Isobars

Isobars are drawn onto a map and used to describe anything from wind speed and patterns to in drop in barometric pressure, like this map is showing here. This particular on shows a dramatic drop within a closed system of Low pressure.
http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Closed_Isobars/Closed_Isobars.html