The UV index is not the warm sensation of sunlight on bare skin

原始链接: https://blog.asciitweezers.com/the-uv-index-is-not-the-warm-sensation-of-sunlight-on-bare-skin/

This Hacker News thread discusses the biological mechanisms of sunburn and the relationship between sunlight, skin health, and UV exposure. Participants clarify that sunburn is not merely a reaction to "damaged DNA" but an active immune response where cells detect damage and signal to be recycled to prevent cancer. Users note that UV-induced DNA damage—specifically thymine dimerization—occurs at a much higher frequency than many realize, with enzymes constantly working to repair these errors. The conversation also critiques the common belief that morning and late-afternoon sun is always "safer." While the UV index is lower, some argue that this does not necessarily mean the sun is safer, as the window for safely acquiring Vitamin D without risking erythema (sunburn) narrows significantly during these times. Other topics covered include: * The mechanics of how UV indexes are forecasted by agencies like NOAA. * Anecdotal evidence that high-UV days can often be identified by the "look" of the sky. * The observation that sunburns are often most severe when people feel cool (e.g., while in water), as the lack of heat sensation removes the body’s natural warning system.

原文

13 Jul, 2026

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.

How does sunburn happen

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 structure.

Diagram of DNA illustrating damage caused by UV-B radiation - a kink in the strand.

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 painless.

Why does the sun feel warm

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.

Different mechanisms, different scales

Not only are the mechanisms completely different for sunburn and solar heating, the orders of magnitude of energy are as well.

Intensity of sunlight (W/m^2/mm) at different wavelengths during solar noon. UV-B is 10-100x less intense than visible or near-IR light.

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 DNA.

For some reason, knowing this makes it more intuitive to me that sunlight's warmth is disconnected from the rate of sunburn. Even if you were absorbing all that UV-B as heat, the resulting warmth would be completely imperceptible.

Time of day affects UV and IR differently

The sun is by far the most intense when the sun is overhead - solar noon. This is true regardless of wavelength of light. However, UV intensity drops off more rapidly than IR as you get further away from noon, as it gets scattered more heavily by the atmosphere.

Plot showing relative intensity of UV-B, UV-A, visible, and near-IR sunlight over the course of a day. Each peak at solar noon, but UV-B has a considerably narrower peak than the other bands.

So, the sun may still feel pretty intense in the morning or late afternoon, but the risk of sunburn is considerably lower.

Wait, what about the weather?

Clouds are made of water, which scatters UV and absorbs IR. On a uniformly cloudy day, both UV index and perceived warmth from the sun drop substantially. However, if there are gaps in the clouds, it can actually amplify the UV intensity at ground level. Snow and ice contribute to this effect as well.

Where did these plots come from?

These plots are simulated measurements created using pvlib, an open-source Python library for modeling solar energy, using its implementation of the SPECTRL2 model to estimate the spectrum of sunlight reaching the ground throughout the day.

Anderson, K., Hansen, C., Holmgren, W., Jensen, A., Mikofski, M., and Driesse, A. “pvlib python: 2023 project update.” Journal of Open Source Software, 8(92), 5994, (2023). DOI: 10.21105/joss.05994.

Some caveats:

pvlib is designed for solar panels, not human bodies
assumes a perfectly clear sky over Toronto (43.8°N, 79.4°W) on 2026-07-12, with the sun hitting a horizontal object
other model parameters are set to reasonable defaults and do not come from historical measurements
SPECTRL2 cuts off at 300 nm, so UV-C and a part of UV-B are not shown

#health #science