Vitamin D in winter: location matters.
A clear January day brings different sunlight in London and Sydney. Season, latitude and solar elevation change the picture.
Written by the Taiyo team · UtropiaSources checked: 25 September 2026 · Clinical review pending
01 / The short answer
UVB does not follow one winter calendar.
At high latitudes, a low winter sun can provide very little UVB for vitamin D production, even under clear skies. There is no universal set of months that applies to every country. Webb et al., 1988 ↗
The NHS describes roughly October to early March as the period when UK sunlight is insufficient to make the vitamin D needed. This is UK guidance, not a calendar to copy for Spain, Mexico or Australia. NHS ↗
02 / The geometry of sunlight
Lower sun, a different path.
Latitude and season change the sun’s elevation. Its light takes a different path through the atmosphere, changing the radiation reaching the surface. Outdoor temperature cannot tell you that combination.
The classic Webb, Kline and Holick study compared winter conditions in Boston and Edmonton and found periods without synthesis in exposed samples. It helps explain why place matters, but it is not a personal exposure timetable for all skin types. Webb et al., 1988 ↗
03 / Explore the seasons
One year, seen from seven latitudes.
Modelled UV index at solar noon on the 15th of each month in 2026. The table uses Taiyo’s API clear-sky model. It compares seasons; it does not identify months “suitable” for making enough vitamin D.
| Latitude | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 60°N | 0.1 | 0.6 | 1.8 | 3.8 | 5.6 | 6.4 | 6.0 | 4.5 | 2.5 | 1.0 | 0.2 | 0.1 |
| 50°N | 0.8 | 1.8 | 3.5 | 5.8 | 7.6 | 8.3 | 8.0 | 6.6 | 4.4 | 2.3 | 1.0 | 0.6 |
| 40°N | 2.0 | 3.5 | 5.5 | 7.9 | 9.3 | 9.8 | 9.6 | 8.5 | 6.4 | 4.2 | 2.4 | 1.7 |
| 20°N | 5.9 | 7.6 | 9.5 | 10.8 | 11.0 | 10.9 | 10.9 | 10.8 | 10.0 | 8.3 | 6.3 | 5.4 |
| 0° | 9.8 | 10.9 | 11.4 | 10.8 | 9.6 | 8.9 | 9.1 | 10.2 | 11.1 | 11.0 | 10.1 | 9.5 |
| 20°S | 11.6 | 11.3 | 10.1 | 7.9 | 6.0 | 5.0 | 5.4 | 7.0 | 9.1 | 10.8 | 11.5 | 11.6 |
| 40°S | 10.2 | 8.6 | 6.4 | 3.9 | 2.2 | 1.6 | 1.8 | 3.0 | 5.2 | 7.7 | 9.7 | 10.5 |
Colours: UV < 3, 3 to < 6 and ≥ 6. Numbers remain readable without colour. Assumptions: sea level, longitude 0°, constant 300 DU ozone and 0.9 aerosol factor. Excludes clouds, snow, local pollution and observed ozone. Allaart model; NOAA solar position. Not a forecast or measured climatology. Method ↗
04 / From the year to your day
Check the actual place and date.
The table shows how seasons reverse between hemispheres. For today or tomorrow, use the local forecast: it includes available cloud data and shows when it falls back to a model. See today’s sun ↗
When the calculator reports insufficient UV, it does not propose compensating with a long exposure. A high-UV day does not prove your vitamin D levels are adequate either. Diet and individual requirements belong to a different part of the assessment.
Make your own comparison
Give it a place and time.
Explore a scenario with your location, skin type and clothing. The calculator shows its assumptions, an illustrative range and when it proposes no minutes.
Open the calculator ↗Follow the sources
Evidence and limitations
- Webb, Kline & Holick (1988) · Season, latitude and cutaneous vitamin D synthesis
- NHS · Vitamin D
- Allaart et al. (2004) · Clear-sky UV model
- WHO · Ultraviolet radiation
Educational content: it does not determine your vitamin D levels, a diagnosis or a supplement dose. Draft awaiting qualified review. Checking sources is not the same as medical review.
Our editorial method ↗