There are lots of little particles in the air that are smaller than the wavelength of light, which cause light to ‘scatter’ (randomly change direction) as it passes through the atmosphere. Shorter wavelength light (Blue) is affected more, and so more blue light is scattered than red. When you look up into the sky you’re seeing all this scattered blue light from sunlight that is passing overhead.
For the same reason, sunset and sunrise is really red, as all the blue light is being scattered as the light as to go through so much atmosphere to reach you.
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