news

"Why does it always start raining in the evening?" Many citizens have discovered that rainfall in Beijing during the flood season often occurs in the evening or at night. Could it be that "night rain" is God's preference during the flood season?

This feeling is indeed supported by scientific data. And this "night rain" phenomenon in summer is not only seen in Beijing, but also in the entire Beijing-Tianjin-Hebei region.By analyzing precipitation data of cities in the Beijing-Tianjin-Hebei region from 2005 to 2023, the meteorological department found that during July and August, the period of peak precipitation in most cities was concentrated between 19:00 and 22:00.Among them, precipitation in cities along the Taihang Mountains such as Chengde, Zhangjiakou, and Shijiazhuang mostly occurs in the morning to noon; while in cities located in plain areas such as Beijing, Tianjin, Cangzhou, and Tangshan, the "night rain" phenomenon is more obvious.

The reporter learned that through the statistics of the meteorological department on the cumulative number of days with rainfall in different periods of July and August in various cities in the Beijing-Tianjin-Hebei region, it can be clearly seen that frequent "night rain" is one of the important characteristics of summer rainfall in the North China Plain.So why is the phenomenon of “night rain” so frequent in summer?

The meteorological department explained that during the rainy season, rainfall in the Beijing-Tianjin-Hebei region is mainly affected by the East Asian summer monsoon. After the summer monsoon arrives, the position of the Northwest Pacific subtropical high pressure (hereinafter referred to as "subtropical high") is biased to the north, and a low-level jet stream will be generated on its west side.The strengthening of low-level jet streams at night is the key reason for the occurrence of "night rain".

During the day, affected by solar radiation heating, the ground temperature is high, the atmospheric turbulence activity in the boundary layer is relatively vigorous, and the mixing effect with the upper air is strong. This turbulence is equivalent to exerting frictional resistance on the air, causing the intensity of the low-level jet stream to weaken. At night, the boundary layer air is relatively stable, the turbulent activity is weak, and the friction is significantly reduced, so the low-level jet stream is strengthened.When the low-level jet stream strengthens at night, the Beijing-Tianjin-Hebei region, which is located at the northwest edge of the subtropical high and in front of the left side of the low-level jet stream, is located in the low-level air convergence area. Due to the rising air, the phenomenon of "night rain" is likely to occur.

In addition, the terrain distribution also has a certain impact on the intensity of the low-level jet stream. From southern Hebei to Beijing, there are Taihang Mountains to the west and plains to the east. Therefore, a thermal circulation similar to valley wind will be formed. At night, the air in the mountains sinks and the air in the plains rises, forming a temperature gradient. Through the effect of atmospheric thermal wind, the low-level jet stream is enhanced, which in turn promotes the occurrence of night rain.

Although we have bid farewell to the main flood season from the end of July to the beginning of August, which is the main rainfall period, the Beijing-Tianjin-Hebei region is still in the flood season.According to the forecast, Beijing will have frequent thunderstorms this week, so we still need to be alert to the impact of rainfall. Please pay attention to the weather forecast and warning information released by the meteorological department, prepare rain gear in advance, and pay attention to the impact of nighttime precipitation on traffic. When walking, be careful to avoid flooded sections of the road. When driving, be careful to reduce speed, drive slowly, keep a safe distance, and try to park in a high place. Do not park your vehicle next to tall objects such as trees, billboards, and telephone poles.

Source: Beijing Daily Client

Reporter: Luo Qianwen

Process Editor: U022