Friday, Nov. 2, 2012

Three songs from Iron and Wine this afternoon: "Freedom Hangs Like Heaven", "The Night Descending" and "The Devil Never Sleeps".

Quiz #3 has been graded and was returned today.  As always be sure to carefully check for grading errors.

The revised Experiment #1 reports have finally been graded.  Thanks for being patient.  You can click here to see if you have a graded report waiting to be picked up.

There was a short In-class Optional Assignment today.  If you'd like to download it and turn it in at the start of class next Monday you can earn credit for the assignment.

We'll be covering a lot of topics in the next 3 weeks or so before the last quiz of the semester: forces that cause the wind to blow the way it does in the northern and southern hemispheres; thunderstorms, tornadoes, and lightning; and hurricanes.

Today and Monday 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.  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.

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.  To understand why this is true we need to learn something about the earth's global scale pressure and wind patterns.  This is a topic we will be getting into the middle of next week.

I've borrowed some more carefully drawn figures below from a previous semester.  Step #1 is found on 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.  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 example of 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.  The frictional force is what causes this to occur.  He is an example of what you will be able to say about surface winds blowing around low pressure in the southern hemisphere.

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.  The only time there is no net force is when something is stationary (example (e) above) or moving in a straight line and at constant speed (example (a) above).  Here are a couple more figures of these two kinds of situtations (that weren't shown in class)

The two objects above are stationary.  In both cases there is no net force.  At left there aren't any forces at all.  At right, forces are present but that cancel each other out and the total or net force is zero.  With zero net force both objects will remain stationary.

Here an object is moving in a straight line at constant speed.  For this to be true the net force must be zero in both cases (otherwise the object would speed up, slow down, or change direction).  As long as the net force remains zero both objects will continue to move in a straight line at constant speed. 

Another important point to take from Step #2 is that a net inward force is needed anytime an object is moving in a circular path even if the speed is constant.  It doesn't matter what direction the object is moving and it doesn't matter what the object is circling around.

A net inward force is needed to keep winds spinning around a center of low pressure, an inward force is needed to keep air moving in a circular path around high pressure, and a net inward force (gravity) is needed to keep a satellite in a circular orbit around the earth.  It wouldn't matter what direction the satellite is moving.

Now we'll start to look at the forces that cause the wind to blow.

Pressure Gradient Force (PGF)

Air moving inward toward low pressure or outward away from high pressure is similar to a rock rolling down and away from the summit of a hill or inward toward the bottom of a depression.  The pressure gradient force always points perpendicular to the contour lines on a map and toward low pressure.  The PGF will cause stationary air to begin to move (it will always move toward low pressure).

Use the following figure (not shown in class) 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.

Coriolis Force

The Coriolis force is caused by the rotation of the earth.  We'll learn more about what causes the Coriolis force next Monday.  The CF points perpendicular to the wind and can only change the wind's direction.  It can't cause the wind to speed up or slow down.  The direction of the CF depends on whether you're in the northern or southern hemisphere. 

Hurricanes don't form at the equator because there is no Coriolis force there.

Time now to begin applying what we've learned.

We start with some stationary air at Point 1.  Because the air is stationary, there is no Coriolis force.  There is a PGF force.  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 didn't have time to do this in class.

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

Below is the answer to the question embedded in the notes about the PGF.

The mistake most people make is to forget to draw the PGF perpendicular to the contours.  It must be perpendicular and point toward low pressure.

And here's how winds blow around Low pressure in the Southern Hemisphere.