From someone with a small amount of knowledge about 3d stuff, I would recommend using a cloudless map and then placing the clouds as a separate visual layer, as well as using diffuse/glossy/rough/bump maps. If your map uses terrain shading, you can do this fairly easily: for the glossy map, just use an image editing program to black out everything that isn't blue and then white out everything that is, etc. Find a fairly high res map, and spend some time doing manual painting. Happy to help with pbrs for free.