We make vitamin D in our skin using a type of solar radiation called ultraviolet B (UVB). Unfortunately, UVB is filtered out by the atmosphere. As the Earth tilts on its axis in winter, the sun’s rays travel through more atmosphere to get to us, and more and more UVB is filtered out. Some estimates have it that, in our part of the country, we simply do not make vitamin D during 4-5 months of every year. Even in summer, morning and afternoon sun has to angle through more atmosphere, filtering out most if not all UVB.