Thursday Nov. 13, 2014

Some great music (in my opinion) to get things rolling again in ATMO 170: Dave McGraw and Mandy Fer "Grow", "Serotiny", "Comin' Down".  

Quiz #3 has been graded.  Not only that but the Experiment #3 reports and the Expt. #2 reports that were handed in with Expt. #3 have all been graded.  You are allowed to revise your report if you want to (it's not required).  The revised reports are due in two weeks which turns out to be Thanksgiving Day.  So you can have until the following Tuesday (Dec. 2).  Though if you could turn in the revised reports before Thanksgiving that would be a big help to me and would be appreciated.  Please return your original report with your revised report.

The revised Expt. #2 reports haven't been graded yet.

The 1S1P reports on Rainbows, Mirages, and the Green Flash have been graded (all 130+ of them, that was quite a job).  I haven't had time to look at the Earth's Changing Temperature reports yet.

The 1S1P reports on Tucson Fog were collected today.  The Atmospheric Stability reports are due next Tuesday.  Two new topics are also now available.  They are both due a week from today.

An Optional Assignment was handed out in class today.  I forgot to mention it is due next Tuesday (Nov. 18), many people turned it in at the end of class (which is certainly OK, and we covered enough material that you should have been able to answer all the questions).  You can download a copy of the assignment here.

By my count we have 5 full class periods between now and the last quiz.  We'll be covering a variety topics in that time: forces that cause the wind to blow the way it does in the northern and southern hemispheres; thunderstorms, tornadoes, lightning, and hurricanes.

Today and briefly next Tuesday we will be looking at how and why surface and upper level winds blow the way they do.

Some real world examples of where this occurs are shown in the figure below (found on p. 121 in the ClassNotes).  The two largest types of storm systems, middle latitude storms (extratropical cyclones) and hurricanes (tropical cyclones), develop around surface centers of low pressure (the term cyclone refers to winds blowing around a center of low pressure).  Winds spin counterclockwise around cyclones (centers of low pressure) in the northern hemisphere and clockwise in the southern hemisphere.

Winds spin clockwise around "anticyclones" (high pressure) in the northern hemisphere and counterclockwise in the southern hemisphere.

Why do winds blow in opposite directions around high and low pressure.  Why do the winds change directions when you move from the northern to the southern hemisphere.  These are the kinds of questions we'll be addressing.

 

Something else to notice in the figure.  Storm systems in the tropics (0 to 30 degrees latitude) generally move from east to west in both hemispheres.  At middle latitudes (30 to 60 degrees), storms move in the other direction, from west to east.  That's not something we will cover in class, though it is covered in the new 1S1P topic on Thermal Circulations and the 3-cell Model of the Earth's Global Scale Circulation.

We'll be able to learn most of what we need to know about surface and upper level winds in 10 relatively easy steps (though I've broken several of the steps into smaller parts)

Step #1 - Upper level and surface winds - summary

Here's a slightly different version of p. 122a in the ClassNotes.



Upper level winds spinning around high and low pressure in the northern and southern hemispheres are shown in the first set of four pictures.  All the possibilities are here.  The first thing to notice is that upper level winds blow parallel to the contours.  Just 2 forces, the pressure gradient force (PGF) and the Coriolis force (CF), cause the winds to blow this way.  Eventually you will be able to draw the directions of the forces for each of the four upper level winds examples.  Here is an upper level wind example showing what you will be able to do. 

The four drawings at the bottom of the page show surface winds blowing around high and low pressure in the southern hemisphere.  These winds blow across the contour lines slightly, always toward low pressure.  A third force, the frictional force is what causes this to occur.  He is an example of what you will be able to say about surface winds.


Step #2 - Newton's 1st law of motion



If there's no force being exerted on an object it will either be stationary or moving in a straight line at constant speed.  Anytime an object is slowing or speeding up or changing direction (or both) a net force is present.

You should be able to look at an object's (or the wind's) motion and tell if there is a net force or not.  There are two situations where there is no net force.  The first is shown below.


The two objects above are stationary.  In both cases there is no net force.  That could mean there aren't any forces at all (left figure).  Or forces may be present but they cancel each other out and add up to zero (the total force is zero).  With zero net force both objects will remain stationary.

The other possibility is that something is moving in a straight line at constant speed.




As long as the net force remains zero both objects will continue to move in a straight line at constant speed.  Note in the bottom figure the two forces could have been pointing right and left.  If they cancel each other out the total force is zero and the object would continue to move in a straight line at constant speed.

Here are some more examples of straight line motion (this and 3 additional figures was on a handout distributed in class).
 


Straight motion in all 6 cases.  The speed is also constant in examples (a) and (f) so there is no net force in those two cases only.  The speed is changing in all the other situations.  A net force is present in (b) - (e) and you should be able to determine its direction.  Maybe you can figure that out just by looking at the pictures, maybe not.  What you might do is think of a situation you are familiar with.




Here's a picture of someone throwing a ball upward.  You know from personal experience that the downward force of gravity will cause the ball to slow down as it rises, come to a stop, and then begin to fall picking up speed as it falls.  Gravity is present during both the rise and fall of the object.

We can use this motion that we understand to figure out the direction of the net forces in the earlier picture.




Notice how the motion in (b) resembles that of a falling ball.  The motion in (e) is just like the motion of the ball thrown upward in the air.  In this kind of way you can figure out and draw in the directions of the forces present in all 4 examples (b) - (e).




The next figure shows circular motion at constant speed.  Is there a net force in any of these examples?  The answer is yes, a net force is present in all three cases because the motion is not straight line motion at constant speed.  What is the direction of the net force?


My sister can help with this question



She's out longeing ones of her horses, training it to run in a circle and to obey her commands.  At least initially the horse doesn't really want to do that and you must pull with a lot of force to keep it moving in a circle (without a force it would move in a straight line).  It's relatively easy to understand when you imagine a strong heavy horse at the end of a rope but a net inward force is needed anytime anything moves in a circular path.



It doesn't matter what the direction of spin is, it doesn't matter what's in the middle of the picture, anytime something is moving in a circle there is a net inward force (gravity is the inward force keeping the satellite in orbit in the 3rd picture).
Here's a question I know many people will have trouble with (but better to have trouble here in the online notes than in the middle of a quiz).  Keeping an image of a horse on the end of a longe line might help.





Is there a net inward or outward force in each case?  You should now know that there is a net inward force in the 1st example because that's what we've just been covering.  What about the next two.  The 3rd example is the one that causes people the most trouble.  You'll find the answer to this question at the end of today's notes.

Now we'll start to look at the forces that cause the wind to blow.
Step #3 Pressure Gradient Force (PGF) 
The figure below is on p. 123a in the ClassNotes)


Isobars on a weather map are very much like height contours on a topographic map.  A center of low pressure on a weather map is analogous to a circular valley on a topographic map (high pressure on a weather chart is like a circular hill).

The PGF always points in a direction that is perpendicular to the contour lines and toward low pressure.  The PGF can start stationary air moving.  The air will always start moving toward low pressure.  Air moving inward toward low pressure or outward away from high pressure is similar to a rock rolling downhill into the center of a depression or downhill and outward away from the summit of a hill.


It's good to understand the pressure gradient force as best you can.  But at the same time it's nice to have some rules that you can fall back on and apply.  The rules for the PGF are shown above.  Follow them (especially the one for direction) to the letter and you'll never go wrong.

Use the following figure to test yourself.  With an arrow draw the direction of the PGF at each of the points in the figure.  You'll find the answers at the end of today's notes.





Step #4 - Coriolis force (CF)
The rules for the CF are shown below.



The top of p. 123b in the ClassNotes has several examples showing the direction of the CF.  You can use them to check and make sure you understand how to apply the direction rule above.




The Coriolis force is caused by the rotation of the earth.  I've put a little information about the cause of the Coriolis Force at the end of today's notes.  We didn't cover that in class today.  The CF points perpendicular to the wind and is to the right or left depending on hemisphere.  Be sure you are looking in the direction the wind is blowing, looking downstream when determining the direction of the CF. 

The CF can only change the wind's direction.  It can't cause the wind to speed up or slow down. 

There isn't any Coriolis force when the wind is calm.  Coriolis force is zero at the equation because that's where the CF changes direction.  Hurricanes don't form at the equator because there is no Coriolis force there.
Time now to begin to apply what we've learned so far.

We'll consider the simplest possible situation - upper level winds with straight contours.  We'll do a Northern Hemisphere (NH) example.




We start with some stationary air in the lower left corner of the picture.  Low pressure is at the top and high pressure at the bottom of the picture.

The PGF can start stationary air moving.  The PGF will point toward the top of the picture (perpendicular to the contours and toward the low pressure at the top).  There won't be any Coriolis force when the air is stationary.

Once the wind starts to blow (blue line above) the CF will appear.  The CF will be weak at first because the wind speed is low but the CF will begin to turn the wind to the right.  As the wind picks up speed the CF will increase in strength.  Eventually the wind will be blowing parallel to the contours from left to right.  The PGF and CF point in opposite directions and are of equal strength.  The net force is zero and the wind will continue to blow to the right in a straight line at constant speed.

Here's a simpler less cluttered way of depicting what we have just figured out.



The dots show the direction of the initial motion.  That will always be toward low pressure.  Then you look in the direction the wind starts to blow and look to see if the wind turns right or left.  It turned right in this case.  That's the effect of the Coriolis force and means this is a northern hemisphere map.

Here's one last example to test your understanding.

The direction of the initial motion is shown with dots.  Where is the high and low pressure in this case?  Is this a NH or SH chart.  You'll find the completed map at the end of today's notes.

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.

Step #9 - Friction

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 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.


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.  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

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.  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).

I've changed my mind.  These lectures notes have gone on long enough.  I'll put the explanation of the cause of the Coriolis force in a separate section by itself (see the Lecture Notes Table of Contents, you'll find it there). 

Here are the answers to the questions embedded in today's notes.





A net inward force is needed in all three cases.  The thing that changes is the strength of the inward force.





This figure shows the directions of the PGF at each of the highlighted points.  The mistake many people make is to draw the arrow pointing straight toward the L.  But the PGF arrow must also always be perpendicular to the contour lines.



The initial motion is always toward L pressure which must be at the bottom of this map.  Then if you turn the map so that you can look downstream in the direction of the initial motion you'll notice the wind turning to the left indicating this is a southern  hemisphere map.