Isohyets

Isohyets are lines drawn through points of equal precipitation. Because different parts of the world experience their "rainy season" at different points in the year isohyets will occur at different point through out the year. Isoheyts are usually drawn in bands that are measured in mm.

Link: http://www.springerimages.com/Images/Environment/1-10.1007_978-1-4020-3995-9_633-0