Tue., Nov. 12 notes

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

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. There's a good chance that we won't cover this in class. Rather it will be the subject of a future optional assignment.

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. 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 blowing around low pressure in the southern hemisphere.

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 the object is, what direction the object is moving, or 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.

What about these three examples. 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. What about the next two. The 3rd example usually causes people the most trouble. You'll find the answer to this question at the very end of today's notes.

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 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 the Coriolis force on Thursday. 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 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, 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 needed to keep anything moving 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. Now we'll try to understand why friction causes surface winds to blow across the contour lines (always toward low pressure).

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 can only slow the wind down. The strength of the frictional force depends on wind speed (no frictional force if the wind is calm) and the type of surface the wind is blowing over (less friction when wind blows over a smoother surface like the ocean, more frictional force when the wind is blowing over a rougher surface like land).

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. Eventually the CF and Frictional force, working together, can balance out the PGF. The net force would again equal zero and the wind would blow in a straight line at constant speed across the contours toward low pressure.

This is where we finished in class. But I've added Step #10 and a little information about the Coriolis Force below.

What we've learned from the straight contour example, namely that the winds will blow across the contours toward low pressure can be applied to a curved contour pattern. This figure wasn't shown in class.

If you take a small little piece of a curved pattern and magnify it, it will look straight. We can mostly apply what we learned about straight contours to curved contours.

Now our last step, surface winds blowing around H and L in the NH and SH.

It is easy to figure out which of the figures are centers of low pressure (the wind blows inward toward the center of the picture) The winds are spiraling inward in the top and bottom examples (1 and 3). These must be surface centers of low pressure. The winds are spiraling outward from the centers of high pressure (2 and 4).

Now you probably don't want to figure out which of these are northern and which are southern hemisphere pictures. It is probably best to remember one of the pictures. Remember in 1, for example, that surface winds spin counterclockwise and spiral inward around centers of low pressure in the northern hemisphere (something we learned early in the semester). Then remember that winds spin in the other direction and blow outward around high pressure in the northern hemisphere (2). The spinning directions of the winds reverse when you move from the northern to the southern hemisphere. Thus you find clockwise spinning winds and inward motion around low pressure (3) and counterclockwise and outward spiraling winds around high pressure in the southern hemisphere.

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's something else not yet covered in class, an explanation of the cause of the Coriolis force. You'll probably find this a little confusing. The pictures that follow aren't in the ClassNotes.

Imagine something flies over Tucson. It travels straight from west to east at constant speed. You would, more or less subconsciously, plot its path relative to the ground. The next figure shows the path that the object would appear to follow as it passed over the city.

Here's the path the moving object would appear to follow relative to the ground. Based on this straight line, constant speed trajectory you'd conclude there was no net force acting on the object (and again no net force doesn't mean there aren't any forces, just that they all cancel each other out so the total force is zero).

In this second picture the object flies by overhead just as it did in the previous picture. In this picture, however, the ground is moving (don't worry about what might be causing the ground to move).

It's kind of like walking across the moving sidewalk at an airport with wet paint on your feet. What sort of track would you leave behind?

Even though you moved perpendicularly to the sidewalk, the foot prints would appear at an angle.

In the case of the object flying by overhead

The path, relative to the ground, would look something like this. It would no longer appear to be moving from W to E but rather from the NW toward the SE. It's still straight line motion at constant speed, though, so you conclude there was no net force acting on the object.

Now the ground is moving and also spinning. The object's motion hasn't changed.

The path of the object plotted on the ground appears to be curved. But remember that's relative to the ground and the ground is spinning. We could take the ground's motion into account or just ignore it. In the latter case you'd conclude that there was a net force perpendicular and to the right of the moving object. This net force would be needed to explain the curved path that the object appears to be following. And that's what the Coriolis forcedoes.

At most locations on the earth the ground IS rotating (and this is what Foucault's Pendulum demonstrates). This is most easily seen at the poles.

Imagine a piece of paper glued to the top of a globe. As the globe spins the piece of paper will rotate. A piece of paper glued to the globe at the equator won't spin, it will flip over. At points in between the paper would spin and flip, the motion gets complicated.

The easiest thing for us to do is to ignore or forget about the fact that the ground on which we are standing is rotating. We do still need to account for the curved paths that moving objects will take when they move relative to the earth's surface. That is what the Coriolis force does.

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

A net inward force is needed in all three cases. It's just the strength of the force required.

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). The wind ends up spinning clockwise around L 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.