HN blog post explains why sunlight's warmth doesn't predict UV sunburn risk
A blog explainer breaking down the physics and biology behind why perceived heat from the sun is unrelated to UV-driven sunburn risk sparked an extended technical discussion on Hacker News.
What to know
- The core claim: sunburn (UV-B DNA damage) and the felt warmth of sunlight (visible/IR heating of water and melanin) are driven by different wavelengths and vastly different energy levels, so one doesn't predict the other.
- UV intensity falls off faster than infrared away from solar noon, and clouds, gaps in clouds, snow and ice can all change UV exposure without changing perceived warmth.
- Commenters added and disputed biological detail — thymine dimerization rates, whether sunburn pain comes from cellular 'self-destruct' signaling or from inflammation triggered by cell death — and traded personal cues (sunblock sensation, shadow length, sky color) for judging UV risk without relying on heat.
The dispute Whether sunburn pain originates from a protective cellular 'self-destruct' warning signal (as the article implies) or is simply the downstream inflammatory consequence of tissue death, as Terr_ and EA-3167 argue. · positions read across 13 posts and comments
The article's biology is basically right but incomplete or slightly mischaracterized, and needs correction/refinement.
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“If you're in the sun, each skin cell gets 50-100 of these pairs created per second (which is much higher than I expected).”
kens · Hacker News ↗
People report being able to sense UV exposure directly through non-heat cues (sunblock gaps, shadow length, sky color).
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“I can feel where there are holes in my sunblock (which only block UV) which lets me know where to apply it, and the sensation is not one of heat of the sun, it's different.”
nomel · Hacker News ↗
The article is unnecessary and adds nothing beyond existing reference sources.
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“This whole article shouldn't even exist, and should just point to wikipedia or mayo clinic.”
skullone · Hacker News ↗
evakhoury Hacker News submitterkens HN commenterTerr_ HN commenterEA-3167 HN commenter
How it unfolded 4 developments, newest first · click a bar or a number to jump articlespostscomments
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EA-3167 disputes the article's cell-signaling framing
A commenter argues sunburn pain is not a warning signal from cells but inflammation from tissue death and cytokine response following DNA damage or apoptosis.
“The sunburn you feel isn't a distress signal from cells warning that they can't be trusted, it's tissue death and an inflammatory reaction triggered by a cytokine response.”
— EA-3167 -
DNA is the instruction set for protein synthesis. If your DNA is damaged beyond repair that's an issue, and sure apoptosis is the usual result and not cancer. At the same time however when cells die either from the direct impact of DNA damage or apoptosis they both stop working and spill their contents into the area... including signalling…
2 more of the top 3 · 4 posts in this stretch
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Another interesting way to consider it is that a sunburn is a really mild case of radiation sickness.Or the other way radiation sickness is the deepest worst sunburn you have ever had.
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Makes me think of the nice shade of pink I get from hitting the ski-slopes. Cold, but daunting.
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Readers share personal cues for detecting UV exposure
Commenters describe non-heat signals they use to gauge UV risk, including a distinct sensation from unblocked skin under sunblock, the shadow-length rule of thumb, and changes in sky color on high-UV days.
“I can feel where there are holes in my sunblock (which only block UV) which lets me know where to apply it, and the sensation is not one of heat of the sun, it's different.”
— nomel -
I don't know about this. I can feel where there are holes in my sunblock (which only block UV) which lets me know where to apply it, and the sensation is not one of heat of the sun, it's different. I'm not sure about the mechanism, but the presence of the sunblock stops the "UV" type sensation. This from the moment I put it on to long after the…
2 more of the top 3 · 3 posts in this stretch
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Good post - the UV index is related to how short the "column of air" that the sun traverses to get to your skin - noon is the shortest distance. Scattering is a density function (more air, more scattering), so this is why it looks like cos^p when compared to normal irradiance (cos). It also explains why it is hard to get a sunburn in the temperate…
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I've heard a quick way to determine if the UV index is dangerous is if your shadow is shorter than you. Though location matters too.I moved from Vermont to Texas and had to purchase an umbrella with UV protection to walk around during the day. The highest the UV index hits in the Vermont's summer is 6. In Texas, it's 11+ !!! 15 minutes and you're…
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Commenters refine the article's DNA-damage explanation
HN users add biological detail on thymine dimerization rates and push back on the article's framing of how damaged cells trigger sunburn symptoms.
“If you're in the sun, each skin cell gets 50-100 of these pairs created per second (which is much higher than I expected).”
— kens -
> Sunburn is triggered when your DNA is damaged from light absorption [...] This interferes with protein synthesis and wreaks havoc in your body over timeNot a skin-scientist, but this seems misleading: Sunburn is not caused by some bad DNA floating around, nor damaged cells making making mistakes or pumping out wrong proteins to inflame their…
2 more of the top 3 · 6 posts in this stretch
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The details of UV damage are interesting. You can think of DNA as a sequence of four letters: C, G, A, and T. If there are two neighboring T's, UV can move a bond, linking the two T's together (i.e. thymine dimerization). If you're in the sun, each skin cell gets 50-100 of these pairs created per second (which is much higher than I expected)…
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Does anyone know how UV indexes reported by weather apps (including forecasts) work?I've found that some sources report lower UV index on cloudy days, whereas some seem to indicate the same index as a sunny day.It's not clear if these sources all have a different model for UV, some being more sophisticated or conservative, and whether any of them…
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Blog post separates UV index from perceived sunlight warmth
blog.asciitweezers.com publishes an explainer arguing sunburn (UV-B DNA damage) and the warm sensation of sunlight (visible/IR heating) are driven by unrelated mechanisms and very different energy magnitudes, illustrated with solar-spectrum simulations.
“the sun can feel scorching hot on your skin in the morning and yet you won't sunburn (very quickly)…”
— blog.asciitweezers.com -
first by HN Frontpage, 1d ago
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What people are saying 4 voices from 1 site · best of 13 · verbatim
- How exactly do consumer weather apps calculate and forecast UV index, and why do different sources disagree on cloudy-day readings?
- Is the sensation some people report of directly feeling UV (via sunblock gaps or sky color) a real physiological signal or a placebo/secondary cue?
- Sep 22
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Our immune system is _so cool_. Seems like every week I learn something new about it.
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This whole article shouldn't even exist, and should just point to wikipedia or mayo clinic.
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I think I got the worst sunburns when I didn't feel hot at all.Usually, it involves water, water cools you down, so you don't feel the heat, but it doesn't block UV, so you don't get the warning.
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I am curious if anyone else can tell whether it is a high UV day just by looking outside.There is a certain look to the sky (slightly whiter-bluer) looking against the clouds, and I am not sure if it is because I am directly seeing UV, whether there is some fluorescence, or whether it’s placebo effect.