"It's overcast, I don't need sunscreen." It's the most common sunburn alibi in dermatology clinics. The truth: up to 80 % of UV radiation passes through light cloud cover, and certain cloud configurations actually amplify ground-level UV beyond clear-sky values.
Why clouds don't stop UV the way they stop light
Visible light and UV behave differently in the atmosphere. Water droplets and ice crystals scatter visible wavelengths efficiently — that's why a cloudy sky looks dim. UV-A and shortwave UV-B, however, scatter rather than absorb: they bounce around inside the cloud and a large fraction still reach the ground, just from many angles instead of one.
Cloud type matters
| Cloud type | UV transmission | What you should do |
|---|---|---|
| Thin cirrus (high, wispy) | 80–95 % | Treat as clear sky |
| Scattered cumulus (puffy, blue gaps) | 90–110 % (enhancement) | Often worse than clear sky |
| Broken stratocumulus | 50–70 % | Reduced but still high |
| Thick stratus / overcast | 20–40 % | Genuinely lower |
| Storm clouds (cumulonimbus) | 10–20 % | Brief reprieve |
The cloud enhancement effect
On partly cloudy days, sunlight reflecting off the sides of nearby cumulus clouds adds to the direct beam. UV index readings up to 25 % higher than clear-sky values have been measured during these conditions. This is why people often get their worst sunburns on "not too sunny" days — they underestimated.
What about fog and haze?
Fog typically blocks 30–60 % of UV. Urban smog blocks slightly more (it absorbs rather than scatters), but should never be relied on as protection — composition varies daily.
Practical rules
- If you can see your shadow, even faintly, UV is reaching you.
- Apply sunscreen based on the forecast UV index — not how the sky looks.
- Reapply on partly cloudy days as you would on a sunny day; the dose accumulates the same way.
- Check Sun Day before going out: the forecast already accounts for cloud cover from the weather model.
How Sun Day handles it
Sun Day pulls hourly UV forecasts from Open-Meteo, which already integrates cloud cover, ozone, and elevation into the index. You see the actual ground-level UV, not "UV minus clouds you happened to see out the window."
Stop guessing the UV
Get accurate, hourly UV predictions for your exact location — clouded or not.