A couple of years ago, I think, I looked into sun-driven vitamin D availability in winter in the LA area. I recall reading that even in very high latitudes, where the sun never reaches 45 degrees, there is at least some vit D made, but I didn't keep the references. If you have a reference for the 30 degree thing, I'd appreciate seeing it.

My city, next to Los Angeles, is 34 degrees N latitude, but I'm not sure how good the D dose is in the winter months. During the spring and summer I sunbathe on my patio in shorts, so nearly full body exposure 15-20 minutes front and back. Also big egg yolk eater daily, with about weekly liver and fish servings. Light supplementation in the fall and winter. Should get tested, but so far I've been disease free.