I currently create fog (in my world) by modifying the RenderSettings in real-time. It works. It caused issues with some gameobjects however, most notably a rotating Earth which rendered completely white. I was able to adjust the mode and lower the fog. But now I have a beam of light coming from a lighthouse (using the particle system) and that gets all "frayed" and is no longer a beam. So I'm looking for whatever solution might exist to generate fog that doesn't automatically affect everything and/or what I might do to make some objects immune to fog.