Look a little closer
A clear daytime sky looks blue because tiny molecules in the atmosphere scatter the shorter wavelengths of sunlight more strongly. Sunlight appears white, but it contains a broad mixture of visible colors.
As sunlight enters the atmosphere, it encounters molecules such as nitrogen and oxygen. Rayleigh scattering occurs when the scattering particles are much smaller than the wavelength of light, and its strength rises sharply as wavelength gets shorter. Blue and violet light are therefore redirected much more than red light.
The blue light reaching your eyes from a patch of sky did not simply travel straight from the Sun. Air molecules changed its direction, sending it toward you from across the sky. This scattered light makes the atmosphere itself appear luminous and blue.
Why not a violet sky, since violet has an even shorter wavelength? The answer combines several effects: the Sun supplies different amounts of light at different wavelengths, human vision is less sensitive to violet than to blue, and some of the shortest wavelengths are absorbed high in the atmosphere.
Near sunset, sunlight crosses a longer path through air. Much of its blue light is scattered out of the direct beam before it reaches an observer, leaving a larger share of orange and red wavelengths. The same physics helps create both a blue noon sky and a red sunset.
Cloud droplets and dust are larger than ordinary air molecules and can scatter visible colors more evenly. That is why clouds often look white and a hazy sky can appear pale. Sky color records how sunlight interacts with particles of different sizes in the atmosphere.
