Across the year, 150 days reach the 45° solar elevation needed for optimal UVB synthesis — roughly April to August. Peak windows last up to 366 minutes. The monthly calendar below shows the exact picture.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 0.58 | — | — | — |
| Feb | 1.39 | — | — | — |
| Mar | 2.92 | — | 10:44 – 14:57 (252 min) | Reduced |
| Apr | 5.03 | 12:21 – 15:03 (162 min) | 10:09 – 17:15 (426 min) | Optimal |
| May | 6.83 | 11:00 – 16:16 (316 min) | 09:17 – 18:00 (523 min) | Optimal |
| Jun | 7.94 | 10:39 – 16:45 (366 min) | 09:00 – 18:25 (565 min) | Optimal |
| Jul | 8.05 | 10:56 – 16:40 (344 min) | 09:14 – 18:20 (546 min) | Optimal |
| Aug | 6.88 | 11:46 – 15:45 (239 min) | 09:51 – 17:40 (469 min) | Optimal |
| Sep | 4.61 | — | 10:51 – 16:22 (331 min) | Reduced |
| Oct ← | 2.33 | — | — | — |
| Nov | 0.9 | — | — | — |
| Dec | 0.45 | — | — | — |
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 7.94, 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 Rotterdam 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 Rotterdam hits 8.05 around June — high enough to burn fairly quickly. Stick to your synthesis window (~8 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 Rotterdam 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.