Why is the clear sky always blue? More detail

This is because the sun's rays are scattered by atmospheric molecules and particles suspended in the atmosphere after being emitted into the atmosphere. Violet, blue and cyan light waves with shorter wavelength are the easiest to scatter, while red, orange and yellow light with longer wavelength have stronger transmission ability. They can pass through atmospheric molecules and particles and keep their original direction, and are rarely scattered by air molecules. For lower air molecules, mainly blue light is scattered, so the sky is blue.

The blue of the sky can only be seen at low altitude. With the increase of height, as the air becomes thinner and thinner, the number of atmospheric molecules decreases sharply, the light scattered by molecules gradually weakens, and the brightness of the sky becomes darker and darker. At an altitude of more than 20 kilometers, the scattering effect is almost invisible and the sky turns black.

When the weather is clear, there will be many tiny dust, water droplets, ice crystals and other substances in the air. When sunlight passes through the air, long-wavelength red light, orange light and yellow light can penetrate the atmosphere and directly hit the ground, while short-wavelength colors such as blue, purple and indigo are easily blocked by particles suspended in the air, thus scattering light in all directions and making the sky appear blue. In fact, the scattered blue light is only a small part, and most of the blue light and purple light that have not encountered particles still hit the earth directly, so the white light that hits the earth is still red, orange, yellow, green, blue, indigo and purple. Have you noticed that the sky will become bluer after the heavy rain? The clearer the weather, the bluer the sky. This is because there are a lot of dust particles, water droplets and ice crystals in the air in such weather.