Why is the clear sky always blue?

In sunny weather, there will be many tiny dust, water droplets, ice crystals and other substances in the air. When sunlight passes through the air, red light, orange light and yellow light with longer wavelength in sunlight can penetrate.

The atmosphere, directly to the ground, and the shorter wavelength of blue, purple, indigo and other colors of light, it is easy to be suspended in.

Particles in the air block light, thus scattering light in all directions, 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.

Blue light is the shortest wavelength in visible light. It is most easily reflected by extremely fine particles. When you see that the sky is blue, it means that the suspended particles in the air are very fine; When the sky is gray, there are more suspended particles in the air and the air quality is poor. In addition, even if there is no floating dust in the air, the thermal movement of air molecules is one of the reasons for the uneven air density, which will also make the air scatter blue light. The particles scattering blue light in the air must be smaller than one micron, mainly solid particles, so that small water droplets will evaporate quickly, while large water droplets will continue to form visible clouds and fog. Such water droplets can reflect not only blue light, but also all kinds of light.