As the days are getting shorter and cooler, it is getting harder to get sufficient vitamin D. One of the best ways to combat this is whenever a warm day does come along, get outside and pull your shirt off to maximize sun exposure. The best time of day to get vitamin D is around solar noon (
when is solar noon?) any way because the ratio of vitamin D making wavelengths to cancer causing wavelengths is better[
1], but this is even more important in the late fall and early spring. If it feels chilly and you are in reasonably good shape, I suggest running hard enough to shed the shirt and still stay warm. Trust me, if you run hard enough, you will be surprised how much body heat you can generate.

Ok, this may be an extreme example, but you see my point.
Getting that extra vitamin D is important, because optimal levels of vitamin D help prevent osteoporosis, cancer, autoimmune diseases, and high blood pressure[
2]. Not to mention that sunlight (and exercise) can help elevate mood, fighting depression.