Fri., Apr. 5 notes

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 toward the end 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 situations.

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.

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 details at the end of today's notes.

Next Monday we'll look at upper level winds that blow around high pressure in the northern and southern hemisphere (Steps 7 & 8). Then we'll look at how the frictional force causes surface winds to blow across the contour lines toward low pressure (Steps 9 & 10). I've put most of that online just in case you want to take a look ahead.

Below is the answer to the earlier question about the PGF. The green arrows show the direction of the PGF, always perpendicular to the contours and pointing toward low pressure.