Grain Rain is coming: the last solar term of spring, what is its connection with rainfall?

Before we know it, the Grain Rain solar term is almost here this year. This is the last solar term in spring, which means summer is about to begin. Do you feel that spring is not enough, or are you looking forward to the arrival of summer more?

After the Spring Equinox, the sun's direct point moves northward by about 8 degrees of latitude every month. Therefore, from Qingming to Guyu, the sun's direct point moves northward by 4 degrees of latitude (about 444 kilometers). Since the heat capacity of land is much smaller than that of the ocean, the land warms faster in spring. In this half month, the 15-degree Celsius line of average temperature in my country ran northward from the Yangtze River Basin for about 1,000 kilometers to the northern part of North China, and the 10-degree Celsius line of daily average temperature ran westward from the Taihang Mountains for about 1,000 kilometers to the Hexi Corridor. The daily maximum temperatures in Dunhuang and Yumenguan exceeded 20 degrees Celsius, and the strong spring atmosphere was already felt.

Grain Rain falls between April 19 and 21 every year. It is the sixth solar term in the twenty-four solar terms and the last solar term in spring. As the saying goes, "Snow stops at Qingming and frost stops at Guyu." During the Grain Rain solar term, temperatures across the country rise faster and even if there is cold air, the lowest temperature seldom drops below freezing.

During the spring, the continent of the Northern Hemisphere warms up as a whole, and the Qinghai-Tibet Plateau warms up significantly as well. The warming Qinghai-Tibet Plateau heats the atmosphere in the form of sensible heat. At the same time, the subtropical high pressure grows over the ocean. In the Andaman Sea in the southwest of the Indochina Peninsula, the Asian monsoon is at a critical time of outbreak. These factors jointly push the large-scale wind system in the Northern Hemisphere northward, and the warm and humid water vapor from the tropics can reach areas further north, where it meets the still strong cold air, forming the Jiangnan spring rain in the Jiangnan region. However, precipitation in the north is still scarce, so "spring rain is as precious as oil", but the rainfall is obviously more than before.

Most of the rainfall in the northern region during this period is related to the Mongolian cyclone, which is also called the northern cyclone. It originates and develops in the central and eastern plateau areas of Mongolia. It is most abundant in spring, followed by autumn, and least in winter. The reason why there are more Mongolian cyclones in spring is mainly due to the strong southerly airflow, which merges with the northern branch of the westerly airflow and easily causes the development of the baroclinic system.

The picture comes from Tuchong.com

When the Mongolian cyclone develops strongly, it will not only bring strong winds and sandstorms, but also bring precipitation to the cyclone center and the southern part of the region. The water vapor of the precipitation comes from the southern branch of the westerly airflow, as well as the southern branch of the Indian Ocean and the southwest Pacific Ocean. In the mid-latitude areas of my country, the north and south cold and warm air converge, bringing rainy weather in spring.

The name Guyu means "rain brings forth all kinds of grains", which shows that it contains the yearning of the working people for a happy life over thousands of years. Generally, at this time, the whole country from south to north is green, and all things need rain to grow. When spring rain falls, all kinds of grains grow. If the rainfall is timely and sufficient, all kinds of grains can thrive.

During the Grain Rain season, the southern part of my country is still in the spring rain period, with more cloudy weather and less sunshine. The spring flowers fall in the wind and rain, so "there are wind and rain at night, how many flowers have fallen", or "the flowers in the forest have faded, too quickly, but the cold rain in the morning and the wind in the evening" (Li Yu's "Meeting Joy: The Flowers in the Forest Have Faded Spring"), so the falling of flowers in the south will cause many people's spring melancholy and sadness, while for the north, "a spring rain brings warmth", if it rains heavily, you should sow quickly while the soil is moist. "Before and after Grain Rain, plant melons and beans", "Every family is busy planting cotton in Grain Rain", which shows that this time is the critical period for sowing and growth of crops.

However, "spring rain is as precious as oil, and if it rains three times in a row, there will be no harvest." If it rains too much, it will not be good for agriculture, because long periods of rain in the north will cause low ground temperatures, affecting the greening of plants and the sowing of spring crops, especially when plants have just sprouted, fruit trees are in full bloom, and bees are flying. After a few rainy days, the temperature drops and causes "late spring cold", and agricultural losses will be more serious, which is particularly obvious in the Northeast. In North China and the Central Plains, if the weather system is strong and easily causes hail, the harvest of the year may be ruined.

The 24 solar terms are determined according to the position of the sun. The seasons are divided according to the 24 solar terms, which are basically fixed every year and can be regarded as the four seasons in the astronomical sense. The meteorological standard of the four seasons is different. The meteorological standard of the four seasons was first discussed by Mr. Zhang Baokun. In 1934, he published an article "The Distribution of Seasons in China" in the "Acta Geographica Sinica", pointing out that 10°C is the dividing line between winter and spring, and above 10°C is the beginning of spring, above 22°C is the beginning of summer, and when the temperature drops steadily from above 22°C to below 22°C, it is the beginning of autumn. This standard has been used as the starting time of the four seasons in various parts of my country.

It can be seen that according to meteorological standards, the time when different regions of my country enter the four seasons is not the same. The south enters spring and summer earlier, while the north enters autumn and winter earlier. During the Grain Rain season, the southern part of my country has already entered summer, at least one solar term earlier. The daily minimum temperature in the Central Plains and the northern regions will occasionally drop below 10 degrees Celsius, and it is still relatively cool in the morning and evening. But no matter what, it is now the last solar term of spring and the best period of spring. As the saying goes, "A year's plan begins in spring." Let's work together for a good harvest this year.

References

Duan Anmin, Xiao Zhixiang, Wang Ziqian. 2018. Research progress on the influence of winter-spring snow cover and surface heat source on the Qinghai-Tibet Plateau on the Asian summer monsoon[J]. Chinese Journal of Atmospheric Sciences, 42(04): 755-766.

Duan Anmin, Zhang Ping, 2022. Interannual variability of atmospheric heat sources over the Qinghai-Tibet Plateau and its driving factors[J]. Chinese Journal of Atmospheric Sciences, 46(02): 455-472.

Zhang Shangyin, 1989. Relationship between cyclone activity and rainfall in northern my country in spring[J]. Chinese Journal of Atmospheric Sciences (02): 247-251.

Zhang Shangyin, 1984. Some statistical characteristics of northern cyclones and meteorological analysis of their development[J]. Plateau Meteorology (03): 83-89.

Lin Zhiguang, 2003. Spring rain is as precious as oil. Too much of it is a problem. [J]. Science News (11): 48.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Wei Ke

Reviewer: Cui Guohui (Associate Researcher, Meteorological Publicity and Science Popularization Center, China Meteorological Administration)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.