Isopleth maps are used in geography and in meteorology. In geography, isopleths maps are used to generalize data with a continuous distribution.
In meteorology, isopleths is the term used for any type of contour line.
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This isopleth map shows the different pH concentration levels across the US. The levels are dotted and the surrounding colors correlate to the different concentration levels.
http://dwb4.unl.edu/chem/chem869v/chem869vlinks/weather.about.com/newsissues/weather/library/weekly/aa071600a.htm