21. Isobars

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

Isobars are a type of contour line used specifically in meteorology to show air pressure. Areas of equal pressure are connected with an isoline. This map shows a low pressure cell over the Great Lakes and can be used to predict weather patterns.