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: