Why is the sky blue?

Daylight sky

The blue color of the daylight sky is caused by Rayleigh scattering in the atmosphere.

Rayleigh scattering occurs on particles much smaller than the wavelength of electromagnetic radiation hitting the particle. The molecules (N2, O2, etc.) in the earth's atmosphere are about 1000 times smaller than the wavelength of visible light and thus cause scattering of sunlight. Since Rayleigh scattering is inversely proportional to the fourth power of the wavelength, more light of shorter wavelengths (violet and blue) is scattered than light of longer wavelengths (red and orange).

That's why the daylight sky is blue and, to a lesser degree, green.

Twilight sky

The twilight sky is a deep blue color for a different reason. It is caused by Chappuis-band absorption^[1] of orange light in the ozone layer of the earth.

After sunset and before sunrise, sunlight traverses a longer way through the stratosphere than during the day. Along its path, light in a broad band of wavelengths around 600 nm, i.e. orange light, is absorbed by ozone.

This process, photodissociation of ozone in the Chappuis band, splits the ozone molecule: O3 + light → O + O2. The loss of ozone in this process is balanced by the termolecular reaction O + O2 + M → O3 + M, where M is either an O2 or an N2 molecule that carries away the excess energy.

Light of shorter wavelengths, i.e. blue light, is not absorbed. After orange light has been filtered out, blue light remains. Due to Rayleigh scattering, this blue light becomes visible on earth. The filtering effect of Rayleigh scattering increases the spectral pureness of the blue light.

That's why the twilight sky is more blue than the daylight sky.

1. Brion et al. Absorption Spectra Measurements for the Ozone Molecule in the 350-830 nm Region