Why is the sky blue in clear and cloudless weather?

Hello! This is because sunlight is composed of seven colors: red, orange, yellow, green, cyan, blue and purple. The wavelengths of these seven colors are different. Compared with other photons with longer wavelengths, dust and other particles in the atmosphere scatter blue light more, so the sky looks blue.

The atmosphere itself is colorless, and most of the suspended matter in the atmosphere is not blue. The blue of the sky is a picture created by atmospheric molecules, ice crystals, water droplets and sunlight. When sunlight enters the atmosphere, long-wavelength colored light, such as red light, has great transmission power and can penetrate the atmosphere and shoot to the ground; However, violet, blue and cyan light with shorter wavelength will easily scatter when it touches atmospheric molecules, ice crystals and water droplets. Scattered purple, blue and cyan light fills the sky, making it appear blue.

When sunlight shines from the sky, the air that must penetrate contains many tiny particles, besides nitrogen and oxygen, there are other gas particles and tiny floating particles, which come from automobile exhaust, factory smoke, forest fires or volcanic ash. Although oxygen and nitrogen particles are only one millionth of a drop of rain, they can still block the path of sunlight. The light bounced back from these small stumbling blocks and naturally changed direction.

According to the determination of scientists, the wavelengths of blue light and purple light are relatively short, which is equivalent to "wavelet". The wavelengths of orange light and red light are relatively long, which is equivalent to "big waves". When encountering obstacles in the air, blue light and purple light are "scattered" everywhere because they can't cross those obstacles, covering the whole sky-the sky, so they are "scattered" into blue. The scientist who discovered this "scattering" phenomenon is called Rayleigh. He discovered it about 130 years ago, and he is also a Nobel Prize winner. This scattering is called Rayleigh scattering.

With the scattering phenomenon, the following astronomical phenomena can be explained:

For example, the sky above you is blue, but on the horizon, where heaven meets the earth, the sky looks almost white. Because sunlight travels far from the horizon to where you are in the air than it falls directly from the air-and it will naturally eat more particles along the way. These large particles scatter light many times in this way, so it appears light blue in white. You can do a small experiment to verify it: take a glass of water, put it in a dark background, put a drop of milk, and then illuminate one end of the cup with a flashlight and get close to it. The light of a flashlight will appear light blue in water. If you put more milk into water, the water will be whiter, because the light is repeatedly scattered by many milk particles, and the result is white. It is as white as the horizon. This kind of scattering is called "Dindar scattering". Unlike Rayleigh scattering, it is still white light after scattering.

When the sun goes down in the evening, the sky turns red instead of blue, and the setting sun turns dark red. As the sunset met countless particles on the way to your place, most of the sunlight was scattered in all directions, leaving only a little orange-red light visible to the naked eye-because of their long wavelength, they have crossed the obstacles on the road.

In addition, the sky will appear dark blue after sunset. This used to be a strange thing that scientists cared about, but several physicists solved the mystery 50 years ago: the blue color that caused the dusk sky was a special substance. This special substance gathers into a thick layer at a height of 20 to 30 kilometers from the earth's surface, which is called the ozone layer. This gas acts as a color filter for the falling sunlight: it intercepts the yellow and orange parts of the sunlight, but lets the blue part pass almost unimpeded. When the last light disappears, all the colors disappear in the night.

The sky is blue. In fact, it is the same from the outside of the earth: the ocean, even the atmosphere, emits blue light. Although the land is brown and the forest is green, the sky is always blue-from the perspective of the universe, the whole earth is wrapped in a soft blue veil.