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.
UV index weighs UVB more heavily than UVA. UVB falls much faster than visible sunlight as the sun gets lower because photons travel through increasingly more ozone and atmosphere. So the higher in the sky the sun is, minus cloud clover, the higher the UV index is.
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.
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:
I really appreciate learning this fact, but I can't tell how it makes the quoted sentence misleading. The author is just talking about the surprising disparity between how warm it feels and how fast you will burn. They don't mention vitamin D.
I know they don’t mention vitamin D, but I think it’s important for people to know the context and not think that late day sunlight automatically = better
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.
Sunburn is triggered when your DNA is damaged from light absorption
This isn't correct. The sunburn you feel is not from bad DNA floating around or being used, and (while very unlikely) you could even get a sunburn while zero DNA is damaged.
Rather, your skin-cells have techniques to measure their exposure, and when they detect that they've gotten enough that DNA damage might have occurred, they self-destruct to mitigate the risk.
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.
UV index weighs UVB more heavily than UVA. UVB falls much faster than visible sunlight as the sun gets lower because photons travel through increasingly more ozone and atmosphere. So the higher in the sky the sun is, minus cloud clover, the higher the UV index is.
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...
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...
Isn't think only true if getting enough vitamin D is as important to you as cancer?
I really appreciate learning this fact, but I can't tell how it makes the quoted sentence misleading. The author is just talking about the surprising disparity between how warm it feels and how fast you will burn. They don't mention vitamin D.
I know they don’t mention vitamin D, but I think it’s important for people to know the context and not think that late day sunlight automatically = better
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
This isn't correct. The sunburn you feel is not from bad DNA floating around or being used, and (while very unlikely) you could even get a sunburn while zero DNA is damaged.
Rather, your skin-cells have techniques to measure their exposure, and when they detect that they've gotten enough that DNA damage might have occurred, they self-destruct to mitigate the risk.