I’m a little puzzled. In reply #13, it states UV is measured at 320 nm, which is in the UVA spectrum and, I believe, has relatively little physiological effect.
https://en.wikipedia.org/wiki/Ultraviolet_index states that the erythemal (sunburn) effect of UV at ± 325 nm is 0.3% as damaging as at 295 nm. I understand (possibly incorrectly) that the Diffey-weighted UV irradiance , or DUV, integrates the erythemal effect over the UVA and, essentially, the UVB parts of the spectrum. I suggest that expressing only the measured UVA values as a numeric index (eg 0-20) may be a cause for confusion because the erythemal effect may be totally different from the value shown in a similar index. In practical terms, if WF shows UV as 8, does the layman plaster him/herself with cream, even though the sunburn effect may be much lower due to high absorption of UVB in the ozone layer?
As I understand it, as a layman, the effective UVA/B is a function of the ‘ozone layer’ density and is measured in Dobson Units which is the thickness of the stratospheric ozone expressed in mm, when it is compressed down to Normal Pressure.
Obviously, it is ridiculous to measure both UVA and UVB and translate them to ‘sunburn effect’ in an economical device such as WF, but can it be made clearer that the UV measured is just an arbitrary value, not necessarily related to sunburn risk, please?