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.
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?
... 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.
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?