The sun is blue again over Beijing! Why does the sun turn blue when there is a sandstorm?

In recent days, the sky in Beijing has been gray. China Weather said that the largest sandstorm in the north this year may hit . When you went out this morning, did you feel the "wind blowing up the sand"? If you look up at the sun, you will find that the sun, which originally emitted strong white light, is emitting blue light, becoming a "blue sun"**. What's going on?

The blue sun over the Beijing Science Center this morning

1

Why does the sun turn blue?

This has to start with the scattering of light. We all know that light will scatter when it propagates in a medium. When sunlight hits the atmosphere, the seven colors of visible light it contains (red, orange, yellow, green, cyan, blue, and purple) will be scattered to varying degrees. When the diameter of the medium particles is smaller than the wavelength of light, the light wave will undergo Rayleigh scattering , and the scattering intensity is inversely proportional to the fourth power of the wavelength, and the intensity is equal in the forward and backward directions. That is to say, relative to red light with a longer wavelength, blue light with a shorter wavelength will be scattered more strongly, which is why a clear sky appears blue.

Source: pixabay

If there are a large number of particles in the atmosphere whose diameters are close to or larger than the wavelength of light, Mie scattering will occur in visible light. The scattering intensity of Mie scattering in the front and back directions is not equal (the scattering intensity in the front is stronger), so it will show obvious directionality. At the same time, it is also colorless (various colors of light will be superimposed together to appear white). So when the sky is full of tiny water droplets, the sunlight will make the entire sky appear grayish white due to Mie scattering - that is, the color of the sky on a cloudy day.

Source: Rendering Outdoor Light Scattering in Real Time

The sky is also gray during sandstorm weather, but when the diameter of the sand is in the range of 0.6~0.8mm, the scattering and absorption ability of the sand for long-wavelength colored light such as red and orange light will be enhanced, greater than blue light . Therefore, around the "big light bulb" of the sun, the blue light will remain and be captured by our human eyes.

In other words, the appearance of Blue Sun is often accompanied by sandstorm weather. So friends who like to use Blue Moon may not like Blue Sun anymore, after all, it may dirty the clothes that were just washed with Blue Moon last night.

2

Mars also has the same "blue sun"

In fact, the rare "blue sun" we see on Earth also exists on Mars.

As we know, Mars is called the "Red Planet" . If you are on Mars, you will see an endless expanse of orange-red Gobi. The surface of Mars is covered with thick iron oxide minerals, which reflect red-orange light, which is why the surface of Mars appears red.

Source: pexels

Due to the lack of vegetation coverage and reinforcement, these red-orange sands will be blown up by the strong winds on Mars to form large-scale sandstorms, which will throw these coarse sands into the sky ten thousand meters high. Even if there is no wind, the sands will not fall, but will form a thick barrier over Mars, making the Martian sky appear gray.

As we have learned in the previous article, in such a climate environment, sunlight will be scattered by the dust in the sky, leaving a mysterious blue light, dyeing a large area of ​​the sky blue. The photo of the blue sun on Mars sent back by Curiosity has caused a heated discussion on the Internet. After everyone has learned about the climate of Mars and the causes of the blue sun, the appearance of the blue sun is naturally linked to environmental degradation. Therefore, every time a blue sun appears on Earth, it will more or less cause some discussion on environmental issues.

Martian Sunset

Source: Sina Exploration

3

Will the sun still be green?

In addition to the "blue sun", there is also the "green sun". But because the green color appears for too short a time each time, this phenomenon is also called green flash .

In November 2021, a "green flash" spectacle appeared in Xiamen, Fujian, which was hotly discussed among netizens for a while.

"Green Sun"

Source: Xiamen Radio and Television

Green flash is indeed rare, so rare that many people have never even heard of it, but its cause can also be explained by current science.

Newton once conducted a famous lens experiment. He used a prism to decompose white sunlight into seven different colors of light. This is called the dispersion of sunlight. The earth's atmosphere can actually be regarded as a huge lens that can refract sunlight and disperse it. When the sun is at the edge of the horizon, the light it emits is almost at the same level as our line of sight, and the dispersion is most obvious at this time. At this time, short-wave light such as blue and purple light is scattered by the atmosphere and is difficult to reach our eyes directly. Only red and orange light with a longer wavelength can penetrate the dust and atmosphere, making the sky appear warm orange-red.

At sunset, the red and orange light will sink into the horizon first. Among the remaining colored light, short-wave light such as blue and purple light is difficult to be detected by the human eye due to scattering. Only green light can still maintain a certain degree of penetration. Therefore, when the sun is about to sink, green light will briefly appear at the top of the sun , flashing for a few seconds and then disappear.

This optical phenomenon is extremely rare and the conditions for its occurrence are very harsh. It can only be observed with a small probability in weather with extremely high air transparency and in places with an open and unobstructed field of vision. If anyone has ever seen this strange sight, it can only be said that they are really lucky.

As an uncommon optical phenomenon, the appearance of the blue sun is often accompanied by dust all over the sky, which looks mysterious and annoying, while the phenomenon of the sun's green flash indicates that the local environment is clean and the air quality is good. As a "great contributor" to the shaping of the entire biosphere of the earth, every subtle change on the face of the sun may have secrets worth studying.