Isotach

http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/water_nitro/water_and_nitrogen_cycles.htm

An Isopleth is used to contour lines that depict a variable which cannot be measured at a point, but which instead must be calculated from data collected over an area. An isopleth map generalizes and simplifies data with a continuous distribution. Isopleth maps are more common for mapping surface elevations, amounts of precipitation, atmospheric pressure, and numerous other measurements that can be viewed statistically as a third dimension.
This is a map of the United States showing regions of sensitivity to acid rain. The isopleths are of the pH of precipitation. In this illustration all of the eastern U. S. has an average pH of rainfall between 4 -5.