pressure in both hemispheres.  Diverging winds created sinking wind motions and result in clear skies.


Somethings change when you move form the northern to the southern hemisphere (direction of the spinning winds).  Sometimes stay the same (winds spiral inward around centers of low pressure in both hemispheres, rising air motions are found with centers of low pressure in both hemispheres).


This is as far as we could get in class today.  We'll finish up the remaining material on this topic in class on Wednesday.