Above ~37° latitude, the winter sun angle is too low for meaningful UVB (290–315 nm) to reach the earth’s surface. That means cutaneous vitamin D production from sunlight is negligible to nonexistent for several winter months - even on clear, sunny days. This isn’t debated in the research. It’s been demonstrated repeatedly using: • solar zenith angle modeling • atmospheric UVB measurements • human vitamin D synthesis studies HOWEVER- You should still go outside in winter. Even without strong UVB