Everyone sees the UV number in their weather app; almost nobody knows what it means. Here’s the short version.

The index runs from 0 to 11+. Under 3, you’re fine without protection. From 3 to 7, sunscreen and shade at midday are worth it. Above 8, unprotected skin can burn in under 15 minutes — even on a cloudy day, since clouds only block about 20% of UV.

The counterintuitive part: temperature has nothing to do with it. A crisp 15°C September afternoon can carry a higher UV index than a humid 30°C evening in July.