Way back I though I saw a mod that lets you paint terrain textures on the map. They are just like the rain mod, effects going away after you leave the city. Does this ring a bell for anyone?
Edit: looks like I may have found it, brb.
Edit: I did :) SMP Terrain Texture Brush Pack 01 1 (http://community.simtropolis.com/files/file/18216-smp-terrain-texture-brush-pack-01/)