Richard, according to NIWA the maximum summer value for UVI in New Zealand is generally about 12. Published data from Lauder shows that Ozone atmospheric concentration is lowest in March and the latest measurement for March 2024 is close to the historical average for this time of the year. However, Whangarei recorded only 7.8 maximum today 22nd March and yesterday 21st. Why were these measurements for Whangarei today and yesterday much lower than 12?
This is part of the usual seasonal cycle, which is driven mainly by changes in sun elevation at noon. At the equinoxes, the elevation angle is 23 degrees lower than at mid summer, when the UVI is highest. At mid winter (21 June), its lower by another 23 degrees and the peak UVI at Whangarei will be around 2.5. You can check this out using the 'Geeks' tab on the GlobalUV smartphone app. Enjoy :)
This is great info, I was actually just looking into this. I read that there is a difference between prescription glasses using glass or using plastic. Plastic blocking UV rays and glass not really blocking them. I wonder if your test implies that all these glasses were made of some sort of plastic, or if there also we're actual glass lenses among them which only block UV rays due to whatever custom coating they had.
Furthermore I noticed the remark about SPF 50 implying UVA has to get blocked as well.
This is news to me, I understood SPF to only refer to UVB rays. Could you elaborate on the connection here?
Can you make any kind of deductions about the amount of UVA being blocked from the SPF value? Since sunscreen manufacturers don't tell us about the amount of UVA being blocked specifically.
Thanks. Some of the glasses I tested were plastic. I thought others were glass but have just check red with my optometrist who tells me that about 98% of their prescriptions use plastic lenses (apparently only grumpy old men want glass). The prices ranged from $2 to hundreds of dollars per pair.
SPF is a protection rating against erythema, which is mainly from UVB wavelengths. But for high sun conditions, there is a contribution of about 5% from the UVA. The fraction is small because the weighting function gets smaller and smaller at longer wavelengths. Because 5% is at UVA wavelengths, you need an SPF of more than 20 to block much UVA. An SPF of 50 or more implies there must be good broad band blocking well into the UVA region. The term SPF is usually applied only to sunscreens, but the principle applies equally well to other materials, including clothing.
I might need to write a post on the subject of SPF with an explanatory diagram.
Richard, according to NIWA the maximum summer value for UVI in New Zealand is generally about 12. Published data from Lauder shows that Ozone atmospheric concentration is lowest in March and the latest measurement for March 2024 is close to the historical average for this time of the year. However, Whangarei recorded only 7.8 maximum today 22nd March and yesterday 21st. Why were these measurements for Whangarei today and yesterday much lower than 12?
Hi Michael
This is part of the usual seasonal cycle, which is driven mainly by changes in sun elevation at noon. At the equinoxes, the elevation angle is 23 degrees lower than at mid summer, when the UVI is highest. At mid winter (21 June), its lower by another 23 degrees and the peak UVI at Whangarei will be around 2.5. You can check this out using the 'Geeks' tab on the GlobalUV smartphone app. Enjoy :)
Richard
This is great info, I was actually just looking into this. I read that there is a difference between prescription glasses using glass or using plastic. Plastic blocking UV rays and glass not really blocking them. I wonder if your test implies that all these glasses were made of some sort of plastic, or if there also we're actual glass lenses among them which only block UV rays due to whatever custom coating they had.
Furthermore I noticed the remark about SPF 50 implying UVA has to get blocked as well.
This is news to me, I understood SPF to only refer to UVB rays. Could you elaborate on the connection here?
Can you make any kind of deductions about the amount of UVA being blocked from the SPF value? Since sunscreen manufacturers don't tell us about the amount of UVA being blocked specifically.
Thanks. Some of the glasses I tested were plastic. I thought others were glass but have just check red with my optometrist who tells me that about 98% of their prescriptions use plastic lenses (apparently only grumpy old men want glass). The prices ranged from $2 to hundreds of dollars per pair.
SPF is a protection rating against erythema, which is mainly from UVB wavelengths. But for high sun conditions, there is a contribution of about 5% from the UVA. The fraction is small because the weighting function gets smaller and smaller at longer wavelengths. Because 5% is at UVA wavelengths, you need an SPF of more than 20 to block much UVA. An SPF of 50 or more implies there must be good broad band blocking well into the UVA region. The term SPF is usually applied only to sunscreens, but the principle applies equally well to other materials, including clothing.
I might need to write a post on the subject of SPF with an explanatory diagram.
R
Thanks for all these great infos. I did have that suspicion, that most glasses might be plastic now anyway. That sounds logical then.
And yes I would very much appreciate a post on SPF