Brian,
Regettably that is not available. All of those 9 witnesses heard or felt the crash. Thus it is almost certain that their recollections of the weather are linked to the crash.
The same cannot be said of the yachtsman who would not have known of the crash until significantly later; most likely not until after he had docked and returned to shore. My own recollection is that, after being informed by STC Ops about the crash, I turned on the TV just as the first "newsflash" gave basic details of the crash at about 1910A.
The yachtsman told the BOI that, when some 2nm SW of the lighthouse he had seen the Chinook fly past his yacht, "The visibility was about one mile and limited by haze". Some 19 months later he told the FAI "I recall the conditions of visibility at sea level as being fine, perhaps as much as five miles. I think at that point I could even see the Antrim coast so it may have been as much as six or seven miles". Those statements are incompatable with one another.
The yachtsman also recalled that he was manouvering around some fishing vessels at the time and that he had called his "cabin boy" up onto the deck to look at the helicopter. Neither the fishing vessels nor the cabin boy had been mentioned to the BOI (the yachtsman was referred to as "the lone yachtsman"), yet they could have been important witnessses: particularly the fishermen who would be used to assessing weather conditions at sea..
Whilst it is possible that he may have seen the lighthouse at times during the afternoon, no less than 4 eyewitnesses are adamant that the visibility at the lighthouse at the time of the crash was exceedingly poor (15-20 metres).
Regretably, human recollection is an imperfect thing. I would, however, suggest that, in this case, the evidence of the 9 witnesses on the Mull is compelling.