My understanding was you wanted this fog at certain times? So find that time in the existing weather, then follow the advice above....
- Change the ambient section that time section calls to a new one... [env_ambient_egg's_spooky_fog]
- Then add that new section to env_ambient and start from there.
That was only a suggestion for the section name by the way

You can also spawn particles on anomolies, one way Utility did his work was to create new anomolies, with no damage or attributes, that then called a particle effect. I don't know if you could make those only go online at certain times though using all.spawn.
Maybe if you used scripts to bring these anomolies online and take them offline? I understand that is basically the way the changing positions of AMK's and OL's anomolies works....
Ooh, just had an idea....