Isopleths

Isopleths maps is continuous distribution that show third dimensions on a map helping to simplify and generalize specific data. The third dimension is shown by a series of lines called isopleths that connect points of an equal value. The difference between the two adjacent isopleths is called the isopleth interval. These maps are common when mapping surface elevations, atmospheric pressure, amounts of perception, and other statistical elements that can be viewed as a third dimension on a map. The map above is the mean annual snow fall which helps to show the impact of the winter.

http://www.weathernotebook.org/features/winters.html