Isobars

http://theweatherwiz.com/school/chpt2a.htm
Isobars are lines of constant or equal pressure on a weather map. They can be used to find areas of low or high pressure over a broad area (like the U.S.), and they can tell us how intense the system may be. On weather maps, you may have noticed areas that have a large “L” or “H” over a region with lines circling around them.