SunkatCan you tan through clouds?
Yes, and you can burn too. Clouds feel protective because they block heat, but UV is a different part of sunlight.
Short answer: light and broken cloud lets most UV through (WHO says up to 80% through light cloud), so you can tan and burn on a grey day. Thick, dark overcast cuts UV much more, often to around half. Window glass blocks most of the UVB that tans and burns. Water does not protect you: about 40% of UV still reaches half a metre down.
How much UV gets through
| Situation | UV reaching your skin | Can you tan or burn? |
|---|---|---|
| Clear sky | All of it | Yes |
| Light or broken cloud | Up to about 80% (WHO) | Yes, and it is easy to underestimate |
| Thick, dark overcast | Roughly half or less | Slower, but still possible at high UV |
| Behind a window | Most UVB blocked, much UVA passes | No real tan or burn, some skin ageing |
| Half a metre under water | About 40% (WHO) | Yes |
| On fresh snow | Up to 80% extra by reflection (WHO) | Yes, faster than the index suggests |
Why cloudy days still burn
Your skin senses heat, which is infrared, not ultraviolet. Clouds absorb a lot of the heat, so a cloudy day at UV 7 feels mild while delivering most of a clear day's UV. Broken cloud can even raise UV for short spells when the sun shines between clouds and light scatters off their edges.
That is why the UV index, not the temperature or the sky, is the number to go by. Check it in your weather app: if it says 3 or more, the same burn times apply as on a sunny day at that UV. See UV 6, UV 8 or the burn time calculator.
How the Sunkat app handles cloud
Sunkat reads the hourly UV and cloud cover for your location and adjusts the UV down only as cloud thickens, so a full overcast sky roughly halves it, while thin and scattered cloud barely changes your time. That matches what UV meters show far better than treating every cloud as shade.
Frequently asked
Can you tan on a cloudy day?
Yes. WHO notes that up to 80% of UV can pass through light cloud. Only thick, dark overcast cuts it substantially, and even then some gets through. Because cloudy days feel cooler, people stay out longer and burn more often.
Can you tan through a window?
Hardly. Ordinary window glass blocks most UVB, the part of sunlight that causes burning and most tanning. UVA does get through, so long hours next to a window can slowly darken existing pigment and age the skin, but you will not get a real tan or a sunburn.
Can you tan in the water?
Yes. WHO notes that about 40% of UV still reaches half a metre below the surface, and the water's surface reflects more UV onto your skin. Snorkellers often burn on the back of the legs and neck.
Does wind or cold weather reduce UV?
No. Temperature and wind have nothing to do with UV. A cool, breezy day at UV 8 burns as fast as a hot one at UV 8, which is why people burn on windy beaches and on ski slopes.
Cloudy or clear, know your minutes
Sunkat adjusts your time per side for the live UV and the cloud above you. Free on iPhone and Android.
