The beautiful sunset can actually be predicted. When do you think it will appear again?

On May 14, the news that "a beautiful sunset appeared in Beijing" became a hot search and sparked heated discussions. On the way home from get off work, people suddenly saw the sunset and stopped to watch it, instantly healing their fatigue from the whole day.

However, this kind of romance is not a coincidence.

It is predictable

After reading this article

Let’s try to see if we can squat until the sunset~

01

Classification of Xia

The colors that appear in the sky, clouds, mountains, or buildings at sunrise and sunset are called rosy clouds. The rosy clouds at sunrise are called morning rosy clouds, and the rosy clouds at sunset are called evening rosy clouds. Depending on the object that emits the rosy clouds, they can be divided into sky rosy clouds, cloud rosy clouds, and ground rosy clouds.

Sky clouds

The glow is formed by the scattering of the sun's afterglow by air molecules and the dust, impurities and other particles suspended in them. At this time, there is no need for clouds in the sky, the whole sky will be filled with glow.

Photo by Li Jun of Tiankongxia

Clouds

The clouds are the color formed by the diffuse reflection and scattering of the sunlight scattered by the atmosphere by the clouds in the sky. Clouds covered with red glow are often called fire clouds.

Photo by Li Haisheng

Landforms

Wang Pengfei, a famous Chinese meteorologist, specifically emphasized in his treatise that the color formed by the diffuse reflection of the sun's afterglow by mountains, rivers, buildings, etc. is also the glow. The color of the glow is mainly determined by the color of the sun's afterglow, which we call it ground glow here.

Photo by Dai Yunwei

Around the winter solstice every year, the setting sun shines through the Seventeen-Arch Bridge in the Summer Palace in Beijing, forming a haze. All the bridge holes are dyed golden by the setting sun, presenting a magnificent landscape commonly known as "golden light shining through the holes."

Landforms and Haze (Golden Light Passing Through the Hole) Photographed by Wang Xiuli

02

Causes of the Haze

To understand the cause of haze, we must first understand an optical effect in the atmosphere - scattering.

When sunlight penetrates the atmosphere, all the air molecules and various particles suspended in them that are illuminated by the sunlight will become independent light sources, just like "light bulbs" that illuminate the surroundings. Because they are too small and their brightness is too weak, we cannot visually distinguish each light source and can only see the bright sky.

Without these "light bulbs" in the atmosphere, the sky outside the sun would look dark even during the day. Air molecules scatter different light differently, and the ability to scatter blue light is 3.44 times that of red light, so the sky we usually see is blue.

Scattering effect

At sunrise and sunset, the sunlight undergoes atmospheric scattering over a longer distance than at noon. Therefore, when it reaches the low altitude, more reddish components remain in the sunlight, and the orange-red afterglow permeates the low-altitude atmosphere. This is why haze appears in a clear sky.

Simple diagram of the causes of haze drawn by Dai Yunwei

In addition to the relatively simple scattering effect of air molecules in the atmosphere, there are also aerosol particles such as water droplets, ice crystals, dust, smoke particles, spores, pollen, bacteria, etc. suspended in the atmosphere. The scattering effect of these particles on light is relatively complex, and they will scatter light of different colors depending on the size of their own diameters.

When there are clouds, they play a more important role as "magicians" in the formation of the glow. They constantly change the color of the glow by changing their own shape, thickness, and the size of water droplets or ice crystals, making each glow unique and colorful.

In addition, the reflection of clouds also plays a supporting role. The sky, clouds and ground objects complement each other through various scattering and reflection, thus forming a more complex and diverse haze. At this time, it is difficult to distinguish who is reflecting and who is scattering. The complex haze is like aged wine, you can no longer tell which specific component is emitting the fragrance.

Schematic diagram of the complex causes of haze drawn by Dai Yunwei

03

How to predict the appearance of Xia?

As a weather phenomenon that occurs in the troposphere, haze, like other phenomena in the atmosphere, can be predicted in advance by combining atmospheric science theories.

In a cloudless sky, if the number of aerosol particles suspended in the air continues to increase and visibility drops below 10 kilometers, haze will form. As long as the weather forecast predicts haze during sunrise or sunset, it can be basically determined that there will be haze in the sky.

Clouds are the "outer coat" of the weather system. When a weather system that brings weather changes passes through an area, a series of clouds often appear in different locations of the weather system.

The corresponding positions of various clouds in the weather system. Rendering by Dai Yunwei

When a weather system is about to arrive or move out of the local area, it happens to appear during sunrise or sunset, so we can predict the appearance of clouds based on the movement pattern of the weather system.

At noon on May 14, the author predicted that there might be clouds in the evening based on the movement pattern of the weather system. This time, the clouds appeared on the stratus clouds at the end of the passage of a weather system.

04

The significance of haze in predicting weather

Since ancient times, humans have been paying attention to the haze and trying to find out its relationship with weather changes. Through long-term experience, many meteorological proverbs with omen significance have been summarized to predict the weather.

Because the glow is mainly formed by the scattering of light by air molecules or suspended particles (such as water droplets, ice crystals, dust, smoke particles, spores, pollen, bacteria, etc.). When the weather changes, the water vapor and clouds in the sky will change accordingly. Therefore, different glows can indirectly reflect weather changes and air quality to a certain extent.

When the weather changes, the amount of water vapor in the air increases, and some suspended aerosol particles tend to absorb water vapor to become small water droplets and continue to float in the air. The greater the water vapor content, the brighter the color of the haze, and the richer the red. The more aerosol particles there are in the atmosphere, the weaker the brightness of the haze. Therefore, the color of the haze can reflect the nature and state of the atmosphere.

"If there is morning glow, don't go out; if there is evening glow, you can travel a thousand miles." This proverb is well known to us. The morning glow here mainly refers to clouds . In the morning, the atmosphere is stable and there is little dust. If there are clouds all over the sky, it is a sign that the weather system is about to affect the local area.

The sunset glow mainly refers to the glow in the sky , which is usually formed by the moist and clean air after the passage of a weather system, so it means that the weather will be sunny.

However, sometimes when a weather system passes through, the altocumulus and stratocumulus clouds that often appear at the back of the weather system can also form colorful clouds.

The glow is mostly red, but it can also appear in other colors such as purple, gold, cyan, and green as the composition and content of aerosol particles in clouds or the atmosphere change. For example, when the fine aerosol particles emitted by industries increase, purple and other colors of glow are likely to form. Therefore, the glow can also indirectly reflect the air quality to a certain extent.

Purple rosy cloudsPhotographed by Dai Yunwei

The colors of the glow formed by different components and contents vary greatly, especially after multiple scattering and mutual reflection of particles, which makes the glow richer and more colorful. However, no matter what kind of glow appears, its essence is the initiator of the several element processes mentioned above.

Supporting Unit: Beijing Meteorological Society

Author: Dai Yunwei, Meteorological Film and Television Center, China Meteorological Administration

Editor: Dong Xiaoxian