Why is the sunset sometimes red and sometimes golden?

When sunlight passes through the atmosphere, violet light with shorter wavelength will be attenuated more, and the color of "remaining" sunlight after transmission tends to red light with longer wavelength. Therefore, at sunrise and sunset when the sun's altitude angle is very low, the sun disc we see is orange-red. After being scattered by scattered particles in the sky, this reddish sunlight is still mostly light with longer wavelength. Therefore, the luminescence tends to be red, orange, yellow and other colors. And the closer to the horizon, the redder the color of Xiaxia. Near the zenith, sunlight passes through the lower atmosphere less, and the light with shorter wavelength attenuates less. There is still some blue-green light in the "surplus sunlight", so sometimes you can see the blue-green glow. Sometimes, the blue light scattered by the upper atmosphere and the red light scattered by the lower atmosphere "overlap" into people's eyes and they will see a purple sky. Generally speaking, in the direction of sunrise and sunset, from the ground to the zenith, the color arrangement of summer is red near the ground and gradually becomes orange, yellow, green and blue.

When the humidity in the atmosphere is high, or the system cloud system is close, many large water droplets will be suspended in the air, and these water droplets with different sizes have different scattering effects on various colors of light. For example, water droplets with a radius smaller than the wavelength of light waves mainly scatter blue light; While water droplets with a radius of 0.5 micron to 1 micron mainly scatter red light. Therefore, sometimes purplish red and brownish red are formed in the sky near the ground. The more water vapor in the atmosphere, the brighter the color of Xiaxia.

Aerosol particles in the atmosphere also have an important influence on glow. When the particles are large, the color of each light will become complex and will be greatly attenuated. When the particles are large (much larger than the wavelength of light, such as more than 5 microns), all colors of light have the same scattering ability, and the scattered light is still white. At this time, the glow will appear very weak, showing light yellow, reddish and gray colors. The more dust in the atmosphere, the weaker the brightness of the glow.

Clouds in the sky can reflect the glow to the ground, thus presenting a beautiful scene of "dyeing" the glow on the white clouds.

The working people in our country have always attached importance to the relationship between sunset glow and weather, and summarized a lot of experience in predicting sunny and rainy days in the future with sunset glow. For example, Xu Guangqi wrote in the Encyclopedia of Agricultural Administration:

"As the saying goes, morning and evening, no water to make tea, drought is the main reason. This is a long sunny summer. As the saying goes, "Don't go out at sunrise, but go to Wan Li Road at sunset. All this means that it will be fine after the rain. "A long time ago, there was a strong convection in the air under the sun, and there was more dust in the atmosphere. At this time, a glow appears, and the red light appears pure, indicating a sunny day. The rainy weather in most parts of China mainly comes from two directions. First, influenced by the westerlies, the systematic weather process moves from west to east. If there is morning glow in the east and thick clouds in the west, there will be systematic precipitation. The other is the convective precipitation process. With the strengthening of sunshine, air convection is enhanced, so it often moves with the sun, forming local rainfall around noon. If it is sunny after the rain, the morning glow appears in the east, indicating that the humidity in the atmosphere is relatively high. This is a sign of rain. The appearance of sunset glow generally indicates that there is no large-scale cloud in the western sky, which is a sign of systematic weather crossing the border. Moreover, due to the weakening of thermal convection at night, it is difficult to form rainfall. So there is a proverb "Don't go out at dawn, travel thousands of miles at sunset".

However, the relationship between smog and weather is extremely complicated. For example, there is a sunset glow on a sunny day after the rain, and the coastal areas are affected by typhoons. When the weather system moves from the east, rain will still form despite the sunset glow. It is impossible to fully understand the weather changes based on sunlight alone. If we can combine other meteorological elements for comprehensive analysis and accumulate a large amount of data, it will also help to forecast the weather more accurately.