Why does the sky turn purple in the evening?

The sky turns purple because of the sunset.

Sunset glow refers to the colorful clouds that appear on the horizon before and after sunset in the evening. The formation of sunset glow is due to the scattering of light by air.

When sunlight enters the atmosphere, it is scattered when it encounters atmospheric molecules and particles suspended in the atmosphere. These atmospheric molecules and particles themselves do not emit light, but because they scatter sunlight, each atmospheric molecule forms a scattered light source.

Principle of formation:

The formation of sunset glow is due to the scattering of light by air. When sunlight enters the atmosphere, it will be scattered when it encounters atmospheric molecules and particles suspended in the atmosphere. These atmospheric molecules and particles themselves do not emit light, but because they scatter sunlight, each atmospheric molecule forms a a diffuse light source.

According to Rayleigh's law of scattering, light in the solar spectrum with shorter wavelengths such as purple, blue, and cyan is the easiest to scatter, while light with longer wavelengths such as red, orange, and yellow is transmitted Very capable.

Therefore, we see that the clear sky is always blue, and the light above the horizon is only yellow, orange, and red light with longer wavelengths. After these rays are scattered by impurities such as air molecules and water vapor, the sky there takes on brilliant colors.

Extended information:

Related phenomena:

Caixia

Colorful clouds. A light phenomenon similar to a rainbow that occurs in the morning and evening. It is not as regular as a rainbow, but as random as spilled paint.

It is like an abstract painting, with a hazy beauty. The clouds in the morning are generally called "Zhaoxia". The clouds themselves are dim in color and huge in shape, but the sky shows an elegant rose color; the clouds in the evening are called "Wanxia", also known as "Huoshaoyun", which are red in color and changeable in shape. The cloud body is small.

In ancient times, there was a saying that "the morning glow does not go out, but the sunset cloud travels thousands of miles", because the morning glow is mostly caused by cumulus clouds, which can easily develop into cumulonimbus clouds; while the sunset glow is mostly caused by light cumulus clouds, which are light cumulus clouds. Clouds will not cause precipitation, and generally indicate that the weather will continue to be sunny in a certain range in the next few days, which is conducive to travel.

Fire clouds

In the early morning, when the sun has just risen, or in the evening when the sun is about to set, the clouds in the sky are often red, like burning. People call this kind of red clouds Huoshaoyun, also called morning glow and sunset glow. Sometimes, if there are no clouds, a fiery red color will appear on the horizon. This is called Huoshaoyun.

Huoshaoyun is the red cloud that appears at sunrise or sunset. Fire clouds belong to the category of low clouds and are one of the phenomena of atmospheric changes. It often appears in the summer, especially around sunset after a thunderstorm, in the western part of the sky. Due to the strong evaporation on the ground, the updraft in the atmosphere has a greater effect, making the shape of the fire cloud ever-changing.

The color of Huoshaoyun is generally bright red. The appearance of fire clouds heralds the coming of a period of warm weather, abundant rainfall, and luxuriant biological growth.