Because there is some misunderstanding whether or not fog is possible (it is), I created a new vote here:
https://forums.flightsimulator.com/t/please-add-visibility-rvr-slider/440825
Fog is possible (via live weather or presets), but what we really need is a Visibility / RVR slider in the Weather Settings dialog so we can set accurate fog at any airport without having to use workarounds or live weather.
If this is what you want, please vote at the new thread.
Edit:
Actually, the request already existed: