CHPT. 2: 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.
Now let’s analyze a low pressure system. In this case you look for the lowest pressures on the map. You find an 842. No isobars are drawn for 842but one is drawn for 844. Since 842 is lower than 844, the 842 must be kept on the inside (toward lower pressure) of the 844. Next isobar is the 880. All pressures lower than 880 must be kept on the inside (toward lower pressure) of the 880 isobar while pressure higher than 880 must be kept outside the 880 isobar. Next look for the 992 pressure reading. Again lower pressure inside the 992 isobar, higher pressures outside. When you are finished, label the center of the system with a large red “L”.
Try to analyze the three pressure systems below. Read the directions carefully.