Tue., Nov. 6 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. 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.

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.

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.

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.

Now what about 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 the ocean, more frictional force when the wind is blowing over 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 acrosss the contours toward low pressure.

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. The figure below wasn't shown in class.

If you take a small little piece of a curved pattern and magnify it, it will look straight.

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 spiralling 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.

We had time, barely, for one more topic today. There's a common misconception involving the Coriolis force. You might have heard that water spins in a different direction when it drains from a sink or a toilet bowl in the southern hemisphere than it does in the northern hemisphere. You might also have heard that this is due to the Coriolis force or the Coriolis effect.

The Coriolis force does cause winds to spin in opposite directions around large scale high and low pressure centers in the northern and southern hemisphere. The PGF starts the air moving (in toward low, out and away from high pressure) then the Coriolis force bends the wind to the right (N. hemisphere) or to the left (S. hemisphere).

Here's what you end up with in the case of low pressure (you'll find these figures on p. 130 in the photocopied ClassNotes):

Air starts to move inward toward low pressure (the dots show this initial motion). Then the Coriolis force causes it to turn to the right or left depending on which hemisphere you're in. You should be able to say which of the pictures above is the northern hemisphere and which is the southern hemisphere picture.

The same kind of idea applies to high pressure except that the air starts moving outward (the dots aren't included here). The Coriolis force then turns it to the right or left.

There are situations where the PGF is much stronger than the CF and the CF can be ignored. A tornado is an example. The PGF is much much stronger than the CF and the CF can be ignored. Winds can blow around Low pressure because the PGF points inward.

The wind can spin in either direction in either hemisphere.

Note that without the CF, winds can't spin around High pressure because there is nothing to provide the needed inward force.

OK, what about water draining from sinks, buckets, toilets etc.

There's just an inward pointing PGF, no CF. Water can spin in either direction in either hemisphere. What causes the inward pointing PGF? The water at the end of the spinning water is a little deeper than in the middle. Since pressure depends on weight, the pressure at the outer edge of the spinning water is higher than in the center. This creates the inward pointing pressure gradient (pressure difference) force.

Water draining from a sink or toilet can spin in either direction. It doesn't matter where you're located.

But this something we should probably checkout for ourselves, so here is one of my favorite optional assignments of the semester.

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

The air starts to move outward and then turns to the left. Winds blow in a counterclockwise direction around high pressure in the southern hemisphere.