The sun can feel scorching hot while your sunburn risk stays low
The UV index is not the warm sensation of sunlight on bare skin
Sunburn comes from UV-B damaging your DNA, not from the warmth you feel. Visible and infrared light heat your skin but don't directly harm DNA. UV-B is hundreds of times less intense than near-infrared, so its warmth would be imperceptible. UV also drops off faster than infrared away from solar noon, and clouds can even amplify UV through gaps. The author uses pvlib simulations to make this intuitive.
For some reason, knowing this makes it more intuitive to me that sunlight's warmth is disconnected from the rate of sunburn.
- Terr_
> Sunburn is triggered when your DNA is damaged from light absorption [...] This interferes with protein synthesis and wreaks havoc in your body over time
Not 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 neighbors.
Instead, your cells have techniques to measure a variety of risk factors and signals (which does include DNA-molecule weirdness [0]) and when things get too risky the well-behaved ones will self-destruct, to avoid the chance of turning into a cancer.
So the sunburn you see and feel is instead due to cells signalling "something damaged me and I can't be trusted, I'm shutting down, recycle me" to your immune-system, plus the attendant bustle of hazmat-cleanup and increased security-sweeps for cancer-leftovers and other potential causes of the damage.
- 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 water component of the sunblock has evaporated. And, the type of sunblock doesn't matter (the drier waterproof kind or various lotion varieties). And, I do notice a difference between the different SPF, along with some "bad" sunblock that I just kept applying but couldn't make the feeling go away (it had apparently separated, and everyone that used it was burned).
I think the "real" test would be to just shine a UV light in a dark room, to see if the source can be identified while blindfolded, or something.
- kens
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). Enzymes fix most of these errors, but sometimes the errors will cause problems during DNA replication and you can end up with mutations.
Details: https://pdb101.rcsb.org/motm/91
- GuB-42
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.
- xattt
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.