Certainly in terms of service provision at Heathrow, the number of days
affected by fog (which means Cloud ceiling 200ft or RVR 600m or less) has
reduced dramatically in recent years. There always seemed to be fog when the
Tridents were doing their autolands.....
What has increased is the number of days affected by wind...(unsurprisingly
less fog then)...
Affect of Global warming ?

Phil