[aqhl]

In my country, the sky is clearly blue in autumn. Someone can explain me why ?

[Yggdrasil]

Basically, the sky is blue because blue light has a shorter wavelenght than red light and is scattered more easily.  For a more detailed explanation, see

http://www.sciencemadesimple.com/sky_blue.html

or just google "why is the sky blue?"

[pantone159]

Ozone absorption also contributes to the blue color, specifically the very vivid blue colors that I usually notice around twilight.

Rayleigh scattering only produces a pale blue color, the more vivid hues need the ozone absorption to get there.