Across the year, 144 days reach the 45° solar elevation needed for optimal UVB synthesis — roughly April to August. Peak windows last up to 360 minutes. The monthly calendar below shows the exact picture.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 0.49 | — | — | — |
| Feb | 1.25 | — | — | — |
| Mar | 2.73 | — | 10:38 – 14:29 (231 min) | Reduced |
| Apr | 4.81 | 12:19 – 14:30 (130 min) | 09:55 – 16:54 (419 min) | Optimal |
| May | 6.62 | 10:48 – 15:54 (306 min) | 09:01 – 17:41 (521 min) | Optimal |
| Jun | 7.73 | 10:25 – 16:25 (360 min) | 08:43 – 18:07 (565 min) | Optimal |
| Jul | 7.81 | 10:42 – 16:18 (336 min) | 08:58 – 18:03 (545 min) | Optimal |
| Aug | 6.62 | 11:38 – 15:19 (222 min) | 09:36 – 17:20 (464 min) | Optimal |
| Sep | 4.35 | — | 10:40 – 15:58 (317 min) | Reduced |
| Oct ← | 2.13 | — | — | — |
| Nov | 0.78 | — | — | — |
| Dec | 0.37 | — | — | — |
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 7.73, someone with medium skin (type III) typically needs about 8 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 Bremen 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.
UV peaks around 7.81 in June — moderate to high. Get your synthesis window in, then apply SPF 30+. It's not dangerously intense, but a bit of protection after your dose makes sense.
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 Bremen 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.