Isopleths

http://www.cbr.washington.edu/papers/jim/draft.html

Isopleths are contour lines on a map which connect points of the same value. The map above is depicting isopleths of north-south coastal wind velocity by month and latitude. By looking at the isopleths you can determine that the positive velocity is associated with the winter and fall seasons. The negative velocity is associated with summer and spring.