Isobars

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

Isobars are lines that measure equal or constant pressure on a graph or map. Isobars that are closely related in space are representing a similar magnitude and wind speed. These factors can be used to predict future weather patterns. This image is of closed Isobars surrounding a low pressure cell area.