I first started to feel POIS in the winter. I didn't relate it to POIS, just figured that for whatever reason, my bones were more sensitive to weather systems. Arthritis I figured at first. Chile has a marked climate. Summer is clear, I mean sky blue clear for about 4 1/2 mo. Then fall and winter bring weather fronts and my problems started.
But summer always a LOT better. As the POIS got worse, I saw that even summer didn't save me, and began to look for other explanations.
So I think sun (vitamin D) is important for lessening POIS, and maybe weather systems changes of humidiy and air pressure also worsen it. I guess to see if vitamin D has anything to do with making us more sensitive to these temp/pres changes we'd have to hear from someone who is in summer AND a place where there are constant changes of weather systems, like Florida etc.