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I read on https://www.uexpress.com/ask-the-doctors/2018/1/17/no-matter-the-time-of-day

UVA maintains the same intensity all day, which makes dawn-to-dusk sunscreen use important

Whereas on weather.com I can see that the UV index, which mostly tracks the level of UVB, significantly varies throughout the day.

I read that the UV index mostly tracks the level of UVB radiations on https://en.wikipedia.org/wiki/Ultraviolet_index:

Because of the way the UV index is calculated, it technically expresses the risk of developing sunburn, which is caused mostly by UVB radiation.

Why is the level of UVA radiations on the earth surface the same throughout the daylight hours, while the level of UVB significantly varies?

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    $\begingroup$ My guess: Rayleigh scattering? Not an answer because I don't know. Shorter wavelengths are scattered less so may pass unaltered through even a very long atmospheric path, whereas longer wavelengths get scattered a little bit? $\endgroup$
    – gerrit ♦
    Jan 16, 2020 at 8:58

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UVA rays are very penetrating and are not weakened by the ozone layer, so are much the same throughout the day. UVB rays are of shorter wavelength, much less penetrating, and are weakened by passing through the atmosphere. At different times of day there are thicker or thinner layers of atmosphere to pass through. Near sunset, the UVB rays will be oblique, and therefore have a greater thickness of atmosphere to pass through, so will be consequently weakened. UVC rays are of even shorter wavelength and are mostly blocked by the ozone layer.

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