I moved to Boulder a few weeks ago, and we're right on the 40th parallel. I've read that for 40-degree latitudes there's a "Vitamin D Winter" from November through February (a time where the sun doesn't rise high enough in the sky for us to synthesize D in the skin). However, people I've met say it's possible to get a sunburn even in the middle of winter just walking around the city, apparently due to the altitude (5500' above sea level). If you can get a sunburn, wouldn't it stand to reason that you could make Vitamin D? I've read that there's more UVB/Vitamin D production at higher altitudes, so I'm wondering:
Can being at a higher altitude extend Vitamin D production through the winter?