Can You Get Vitamin D on a Cloudy Day?
Yes, you can still make some vitamin D on a cloudy day, but a lot less than under clear skies. Clear skies let through nearly all the sun’s UV. Scattered clouds pass about 89%, broken clouds about 73%, and full overcast only around 31%, going by the figures the EPA uses in the UV index. So a gray, fully overcast day can cut your vitamin D production by roughly two-thirds.
The short version: clouds dim your vitamin D, they don’t switch it off, and how much they cost you depends entirely on how thick they are.
The short answer, with the numbers
Clouds don’t block UV like a wall. They scatter and absorb it, and how much depends on how heavy the cloud cover is. The EPA uses these transmission figures when it calculates the UV index, and they’re the most useful citable numbers for this question:
| Sky condition | UV reaching the ground | Rough hit to vitamin D |
|---|---|---|
| Clear | ~100% | None, full production |
| Scattered clouds | ~89% | Slight |
| Broken clouds | ~73% | Noticeable |
| Overcast | ~31% | Roughly two-thirds gone |
So a few puffy clouds barely matter. A thick, solid gray sky is a different story, and it can push the UV index below the level of 3 your skin needs to make vitamin D at all. (More on that threshold in what UV index you need.)
Why clouds block UVB unevenly
Not all clouds are equal. A thin layer of high haze lets most UV straight through, which is why you can get a surprising amount of sun on a bright but slightly milky day. A thick, dark storm cloud absorbs and scatters far more, dropping ground-level UV sharply.
This is also why a cloudy day can be deceptive in both directions. You can make decent vitamin D under light cloud while assuming you’re getting nothing, and you can make almost none under heavy overcast while still feeling plenty of daylight. UV strength and brightness aren’t the same thing, and your eyes are a poor judge of how much UVB is actually getting through.
How bright a day looks tells you about visible light, not UVB. A hazy-bright sky can carry real UV, and a luminous gray overcast can carry very little. The only reliable read is the UV index, not how sunny it feels.
The catch: you can burn on a cloudy day too
If clouds let through enough UV to make vitamin D, they also let through enough to burn you. Plenty of people get caught out this way, spending hours outside on an overcast summer day and ending up red, because the UVB was still strong enough to do damage even though the day felt mild.
So a cloudy day isn’t automatically a safe day. On a bright overcast day in summer, especially at altitude or near water, the UV index can still be high. Cloud cover lowers your risk, it doesn’t remove it.
How much longer you’d need on an overcast day
If overcast skies cut ground-level UV to about a third, the rough implication is that you’d need substantially longer outside to make the same vitamin D you’d get on a clear day, on the order of two to three times as long. The problem is that the math only works if the UV index is still 3 or above to begin with. If heavy cloud has dragged the index below 3, no amount of extra time helps, because the UVB simply isn’t reaching you.
That’s the real takeaway for cloudy days: it’s not “stay out longer,” it’s “check whether there’s any usable UV at all.” On a thin-cloud day there often is. On a heavy-overcast winter day there often isn’t.
How Bask factors cloud cover into your reading
This is exactly the kind of thing that’s hard to eyeball and easy to get wrong. Bask doesn’t ask you to judge the clouds. It reads your live, local UV index, which already reflects current cloud cover, and tells you whether you’re above the level of 3 where vitamin D production happens, and how long you’d need given today’s conditions and your skin.
So on a patchy day, instead of guessing whether those clouds matter, you get a straight answer: there’s enough UV right now, here’s your window, or there isn’t, skip it and rely on food or a supplement today.
Bask’s live UV reading already accounts for cloud cover, so you don’t have to judge whether a gray day is worth it. It tells you if you’re above the vitamin-D threshold and for how long.
Frequently asked questions
Can you get sunburned on a cloudy day? Yes. Clouds reduce UV but rarely block it completely, and on a bright overcast summer day the UV index can still be high enough to burn unprotected skin within a reasonable time outdoors. Don’t treat clouds as sunscreen.
Does fog block vitamin D? Mostly, yes, while it lasts. Thick fog and heavy low cloud scatter a large share of UVB, so vitamin D production during dense fog is minimal. Once it burns off and the sun is high, your window can reopen.
How long should you stay in the sun on a cloudy day? It depends on the UV index, not the clouds directly. If the index is still 3 or higher under light cloud, you may need roughly two to three times longer than on a clear day. If heavy overcast has pushed it below 3, no length of time will produce meaningful vitamin D.
Is winter sun worth it when it’s overcast? Often not. Winter sun is already weak at higher latitudes, and adding heavy cloud usually pushes UV below the threshold for vitamin D. On those days, food and supplements are the dependable sources.
Where to go next
- The big picture: How much sun do you need for vitamin D?
- The threshold clouds can push you under: What UV index do you need to make vitamin D?
- Another common myth: Can you get vitamin D through a window?
Sources
- US EPA, “Learn About the UV Index”: cloud-cover UV transmission figures (clear ~100%, scattered ~89%, broken ~73%, overcast ~31%).
- NIH Office of Dietary Supplements, Vitamin D Fact Sheet: UVB requirement for cutaneous vitamin D synthesis.
- World Health Organization, “The ultraviolet (UV) index”: clouds reduce but do not reliably eliminate UV exposure.
This article is educational, not medical advice. If you’re treating a known deficiency or have skin-cancer risk factors, talk to a clinician and consider a blood test to know your actual level.