How to add clouds or fog to an existing map in GIMP?

[jfrazierjr]

Granted, it would be easy enough to make a new layer on top of your map, render some random noise, and reduce the opacity somewhat (depending on how thick you wanted the fog). Maybe change the layer mode to overlay as well?

Well... one good thing about the foggify filter is that you don't have to Delete stuff. Normally. noise is either black or white and covers every single pixel in your image in some shade. The foggify filter also addes in alpha to this, so you have patches of (mostly) white colored fog and the rest is alpha instead of black.