Thread

The UV index is not the warm sensation of sunlight on bare skin

99 points · 79 comments · evakhoury

herf · 1790117121 · focus · HN ↗
Good post - the UV index is related to how short the "column of air" that the sun traverses to get to your skin - noon is the shortest distance. Scattering is a density function (more air, more scattering), so this is why it looks like cos^p when compared to normal irradiance (cos). It also explains why it is hard to get a sunburn in the temperate winter, since the sun always appears at an angle, it doesn't go directly overhead.
1. xdavidliu · 1790159195 · focus · HN ↗
  i'm not an expert but i think you might be conflating uva and uvb. uvb is the one that goes to zero in winter and near sunrise and sunset, is completely blocked by glass, and requires being near solar noon to have maximum effect (i.e. highest when your shadow is shorter than you). It is also the one that is responsible for vitamin D synthesis.
  on the other hand, uva is the one that is still substantial in winter and away from solar noon, goes through glass easily, and can cause sunburns and skin cancer.

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