There's a specific kind of frustration people describe when they get winter blood work back. They went outside. They walked at lunch, they took the dog out, they got sun on clear cold days — and the number came back low anyway. The usual conclusion is that they didn't try hard enough.
They almost certainly did. For several months a year, above a certain latitude, effort is simply not the variable. The sun is in the wrong place, and no amount of time outdoors changes that.
What "vitamin D winter" actually is
Vitamin D synthesis begins when UVB photons — specifically the short end of the UVB band — strike 7-dehydrocholesterol in your skin. Those wavelengths are also the ones the atmosphere is best at removing.
When the sun is high, its light takes a short, steep path down through the atmosphere and enough UVB survives. When the sun is low, the same light travels a long, oblique path through far more air and ozone, and the short wavelengths are absorbed almost completely on the way. What reaches the ground is still light — still bright, still visible, still capable of feeling warm on your face — with the vitamin-D-relevant slice removed.
Once the midday sun stops climbing above roughly 45° for the whole day, that removal is close to total. Photobiologists call the resulting period vitamin D winter, and it's a threshold phenomenon rather than a gentle decline: production doesn't taper toward zero so much as switch off.
Webb, Kline and Holick demonstrated this experimentally back in 1988, in a study whose title is still the cleanest summary anyone has managed — that winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Nearly four decades of atmospheric modelling later, the finding has only sharpened.
How long yours lasts
The duration depends almost entirely on latitude. A 2022 analysis of European UVB data by Khanna and colleagues put numbers on it: below 40° north, vitamin D winter runs about two months. Above 59° north, it runs more than six. At the extremes the range is startling — roughly four days a year in Nicosia, and 215 days in Reykjavik.
| Roughly where you are | Example cities | Vitamin D winter |
|---|---|---|
| Below ~35°N | Los Angeles, Atlanta, Tel Aviv | Little to none |
| ~35–40°N | San Francisco, Washington DC, Athens | ~2 months |
| ~40–50°N | New York, Chicago, Madrid, Paris | ~3–4 months |
| ~50–59°N | London, Berlin, Vancouver, Warsaw | ~4–6 months |
| Above ~59°N | Stockholm, Oslo, Anchorage, Reykjavik | 6+ months |
Directional ranges drawn from published European modelling; your exact window depends on local ozone, altitude and terrain as well as latitude.
Three things that make it worse than people expect
It starts earlier than the weather does
People associate vitamin D winter with deep winter, and it's already running by then. Because the trigger is solar angle, the shutdown tracks the sun's declination rather than the temperature — so at mid-to-high latitudes it typically begins somewhere in September or October, during a stretch of the year that still feels perfectly pleasant. The last genuinely productive weeks of sun are the ones nobody notices passing.
The daily window shrinks before it closes
Before the season shuts off entirely, the hours in each day that count contract sharply. In midsummer you might have five or six hours around noon with the sun high enough to matter. By late September that can be down to a narrow band around solar noon — which, for anyone working indoors, is precisely the period they're least likely to be outside. Effective exposure often collapses before the calendar closes the door.
Bright and sunny is not the same as productive
This is the one that catches people. A crisp cloudless January day at 52° north looks like excellent sun. It is excellent light. It contains essentially no vitamin-D-effective UVB. The visible spectrum and the warmth get through the long atmospheric path perfectly well; the short UVB doesn't. Your eyes have no way to tell the difference, which is why intuition performs so badly here.
Two smaller versions of the same trap: window glass blocks virtually all UVB, so a sunny room contributes nothing, and the UV Index itself is an imperfect guide — it's weighted for sunburn, not synthesis, and the gap between the two widens exactly when the sun is low.
The one useful consequence: circulating 25-hydroxyvitamin D has a half-life measured in weeks, so your body genuinely carries stores across the seasonal boundary. That means the level you build through August and September is doing work for you in November. Vitamin D winter is one of the few health problems where the intervention window opens months before the problem does.
What to actually do
Three things, in the order they matter.
Bank what's left of the productive season. Late summer and early autumn are the last weeks your skin can do the job, and they're worth spending deliberately — sensible midday exposure, without burning. This isn't about a heroic push; it's about not sleepwalking through the final productive fortnight because it doesn't feel urgent yet.
Know your actual date. "Winter" is too coarse. The month your local window closes depends on your latitude, and the specific hours that count shrink week by week before it does. Knowing when yours arrives is what converts this from a vague worry into a plan.
Then switch sources. Once the window shuts, sunlight is off the table and dietary intake and supplementation carry you — fatty fish, fortified foods, egg yolks, and for many people a supplement. Dose and testing are worth discussing with a healthcare professional rather than guessing; this is where individual circumstances genuinely differ.
How SolGlow handles the dark months
Most sun-tracking apps become useless in October, which is a strange design choice for a category whose users need help most in winter. SolGlow 1.6 was built specifically around that gap — the release we internally called Own the Winter.
It tells you when your local vitamin D winter starts and ends rather than leaving you to infer it, and counts down to it: a free Winter Countdown widget and Watch complication that put the date somewhere you'll actually see it. From September it nudges toward banking the productive weeks that remain — the September head-start — instead of waiting until the window has already closed. Through the dark months it shifts to tracking what's carrying you instead: one-tap supplement check-ins, and UV-lamp session logging for people who use one.
On the lamp feature specifically, we're deliberate about scope: SolGlow logs sessions you choose to record. It never recommends, prescribes or times lamp use — that's a conversation for you and a clinician, not an app.
And the underlying estimate uses True-D, the vitamin-D-weighted UV engine introduced in 1.7, which matters more in winter than at any other time of year: the sun's low angle is exactly the condition under which the ordinary sunburn-weighted UV Index overstates what your skin can do.
Find out when your vitamin D winter starts
SolGlow computes your local window from your latitude and the sun's angle, counts down to it on your home screen, and keeps tracking through the months when there's no sun to track. Free on the App Store.
Explore SolGlowFrequently asked questions
What is vitamin D winter?
The part of the year when the midday sun never gets high enough for meaningful vitamin-D-effective UVB to reach the ground, so your skin makes essentially none no matter how long you're outside. It's caused by the sun's angle, not by cold or cloud.
At what latitude does vitamin D winter start?
It becomes significant above roughly 37°. European modelling puts it at about two months below 40°N and more than six months above 59°N — from around four days a year in Nicosia to 215 in Reykjavik.
Can you get vitamin D on a clear sunny winter day?
Not meaningfully, inside a vitamin D winter. The light and warmth get through; the short-wavelength UVB that synthesis needs does not.
How do you keep levels up in winter?
Because 25-hydroxyvitamin D has a half-life of weeks, the stores you build in late summer carry into autumn. After that, diet and supplementation do the work — discuss dose and testing with a healthcare professional.
References & further reading
- Webb, A.R., Kline, L., & Holick, M.F. (1988). Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology & Metabolism, 67(2).
- Khanna, T., et al. (2022). Comprehensive Analysis of Seasonal and Geographical Variation in UVB Radiation Relevant for Vitamin D Production in Europe. Nutrients, 14(23):5189.
- O'Neill, C.M., et al. (2016). Seasonal Changes in Vitamin D-Effective UVB Availability in Europe and Associations with Population Serum 25-Hydroxyvitamin D. Nutrients, 8(9):533.
- Kazantzidis, A., et al. (2009). On the relationship between erythemal and vitamin D action spectrum weighted ultraviolet radiation. Journal of Photochemistry and Photobiology B: Biology.
Related reading: The UV Index is the wrong number for vitamin D · Vitamin D deficiency: signs, science, and your risk · How much sun do you actually need?
This article is for general education and is not medical advice. The latitude ranges above are illustrative rather than prescriptive. SolGlow provides vitamin D estimates and is not a medical device; its output is an estimate, not a diagnosis. Talk to a healthcare professional about your vitamin D status, testing, or supplementation.