Isobars

http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Closed_Isobars/Closed_Isobars.html

Isobars are used to connect area of equal averaged pressure on a map. The pressure is reduced to sea level and not the surface pressure of an area in an attempt to have a base pressure. This is a weather forecast in the Great Lakes area.