Monday Apr. 14, 2014

I tried out The Soil & the Sun "Like Diamonds" in the 10 am class.  But decided Buena Vista Social Club "Hasta Siempre (Che Guevara)"  and "Chan Chan" might be better for the 2 pm people.

The 1S1P reports on Fog have been graded and were returned today.  All but a handful of students received full credit (8 pts).  The number in red on your paper is the grade on this report.  The number in green is the total number of points you have earned so far.

I hope to have the Scientific Paper and Book reports graded by Wednesday (the ones that were turned in last week anyways).

Part 1 of the Quiz #4 Study Guide is now available even though Quiz #4 is a little over 2 weeks away.  There are several links on the study guide to help you with the material on Newton's 1st law of motion and the material on surface and upper-level winds.  We will be finishing most of that today.  I think you will find that you are able to figure this material out without too much trouble or effort.  And if you find that you aren't understanding the material there's plenty of time to remedy the situation before the quiz.

Here's a quick review of the rules for the direction and strength of the pressure gradient force (PGF) and Coriolis force (CF)


And a quick review of the development of upper level winds in situations where the contour lines are straight.  That's the simplest kind of situation.



Winds in the northern hemisphere are shown at left.  The picture at right shows the development of winds in the southern hemisphere.  You'll find this explained in more detail at the end of the Fri., Apr. 11 notes.

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 Point 1.  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). 

Once the air starts to move, the CF causes it to turn to the right (because this is a northern hemisphere chart).  This is happening at Point 2 (the dots show the initial motion of the air).  As the wind speeds up the CF strengthens.  The wind eventually ends up at Point 3 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.

See if you can figure out what would happen with low pressure in the Southern Hemisphere. 



We start again with some stationary air at Point 1 in this figure.  You'll find the details at the end of today's notes.



Next we'll look at what happens around upper level high pressure



Here initially stationary air at Point 1 begins to move outward in response to an outward pointing pressure gradient force (PGF).  Once the air starts to move, the Coriolis force (CF) will cause the wind to turn to the right.  The wind ends up blowing in a clockwise direction around the high.  The inward pointing CF is a little 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.



This is a southern hemisphere upper level center of high pressure.  You should be able to figure out how the winds will blow in this case.  You'll find the answer at the end of today's notes.

Upper level winds blow parallel to the contour lines.  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 and points in a direction opposite the motion.

The strength of the frictional force depends on 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.


The top figure shows upper level winds blowing parallel to straight contours.  The PGF and CF point in opposite directions and have the same strength.  The total force, the net force, is zero.  The winds would blow in a straight line at constant speed.  Since the CF is perpendicular and to the right of the wind, this is a northern hemisphere chart. 

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. 

One of the key things to remember is that friction causes surface winds to 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.



Winds blow into the centers of low pressure and outward away from centers of 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.  And instead of winds, the arrows represent other cars driving around a 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 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).


Here are the two figures showing how  the figure showing how upper level winds blow around Low pressure in the Southern Hemisphere.


The stationary air starts to move in toward the center of low pressure (just like it did in the northern hemisphere).  But then it takes a left hand turn rather than a right hand turn.  This is because the CF is perpendicular and to the left of the wind (as you look downstream) in the southern hemisphere.  The wind ends up spinning clockwise around L in the southern hemisphere.



Winds that blow around high pressure in the southern hemisphere.  The air starts to move outward but then turns left due to the Coriolis force.  The result is that winds spin counterclockwise around H in the southern hemisphere.