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...

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.

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

> Sunburn is triggered when your DNA is damaged from light absorption [...] This interferes with protein synthesis and wreaks havoc in your body over time

No, your sunburn is not a result of DNA damage or misbehaving cells.

Rather, your skin-cells have other chemistries (not DNA!) to measure their own exposure, and when they detect that they've been hit with UV radiation so that DNA damage might be significant. This causes the well-behaved ones to self-destruct to mitigate the potential risk.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3634415/

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.

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.