With 365 optimal days per year, Hanoi is among the world's best-positioned cities for solar vitamin D. The sun clears 45° — the minimum angle for effective UVB synthesis — for up to 390 minutes on peak days. The monthly calendar below shows exact windows all year.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 6.04 | 11:02 – 13:09 (127 min) | 09:11 – 15:00 (349 min) | Optimal |
| Feb | 7.5 | 10:10 – 14:11 (241 min) | 08:49 – 15:32 (403 min) | Optimal |
| Mar | 9.24 | 09:27 – 14:43 (316 min) | 08:18 – 15:52 (454 min) | Optimal |
| Apr | 10.62 | 08:54 – 14:58 (363 min) | 07:50 – 16:02 (492 min) | Optimal |
| May | 11.27 | 08:40 – 15:05 (384 min) | 07:35 – 16:10 (514 min) | Optimal |
| Jun | 11.75 | 08:42 – 15:12 (390 min) | 07:35 – 16:18 (523 min) | Optimal |
| Jul | 12.5 | 08:48 – 15:16 (388 min) | 07:43 – 16:22 (519 min) | Optimal |
| Aug | 13.07 | 08:54 – 15:07 (374 min) | 07:49 – 16:12 (503 min) | Optimal |
| Sep | 12.34 | 09:02 – 14:40 (338 min) | 07:56 – 15:46 (471 min) | Optimal |
| Oct ← | 10.12 | 09:24 – 13:59 (275 min) | 08:09 – 15:14 (425 min) | Optimal |
| Nov | 7.43 | 10:13 – 13:08 (174 min) | 08:37 – 14:45 (368 min) | Optimal |
| Dec | 5.92 | 11:19 – 12:23 (64 min) | 09:05 – 14:37 (332 min) | Optimal |
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 11.75, someone with medium skin (type III) typically needs about 5 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.
Surprisingly, yes — Hanoi gets enough direct sun for some synthesis even through the colder months. The windows get shorter, but they're there. Check the monthly table above for exact times.
Peak UV in Hanoi hits 13.07 around June — high enough to burn fairly quickly. Stick to your synthesis window (~5 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 Hanoi 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.