Yes, you can burn on a cloudy day, and yes, you can tan on one. Thin or broken cloud typically lets through 70 to 90 percent of the ultraviolet that would reach you under clear sky, and scattered cumulus can briefly push the UV index above the clear-sky value. Only thick, unbroken overcast cuts UV substantially — and even that rarely takes it to zero.
The reason cloudy days cause so many burns is not that the ultraviolet is unusually strong. It is that the two cues people actually use — brightness and heat — both drop, while the ultraviolet largely does not.
How much UV cloud actually blocks
| Sky condition | Approximate share of clear-sky UV reaching you |
|---|---|
| Clear | 100% |
| Scattered / broken cumulus | 70–90%, with brief peaks above 100% |
| Thin high cloud (cirrus) | 80–95% |
| Hazy / light overcast | 50–80% |
| Thick, dark overcast | 10–30% |
The figures vary with cloud thickness, type and how much of the sky is covered, but the shape holds: partial cloud is barely protective.
Two physical reasons:
Ultraviolet is heavily scattered rather than absorbed. Short wavelengths scatter far more than visible light, so a large share of the UV reaching the ground arrives as diffuse skylight from all directions rather than as a direct beam from the sun. A cloud that hides the sun's disc removes the direct component but leaves much of the diffuse component intact. This is also why deep shade is not zero — you are still under an open dome of scattering sky.
Clouds reflect as well as block. Under broken cloud, sunlight can reach you directly and be reflected toward you off the bright sides of nearby cloud. This "cloud enhancement" effect is well documented and can push surface UV briefly above the clear-sky value — measurements of 10 to 25 percent above clear sky are common under scattered cumulus, with larger short spikes recorded.
So the worst case is not overcast. It is a bright day with big scattered clouds, which also happens to be the sort of day that feels comfortable enough to stay out for hours.
Why cloudy days catch people out
The cues that fail:
- Brightness. Clouds cut visible light much more than ultraviolet, so a day can look half as bright while delivering 80 percent of the UV.
- Heat. Cloud reduces infrared, so you feel cooler and stay out longer. Temperature has no relationship to burning power.
- Glare. Without squinting, there is no instinctive prompt to seek shade.
- Wind. A breeze on a cloudy coast removes the last sensory warning. Sea breezes are a classic burn scenario for exactly this reason.
The result is longer exposure at 70–90 percent intensity, which is usually a bigger total dose than a shorter session under clear sky.
Water, sand and snow make it worse
Reflection from the ground adds to whatever comes through the cloud:
- Fresh snow: up to about 80 percent reflected — effectively close to a doubled dose.
- Dry sand: roughly 15 percent.
- Sea foam and water surface: roughly 10–25 percent depending on sun angle.
- Concrete: around 10 percent.
A cloudy day on a beach or a ski slope can therefore deliver more ultraviolet to your face and the underside of your chin than a clear day in a grass field. It is also why a beach umbrella is only a partial defence: it blocks the direct beam while sand keeps reflecting UV underneath it.
Can you tan on a cloudy day?
Yes — at roughly the same rate as the fraction of UV getting through. Under broken cloud at 80 percent transmission, a session that would take 30 minutes under clear sky takes about 37. That is a modest difference, and far smaller than most people assume.
The tanning implication is the same as the burning one: treat a partly cloudy day as very close to a clear one. Use the same limit, the same sunscreen routine and the same timer. The one genuine benefit is that intensity is slightly lower and slightly less spiky, which gives marginally more margin for error.
Only under heavy, unbroken overcast does the picture change — and at 10–30 percent transmission, tanning becomes slow enough that it is generally not worth planning a session around.
Do UV forecasts account for cloud?
Modern UV index forecasts do include cloud cover, which is exactly why they disagree with one another most under broken cloud. Clear-sky UV is largely a function of sun elevation and ozone, both of which models handle well. Cloud is the difficult variable: a forecast grid cell is kilometres wide, cumulus fields evolve in minutes, and the enhancement effect is inherently local and short-lived.
Practical consequences:
- On clear days, trust the forecast index closely.
- On broken-cloud days, treat the forecast as a floor rather than a ceiling — the instantaneous value where you are standing may be higher.
- Under heavy overcast, the forecast is likely a reasonable estimate, though it may lag a clearing sky.
No consumer app measures UV. Your phone has no UV sensor; every number you see is a model forecast, and broken cloud is where the model is weakest.
The practical rule
If the UV index for this hour is 3 or higher, treat cloud as decoration. Same limit, same sunscreen, same timer. Reserve any relaxation for genuinely thick, dark overcast, and reinstate full caution the moment it starts to break up.
Sunlit works from the live UV forecast for your exact coordinates, cloud cover included, rather than from the daily headline number — and converts it into a time limit for your skin type and SPF, with a 10-day forecast so you can see which days are genuinely low. Free on iPhone and iPad.
Related reading
- What Is the UV Index? The Scale, the Chart and What It Means
- How to Tan Without Burning: A Step-by-Step Method
- The Best UV Index Apps, Compared Honestly
General information about ultraviolet exposure, not medical advice. UV values from consumer apps are model forecasts, not measurements, and are least reliable under broken cloud. Sources: WHO/WMO/UNEP/ICNIRP, "Global Solar UV Index: A Practical Guide"; published measurements of cloud-enhanced surface UV irradiance; standard albedo values for snow, sand, water and concrete.
