Every European city has a season when the sun cannot help you

Vitamin D synthesis depends on UVB, the shortest and most easily scattered part of the light that reaches the ground. It only arrives in useful quantities when the sun sits high overhead, because at lower angles the light travels through enough atmosphere that the UVB is absorbed before it gets to you. Heat still comes through. So does the glare. Neither has anything to do with the reaction happening in your skin.

The practical threshold most people can check without instruments is the shadow rule: when your shadow is shorter than you are, the sun is above 45 degrees, and that is where synthesis is quickest. Between 30 and 45 degrees it still works and simply takes longer outdoors, and below about 30 degrees it effectively stops, which is the state usually called vitamin D winter.

We calculated the maximum solar elevation for every day of the year in 38 European cities. Not one of them escapes a vitamin D winter, including Málaga at the southern edge of the continent.

38 cities, one year, by latitude

Each bar is a full calendar year, read left to right as the days the sun is high enough for synthesis to be quick, the days it still works but takes longer, and the days it cannot help at all. The seventh column is the date vitamin D winter begins.

When does vitamin D winter start in your city?

It has already started in the north. Reykjavík crossed the line on 13 September, Helsinki on 23 September, Oslo on 24 September, and Stockholm and Tallinn on 25 September. Riga follows on 1 October, Copenhagen and Edinburgh on 4 October, Vilnius on 7 October, then Manchester, Dublin and Hamburg together on 10 October.

The middle of the continent goes in the fortnight after that. Amsterdam and Berlin on 13 October, Warsaw on 14 October, London on 15 October, Brussels on 17 October, Prague and Frankfurt on 19 October, Paris on 23 October, Munich and Vienna on 25 October. The south holds out considerably longer: Barcelona until 16 November, Madrid until 20 November, and Málaga, the sunniest city in the set, not until 16 December, when it closes for twelve days around the solstice and reopens.

One date is worth putting beside another. The clocks go back across Europe on 25 October 2026, and that is the moment most people register as the arrival of winter. In London the light stopped being useful ten days earlier, on 15 October, and in Manchester a fortnight before that. The change everyone notices is in the hour on the clock. The change that matters is in the angle of the sun, and it is silent.

Vitamin D winter in all 38 cities

Every city in the index has its own two dates: the day the midday sun drops below 30 degrees and stays there, and the day it comes back. Below are both dates for all 38, with the length of the closed season, the window when the sun clears 45 degrees, and how each city compares with the nearest reference point in the set. The cities are ordered by latitude, from the southernmost to the northernmost.

Three things the numbers change

Moving south helps less than people expect. Madrid gets 210 days above 45 degrees against London's 152. That is a real difference, but it is 38 percent rather than the doubling most people imagine when they relocate for the light, and Madrid still spends 64 days a year in vitamin D winter.

The useful hours are not simply the bright ones. Even in the far south the sun clears 45 degrees for under seven hours on the best day of the year, and in Paris on the summer solstice it is 6.3 hours, falling roughly between nine in the morning and half past three. You do not need to spend all of that outside, because a short exposure is enough and gets shorter the higher the sun climbs, but you do need to be outside somewhere inside it, and that is precisely the stretch most people spend indoors.

Latitude sets the calendar, not the weather. Barcelona and Rome sit within half a degree of each other and get 205 and 202 days respectively. Clouds change how it feels; they do not change when the sun is high enough for the reaction to run.

Questions the data answers

How this was calculated

For each city we computed the sun's maximum elevation on every day of the year, which occurs at solar noon and equals 90 degrees minus the absolute difference between the city's latitude and the solar declination for that date. Declination was derived from a standard astronomical approximation accurate to within about 0.2 degrees. The winter start date is the first day of the continuous run on which that maximum elevation falls below 30 degrees, and the solstice window was solved from the same relationship, giving the hours per day the sun exceeds 45 degrees.

Because the underlying approximation carries about 0.2 degrees of uncertainty, a start date should be read as accurate to within a day or so rather than to the hour. The dates are for 2026 and shift by less than a day from year to year.

Both thresholds are borrowed. The 45 degree shadow rule began life as a sun protection signal, the point at which your shadow becomes shorter than your height and the advice is to seek shade. The modern version of that advice is to cover up once the UV index passes 2, which on a clear day corresponds roughly to the sun clearing 30 degrees. We have repurposed both as synthesis thresholds because the same geometry governs both, and because the tension between them is the whole subject: the light that makes vitamin D is the same light that burns you.

What these figures deliberately do not measure is dose. Only a brief exposure is needed to produce a useful amount of vitamin D, the higher the sun sits the shorter that exposure becomes, and in every case it is well short of what would burn a sensible area of unprotected skin. So the solstice window column tells you when in the day to go outside rather than how long to stay there, and a city with a narrow daily window is not thereby short of opportunity on that day. What separates these cities is the number of days, which is what the index counts.

These are astronomical figures describing potential rather than status. They say when the sun is geometrically capable of driving synthesis, not how much any individual will produce, which also depends on skin tone, exposed surface area, altitude, cloud cover, ozone, air quality and sunscreen. People in Madrid are not thereby better supplied than people in London, since what anyone ends up with depends on how they behave inside that ambient light, and southern Europeans often test lower for exactly that reason. Glass blocks UVB entirely, so time spent behind a window counts as none of this.

As a cross-check, UK public health guidance, and discussion with Prof Webb at University of Manchester, puts the period when sunlight cannot supply vitamin D at roughly October to March. Our calculation gives London 135 days below 30 degrees and Manchester 145, which arrives in the same place from a different direction.