Objectives: To determine gain a better understanding of air pressure and upper-level maps

To understand air pressure we first need to be able to decode weather maps. A typical symbol looks like

You have seen these and know that 45 represents the dewpoint temperature and 54 represents the actual temperature both in degrees Fahrenheit. So, what does the 017 represent? It gives the pressure. If we have a surface map then it gives the sea level pressure and is corrected for altitude. We need to learn how to decode this pressure. Fortunately, it is relatively simple to do. If the three digit number is 499 or less then place a 10 in front of it. If it is 500 or greater then place a 9 in front of it. This will then give the pressure in millibars (mb). Recall that 1 atmosphere equals 1031.25 mb. The decoded pressure 017 is equal to 1001.7 mb. If the three digit code were 789 then the decoded pressure is simply 978.9 mb. This technique works 99% of the time since rarely do we have pressures below 950 mb or higher than 1050 mb. Decode the following pressures.

1) 324 = _______ mb

2) 1023.2 mb = ________

3) 932 = _______ mb

4) 978.4 mb = ________

5) 567 = _______ mb

6) 995.6 mb = ________

7) 890 = _______ mb

8) 1031.2 mb = ________

9) 268 = _______ mb

10) 1039.5 mb = ________

You have a surface map with coded pressures. Place the decoded pressures on the blank map provided. You should decode every station. You may have to add stations to your blank map.

Once you finish decoding the pressures, you are to draw (WITH AN ERASABLE PENCIL) isobars (lines of constant pressure). Take extra care when constructing your isobars. If you have questions please ask.

Once you have the isobars drawn label the low and high pressure systems if any are present.

Upper-Level Maps

In surface maps isobars are plotted. Upper-level maps are somewhat different. Instead of plotting lines of constant pressure you will be plotting lines of constant height for a given pressure. For example, when you see a 500 mb map every isoplyth is a line of constant pressure of 500 mb. But, the lines are lines of constant height measured in meters. On the 500 mb upper-level map provided draw the geopotential height contour lines (lines of constant height) in the same way you drew the isobars for the surface map.

To help you better understand how upper-level maps affects the weather go to The Weather Visualizer (http://covis.atmos.uiuc.edu/covis/visualizer/) and click on "Upper-level Observations." Scroll down and click on the statement, "Are you familiar with Constant Pressure Surfaces?" Answer the following questions based on what you find at this web site.

11) What pressure surface is higher? 200 mb or 500 mb--Explain.
 
 
 

12) How are pressure surfaces located?
 
 
 

13) In a column of cold air is a 500 mb surface lower or higher than that of a warm column of air? Explain.
 
 
 

14) Is a 500 mb surface lower or higher in the South compared to the North? Explain.
 
 

15) What is an Upper Air Trough and what type of weather is typically associated with it? Explain.
 
 
 
 
 
 

16) What is an Upper Air Ridge and what type of weather is typically associated with it? Explain
 
 
 
 
 
 

17) Is an upper-level ridge consider a high or low pressure system?
 
 
 
 
 
 

18) Is an upper-level trough consider a high or low pressure system?
 
 
 
 
 

19) Summarize what you have learned about upper-level maps.