Hiya!
I've not experienced this issue so strongly personally, but I figured the least I could do for a fellow Skyrim player was look it up
Unfortunately, it does not appear to be a directly hot topic. So.. I checked for myself to see if I could trigger a weather change in riften, while assigning my Girlfriend to try checking files regarding the weather engine.
It seems you've been extremely 'lucky' to have persistant fog for each visit giving the awesome feeling. I did'nt find any direct regards as to why the weather would be one of the (many) weather options and my Girlfriend agrees. It's PRETTY randomly assigned, although most zones have their very own Rythm's and Riften is among them. Riften does, indeed, appear to have a good chance of being foggy, but it should not be quest related weither it's sunny or foggy in the city.
Try giving it a day and check if it's sunny again and note if it's persistant. If the same weather repeats more than 5 days of full sunshine, try travelling around, come back and check again. If the issue still remains, then it appears the engine does'nt work quite like the way we've come to anticipate.
But that's just our two (or four?) cents on the matter and from what two novices regarding this kind of stuff could come across. If anyone knows anything for certain that we overlooked, I apologize for the faulty response. Untill then, I hope our efforts atleast helped a bit =)