Sun definitly has a positive effect for me.
I live where spring and fall are short and summer is about 5 months of 32C (90F) with cloudless skies. I'm much better in summer.
But there's another factor to consider, especially with soft tissue inflamation, and that is weather systems. I caught on to weather systems in relating to my neck and joint symptoms before I related symptoms to POIS.
A low pressuer system 1 to 2 days out can beging to wreak havoc. As the system approaches, it gets worse, up until it arrives. For me, when the rain actually arrives, I start to feel better already.
Also certain combinations of cold and humidity can have affect.
But, I'd say, that sun has a big part. Even with good weather in winter, I'm more susceptible.