Vitamin D from sunlight in Longyearbyen
Longyearbyen sits at 78.22°N, 1 m above sea level. Between 1 January and 31 December the midday sun never climbs high enough for your skin to make vitamin D at all — 365 days of the year.
How long do you need outside?
Month by month in Longyearbyen
At solar noon, for skin type III with arms and legs exposed.
| Month | Sun height | UV index | 1000 IU in | Burns after |
|---|---|---|---|---|
| January | -9.3° | 0.0 | no synthesis | not reachable |
| February | -0.8° | 0.0 | no synthesis | not reachable |
| March | 9.7° | 0.1 | no synthesis | 2113 min |
| April | 21.6° | 0.7 | no synthesis | 331 min |
| May | 30.7° | 1.5 | no synthesis | 151 min |
| June | 35.1° | 2.1 | no synthesis | 110 min |
| July | 33.3° | 2.0 | no synthesis | 117 min |
| August | 25.7° | 1.2 | no synthesis | 194 min |
| September | 14.7° | 0.3 | no synthesis | 670 min |
| October | 3.2° | 0.0 | no synthesis | 25873 min |
| November | -6.8° | 0.0 | no synthesis | not reachable |
| December | -11.5° | 0.0 | no synthesis | not reachable |
When during the day?
Vitamin D-effective UV by month and hour, in Arctic/Longyearbyen local time (daylight saving included). Darker cells mean faster synthesis.
| Month | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Jan | ||||||||||||||||||
| Feb | ||||||||||||||||||
| Mar | ||||||||||||||||||
| Apr | ||||||||||||||||||
| May | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||
| Jun | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||
| Jul | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||
| Aug | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||||
| Sep | ||||||||||||||||||
| Oct | ||||||||||||||||||
| Nov | ||||||||||||||||||
| Dec |
none strongest
Best time in June: around 12:30 local time.
How much skin matters more than how much time
In June, at solar noon, for skin type III:
| Exposed | Body surface | 1000 IU in | 30 minutes gives |
|---|---|---|---|
| Face and hands | 5% | not reachable | 94 IU |
| Face, hands and forearms | 10% | not reachable | 188 IU |
| Face, hands, arms and legs | 25% | not reachable | 470 IU |
| Swimwear | 80% | not reachable | 1,504 IU |
Exposing more skin gets you there faster, but it also burns faster. See how long before you burn in Longyearbyen →
Note the first row. With only face and hands exposed, 1000 IU is not reachable at any duration: vitamin D synthesis saturates, so the previtamin D3 in your skin converts to inactive products as fast as it forms. Past that ceiling, extra time adds sunburn risk and no more vitamin D. Exposing more skin briefly beats exposing less skin for longer.
Assumptions
- Clear sky. Every figure is an upper bound. Cloud can cut UVB by more than half, and these numbers do not model it.
- Direct exposure. Glass blocks UVB almost completely, so sunlight through a window produces no vitamin D. Sunscreen and clothing block it too.
- A healthy adult. Age, body composition, medication and existing vitamin D status all change the real answer.
- Ozone is modelled, not measured. Column ozone comes from a smooth climatology, so day-to-day variation is not captured.