Across the year, 168 days reach the 45° solar elevation needed for optimal UVB synthesis — roughly April to August. Peak windows last up to 380 minutes. The monthly calendar below shows the exact picture.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 0.89 | — | — | — |
| Feb | 1.84 | — | — | — |
| Mar | 3.5 | — | 10:07 – 15:08 (301 min) | Reduced |
| Apr | 5.67 | 11:38 – 15:20 (222 min) | 09:47 – 17:11 (444 min) | Optimal |
| May | 7.44 | 10:36 – 16:14 (339 min) | 09:01 – 17:49 (527 min) | Optimal |
| Jun | 8.53 | 10:19 – 16:39 (380 min) | 08:47 – 18:11 (564 min) | Optimal |
| Jul | 8.7 | 10:33 – 16:36 (362 min) | 09:01 – 18:08 (548 min) | Optimal |
| Aug | 7.64 | 11:14 – 15:52 (278 min) | 09:33 – 17:33 (480 min) | Optimal |
| Sep | 5.38 | — | 10:22 – 16:25 (363 min) | Reduced |
| Oct ← | 2.95 | — | 11:54 – 14:34 (160 min) | Reduced |
| Nov | 1.3 | — | — | — |
| Dec | 0.73 | — | — | — |
The calendar above shows typical windows based on solar geometry. Below, today's cloud cover and your skin type are factored in for a personalised dose recommendation — updated live.
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In June, when UV peaks around 8.53, someone with medium skin (type III) typically needs about 7 minutes outside — arms and face exposed, no sunscreen. Darker skin takes roughly 2–4 times longer to absorb the same amount. If you're out during the extended window when the sun's a bit lower, budget closer to 30–45 minutes.
Not really. From November to February, the sun stays too low in Strasbourg for meaningful synthesis — the atmosphere filters out most UVB at shallow angles. Supplementation (800–2,000 IU/day) is a practical option during those months.
Peak UV in Strasbourg hits 8.7 around June — high enough to burn fairly quickly. Stick to your synthesis window (~7 min for medium skin), then put SPF 30+ on straight away. More time outside doesn't mean more vitamin D; it just means more burn risk.
Yes, but maybe less than you'd expect. Thin cloud cover barely makes a dent — you're still getting around 90% of UVB on a lightly overcast day. Once you're into heavy overcast territory (75%+ cloud cover), it's a different story: you might need to stay out about twice as long for the same effect. On a genuinely grey day, it's often not worth it. The calculator below uses today's actual forecast for Strasbourg so you don't have to guess.
Sadly, no. Glass blocks UVB almost entirely — the exact wavelength your skin needs to make vitamin D. Sitting in a sunny room with light streaming in feels nice, but it won't do anything for your levels. You need to actually be outside, skin exposed to real, unfiltered sky.
Think of it this way: the optimal window (sun above 45°) is when UVB punches through the atmosphere efficiently — 10–20 minutes is enough for most people with medium skin. The extended window (30–45°) is the sun at a shallower angle, working through more atmosphere. UVB is still there, just diluted. Same result in the end, you just need 30–45 minutes rather than 15. Below 30°, the path is too long and most UVB gets absorbed before it reaches you, regardless of how long you stay out.