Sun angle, vitamin D and UVB
In my previous post on this topic, I summarized the difficulty of maintaining adequate vitamin D levels, and how nearly everyone in Western civilization lacks enough. All of the risk groups for Covid-19 are people who are lower than most in vitamin D, and the average is already catastrophic. Some of the lowest vitamin D levels can be found in seniors, people with darker skin, and the obese. All of these correlate with a much higher rate of Covid-19 infections and complications.
A commonly accepted idea is that the sun needs to be at least 50° above the horizon if you want to generate "any" vitamin D. If you do a quick search, Google will reinforce this. If the sun angle is lower, the UVB portion of ultraviolet light will "reflect" from the atmosphere, and no vitamin D for you!
I've found this study by M. El-Nouby Adam, Effect of The Atmosphere on UVB Radiation Reaching the Earth's Surface: Dependence on Solar Zenith Angle, which suggests such popular guidance is poppycock. A meteorological research station at 26.2°N in Egypt measured UVB over a 5-year period. 4 hours before and after the sun was highest, the quantity of UVB was on average still 18-31% of mid-day maximum:
At this southern latitude, the solar angle at ±4 hours from peak is no more than 36° during summer solstice, 25° at equinox, and 14° during winter solstice. At no time is the angle above 36°. And yet the quantity of UVB at this time, over 5 years, was some 25% of mid-day.
Right now, in October, in Europe and northern US, the sun angle at peak (which could be at noon, or might be 1 pm) is 44°. This suggests the reason our vitamin D levels fall so catastrophically in winter is not that we can't generate it. It's that we cover up head to toe and show minimal skin to the sun at mid-day.