Why is the sky blue?

 

At night, the sky is black. By day it is blue. This is the case only on planet Earth, whose atmosphere, with special optical properties, diffuses sunlight mainly in the blue.

On the Moon, the sky is black: thus, whatever the hour, we can very well distinguish the Earth, the Sun and the other stars. From our planet, the view is different. The sky is blue, or white if there are clouds, or even gray if the storm is not far. These colors are simply due to the presence of our atmosphere, whose molecules absorb certain wavelengths of solar radiation.

Rayleigh for the blue of the sky

Every particle in the atmosphere is an obstacle for solar radiation: light is diffused in all directions. There are two diffusion mechanisms: one comes from the wave nature of solar radiation, the other from the refraction of rays, linked to geometric optics.

If the sky is blue it is because of the Rayleigh scattering. It characterizes the scattering of light by particles whose radius is very small compared to the wavelength of solar radiation. The scattered intensity is, according to Rayleigh, inversely proportional to the wavelength (by a factor to the power of four). The shorter the wavelength the radiation, the more it is scattered. Thus, since the incident light is white, blue (with a wavelength of 400 nm) will be much more diffused in the atmosphere than red (with a wavelength of 800 nm).

Mie for the white of the clouds

The molecules small enough to enable the Rayleigh mechanism are the molecules of the air. Majority in the atmosphere, it is therefore to them that we owe the dominance of blue in the sky. However, in the presence of water drops or aerosols, the Rayleigh diffusion is no longer valid. In this case, it is Mie's broadcast that is valid. This proves that for these dimensions of molecules, the scattered light retains the spectral characteristics of the incident light. This is the reason why the clouds are white.