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.
În zona noastră, vara, soarele luminează direct pământul, dar iarna lumina trece printr-un strat atmosferic imens, pierzându-se mare parte din UV. Lungimile de undă care ating pământul diferă de la vară la iarnă.