Vitamin D and Cloud Cover
Not all clouds are equal. UVB transmission through the atmosphere varies from near-total (thin cirrus) to almost zero (thick storm clouds) depending on optical depth. On a cloudy day, you can be making almost as much vitamin D as a clear day — or almost none.
The transmission spectrum
- Clear sky — 100% UVB reference.
- Thin cirrus (high wispy) — 85–95% UVB transmission. Barely affects synthesis.
- Broken cumulus (scattered puffy) — 60–90% average, with intermittent full sun. Beach cloudiness — actual UV exposure often surprisingly high because scattering from cloud edges augments direct beam.
- Overcast altostratus (grey uniform) — 30–50% transmission. Reduced but not zero.
- Thick stratocumulus / marine layer — 10–30% transmission. Substantially reduced.
- Storm clouds / thick cumulonimbus — 0–20% transmission. Effectively zero synthesis.
"But I still got sunburned on a cloudy day"
Common experience because broken clouds actually enhance UVB via edge scattering — the "cloud enhancement effect." Recorded values 25% above clear-sky at cloud edges. This is why sunscreen is still recommended on partly cloudy days. It also means partly cloudy days at midday in summer can still deliver plenty of UVB for vitamin D synthesis.
Consistently cloudy climates
Chronic cloud cover compounds latitude in producing regional vitamin D deficiency. The Pacific Northwest (Seattle, Portland), UK, Ireland, coastal Norway, and Newfoundland all combine high latitude with high mean cloud cover. Deficiency rates in these regions exceed comparable-latitude sunny climates. Continuous year-round supplementation is standard advice for adults in cloud-heavy temperate zones.
Fog vs cloud
Ground-level fog behaves like a very low cloud — UVB transmission through thick fog is ~10–30%. Marine layer fog (common in coastal California) that burns off by noon has minimal impact on daily UVB budget. Persistent all-day fog reduces synthesis substantially.
Wildfire smoke and aerosol haze
Increasing concern in the western US and Australia. Heavy wildfire smoke can reduce UVB by 30–80% during severe events. Combined with air quality reasons to stay indoors, wildfire seasons dramatically reduce vitamin D synthesis in affected regions. Supplementation becomes essential during multi-week smoke events.
The reflectance surprise — snow doubles UVB
Fresh snow reflects 80–90% of incident UVB (vs 4% for grass). Winter mountain conditions with clear sky and snow cover can nearly double effective UVB dose. This is why ski goggles and lip balm matter even on cold, "safe-feeling" days — and it's a small vitamin D silver lining for winter mountain populations.
Water reflectance
Sand reflects ~25% UVB, water ~10%, concrete ~10%, grass ~3%. Beach environments thus receive both direct and reflected UVB — reinforcing the vitamin D-synthesis case for spending time by the sea, and the sunburn-risk case for the same.
Practical guide
- Cloudy but bright — assume 40–60% UVB, roughly 1.5–2× the exposure time for equivalent synthesis.
- Grey overcast — assume 20–40% UVB, 2.5–5× exposure time.
- Storm dark — assume < 10%; don't count on synthesis.
- Consistently cloudy climate — supplement year-round; don't wait for weather.
- Use our calculator — it doesn't model cloud cover directly, but assume clear-sky output and reduce accordingly.