Why is the sky gray?

Clouds are gray, sometimes yellow, red, black and white.

As long as the atmospheric composition remains unchanged, the sky will always be blue.

The atmospheric composition determines the color of the sky.

Sunlight passes through the atmosphere.

It is equivalent to irradiating the mixed gas of oxygen, nitrogen, carbon dioxide, rare gas and water vapor with a beam of parallel white light (air ratio), but the scale is very large.

Transparent and colorless objects do not reflect light,

Nitrogen and water vapor in the atmosphere have no effect on sunlight, because they are colorless and transparent even in high density (such as liquid nitrogen and liquid water). If there is only nitrogen and water vapor in the air, then each color of sunlight can easily penetrate the atmosphere.

But there is still oxygen in the atmosphere, and oxygen molecules reflect blue light, which can be seen from the fact that liquid oxygen is blue.

There are a lot of oxygen molecules in the air, although the oxygen content

Neutron density is sparse, but they are huge as a whole, and their overall effect on blue light is no less than that of liquid oxygen. And the farther away from them (oxygen groups), the more obvious their overall color is.

So blue comes from the sky.

No matter how the sun changes position, no matter whether the weather is good or bad, the sky will always be blue.

Scattering effect: the cup filled with milk is illuminated by light, and the color is different from different directions.

If the color of the sky is caused by scattering effect, the color of the sky will change when the sun changes its position.

The sun is red in the morning, but when the sun runs at noon, it becomes colorless because of this scattering effect. In fact, the color of the light source changes with the position of the light source.

When this effect occurs, the medium generally has the function of convex lens and concave lens (for example, the milk cup is an irregular convex lens).

So when the sun turns red, it becomes bigger, because the atmosphere also acts as a convex lens when scattering sunlight. Similar to a milk cup, it is also a convex magnifying glass when light is scattered.