Earth Science – Weather Maps: Isobars And Isotherms

Earth Science – Weather Maps: Isobars and Isotherms

DRAWING ISOBARS ON A WEATHER MAP

Surface weather maps have isobars drawn around high and low pressure systems. (Isobars are lines of equal pressure). Here are the rules.

1. See if you can locate the high and low pressure systems. High pressure systems have the highest pressure in the center of the system. Look for such pressures as 1016.0 or 160, 1020.0 or 200, 1024 or 240, 1028 or 280 etc. 2. Low pressure systems have the lowest pressure in the center of the system. Look for pressures such as 1000.0 or 000, 996.0 or 996.0, 992.0 or 920, 988.0 or 880, 984.0 or 840, 980.0 or 800 etc.

3. If you are analyzing a high pressure system, first look for the highest pressure you can find on the map. For example, you find a station that has a pressure of 290. An isobar is not drawn for 290 but one is drawn for 280. That is our first isobar. Since 290 is higher than 280, it must be kept inside (toward higher pressure) the 280 isobars. Everything lower than 280 must be kept outside (toward lower pressure) the 280 isobar. If you see a 270 near a 290 the 280 isobar goes right between the two readings. The next isobar you will draw is the 240. Again, everything higher than 240 must go inside (Toward higher pressure) the 240 line. All pressures lower than 240 must go outside the 240 isobar. Next draw the 200 isobar. Again, all pressures higher than 200 must stay on the inside of the 200 isobar, all pressure lower than 200 must stay on the outside of the 200 isobar.

4. Label the center of the system with a large blue “H” for high pressure and the system is complete. Follow the same procedure for low pressure systems and any other highs.

Important rules to remember:

1. Isobars (or Isotherms) NEVER cross 2. Isobars (or Isotherms) eventually form a circle even if it is not completely happening on your map (maybe they attach off of the edge of the map you are looking at) 3. If you ever draw Isobars AND Isotherms on the same map, they CAN cross since one is measuring pressure and the other temperature. 4. If you don’t see any plotting points for stations between your numbers, you must interpolate where your isolines go. If one station has the pressure 992 and the next closest one is 1001 then you will need the isobar 992 going through that station and then the 996 and 1000 isobars passing between that station and the station that reads 1001. Do you best with interpolation comparing what is nearby and remember rule #1!

Procedure:

1. Please draw your isobars in blue on the map. Find the lowest pressure and label the next isobar up from it. (Say you found a low pressure of 1003.5 mb then you would draw a 1004mb isobar around it) Then start drawing isobars following the station plots in 4mb increments using interpolation. 2. Label any high and low pressure systems you find. 3. Please draw your isotherms in red on the map. Follow the same procedure for temperatures as you followed for pressure.

Questions:

1. Where did you discover High pressure systems? What was the highest reading for that system?

2. Where did you discover Low pressure systems? What was the lowest reading for that system?

3. Where did you find some of the fastest winds? How did you know? 4. How do isobars tell you the direction of wind flow? (How does wind flow with respect to isobars?)

5. Pressure gradient force indicates that wind will flow from high pressure to low pressure to try and equalize the pressures. However wind does not flow like this. Why not?

6. Where can you expect to find cloudiness and rain on your maps? Sunny skies?

7. Please draw green hatched areas on your map where you might expect to find rain. Please draw light blue hatched areas on your map where you might expect to find snow (make sure this hatching can be easily differentiated from your isobars which are also in blue).

8. Assuming that weather and winds generally follows the westerlies for the map we are looking at, where would you expect high winds and precipitation tomorrow or the next day?

9. Where do you find your coldest temperatures? What are they associated with on this map?