One of the most common sun-safety mistakes: judging UV by how hot it feels. Temperature and UV index are driven by completely different factors, and confusing them leads to burns on "cool" days.
What Drives UV vs What Drives Temperature
| UV Index Depends On | Temperature Depends On |
|---|---|
| sun elevation (time of day + season) | air mass temperature |
| ozone layer thickness | wind, humidity |
| altitude | ground heating |
| clouds (partial) | clouds (strong) |
A 5°C spring day at UV 7 burns you faster than a 35°C evening at UV 1. That's because the sun is high at noon regardless of air temperature.
How Clouds Affect UV
- thin cirrus clouds — only 10–20% UV reduction;
- broken clouds — UV can actually spike due to edge scattering;
- thick overcast — up to 70% reduction, but still 30% gets through;
- rain — significant drop but not zero.
You cannot feel UV. It has no heat signature on your skin — that heat is infrared. You only notice UV damage hours later as redness.
Wind Makes It Worse
Wind cools your skin so you don't feel the burn developing. Windy beach days are among the most dangerous — people stay out 3–4 hours and end up with severe burns.
Bottom Line
Always check the UV index, never rely on how hot it feels. A cold but sunny day at UV 6 needs the same SPF as a hot one. Check your city every morning regardless of the forecast temperature.