The UV index is ( effective irradiance from 250-400nm, in watts/m^2 ) x 40.
If the UV index is truly, exactly zero, that means it’s nighttime and there’s no detectable UV coming in.
But keep in mind, if they report the UV index as a whole number, that means it could be as high as 0.499 and they’d round that down to zero…so an index of 0 *could* mean as much as 0.012499W/m^2 of radiation coming in. You could burn in that, but you’d have to have terrible genetics, use no sun protection and be exposed for hours.
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