You can't feel UV radiation. There's no warmth, no tingling, nothing that tells you it's happening. That's what makes it different from every other factor on this site: it's not about comfort, it's about damage that shows up hours later and accumulates over years.
What the UV index measures
The UV index runs from 0 to 11 and above. It expresses how quickly unprotected skin can be damaged at a given moment. In the Netherlands during summer, values between 5 and 8 are common on clear days, which the RIVM classifies as high to very high.
| UV index | Category | Time to sunburn (fair skin) |
|---|---|---|
| 1-2 | Low | 60+ minutes |
| 3-5 | Moderate | 30-45 minutes |
| 6-7 | High | 20-25 minutes |
| 8-10 | Very high | 15 minutes or less |
| 11+ | Extreme | Under 10 minutes |
At UV 6 or above, fair skin can burn in around 20 minutes without protection. That's shorter than most people assume when they're heading out for a walk or sitting at a café terrace.
What makes UV exposure worse than you expect
A few things push the effective exposure higher than the index alone suggests. Altitude adds roughly 15 to 20 percent per 1,500 metres. Reflective surfaces (sand, water, snow) bounce UV back at you from below, hitting areas that shade from above wouldn't protect. And clouds don't block UV the way they block visible light: 50 to 80 percent of UV radiation still passes through a cloudy sky. You can get a significant burn on a day that feels cool and overcast.
Clothing matters here. A regular t-shirt provides some UV protection, but not as much as people assume, especially when the fabric is thin or wet. Dedicated UV-protective clothing has a UPF rating that tells you exactly how much is blocked.
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How KorteBroekAan.nl uses the UV index
The UV index appears as supplementary information alongside the feels-like temperature and clothing advice. It doesn't directly affect whether shorts are recommended, but it gives you a reason to reach for sunscreen or a hat even on days when the temperature advice suggests light clothing. The two factors are independent: a cool, clear spring day can have a surprisingly high UV index, while a hot humid day under cloud cover might have a low one.
The other articles in the Weather Explained section cover the factors that do feed directly into the clothing recommendation: wind, humidity, cloud cover, and the difference between what the thermometer says and what you'll actually feel.
Further reading
Related articles in the Weather Explained section:
Read next
Why cloud cover matters for clothing advice
The same thermometer reading feels completely different under a clear sky versus a grey one. Cloud cover changes how much solar radiation actually reaches you.
Why feels-like temperature matters for clothing advice
The thermometer reads 18°C but you still feel cold. How is that possible? We explain what feels-like temperature is, how it is calculated, and why KorteBroekAan.nl uses it.
Why humidity matters for clothing advice
Humidity works in both directions: it makes hot days feel suffocating and cold days feel more biting. Here's what's actually happening and why KorteBroekAan.nl includes it.
Why rain probability matters for clothing advice
A 60% chance of rain doesn't mean it will rain 60% of the time. Here's what rain probability actually tells you and when it should change what you wear.
Shorts for children: a different threshold than adults
Children aren't just small adults when it comes to cold. Their body proportions and activity levels change the temperature at which shorts are comfortable or too cold.