Vitamin D by Latitude
Latitude is the single largest factor in year-round vitamin D availability. A person in Bogota (4° N) makes vitamin D from noon sun every day of the year; a person in Reykjavik (64° N) makes essentially none for six months. Understanding your latitude tells you whether sun exposure is a plausible year-round strategy or a summer-only one.
The 37° rule of thumb
Below 37° latitude (north or south), synthesising UVB reaches the ground every day of the year. Between 37° and 55°, UVB is present for 6–9 months per year; "vitamin D winter" runs from roughly November through March. Above 55°, UVB is seasonal to strongly seasonal; above 66° (Arctic/Antarctic Circle), months of polar night eliminate UVB entirely.
Sample cities and their vitamin D winter length
- Singapore (1° N): 0 months. Year-round synthesis.
- Bogota, Nairobi, Quito (near equator): 0 months.
- Miami (26° N): 0 months. Year-round.
- Cairo, Delhi (~30° N): minimal winter, brief.
- Los Angeles (34° N): ~1 month deep-winter reduction.
- Atlanta, Phoenix (~33° N): minimal, mostly year-round.
- Rome, Beijing, Nashville (~40° N): ~3 months (Nov–Jan) synthesis limited.
- New York, Madrid (~40–41° N): ~3–4 months.
- Boston, Chicago, Rome (~42° N): ~4 months (Nov–Feb).
- Seattle, Paris (~47–48° N): ~5 months (Oct–Feb).
- London, Berlin, Vancouver (~51–52° N): ~5–6 months (Oct–Mar).
- Edinburgh, Copenhagen, Moscow (~55–56° N): ~6 months (Oct–Mar).
- Oslo, Stockholm, Anchorage (~59–61° N): ~6–7 months.
- Reykjavik (64° N): ~7 months.
- Tromsø, Fairbanks (~65–69° N): polar night (no sun) in December.
- Barrow, AK (71° N): polar night late Nov–late Jan; polar day May–early Aug.
Polar day — midnight sun
Above the Arctic and Antarctic Circles, mid-summer produces continuous sun for weeks — the sun never sets. UVB is present around the clock during these periods, but sun angle is low even at "midday," which limits per-hour synthesis. Total accumulated UVB over 24-hour days can still be substantial.
Polar night
The reverse: months of no sunrise. Vitamin D synthesis is zero. Supplementation is mandatory. Traditional Arctic diets included generous seal, whale, and fish liver — natural cholecalciferol sources. Modern indigenous and non-indigenous Arctic populations without traditional diet or supplementation show near-universal severe deficiency in winter.
Altitude
Higher altitude means thinner atmosphere and more UVB reaching skin. Every 300 m (1,000 ft) increase adds ~4% UVB intensity. Denver (1,600 m) gets meaningfully more UVB than coastal cities at the same latitude. Populations at very high altitude (Andean, Tibetan, Ethiopian highland) generally have better vitamin D status than coastal counterparts at the same latitude — but sunburn risk is also higher.
Ozone and atmospheric variables
Ozone absorbs UVB. Ozone layer thickness varies seasonally and with latitude. The historical ozone hole over Antarctica temporarily raised ground-level UVB in the Southern Hemisphere. Air pollution — particulate matter, aerosols — also absorbs UVB; historically bad air pollution episodes (London smog, Beijing haze) reduce vitamin D synthesis.
Practical guide by latitude
- Below 30° — sun-derived vitamin D year-round; supplementation optional if you're outside regularly.
- 30–37° — mostly year-round; consider supplementation Dec–Jan for indoor workers.
- 37–45° — supplement Nov–Mar (5 months) if not getting deliberate summer sun.
- 45–55° — year-round supplementation reasonable; deliberate summer sun useful but not sufficient.
- Above 55° — year-round supplementation mandatory for most; even summer sun may need supplementation.
- Above 60° — supplementation is the primary strategy; winter polar night makes sun impossible.