The Broken-Cloud Effect: Why Partly Cloudy Skies Hit You With More UV
The most dangerous sky isn't the clearest one — scattered clouds can boost surface UV-B above clear-sky levels, sometimes by 40%.
Scattered clouds can enhance surface UV-B above clear-sky levels by 20% or more, and DNA-damaging UV-B has been measured up to 40% stronger under broken cloud.
Atmospheric scientists describe this 'cloud enhancement' effect in reviews of cloud impacts on UV, explaining that broken clouds can scatter additional sunlight toward the ground so that surface UV briefly exceeds what a clear sky would deliver.
Questions we get a lot
Can a partly cloudy sky give you more UV than a clear one?
Yes. Scattered clouds can enhance surface UV-B above clear-sky levels by 20% or more, and DNA-damaging UV-B has been measured up to 40% stronger under broken cloud conditions. This happens when sunlight reflects and scatters off the bright edges of nearby clouds toward the ground. So a partly cloudy day can briefly be more intense than a cloudless one.
Why do I get sunburned on cloudy days?
Clouds don't reliably block UV the way they block visible light and heat, so a sky that looks gray or overcast can still pass plenty of burning UV. Under broken clouds, scattering can even push surface UV above clear-sky levels. Because the day feels cooler, people often skip protection and burn without realizing the UV was strong.
Should I wear sunscreen when it's cloudy?
Yes. Because clouds are unreliable UV filters and broken clouds can even amplify surface UV-B, an overcast or partly cloudy sky is not a reason to skip protection. Treat cloudy days like sunny ones when UV levels are forecast to be moderate or higher. The lack of obvious sunshine can give a false sense of safety.
r/SkincareAddiction: 'I always thought cloudy days were safe to skip sunscreen, but I got fried on an overcast beach day. Is it possible clouds made the UV worse?'