Sun and health · 22 winters estimated by satellite

The vitamin D winter: when the sun is too weak to make vitamin D, for every town in the UK

In an average year (the mean of the 2004–2025 winters), the sun over London is too weak for vitamin D from 13 October until 11 March: 150 days when the daily dose of ultraviolet light is too small. Below this level, researchers say, skin makes very little vitamin D. Further north it lasts longer. In Glasgow it starts on 30 September and runs to 21 March.

London: in an average year, the vitamin D winter runs from 13 October to 11 March, 150 days.

Under clear skies it would be 131 days; cloud adds 19 days. Shortest winter: 2011–12 (141 days). Longest: 2012–13 (164 days).

What the guidance says for the winter months ↓

Search your town. The ring is the whole year: summer at the top, winter at the bottom. The dark arc is the vitamin D winter of an average year, and the 22 thin rings are the estimated winters of each year from 2004–05 to 2025–26.

vitamin D winter, average yearif the sky were always cleareach winter, 2004–05 in the centre, 2025–26 at the edge

The threshold of 1 kilojoule per square metre per day is a research benchmark for ambient UV. It cannot tell how much vitamin D a particular person makes or whether they have enough: that depends on time outdoors, how much skin is uncovered and the skin itself. The result covers one cell of a 0.25-degree grid (in the UK about 28 kilometres north to south and 16 east to west); places in the same cell get the same dates.

#How long the vitamin D winter lasts across the UK and Ireland

It generally lengthens northwards: 143 days in Plymouth, 150 days in London, 173 days in Glasgow and 175 days in Aberdeen, nearly half the year. Latitude does most of the work: the sun sits lower the further north you go. Cloud adds two to three weeks almost everywhere.

Click anywhere on the map to choose the nearest place.

The NHS says that between October and early March sunlight is not strong enough to make enough vitamin D. That matches London closely. In Scotland the season starts in late September and lasts until late March.

#What clouds add

TEMIS estimates two doses: the one that would arrive under a permanently clear sky, and the one adjusted for the cloud the satellites saw that day. Under clear skies London’s vitamin D winter would last 131 days, from 22 October to 1 March. Cloud adds 19 days.

On the ring, the thin arc is the clear-sky winter and the thick arc is the winter with cloud.

Large UK towns and cities, from the shortest vitamin D winter to the longest

Town	Starts	Ends	Days	Clear sky
Brighton England	16 October	7 March	143	127
Plymouth England	16 October	7 March	143	125
Portsmouth England	16 October	7 March	143	127
Southampton England	16 October	8 March	144	127
Southend-on-Sea England	14 October	9 March	147	131
Bexley England	14 October	10 March	148	131
Bristol England	14 October	10 March	148	129
City of Westminster England	14 October	10 March	148	131
Reading England	14 October	10 March	148	131
Barking England	13 October	10 March	149	131
Cardiff Wales	13 October	10 March	149	130
Swindon England	13 October	10 March	149	131
Archway England	13 October	11 March	150	131
Islington England	13 October	11 March	150	131
London England	13 October	11 March	150	131
Swansea Wales	13 October	11 March	150	131
Luton England	12 October	11 March	151	133
Milton Keynes England	12 October	11 March	151	134
Northampton England	12 October	11 March	151	134
Birmingham England	11 October	12 March	153	135
Coventry England	11 October	12 March	153	135
Leicester England	10 October	13 March	155	137
Derby England	9 October	13 March	156	138
Dudley England	9 October	13 March	156	137
Nottingham England	9 October	13 March	156	138
Wolverhampton England	9 October	13 March	156	137
Birkenhead England	8 October	14 March	158	140
Kingston upon Hull England	8 October	14 March	158	141
Sheffield England	8 October	15 March	159	140
Stoke-on-Trent England	8 October	15 March	159	139
Leeds England	7 October	15 March	160	143
Liverpool England	7 October	15 March	160	140
Manchester England	7 October	16 March	161	140
Bradford England	6 October	16 March	162	142
Oldham England	6 October	16 March	162	141
Preston England	6 October	16 March	162	142
Belfast Northern Ireland	5 October	17 March	164	146
Newcastle upon Tyne England	5 October	17 March	164	147
Edinburgh Scotland	2 October	20 March	170	152
Glasgow Scotland	30 September	21 March	173	152

#Every winter is different

The same rule applied to each winter separately gives very different years. In London the shortest vitamin D winter was 2011–12: 141 days. The longest was 2012–13: 164 days.

London

Across 147 large places in the UK (all with 75,000 people or more), 2012–13 was the longest since 2004, ties included, in 105, and 2011–12 the shortest in 80. Last winter, 2025–26, London’s ran 155 days, from 10 October to 13 March.

#What the guidance says for the winter months

The NHS advises everyone to consider a vitamin D supplement in autumn and winter. The ring shows when ambient UV is below a research benchmark where you live; it does not tell you personally when to take supplements.

Everyone, in autumn and winter. The NHS says children from the age of 1 year and adults need 10 micrograms of vitamin D a day, and that everyone, including pregnant and breastfeeding women, should consider a daily 10 microgram supplement during the autumn and winter.
All year for some people. The NHS recommends a daily 10 microgram supplement all year for people who are rarely outdoors or who usually cover most of their skin, and says people with dark skin should consider taking one all year.
Babies and young children. Babies up to 1 year need 8.5 to 10 micrograms a day. The NHS advises a daily supplement for breastfed babies and for formula-fed babies having less than 500 ml of formula a day, and for all children aged 1 to 4. Ask a health visitor or GP if unsure.
From food. Oily fish such as salmon, sardines, herring and mackerel, red meat, liver (avoid it in pregnancy), egg yolks and fortified foods such as some spreads and breakfast cereals.
More is not better. The NHS says not to take more than 100 micrograms (4,000 IU) a day as an adult, or 50 micrograms for children aged 1 to 10. You cannot overdose on vitamin D through sunlight.
Not sunbeds. The European Code Against Cancer, published by the World Health Organization’s cancer agency, says using sunbeds to raise vitamin D is unnecessary and strongly discouraged.
When the sun returns. The same code says that in spring and summer a few minutes outside at midday (about 10 to 15, with face, arms and legs uncovered) is usually enough, and that extra exposure is unnecessary. Fair skin can burn within 10 to 15 minutes when the UV index is high: protect your skin and avoid sunburn.

This is general information from the documents cited. For an illness, a pregnancy or a treatment, ask a doctor or pharmacist.

#The UK and the rest of Europe

The vitamin D winter generally lasts longer farther north. In an average year it lasts 59 days in Athens, 150 days in London and 214 days in Reykjavík, nearly seven months.

City	Starts	Ends	Days
Nicosia	no vitamin D winter in an average year		0
Athens	28 November	25 January	59
Madrid	20 November	28 January	70
Rome	12 November	9 February	90
Sofia	10 November	10 February	93
Bucharest	4 November	15 February	104
Belgrade	4 November	17 February	106
Chișinău	26 October	25 February	123
Budapest	26 October	1 March	127
Vienna	22 October	26 February	128
Paris	21 October	4 March	135
London	13 October	11 March	150
Berlin	11 October	13 March	154
Dublin	8 October	14 March	158
Oslo	24 September	26 March	184
Helsinki	25 September	28 March	185
Reykjavik	10 September	11 April	214

#How it was calculated

The data come from the TEMIS archive of the Royal Netherlands Meteorological Institute (KNMI) and the European Space Agency: the daily dose of ultraviolet light weighted for making vitamin D in skin (the CIE 174 action spectrum), on a 0.25-degree grid. The dose is computed from the ozone and cloud seen by satellites, from sunrise to sunset; it is an estimate, not a ground measurement. The series starts in 2004 and runs to 30 October 2026.

The threshold is 1 kilojoule per square metre per day. O’Neill and colleagues (2016) proposed it as a guide: below it, blood vitamin D rises very little. Khanna and colleagues applied it to the same TEMIS data in 2022 for 46 European capitals, assuming that production below it is negligible.

As a check, the monthly values for Bucharest in 2004–2021 differ from those Khanna published by at most 0.01 kilojoules per square metre, in all 12 months. For the same period, the number of days below the threshold matches exactly in Reykjavík (215), Oslo (187) and Nicosia (4). Valletta gives 16 days here against 38 in the paper; the difference is unexplained (Malta is a small island and its grid cell is almost all sea).

The average year is the 2004–2025 mean of each calendar day, smoothed over 15 days. The vitamin D winter is the unbroken run of days below the threshold around the winter solstice (21 December), so the start, the end and the count of days come from the same series. The same rule applied to each winter separately gives the thin rings; the studies cited used only the multi-year mean.

On the shortest winter days in northern Europe the satellite has no cloud data. There the clear-sky dose is far below the threshold anyway; as in Khanna’s study, the gap is filled with 70% of the clear-sky dose, which cannot change how a day is classed.

The dose is a whole day’s total on a horizontal surface, not what skin receives in a few minutes. Altitude and snow matter too, and the grid cannot tell a valley from a hilltop.

Places come from GeoNames (the UK, Moldova and European towns of 5,000 or more people) and Romania’s SIRUTA register.