The Sun & Vitamin D Window
When can your skin actually make vitamin D? Pick your city and see the sun's midday height across the year — including the months the winter sun is simply too low to help, and what to do about it.
≈35° sun height
below it, UVB is too weak for skin to make vitamin D
Weeks to months
the higher your latitude, the longer the window
Pure astronomy
the arc is geometry — the same every year
Your skin makes vitamin D from UVB — and UVB only gets through when the sun is high enough. We chart the sun's midday height in your city across the whole year using the NOAA solar-position algorithm. Where the arc dips below roughly 35°, the atmosphere filters the UVB out before it reaches you — no matter how long you're outside.
In much of the northern US and Canada, the midday winter sun never climbs above the synthesis threshold — that's the "vitamin D winter." The question isn't how much time outside; it's diet and supplements until the sun comes back. Pick your city to see your window — and if you're in the sunny south, why you may not have one at all.
This tool is educational, not medical advice, and never advises sun-exposure amounts — sunburn is never the goal. Intake guidance is quoted from the NIH Office of Dietary Supplements (Health Canada reflects the same shared DRIs). Nothing you enter is stored — inputs live only in this page's URL.