On cloudless sunny days, the sky is mostly azure blue. But if you look closely, this color is also changing at any time, sometimes darker, sometimes white. Why? As we all know, there are many tiny dust and water droplets floating in the air, which can disperse the mixed light of the sun, which is called "scattering" in optics. The scattering ability of dust and water droplets in the air is related to the length of light waves, and the shorter the wavelength of light, the greater the scattering ability. Among the sun's mixed lights, violet light has the shortest wavelength and the greatest scattering ability. When solar rays enter the atmosphere, the strongest initial scatter is violet light. However, the violet light has been scattered before it reaches the ground, and there is very little violet light close to the ground, and the blue light is the most, so we see the sky on the ground as blue, and gradually turn purple at high altitude.
If the air contains a lot of large water droplets and dust, it can also scatter yellow, red, green and other colors of light with longer wavelengths, so sometimes the sky color is somewhat whitish. Around noon, the sunlight hits the ground directly, and through the thinner layer of air, the blue light is easily scattered, so the sky is azure blue. If it is in the evening or morning, because the sun is obliquely hitting the ground, through the thicker layer of air, the blue light is scattered before reaching the ground, while the red and orange light is scattered to the ground, so the sky will turn white, and sometimes there will be a rainbow.