Across the year, 212 days reach the 45° solar elevation needed for optimal UVB synthesis — roughly March to September. Peak windows last up to 400 minutes. The monthly calendar below shows the exact picture.
| Month | Max UV | Optimal window (≥ 45°) | Extended window (≥ 30°) | Status |
|---|---|---|---|---|
| Jan | 2.16 | — | — | — |
| Feb | 3.52 | — | 10:10 – 14:19 (248 min) | Reduced |
| Mar | 5.57 | 11:55 – 14:23 (148 min) | 10:00 – 16:18 (378 min) | Optimal |
| Apr | 7.88 | 10:28 – 15:32 (304 min) | 09:04 – 16:57 (473 min) | Optimal |
| May | 9.58 | 09:50 – 16:03 (373 min) | 08:31 – 17:22 (531 min) | Optimal |
| Jun | 10.62 | 09:41 – 16:21 (400 min) | 08:22 – 17:39 (557 min) | Optimal |
| Jul | 10.98 | 09:52 – 16:20 (388 min) | 08:34 – 17:39 (545 min) | Optimal |
| Aug | 10.24 | 10:17 – 15:52 (335 min) | 08:56 – 17:13 (497 min) | Optimal |
| Sep | 8.03 | 11:04 – 14:46 (222 min) | 09:27 – 16:23 (417 min) | Optimal |
| Oct ← | 5.17 | — | 10:15 – 15:16 (302 min) | Reduced |
| Nov | 2.87 | — | 10:51 – 12:38 (107 min) | Reduced |
| Dec | 1.93 | — | — | — |
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 10.62, someone with medium skin (type III) typically needs about 6 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 — Philadelphia 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 Philadelphia hits 10.98 around June — high enough to burn fairly quickly. Stick to your synthesis window (~6 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 Philadelphia 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.