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

> 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.

[0] https://www.nature.com/articles/newbio241074a0

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.

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

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.

I live in New Zealand, and my wife is Spanish so we’ve spent a bunch of time in Spain too. NZ for a combination of reasons has much higher UV than Europe, with the corresponding skin cancer rates. In midsummer the UV is around 150% of what it would be in Europe at the equivalent time of year, but here, especially if it’s windy, it might feel like 20 degrees and you’d quite like to be in the sun, whereas in Spain you’d probably be at 40 degrees and there’s no way you’d want to be out of the shade. The difference is really stark, in general there I can regulate my sun exposure by feel much more than I can here.

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 toast

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 are backed up by a UV meter.

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 winter, since the sun always appears at an angle, it doesn't go directly overhead.

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.

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

This is very misleading. By some coincidence I was just looking into this earlier today.

The lower the UV index, the smaller the window is between the amount of time needed to get enough Vitamin D and the time to get burnt (erythema). So the best thing to do is get sunlight mid-day for a shorter amount of time.

UV Radiation: Balancing Risks and Benefits https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...

  As the UVI becomes smaller, it becomes more difficult to produce sufficient vitamin D without inducing erythema. For low sun conditions typical of midday in the mid-latitude winter (SZA ∼65°), where R∼1, the skin area exposed must be more than 12% (see Eq. 5) This means that for these low sun conditions it is improbable that sufficient vitamin D can be produced from exposing the hands and face alone, since that area is less than 12% of the full body.

  Figure 6 shows that for full body exposures, there is a wide window between the time for sufficient UV and the time for too much UV. As the fraction of body that is exposed decreases, the window of optimum UV exposure times also decreases. If only hands and face are exposed, there is generally only a small window between receiving insufficient UV for vitamin D production, and too much UV for skin damage (erythema).

You would do well to calculate the numbers for your own city, as I did earlier today for mine:

https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da3...

Toasting my DNA is quite a way to put it. It's an informative read, it helps me understand why the noonday sun does feel so intense. Even if it's not hot, the ability to find shade at noon is so poor, it feels inescapable.