[Imp]

If dynamic visibility could be done it would add to the game as suddenly units are visible due to a break in cloud cover or whatever. Simulating weather effects adds nothing though & as it stands its your choice whats causing the low visibility so restricts imagination. Also weather is a big deal to impliment firing penalties for firing into the wind in a rainstorm etc.
Fog is possible if do in the editor by placing an arty barrage on the map & saving it.
Good for doing in a valley or over a stream river as early morning mist. It burns off after about 5 turns so get crossing fast.