// HACKER NEWS — CYBERSECURITY
The UV index is not the warm sensation of sunlight on bare skin
The warmth of sunlight is not a reliable indicator of how quickly you will sunburn in a given situation. On one hand, you can still get sunburned on a cooler cloudy day, while on the other hand, the sun can feel scorching hot on your skin in the morning and yet you won't sunburn (very quickly).
You probably knew this already, but it still feels unintuitive to me. I'm going to look at numbers and think about them until I convince myself, and perhaps you, that this is true.
Sunburn is triggered when your DNA is damaged from light absorption - in particular, UV-B (and to a limited extent, UV-A) is absorbed in a way that alters DNA's molecular structure1.
Image: DNA UV mutation by Mouagip (derivative of a NASA / David Herring original), released into the public domain.
This interferes with protein synthesis and wreaks havoc in your body over time, but the DNA damage itself is painless2.
Sunlight feels warm because visible and infrared light excites vibrational modes in various molecules in your skin, especially water and melanin. For physics reasons, this is strictly a form of heating and never directly damages molecular structure.
And if you have a darker skin tone, the sun will feel warmer to you, yet you'll sunburn much more slowly.
Visible light, near-IR and mid-IR are absorbed to different degrees and at different depths of the skin, but they all contribute to that feeling of warmth.
Not only are the mechanisms completely different for sunburn and solar heating, the orders of magnitude of energy are as well.
At the Earth's surface, the UV-B band is hundreds of times less intense than near-IR. Reasonably, it takes a lot more power to toast your buns (and the rest of your body) than it does to toast your DNA3.