Isobars

This visual shows the lines of constant pressure around a Low pressure somewhere over the Great Lakes. These visuals are useful in predicting weather movement and how either a High pressure of Low pressure will react with adjacent climate patterns.

http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/
Closed_Isobars/Closed_Isobars_fig02.jpg