Currently, there is a single default fog environment used for all rendering in all Ages, and its values are reset when loading an Age via console commands in the .fni files. There is also a Python API that can be used to change the values.
Support exists (but is not exposed or ever used) for per-object fog environments via
plFogEnvironment objects and references in the DrawableSpans.
However, there is currently no way to animate fog changes or transitions between fog values without resorting to Python hacks.