In Zhaosu, Xinjiang, be a "rainbow chaser"

The rainbow has been a symbol of beauty since ancient times. The rainbow is the general name for the weather phenomena of rainbow and neon. A rainbow is a circular band of light with red outside and purple inside, which is formed when sunlight shines on water droplets and undergoes refraction, splitting, total reflection, and then refraction. The maximum solar altitude angle for a rainbow is 42°22′. The smaller the solar altitude angle, the bigger and higher the rainbow; the bigger the raindrops, the brighter the rainbow. A neon is a circular band of light with purple outside and red inside, which is formed when sunlight shines on water droplets and undergoes refraction, splitting, total reflection, total reflection, and then refraction. It is darker than a rainbow, and the maximum solar altitude angle for a neon is 50°24′.

There are records of rainbows in the oracle bone script of the Yin Dynasty in my country, written as "=". Kong Yingda of the Tang Dynasty wrote in "The Book of Rites: Monthly Orders": "If the clouds are thin and the sun leaks through, the sun shines on the rain and leaks through, then a rainbow will appear." This roughly reveals the cause of the rainbow. In Europe, British scientist Roger Bacon first pointed out that rainbows are the reflection and refraction of sunlight shining on raindrops in the air. Later, Newton used a glass prism to scatter sunlight into colors and discovered the optical principle of the rainbow.

Due to thermal effects, when the sun rises in the west and the rain falls in the east, rainbows are more likely to form in the east. On the contrary, when the sun rises in the east and the rain falls in the west, rainbows are less likely to form in the west. The reason is that convection occurs in the west in the morning, and there is usually a strong typical cold front system moving eastward. The visible sky is in the ascending movement area, and the blue sky and sun are less visible in the east. There are more rainbows in the afternoon and fewer in the morning. At noon, the solar altitude angle is greater than 42°, so no visible rainbows can be formed.

Thunderstorms determine the probability of rainbows. Zhaosu is the only county in Xinjiang without deserts. It has frequent convective weather and is the area with the most thunderstorms and hail in Xinjiang. Zhaosu County is located in the Tianshan Mountains at an altitude of about 2,000 meters and a latitude of about 43°N. In summer, there are two mountain passes with inlet airflow on the east and west sides of the mountainous area. Weak cold air from the two directions often converge in Zhaosu, causing abnormally frequent convective weather. Thunderstorms mostly occur in the afternoon and evening in the summer half of the year. The higher the latitude, the lower the solar altitude angle, the more rainbows are formed, and there are more rainbows in mountainous areas than in plains.

Rainbows often appear after short-term heavy rainfall caused by strong convective clouds, that is, when a certain place is in the cold air control area in front of the trough, or in the strong front zone, and cold air continues to move southward, it is easy to form strong convection and short-term heavy rainfall, thus rainbows may appear. In Zhaosu, when the Caspian Sea and Aral Sea ridges are maintained, there are fluctuations on the strong front zone moving eastward, or when the Central Asian region maintains a low vortex, the Caspian Sea and Aral Sea ridges guide the cold air southward, it is easy to form strong convective weather, produce short-term heavy rainfall, and form rainbows.

Zhao Xizhang, 86 years old this year, still can't get over the Zhaosu rainbow. When he first started working at the Zhaosu County Meteorological Bureau in the summer of 1963, he was amazed when he encountered a rainbow for the first time. After working for a long time, he found that rainbows were commonplace here.

Unfortunately, since rainbows are not an observation item in the meteorological department's observation business specifications, no relevant records of rainbows have been found in historical archives. The local government uses the Zhaosu National Basic Meteorological Station with a wide field of view as a rainbow observation point, and the county's Lighthouse Ranch and Grassland Stoneman Scenic Area as auxiliary observation points. It also collects rainbow data through 21 regional meteorological stations; it has formulated the country's first "Rainbow Observation Specifications (Trial)", which will be released as a local standard in 2023 and will be implemented on August 17.

The local area has been conducting rainbow observations since May 2017, and has formed six years of fixed-point observation data. These data are hard-won: on the one hand, unlike precipitation, temperature, lightning, etc., which can be directly recorded by observation instruments, they can only be observed manually by observers climbing to the top of the mountain or on the tower, and are exposed to wind and rain; on the other hand, rainbows are small-scale meteorological landscapes. Sometimes a rainbow can only be seen within 5 square kilometers, and the time is short. Observers who drive to chase rainbows often return disappointed, and "catching" rainbows has become a problem. For this reason, the Propaganda Department of the County Party Committee encourages people of all ethnic groups to join in the filming and feedback work, and the "National Rainbow Chasing" activity has enriched the rainbow observation data.

According to observation records, the local meteorological department found that rainbows mainly appear from May to August, accounting for 90% of the annual number, with the most in July, mostly after 21:00. They most often appear in Xiata, Military Horse Farm, Lighthouse Ranch, Kashagar Town and other places. They are basically the moving path of rainfall clouds, forming a line from southwest to northeast.

The 30-year historical meteorological data from 1981 to 2010 from 2,381 national meteorological stations across the country were investigated, with a focus on the number of thunderstorm days, hail days, days with precipitation ≥ 0.1 mm, precipitation, sunshine hours, etc. from May to September. It was found that the meteorological conditions for rainbows in Zhaosu County are higher than other places in the country, and meet the best conditions for double rainbows.

By comparing data such as solar altitude angle, number of thunderstorm days, and air cleanliness, it is determined that Zhaosu is currently the place in the country where rainbows appear most frequently. On December 24, 2019, Zhaosu County was awarded the title of "China's Rainbow Capital" by the Public Meteorological Service Center of the China Meteorological Administration, becoming the only place in the country to receive this honor.

Thunderstorms are small- to medium-scale weather systems that are highly localized, short-lived, and rapidly evolving, making them difficult to predict. Summer afternoon thunderstorms are often accompanied by rainbows, so rainbow forecasts must be based on thunderstorm forecasts.

"Capturing" thunderstorm weather is the key to "capturing" rainbow weather. The main means of short-term weather forecasting relies on numerical forecast models, especially mesoscale numerical forecast models. It is also necessary to pay attention to information from weather radars, meteorological satellites, ground meteorological stations, etc., and grasp the subtle changes in high-altitude atmospheric circulation.

In 2018, the Rainbow Forecasting Team was established, and a forecasting technology route was formed based on the country's first rainbow database. Through numerical forecasting models, the atmospheric circulation background is mastered, and attention is paid to the occurrence, development, and evolution of summer convective weather. When judging that convective weather may occur in the next three days, attention is paid to the changes and developments of the weather influencing system. According to the weather conditions and topographical conditions for the formation of rainbows and the statistical results of historical data census, the time and place where strong convective clouds are most likely to occur are judged through weather radar and satellite cloud maps. By calculating the solar altitude angle in Zhaosu in summer, the specific time of the day when the angle is less than 42° is determined. The correlation between rainbow data analysis and weather influencing system is used to condense indicators to make a probability forecast of the time and place of rainbow appearance.

On July 5, 2019, the WeChat public account "Zhaosu Weather" released a rainbow forecast, creating a "Rainbow You Can Chase" business card. Zhaosu took this opportunity to deepen the whole-region tourism and release more energy in improving people's livelihood and helping to fight poverty.