Wonder! The sky appeared strange green. What is the reason?

Earlier, a severe storm swept through Sioux Falls, South Dakota, USA, and the sky turned a strange doomsday green.

Many people posted the photos of the “green” Amaterasu they took online, and those who didn’t know thought the end of the world had come.

Why is there a green sky?

The reason why the sky has color is that it is not completely "pure". There are many tiny dust particles, ice crystals, water droplets, etc. floating in the air. Because of the existence of these "impurities", when the sunlight penetrates the atmosphere and shoots to the ground, it will be scattered by the "impurities". If the sky is clear, there are not too many particles in the atmosphere (the diameter of gas molecules is much smaller than the wavelength of light, Rayleigh scattering occurs), and it is relatively pure, which is conducive to the scattering of short-wave light. Therefore, the shorter wavelength blue and purple light in the sunlight are easily scattered through the atmosphere and spread on the entire sky background. Since the human eye is not very sensitive to purple light, the sky looks blue.

If there were no atmosphere in the sky and no "impurities", there would be no scattering. Then during the day, apart from the scorching sun, the background of the sky would be dark.

Compared to the common blue sky, the green sky is much rarer. Of course, it is also caused by scattering, but the weather at that time was before the storm, and the scattering medium was different from the tiny impurities in the blue sky, but ice particles and water droplets.

The density of water droplets and ice particles in the storm is extremely high, almost connected into one piece, blue light and some green light, the light wave is relatively short, the penetration is poor, and it is easy to scatter. At this time, the water droplets and ice droplets in the storm are blue, as shown below:

Moreover, the sun must be setting at this time, and the setting sun on the horizon is not blocked by clouds, scattering the yellow color of the sunset. The blue of water droplets and ice droplets superimposed on the yellow color of the sunset makes the sky look green.

"Green sky" is certainly not a good sign. After all, it contains a large number of water droplets and ice particles, which means that heavy rain or hail is about to come.

This reminds me of the blood-red sky that appeared in Zhoushan in May this year. At around 8 pm on May 7, many people in Zhoushan, Zhejiang Province took photos of the magical scene of a sky full of red light.

There were many different opinions at that time, but in the end it can still be explained by the scattering of light. At that time, Zhoushan was foggy and the air was full of water droplets.

At this time, an ocean-going vessel fishing for Northern Pacific saury was anchored in Zhoushan. In order to catch saury, a red "fish-attracting light" was used, and the saury would swim towards the red light source.

The stronger the light source, the easier it is to catch a large number of saury, so the fishing boat emits a strong and bright red light. Because this is an ocean-going ship, it is about to go out to sea, so before that, the equipment is tested, including red light fish attracting lights. It happened that day that there was heavy fog, and it happened to be an ocean-going ship that was about to go out to sea. The red light shone in the fog composed of water droplets, scattering the concentrated light source and turning the sky blood red.

A few days later, a blood-red sky appeared in Fuzhou as well. On the same day, there were also distant-water fishing boats and heavy fog.

In fact, whether it is the doomsday green sky in Sioux Falls or the blood-red sky in Zhoushan, they can be explained by optical principles based on the environmental conditions at the time.

Source: Bringing Science Home, written by Su Chengyu