Hiya Pete, welcome to the Guild! I don't know about any tutorials that discuss this, but you could always try using some random clouds to do this, though I don't know off hand of any tutorials that suggest how. But basically you would generate a random set of clouds, use your "Select By Color" tool (in GIMP, I don't know what its name is in PS), experiment with different Threshold settings (start with 50 and work your way higher), select the whitest portion of the clouds layer, inverse the selection and then delete out the gray and black portion of the clouds, leaving the white portions. You might create several different clouds layers, using this method on each of them until you have an assortment of cloud patterns. Then just arrange them (i.e. cut and paste them to a "Fog" layer) however you like, varying the layer opacity to achieve the affect you want. At least, this method might work for a more photorealistic map.