Thanks for your response, but that was not what I had in mind when I asked it. I usually fly with automatic weather loading from the Internet, and occasionally when it rains (because I suppose the METAR says it's raining in that location) the visibility does not always change (reduce) accordingly. In the real world whenever we get some rain the visibility will reduce (if it's not already bad enough), but it does not happen using AS4 (latest version confirmed installed).
Is "Fog layer generation" enabled in ASP4 Options\Visibility options? You can also adjust maximum surface visibility and maximum upper visibility to create IFR like conditions without having to restart ASP4.