Fri., Feb. 15 notes

Friday Feb. 15, 2008

The first Optional Assignment was returned in class today. If you don't have a grade marked on your paper, you received full credit (0.5 pts to be added to your overall average at the end of the semester, before the final exam). It would be a good idea to compare your answers to the answers online even if there aren't any questions marked wrong on your paper.

A second Optional Assignment is due at the beginning of class on Monday.

You can keep track of progress being made on the grading here.

Today we will look again at upper-level charts.

Hopefully you still remember the trought (u-shape) and ridge (n-shape) features, the fact that cold air is found under an upper level trough and warm air below a ridge, and that the upper level winds blow parallel to the contour lines and from west to east.

By the end of today's class you should understand what the title "850 mb Chart" on the upper level map above refers to. You should also understand what the numbers on the contour lines represent and what their units are. Note that the values on the contours decrease as you move from the equator toward higher latitude. You should understand why that happens (temperature also decreases as you move toward higher latitude, maybe that is the explanation). You should understand why troughs and ridges are associated with cold and warm air, respectively.

You really only need to remember two things from earlier in the semester (you'll find the figure above at the bottom of p. 115 in the photocopied Classnotes): (1) pressure decreases with increasing altitude, and (2) pressure decreases more rapidly in high-density air than it does in low density air. We yet haven't learned about the association cold air = high density air and warm air = low density air.

Pressure drops from 1000 mb to 800 mb when moving upward 1500 meters in the cold air. It decreases from 1000 mb to 900 mb in the same distance in the warm low density air.

One way of depicting upper level conditions would be to measure pressure values at some fixed altitude above the ground. This approach is shown above. Pressures range from 800 mb to 900 mb at 1500 meters altitude. The pressure pattern could then be plotted on a constant altitude chart using isobars (figure below). Note the lowest pressures are found in the cold air, higher pressures would be found in the warm air.

That would seem to be a logical way of mapping upper level atmospheric conditions. Unfortunately that isn't how things are done.

Just to make life difficult for NATS 101 students meterologists do things differently. Rather than plotting conditions at a constant altitude above the ground, meterologists measure and plot conditions at a particular reference pressure level above the ground.

In the picture above you start at the ground (where the pressure is 1000 mb) and travel upward until you reach 850 mb pressure. You make a note of the altitude at which that occurs. In the cold dense air at the left pressure decreases rapidly so you wouldn't need to go very high, only 1200 meters. In the warm air at right pressure decreases more slowly, you would have to going higher, to 1800 m.

Every point on the sloping surface above has the same pressure, 850 mb. The altitude above the ground is what is changing. You could draw a topographic map of the sloping constant pressure surface by drawing contour lines of altitude or height.

The two kinds of charts (constant altitude or constant pressure) are redrawn below.

The numbers on the contour lines have been left off in order to clearly see that both types of maps have the same overall pattern (they should because they're both depicting the same upper level atmospheric conditions).

In the example above temperature changed smoothly from cold to warm as you move from left to right (west to east).
See if you can figure out what temperature pattern is producing the wavy 850 mb constant pressure surface below.

It shouldn't be too hard if you remember that the 850 mb level will be found at relatively high altitude in the warm air where pressure decreases slowly with increasing altitude. The 850 mb level will be found closer to the ground in cold air where pressure decreases rapidly with increasing altitude. Click here when you think you have it figured out.

In the next figure we are going to add south to north temperature changes in addition to the west to east temperature gradient.

Here's what the temperature pattern will look like.

Temperature drops as you move from west to east (as it did in the previous pictures) and now it drops as you move from south to north. What will the wavy 850 mb constant pressure surface look like now? Click here when you think you know (or if you just want to see the answer and would rather not think about it).

Now let's go back to the figure at the top of p. 115 in the photocopied Classnotes.

1. The title tells you this is a map depicting the 850 mb constant pressure level in the atmosphere.

2. The numbers on the contour lines (height contours) are altitudes (the units are meters)

3. The numbers get smaller as you head north because the air up north is colder. The 850 mb level is closer to the ground.

There was enough time left in the period to look at a short video. The tape described Auguste and Jacques Piccard's descent to a depth of 10,000 ft. in the ocean in a bathyscaph. Here are some notes written down during the video.

At 10,000 ft. depth, the pressure on the bathyscaph was about 5000 psi (remember normal atmospheric pressure at sea level is 14.7 psi). The video had a demonstration of what would happen to a human head if it were exposed to a pressure that large.

Jacques Piccard would later descend to a depth of more than 35,000 ft.

Here's another figure with some questions to test your understanding of this material (this wasn't shown in class)

This is a 500 mb constant pressure chart not an 850 mb chart like in the previous examples.

Is the pressure at Point C greater than, less than, or equal to the pressure at Point D (you can assume that Points C and D are at the same latitude)? How do the pressures at Points A and C compare?

Which of the four points is found at the lowest altitude above the ground, or are all four points found at the same altitude?

The coldest air would probably be found below which of the four points? Where would the warmest air be found?

What direction would the winds be blowing at Point C?

Click here for all the answers.

Finally we will compare upper level charts in the northern and southern hemisphere (something we didn't cover in class)

The contour values get smaller as you move toward colder air. The cold air is in the north in the northern hemisphere and in the south in the southern hemisphere. The winds blow parallel to the contour lines and from west to east in both hemispheres. The winds are something we will look at again when we get to Chapter 6.