This phenomenon requires enough light on the ground, otherwise there may be light pollution. It is not enough to only have the city's ground light. Only rely on the scattering of air molecules. If there are clouds, long-wavelength red light will be reflected a lot, and you will see a red sky.
Light pollution is a new environmental pollution source after waste gas, waste water, waste residue and noise pollution, which mainly includes white light pollution, artificial daytime pollution and colored light pollution. Light pollution is threatening people's health.
Extended data:
main feature
Locality: Light pollution decreases rapidly with the increase of distance.
No residue: when the light source disappears in the environment, the pollution disappears.
Relativity: Relativity is divided into two aspects. Only under certain environmental background will there be light pollution, which is relative to the background. However, different people have different conclusions about whether some artificial light is polluted.