But it does seem to me that Tolkien often LIKES to be ambiguous about these things!

I wonder if Tolkien worked this way (or in a similar manner)? I can certainly imagine him trying to figure out whether certain events should be coincidence or not.

As you say, a lot of what Tolkien writes could be supernatural or natural. Off the top of my head, another weather event where he has been rather ambiguous is when Glorfindel saves Frodo at the River Bruinen. Admittedly, Tolkien does say that it was Elrond who unleashed the flood, but could it also have been entirely natural. There had been considerable rainfall throughout this part of Middle-earth, so perhaps a natural dam broke upstream? It's naturally possible, even though Tolkien gives as the definitive cause.

So I'm debating: was this simple radiation fog, which I'm sure as a resident of the UK and a man aware of weather JRRT was aware of, compounded by the chill area proximal to the Barrow?

OR - an intervention by the bad guys

Standard Barrow Wight entrapment technique? (With or without enhancement by the Witch King, who might figure he could use the wights to seal off this route, and collect the Ring from the Wight later)

Seest thou not how here in this little realm in the Deeps of Time Melkor hath made war upon thy province? He hath bethought him of bitter cold immoderate, and yet hath not destroyed the beauty of thy fountains, nor of my clear pools... Melkor hath devised heats and fire without restraint, and hath not dried up thy desire nor utterly quelled the music of the sea. Behold rather the height and glory of the clouds, and the everchanging mists; and listen to the fall of rain upon the Earth! And in these clouds thou art drawn nearer to Manwė, thy friend, whom thou lovest.

Tolkien certainly gives a good and accurate description of the sort of weather that we get in Southern Britain during September. I live and work in Down lands very similar to those that Tolkien describes and mists certainly do form quite quickly later in the day (I always assume that the Barrow Downs are based on the Berkshire Downs, but they are a little further from the sea than my area) And even without barrows, the landscape can seem creepy enough when that happens- part of this being the suppression of sounds in these atmospheric conditions. I am sure that walks in the Downs in these conditions probably inspired Tolkien originally. After that, it seems to me that he set up the climactic conditions appropriate to that setting and added a dash of the supernatural. I agree with NoWiz, that this ambiguity was intentional on Tolkien's part.

Thanks for an interesting essay Daniel and for reminding me of O level geography and A level physics!!