Isopleths

 http://www.nature.nps.gov/air/edu/lessons/figure3.cfm

 

An isopleth is an isoline that shows the frequency of a phenomenon or event based on constant values. Isopleths are open-ended and used for a myriad of cartographic techniques. In the map above, USA rainfall for the year 1996 is plotted using isolines. Precipitation is most dense on the northwestern coast.