Mon., Apr. 9 notes

Monday Apr. 9, 2007

The Expt. #4 reports (for most people) were returned in class today. The revised reports are due in 2 weeks, on or before Mon. Apr. 23. Please return your original report with your revised report. If you didn't get your report back today it means I am trying to repeat some of your measurements and you should get your report back on Wednesday. There are several people that haven't returned their materials or turned in a report.

Today was the 2nd and last 1S1P makeup report due date.

Wednesday this week is the 1st of the 1S1P Assignment #3 due dates. If you plan to turn in two reports in Assignment #3, at least one report must be turned in on Wednesday.

An optional assignment handed out in class on Friday was collected at the beginning of the period.

Now we'll use what we know about the pressure gradient and Coriolis forces to determine why upper level winds blow the way they do. You only need to worry about the PGF and CF when considering upper level winds. When we get to surface winds we will need to include the frictional force.

We'll start with a simple upper level contour pattern, straight parallel contour lines. And we'll start with the analogous situation shown at the top of p. 123 in the photocopied Class Notes.

Placing a volume of air in a pressure gradient is just like putting a rock on a ramp. The rock will begin to roll downhill. The PGF will start the stationary air moving and cause it to move toward low pressure. The wind will pick up speed as it goes.

On the larger weather chart at the bottom of p. 123, we start with a stationary volume of air at Point 1. The PGF (perpendicular to the contour lines and pointing toward low) will start the air moving toward low pressure. The dots show this initial motion.

At Point 2 the air is moving and the Coriolis force appears. It is perpendicular and to the right of the wind. It is weak because the wind speed is low. The CF begins to bend the wind (it is bending to the right if you look in the direction the wind is blowing).

The wind picks up speed and, in Points 3 and 4, the CF is getting stronger and the wind is continuing to bend.

At Point 5, the wind is blowing parallel to the contours, and the wind speed is high enough that the CF is able to balance the PGF. The net force is now zero. From this point on the winds will blow in a straight line at constant speed parallel to the contour lines. This is known as a geostropic wind or geostrophic flow.

Some more examples from p. 124 in the photocopied Class Notes.

Some air is placed at Point 1 in the two figures above. The PGF force starts the stationary air moving (the dots show the direction of this initial motion). Low pressure would be found at the top of both maps in this figure. Then if we watch the motion carefully we see the air beginning to turn to the right at Point 2 in the left figure and turning to the left in the right hand figure. This is caused by the Coriolis force. The CF is to the right of the wind in the left figure, this is a northern hemisphere (NH) chart. The CF is to the left of the wind in the right figure, this is a southern hemisphere (SH) chart.

The CF and PGF again balance by the time you get to Point 3.

The two figures above (middle of p. 124) show maps with strong and weak pressure gradients. The wind in the left figure ends up blowing much faster than the wind in the right figure (much as a rock would roll quickly down a steep ramp and slowly down a more gradual slope). The fast wind in the left figure produces a strong Coriolis force that is eventually able to balance the strong PGF. The slow winds at right produce a weaker CF. The CF is to the right of the wind in both examples, so these are both in the northern hemisphere.

In the left figure the direction of the initial motion (the dots) is toward the bottom of the figure. The initial motion is caused by the PGF. The PGF points toward low pressure at the bottom of the chart.
In the right figure the wind takes a left turn once it begins to blow (turn the page upside down so you are looking in the direction the wind is blowing). That identifies this as a southern hemisphere chart.

Now we'll look at upper level circular centers of low and high pressure.

The rock rolling down a hill vs air moving in a pressure gradient analogy is shown again at the top of the figure.

By now you should be understanding what is shown in the bottom two figures. The dots tell you the direction of the initial motion. They tell you the direction of the PGF, inward toward low pressure in both these figures.

In the middle figure the wind takes a right turn (identifying this as a northern hemisphere chart) and eventually ends up blowing parallel to the contour lines in a counterclockwise direction.

In the bottom figure the wind turns to the left and ends up blowing parallel to the contour lines in a clockwise direction. The CF points to the left of the wind in this figure, this is a southern hemisphere chart.

Note in both charts that the PGF and the CF point in opposite directions but they are no longer equal in strength. The inward point PGF is stronger than the outward CF. The difference provides the net inward force (the purple arrow) needed to keep the wind blowing in a circular path.

Because of the Coriolis force, winds blow counterclockwise around low pressure in the NH and clockwise around low in the SH.

Here's what happens with upper level high pressure centers.

The initially stationary air begins to move outward and away from the high pressure in the center (the dots show this initial motion).

In the middle figure the CF is to the right of the wind. This is a northern hemisphere map. Winds blow parallel to the contours and spin clockwise around high in the northern hemisphere.

The winds blow parallel to the contours and in a counterclockwise direction around circular centers of high pressure in the southern hemisphere.

Note the net force is inward in both cases.

Now before you get the idea that all winds change directions in the NH and SH we'll look at the next figure.

The winds are blowing from west to east in both hemispheres even though the CF changes directions in the NH and SH. How is this possible. If you look closely you will notice that the pressure pattern is also "flipped." Low pressure is found at the top of the map in the NH and at the bottom of the chart in the SH. The direction of the CF changes directions in the NH and SH hemisphere, the PGF also charnges directions and the winds blow in the same direction.

The spacing of the contour lines and the strength of the PGF stays about constant on this chart. If you look closely at the figure you will notice that the CF force is sometimes stronger and sometimes weaker than the PGF. This changing imbalance results in a net force needed for the right and left turns that the winds take as they blow through this pattern. If you remember that the strength of the CF depends on latitude (as well as wind speed) you can understand why the CF changes strength. The CF is strongest when the winds are far from the equator, weakest when the winds are close to the equator.