Isobars

Isobars are contouring lines that connect the same values of atmospheric pressure. These lines tend to display trends that can be used by people to find areas of Low and High pressure. In the example above, one is able to see the isobars that relate to the low pressure that can be seen over the Great Lakes. 
http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Closed_Isobars/Closed_Isobars.html