If you live up here, in summer the sun is coming fairly directly down, but in winter it's coming through a huge amount of atmosphere, and much of the ultraviolet is weeded out, and the range of wavelengths that hit the Earth are different from summer to winter.
如果你住在 太阳光在夏季几乎是直射的 而在冬季就会斜穿厚厚的大气 很多紫外线无法到达地面 而到达地面的波长范围 夏天和冬天是不同的