I read on https://www.uexpress.com/ask-the-doctors/2018/1/17/no-matter-the-time-of-day
UVA整天保持相同的强度,这使得从黎明到黄昏使用防晒霜很重要
然而在weather.com我可以看到紫外线指数,主要跟踪中波紫外线的水平,在一天中有显著的变化。< / p >
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?