Sun
There is a period each year, in most of the populated northern hemisphere, when standing outside all day produces exactly as much vitamin D as staying indoors. The reason is geometric, not meteorological.
Boston in January is not a weaker version of Boston in July. For the purpose of cutaneous vitamin D synthesis, it is a different planet. The sun is present, it is bright, it casts shadows and warms south-facing walls — and it delivers essentially zero usable UVB.
Ultraviolet B is absorbed by stratospheric ozone with strong wavelength dependence. Absorption climbs steeply as wavelength drops below about 315 nanometres, which is precisely the band that vitamin D synthesis requires.
When the sun is directly overhead, sunlight passes through one atmospheric thickness. As solar elevation falls, the path length through the atmosphere increases roughly as the secant of the zenith angle. At a solar elevation of 30 degrees the path is twice as long. At 20 degrees it is nearly three times. At 10 degrees it approaches six.
Because ozone absorption is exponential in path length, the UVB fraction collapses far faster than total sunlight does. This is why winter light can feel bright and remain photochemically useless: the visible and UVA components pass through the longer path with modest attenuation while the UVB component is nearly eliminated.
That last version is the field test. If your shadow at midday is longer than you are tall, the sun sits below 45 degrees and the UVB fraction is marginal. It requires no instrument and works anywhere.
Latitude sets the maximum solar elevation achievable on any date. Below roughly 35 degrees, midday sun in December still clears the threshold on clear days. Above it, a widening winter window closes entirely.
| Latitude | Representative cities | Approximate vitamin D winter |
|---|---|---|
| 33°N | Atlanta, Los Angeles | Marginal; brief or none |
| 40°N | Philadelphia, Pittsburgh, Denver, Madrid | Mid-November to mid-February |
| 45°N | Minneapolis, Portland, Milan | Late October to late February |
| 52°N | London, Berlin, Warsaw | Early October to late March |
| 60°N | Oslo, Helsinki, Anchorage | September to April |
These are approximations. Altitude extends the season, since there is less atmosphere overhead. Persistent cloud, urban aerosol and air pollution shorten it. Snow cover raises effective dose through reflection, though the sun still has to clear the elevation threshold for that reflection to carry any UVB.
Vitamin D stored as 25-hydroxyvitamin D has a circulating half-life of two to three weeks, and adipose tissue holds a further reserve that releases slowly. Summer status therefore decays through autumn rather than dropping off a cliff, which is why serum measurements in northern populations typically peak in late August and trough in late February or early March.
The decay is real, though. A person who ends September at a comfortable serum level will commonly be measurably lower by March, and someone who ends September already low will be lower still.
Bank the shoulder seasons. The most reliable lever is arriving at winter with high stores. Regular short midday exposures from April through September, at the substantial skin area covered by summer clothing, does more for a February blood level than anything attempted in February.
Reconsider the source in winter. Dietary vitamin D is limited but not negligible: fatty fish, egg yolks, and fortified dairy and cereals are the practical contributors. Oily fish is by a wide margin the densest natural source.
Supplement deliberately if indicated. Vitamin D3 supplementation is inexpensive and well studied, and winter is precisely the circumstance it was designed for. It is also the one route where overdose is possible, because it bypasses the photodegradation ceiling that makes solar synthesis self-limiting. Dosing should be set against an actual serum measurement in consultation with a clinician rather than guessed.
Do not chase it with tanning beds. Commercial tanning equipment is typically weighted heavily toward UVA to produce cosmetic tanning without burning. UVA produces no vitamin D and carries independent carcinogenic and photoaging risk. It is the worst available trade in this entire subject.
None of this argues for staying in. Winter outdoor time delivers bright-light exposure that anchors circadian timing, cold exposure that appears to carry its own metabolic effects, physical activity, and the psychological benefits associated with natural environments. Those do not require UVB and do not stop in November.
What changes is only the vitamin D accounting. Between roughly November and February at mid-northern latitudes, treat sun exposure as valuable for reasons that have nothing to do with vitamin D, and handle vitamin D through stores and intake instead.
To see the effect for your own coordinates, the vitamin D calculator flags vitamin D winter conditions directly when the entered UV index falls below the synthesis threshold.