A strange cloud in the morning glow in Beijing, did you take a picture of it?

On the morning of December 16, a fiery glow appeared in the sky of Beijing, gently connecting the silent night sky with the awakening city.

In the morning glow, there is a "flying saucer cloud", did you take a picture of it?

Photo by Wang Jianzhong

So, how is the morning glow formed? Why do clouds have so many different shapes and colors? And how are these "flying saucer clouds" formed?

How are morning and evening glow formed?

There are morning glow and evening glow. They are produced when sunlight passes through the thick atmosphere and is scattered by a large number of air molecules. The more impurities such as dust and water vapor in the air, the more obvious the color.

When there are no clouds blocking the sun, it shines directly on the ground. Sunlight is composed of ultraviolet rays, infrared rays and visible light, accounting for 5%, 43% and 52% respectively. Visible light is divided into seven types: red, orange, yellow, green, cyan, blue and purple, with wavelengths from large to small.

After sunlight enters the atmosphere, it encounters clouds, which contain "large particles" such as water droplets and aerosols, and Mie scattering will occur. Light with shorter wavelengths such as green, blue, and purple in the solar spectrum is most likely to be scattered by "large particles", while light with longer wavelengths such as red, orange, and yellow has stronger transmission ability and will be scattered in the clouds. At this time, we see the orange-red color of the morning and evening glow.

The morning and evening glow are not a single orange-red color . Due to the different diameters of water droplets and raindrops in the clouds, the glow can appear in a variety of colors, including red, orange, orange-red, yellow, and pink.

Why do clouds have such varying shapes?

Clouds are aggregates of countless small water droplets or ice crystals that are formed when water vapor in the atmosphere cools. They float in the air and form something visible to our naked eyes.

The shapes of clouds are ever-changing because the water droplets and ice crystals that make up the clouds move with the flow of the atmosphere in the clouds. The flow of the atmosphere is changing all the time, which also causes the shapes of clouds to be different.

In addition to their various shapes, clouds also come in a variety of colors. Why is that? Let’s take a look…

Why are clouds different colors?

In fact, this is related to the scattering of sunlight.

When the diameter of cloud droplet particles in the cloud is larger than the wavelength of sunlight, Mie scattering will occur. The various colors in the sunlight will be scattered evenly and eventually overlap to white, and what we see are white clouds.

But on cloudy days, the upper clouds block the sun, the sunlight reaching the lower clouds is weaker, and the Mie scattering will also be weaker, so the clouds we see will become darker.

When sunlight passes through the atmosphere, the shorter wavelength blue light will undergo Rayleigh scattering when it encounters suspended particles and be scattered into the air. In the morning and evening, because the sunlight is oblique and the propagation path in the atmosphere is longer, more blue light will be scattered.

When sunlight reaches the clouds, only the red light with a longer wavelength is left and is scattered in the clouds, so the morning and evening glow we see at sunrise and sunset are often red.

How are “flying saucer clouds” formed?

The "flying saucer clouds" we usually talk about are also called lenticular clouds. Depending on the altitude at which they appear, they may be lenticular altocumulus clouds or lenticular stratocumulus clouds. Their shape is a bit like a convex lens or an almond, and they got their name because they resemble the appearance of alien flying saucers in science fiction works.

Most UFO clouds are related to the surrounding terrain. Generally, the leeward waves of the mountains are formed when the air flows over the mountains. The leeward waves spread downstream, and the rising and sinking air currents formed work together to form UFO clouds. Some UFO clouds are also caused by the fluctuations before the front arrives.

Most UFO clouds indicate stable atmospheric conditions, which bodes well for the future, but if they are brought by a cold front, such as the lenticular clouds seen near Mount Fuji in Japan, they often herald stormy weather.

Source: Beijing News, Science Popularization China, Tadpole Musical Notation