Why are there ziyun in the sky?

Different colors in the sky are caused by the scattering of sunlight in the atmosphere under different meteorological conditions. The color of the sky we see is actually the color of light scattered by the atmosphere. If the sky is very pure and there is no scattering effect from the atmosphere and other particles, the whole sky background will be dark except the sun, moon and stars.

The scattering effect of the atmosphere on different colors of light is not "equal opportunity". In the same heterogeneous medium, the shorter the wavelength of light, the stronger the scattering. The light intensity of blue light and violet light with shorter wavelength is 654.38+ million times higher than that of red light and orange light with longer wavelength. On the other hand, the scattering intensity is related to the size of particles in the medium. The smaller the particles, the better the scattering of short-wave light, but the worse the scattering of long-wave light.

When the weather is clear, the atmosphere is relatively pure, and atmospheric molecules are extremely fine particles, which is beneficial to the scattering of short-wave light. Therefore, blue light and purple light with short wavelength in sunlight are easily scattered through the atmosphere and scattered on the whole sky background. The sky looks blue because the human eye is not sensitive to purple light.

When there are clouds in the sky, the water droplets in the clouds are larger particles, which can cause the scattering of light of various colors, and the mixed results look like pieces of white flocs.

Before the rainstorm, the water droplets in the cloud were large and dense, with low transparency and little scattered light, so the sky looked gray or dark.