Isobars, in use of weather maps, are lines of equal atmospheric pressure drawn on a meteorological map. Each line passes through a pressure of a given value that never cross eachother. This is an example of  describing the patterns of air pressure, over the Great Lakes, that include one or more closed, circular isobars.

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