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.
Se vives aquí no norte, no verán o sol chega directamente, pero no inverno atravesa unha gran cantidade de atmosfera, e moitos dos raios ultravioleta desfanse, e o rango das lonxitudes de onda que golpean a Terra é diferente no verán e no inverno.