With 324 optimal days per year, Guangzhou 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 394 minutes on peak days. The monthly calendar below shows exact windows all year.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 5.55 | 12:04 – 13:07 (63 min) | 09:50 – 15:22 (332 min) | Optimal |
| Feb | 7.05 | 10:50 – 14:31 (221 min) | 09:24 – 15:57 (393 min) | Optimal |
| Mar | 8.88 | 10:02 – 15:09 (307 min) | 08:51 – 16:20 (448 min) | Optimal |
| Apr | 10.42 | 09:26 – 15:27 (361 min) | 08:21 – 16:32 (492 min) | Optimal |
| May | 11.21 | 09:10 – 15:36 (386 min) | 08:04 – 16:41 (517 min) | Optimal |
| Jun | 11.77 | 09:10 – 15:44 (394 min) | 08:03 – 16:51 (528 min) | Optimal |
| Jul | 12.49 | 09:17 – 15:48 (391 min) | 08:11 – 16:54 (523 min) | Optimal |
| Aug | 12.9 | 09:24 – 15:37 (373 min) | 08:19 – 16:43 (504 min) | Optimal |
| Sep | 11.97 | 09:35 – 15:08 (332 min) | 08:28 – 16:15 (468 min) | Optimal |
| Oct ← | 9.6 | 10:02 – 14:22 (260 min) | 08:44 – 15:40 (416 min) | Optimal |
| Nov | 6.88 | 11:02 – 13:20 (138 min) | 09:14 – 15:08 (353 min) | Optimal |
| Dec | 5.41 | — | 09:45 – 14:58 (313 min) | Reduced |
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 11.77, 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 — Guangzhou 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 Guangzhou hits 12.9 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 Guangzhou 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.