Hello Richard, Yesterday I read a detailed explanation of how UVI is measured and derived by calculation. Now I understand that UVI is a logically calculated index of radiation which has most risk for sunburn. It is obvious to me that UVI represents mostly ultraviolet light in the wavelength range characterized as UVB which is also part of the spectrum which produces vitamin D in the skin. But it is still not clear to me why radiation in this part of the spectrum is absorbed to a greater extent by ozone when the sun is low in the sky versus ultraviolet light in the wavelength range characterized as UVA. Is the other factor, namely ultraviolet light scatter around the atmosphere and indirect radiation a considerable difference between UVA and UVB radiation?
Richard, we need a small dose of UVB for maximum vitamin D production and minimum risk of skin damage. Please can you show us a graph of UVB intensity and variation dutng a clear day in midsummer and midwinter at Lauder or Wellington.
If you download the UVNZ or GlobalUV app, you can see how the UVI varies at any location over the day. If you want to see other dates (or outside NZ), you need to go to the ’geeks’ section of globalUV.
I know how the UVI varies at any location over the day. But I do not understand why UVB intensity diminishes much more than UVA as the sun starts setting. I expected the ratio of UVA to UVB would remain the same as the sun goes down and UV intensities diminish.
Thanks for all these fascinating insights, this is exactly the kind of content I have signed up for here, I really appreciate it.
I do wonder though, the UV index is heavily weighted in favor of UVB isn't it?
I've seen graphs that indicate that UVA is present a lot more evenly during the whole day than UVB which seems to be near zero at 7am and obviously peaks very strongly at midday. The graph I've seen showed the UVA intensity yo only be about 2x stronger at midday than at 7, where as the UVB one seems to be about 12x stronger.
Sorry for the abstract explanation. Essentially the curve was flatter for UVA and started higher up at 7am already where as the curve for UVB was steep and was near zero at 7am.
So my question is, if this is true it would mean that you would indeed still receive plenty of UVA damage at lower sin angels. Just not from UVB which is what the UV index tends to represent mostly?
.... to your other point. An erythemal dose is merely the dose at which damage is large enough to become visible (as visible skin-reddening 24 hours afterwards). Sadly, any UVB photon could be the one that breaks a DNA molecule and leads to a mutation that eventually manifests itself as a skin cancer. Implicit in using erythema as an endpoint is the assumption that while doses are low, repair mechanisms will be able to keep up and avoid long term damage.
I see, very interesting. Thanks for the explanation.
But would that not mean that as long as you don't get red your skin would never actually end up damaged long term? As in you'd have smooth great skin as long as you don't get red, despite getting unlrotected sun exposure, just as long as you don't turn red?
No. That's the point I was trying to make. Any photon can trigger long term DNA damage. So, the fewer photons, the better. (Who knows if the assumption I mentioned above is correct. Certainly not me. I'm only an atmospheric physicist).
Yes, for high sun, more than 90% of the UVI is from UVB wavelengths. But, as you note, the drop-off for lower sun elevations is much more rapid for UVB than for UVA. So, for lower sun elevations, the proportion of UVA does increase. I could write a post about this. So yes, at low sun elevations, there's still a significant amount of UVA. But the overall UVI is still very small (because the weighting function for UV damage is orders of magnitude smaller in the UVA than the UVB).
I'd really like to know more about the significance of UVA when it comes to skin damage. I always understood that most of the skin ageing and skin damage comes from UVA, less so from UVB. Is that not the case?
I just don't get why this seems to be such a small concern in general then. Like for the UV index, why is it weighted so small, or why are sunscreens relatively bad at protecting from UVA in general, or why is there little awareness about UVA more or less constantly being present during daylight unlike UVB, etc. Mostly rethorical questions but it puzzles me.
... like me - a retired teacher of English and Classical Studies.
Hello Richard, Yesterday I read a detailed explanation of how UVI is measured and derived by calculation. Now I understand that UVI is a logically calculated index of radiation which has most risk for sunburn. It is obvious to me that UVI represents mostly ultraviolet light in the wavelength range characterized as UVB which is also part of the spectrum which produces vitamin D in the skin. But it is still not clear to me why radiation in this part of the spectrum is absorbed to a greater extent by ozone when the sun is low in the sky versus ultraviolet light in the wavelength range characterized as UVA. Is the other factor, namely ultraviolet light scatter around the atmosphere and indirect radiation a considerable difference between UVA and UVB radiation?
Richard, we need a small dose of UVB for maximum vitamin D production and minimum risk of skin damage. Please can you show us a graph of UVB intensity and variation dutng a clear day in midsummer and midwinter at Lauder or Wellington.
Hi Mike
If you download the UVNZ or GlobalUV app, you can see how the UVI varies at any location over the day. If you want to see other dates (or outside NZ), you need to go to the ’geeks’ section of globalUV.
Let me know if you need more.
Richard
I know how the UVI varies at any location over the day. But I do not understand why UVB intensity diminishes much more than UVA as the sun starts setting. I expected the ratio of UVA to UVB would remain the same as the sun goes down and UV intensities diminish.
Thanks for all these fascinating insights, this is exactly the kind of content I have signed up for here, I really appreciate it.
I do wonder though, the UV index is heavily weighted in favor of UVB isn't it?
I've seen graphs that indicate that UVA is present a lot more evenly during the whole day than UVB which seems to be near zero at 7am and obviously peaks very strongly at midday. The graph I've seen showed the UVA intensity yo only be about 2x stronger at midday than at 7, where as the UVB one seems to be about 12x stronger.
Sorry for the abstract explanation. Essentially the curve was flatter for UVA and started higher up at 7am already where as the curve for UVB was steep and was near zero at 7am.
So my question is, if this is true it would mean that you would indeed still receive plenty of UVA damage at lower sin angels. Just not from UVB which is what the UV index tends to represent mostly?
.... to your other point. An erythemal dose is merely the dose at which damage is large enough to become visible (as visible skin-reddening 24 hours afterwards). Sadly, any UVB photon could be the one that breaks a DNA molecule and leads to a mutation that eventually manifests itself as a skin cancer. Implicit in using erythema as an endpoint is the assumption that while doses are low, repair mechanisms will be able to keep up and avoid long term damage.
I see, very interesting. Thanks for the explanation.
But would that not mean that as long as you don't get red your skin would never actually end up damaged long term? As in you'd have smooth great skin as long as you don't get red, despite getting unlrotected sun exposure, just as long as you don't turn red?
No. That's the point I was trying to make. Any photon can trigger long term DNA damage. So, the fewer photons, the better. (Who knows if the assumption I mentioned above is correct. Certainly not me. I'm only an atmospheric physicist).
I see, thanks!
Yes, for high sun, more than 90% of the UVI is from UVB wavelengths. But, as you note, the drop-off for lower sun elevations is much more rapid for UVB than for UVA. So, for lower sun elevations, the proportion of UVA does increase. I could write a post about this. So yes, at low sun elevations, there's still a significant amount of UVA. But the overall UVI is still very small (because the weighting function for UV damage is orders of magnitude smaller in the UVA than the UVB).
I'd really like to know more about the significance of UVA when it comes to skin damage. I always understood that most of the skin ageing and skin damage comes from UVA, less so from UVB. Is that not the case?
That's the current wisdom ...
I just don't get why this seems to be such a small concern in general then. Like for the UV index, why is it weighted so small, or why are sunscreens relatively bad at protecting from UVA in general, or why is there little awareness about UVA more or less constantly being present during daylight unlike UVB, etc. Mostly rethorical questions but it puzzles me.