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.
Method & Sources
We draw the sun's midday height across the year for your city with the NOAA solar-position algorithm — pure astronomy, so this data never goes stale. Below roughly 35° above the horizon, the atmosphere filters out so much UVB that skin can't make meaningful vitamin D; we shade the stretch of the year under that line as your "vitamin D winter" (threshold from published photobiology research — the exact figure varies with ozone and season, so treat the dates as approximate). This is geometry, not a personal reading: skin tone, age, cloud cover, sunscreen, and time outdoors all shift real synthesis. This tool never tells you how long to stay in the sun.
Educational, Not Medical Advice
Daily Vitality tools organize information and published guidance — they don't diagnose conditions, and they're not a substitute for care. Talk with your doctor, pharmacist, or another clinician about what's right for you.
Wondering about your own vitamin D level? That's a blood test — talk with your clinician about testing and supplements. And on the sun itself: skin-cancer risk is real, and sunburn is never the goal.
Sources: solar geometry — NOAA solar-position algorithm (Spencer day-angle declination series; public domain). Synthesis threshold: ~35° solar elevation, per published photobiology literature (citation string pending clinician review). Intake guidance: NIH Office of Dietary Supplements vitamin D fact sheet (RDA 600–800 IU/day), retrieved July 2026; Health Canada guidance reflects the same joint US–Canada DRIs. City coordinates: public reference data. This tool stores nothing you enter — your inputs live only in this page's URL.