When in doubt, put it to the t-test.
Wait a moment... The amount of sunlight is the same in spring and summer and the same in autumn and winter. Temperature is not the same thing as sunlight.
From what I can gather, they factored temperature in because of the average amount of clothing people wear. More exposed skin means less time in the sun needed.
I wonder about both sons living above the Arctic Circle. I tend to be opposed to vitamin supplements, but do wonder if extreme situations warrant them.OTOH, they both eat a lot of fish these days.
@Joseph,the solar energy striking the square inch of earth (or skin) varies over the year. The variance is small but measurable; it relates to the angle-of-incidence of the sunlight. (The angle is much better in summertime than wintertime, due to the Sun's apparent movement towards the North sky in the summertime.)However, I suspect the variance in recommended times is split between this answer and BS's. Both factors are in play.On another front, my hometown (Metro Detroit) is at 42.3 N latitude. Is that far enough from the line to be worrisome? Maybe some more detailed research is in order.