If you would like to "plot" a fog, and have the data as ascii, you could try
df3Maker at http://www.geocities.com/m_j_lewin/index.html. It can generate df3
files that others here have mentioned, but you do not have to generate any
extra textures etc. The downside is that you have to use Windows. The site has
some info on using df3Maker, and some examples as well.
MJL
