After extensive debugging I found the bugs: 1.) Integer division instead of floating point division in the latitude from y calculation 2.) Only 6 monsoon neighbor plots (which is Civ5 hex setup) instead of 8 neighbor plots (as it should be in Civ4) 3.) Geostrophic rain deactivated (was commented out but disfunctional with the other bugs anyway) 4.) Bugs in the plot list generation for geostrophic rain (sorted geo map) I have attached the file I debugged it on but since it is an adaption for C2C it has several other changes as well.