Why visibility suddenly to 1 hex?

[Volcano Man]

It looks like it shouldn't go to 0, but in your game turn select to view the weather report. If it says "none" in the dialog then it means that the weather.dat file with your dates in them for 1989 were added later, after you started. In that case there is a known issue where 1/2 way through the scenario it will drop to 1 hex vis.

I am not saying this is what it is, but it could be a possibility which comes from starting a scenario with "programmed weather" selected and with no weather data available.