Friday Nov. 2, 2007

Quiz #3 and the 1S1P Topic #4 reports were returned in class today.

The final 1S1P assignment of the semester is now online.  Note that there are two due dates.   If you plan to turn in two reports (the maximum allowed), at least one of the reports must be turned in on the Wed., Nov. 14 due date.  The second due date, Mon., Nov. 19 is the Monday before Thanksgiving.

We'll cover a variety of topics in the final month of the semester.  We'll start in Chapter 6 by learning more about the forces that cause winds. 


The Coriolis Force is one of these.  You shouldn't leave NATS 101 Intro. to Weather and Climate without having been introduced to the Coriolis Force.  It is part of the reason why winds spin counterclockwise (CCW) around Low pressure and clockwise (CW) around High pressure in the northern hemisphere.   It also explains why winds spin in the opposite directions around Highs and Lows in the southern hemisphere.

You may already have been to the southern hemisphere or you may go there one day.  You'll probably hear about how the Coriolis force or the Coriolis effect causes water to spin in a different direction in the southern hemisphere when it is draining out of a sink and toilet bowl (it's not true).  That's another reason for covering the Coriolis effect in NATS 101. 

In Chapter 7 we'll learn why middle latitude storms move in the same direction, from west to east, in both the northern and southern hemispheres.  Hurricanes move from east to west in both hemispheres, this is also explained in Chapter 7.
Today we will look again at upper-level charts, something we covered earlier in the semester. 

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.

The upper level winds blow parallel to the contour lines from west to east in both hemispheres.  The winds sometimes turn right, sometimes left.  Once we learn more about the forces that govern wind motions, you'll understand what causes this.



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 rapidly in cold high-density air [it decreases more slowly with increasing altitude in warm, low-density air].  This is shown at the bottom of the figure above.  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 find the pressure has dropped to 850 mb.  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 inds of charts (constant altitude or constant pressure) are redrawn at the bottom of the picture above.  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.
Here's another figure with some questions to test your understanding of this material.

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.