Vitamin D and Climate
Two cities at the same latitude can produce very different real-world vitamin D status because climate — cloud cover, temperature, humidity, outdoor culture, dress norms — modifies how much of the available UVB actually reaches human skin. This is the difference between a Mediterranean coast and a rainy northern port at the same 43° latitude.
Mediterranean climates
Hot dry summers, mild wet winters, high sunshine hours. Southern Europe, coastal California, central Chile, western Australia. Excellent UVB availability; historically low vitamin D deficiency rates. However, mid-day heat drives many people indoors in summer, and Mediterranean cultural dress in some populations (Greek/Italian widows in black; conservative Middle Eastern dress) reduces effective exposure. Winter deficiency is real but modest — 15–25% of adults.
Temperate continental / oceanic
Four distinct seasons, moderate rainfall, latitude 40–55°. Much of North America, Europe, East Asia, southern Australia, New Zealand. Vitamin D deficiency is seasonal — 30–60% of adults deficient in winter, 10–30% in summer. Continuous supplementation is standard advice for adults with limited summer sun.
Oceanic / marine (Northwestern Europe, Pacific Northwest)
High cloud cover year-round, mild temperatures, moderate rainfall. UK, Ireland, coastal Norway, Pacific Northwest US, coastal British Columbia. Compound climate penalty: latitude ~50° + persistent overcast. Deficiency rates are among the highest in the developed world — 60–80% of UK adults deficient at some point annually; median 25(OH)D in Seattle adults ~18 ng/mL vs 27 in Sacramento at similar latitude.
Tropical / equatorial
High UVB all year, warm to hot temperatures, high humidity. Central Africa, South America, Southeast Asia, Pacific Islands. Expectation: no vitamin D deficiency — but the reality is more complex. Indoor lifestyles in growing tropical cities, long protective clothing for sun avoidance, and dark skin combine to produce substantial deficiency in some tropical cities. Bangkok, Mumbai, Jakarta all show 40%+ adult deficiency rates in office-worker populations. The lesson: tropical latitude doesn't guarantee sun exposure.
Arid desert climates
Cloudless skies, high UVB, high heat. Middle East, North Africa, southwestern US, Australian outback. Should be sun-replete populations. Instead, desert cultures have some of the highest vitamin D deficiency rates in the world (60–90% in Saudi women, up to 80% in UAE adults) because sun avoidance is cultural and practical — heat drives everyone indoors during productive UVB hours, and conservative dress covers most of the body. Air-conditioned malls, workplaces, and cars complete the enclosure.
Monsoon climates
Wet season, dry season, latitude 10–30°. South Asia, Southeast Asia, parts of West Africa. Monsoon months produce persistent cloud cover, sometimes for weeks — substantial UVB reduction even at low latitude. Combined with dark skin and indoor pursuits during rain, monsoon populations often are surprisingly deficient.
Polar / subpolar
Months of no or minimal sun. Alaska, northern Canada, Iceland, northern Scandinavia, southern tip of South America, Antarctica. Traditional Arctic and subarctic populations solved this with vitamin D-rich diet (marine mammal and fish liver, oily fish); populations without that diet — or modern populations that have adopted low-fat/low-fish Western patterns — show extreme deficiency.
Urban vs rural
Within a given climate, urban adults consistently have lower 25(OH)D than rural counterparts. Indoor employment, less time outside, more air pollution, and taller buildings creating shade all contribute. Air pollution in some cities (particulate matter, ozone, aerosols) can reduce ground-level UVB by 20–40% during severe episodes.
Climate change and vitamin D
Longer wildfire seasons in the western US and Australia may reduce UVB during smoke events. Increased extreme-heat days may push more people indoors during peak UVB hours. Air-quality events from various causes may compound. On the other hand, warmer weather may increase outdoor time in cooler regions. The net effect on vitamin D is uncertain and probably modest compared to the dominant drivers (latitude, culture, supplementation).