I keep thinking about POIS and the weather all the time. If I collect enough evidence of its effect, I would relocate once I have the chance.
For me the worst is a wet rainy spring (like this year). Sunny and pleasant days with wind work the best, sometimes the symptoms just disappear if I go to a mountain in a nice sunny (not too hot) day. Interestingly, however, cold but sunny and dry winter days work well for me as well. It was mentioned a couple of times "cold" days in this thread, but in Sweden and the Netherlands - I think it is cold and wet there during the winter. For me cold and dry (sunny) works well.
A possible explanation is nitric oxide, it has been mentioned many times in the forum (do a forum search for nitric oxide). You generate it from sun's UV rays, not just from the temperature. It's another thing you get from the sun, not just vitamin D. Even one hour of (appropriate) sun exposure can generate nitric oxide (and one hour of sun would have no effect on your vitamin D). Of course, it is very intuitive that one can't substitute sun by just vitamin D, there are plenty of other things one gets from sun. The nitrix oxide then acts as a vasodilator. It has been suggested in the forum that vasodilation can possibly open up some blood vessel or a capillary that allows [something good that has to do with nervous system or hormones to happen]. I like this theory, find it plausible.
One way to test it is through the following question. Say it is summer and you have reduction of symptoms. Do you have this reduction if you spend little or no time in direct sun exposure, just having the benefit of the temperature? If yes, the theory would be wrong. If no, the theory has some more grounds.
Here is then also an explanation of why too hot sunny days do not work well, at least for me: in these days, we avoid the sun altogether, can't get the positive effects from its UV. And also, a day too hot is always complete lack of energy for me, even before my POIS, so now I can't tell if hot days are bad because of POIS or just because of what they are for me.
That said, of course, lack of sun is not the cause of our symptoms (so many people live in pretty dark and gloomy countries and have no POIS); however, it is possible that sun helps through nitric oxide to open up that small blood vessel somewhere and this might be what causes reduction of symptoms.
The mystery for me is that I remember someone in the forum saying that he lives in San Diego, this is the best climate I can imagine, sunny, dry, mild all year round, and he still has POIS.
