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?