Across 3 tracked cities, the national average is ~68 optimal days per year — days when the sun clears the 45° threshold needed for effective UVB synthesis. Oslo leads with 104 optimal days/year. Explore the month-by-month breakdown and all cities below.
The data above shows averages across cities. Your real window depends on where you are today, current cloud cover, and your skin type. Oslo is loaded below — type your town to switch.
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If you had to pick one city in Norway for vitamin D, Oslo would be hard to beat — it gets 104 optimal days a year, well above the national average of ~68. That gap can be the difference between a quick 15-minute top-up and planning a much longer outing.
No — during winter the sun simply doesn't climb high enough anywhere in Norway for meaningful synthesis. The atmosphere filters out most UVB at shallow angles. Supplementation (800–2,000 IU/day) is the practical approach during those months.
UV peaks around 6.4 in Oslo — moderate to high. Get your synthesis window in and then put on SPF 30+, especially in summer. It's not dangerously intense, but a bit of protection after makes sense.
It makes a real difference. Across Norway's ~10° of latitude, the more southerly cities get a few extra weeks of good synthesis windows each year. Not a dramatic gulf, but noticeable — worth checking the city pages if you're in the northern part of the country.
Sadly, no — same answer wherever you are in Norway. Glass blocks UVB almost entirely, which is the exact wavelength needed for vitamin D synthesis. Sitting in bright sunshine through a window feels warm, but it won't move your vitamin D levels. You need real, unfiltered sky on your skin.
Yes, but maybe less than you'd expect. Thin cloud cover barely makes a dent — around 90% of UVB still gets through on a lightly overcast day. Heavy overcast (75%+ cloud cover) is more significant: you might need to stay out about twice as long for the same effect. On a genuinely grey day, it's usually not worth it. SunDose uses today's cloud forecast for your exact location so you always get a realistic estimate.