What is the climatological significance of vernal equinox?

Every year around March 2 1 in Gregorian calendar, the sun reaches 0 degrees of the Yellow Meridian, which is the vernal equinox of the 24 solar terms. A day is half of spring, so it is called vernal equinox. At the vernal equinox, the sun is directly above the equator, and the duration of day and night is almost equal, each being 12 hour. After the vernal equinox, the position of the sun gradually moved northward, and the days began to get longer and the nights began to get shorter. Therefore, the vernal equinox was also called "Japan-China", "Day and Night Equinox" and "Midspring Moon" in ancient times.

"Twenty-four solar terms" also records: "In mid-February, it is divided into 90 days and a half, so it is called minutes." Another article, Spring and Autumn Stories, Yin and Yang in and out, said: "At the vernal equinox, Yin and Yang are also half-phase, so day and night are cold and summer." Another "Li Mingshi Yi" said: "The dividing line, the point where Huang Chi meets, the sun is here, and the day and night are equally divided." Therefore, the significance of the vernal equinox has two aspects. One is to divide the day and night equally; Second, in ancient times, it was spring from beginning of spring to long summer, and the vernal equinox happened to be in the three months of spring, and spring was equally divided.

Ouyang Xiu once had a wonderful description of the vernal equinox: "At half past the vernal equinox in Nanyuan, the wind and the smell of Ma Si are like peas and willows, and butterflies are flying." Whether in the north or the south, the vernal equinox and solar terms are good seasons with strong spring, and Taiwan Province Province in China is the season of orchids in full bloom.

In ancient China, the vernal equinox was divided into three stages: "The first stage was when the mysterious bird arrived; Second, wait for Lei Nai to make a sound; Third, wait for electricity. "That is to say, after the vernal equinox, swallows will fly back from the south, and when it rains, the sky will thunder and lightning. The vernal equinox is recorded in the ancient Chinese calendar as: "Three days before the vernal equinox, the sun was at the equator. "

In the spring equinox and solar terms, the East Asian trough is obviously weakened, and the trough ridge activity in the westerly belt is obviously enhanced. From Mongolia to the northeast, there are often low-pressure activities and cyclone development. Low-pressure movement causes cold air to go south, and it is windy and windy in the north.

As soon as the spring equinox arrives, the rainfall increases obviously, and the national average temperature has stabilized at 10℃, which is the spring temperature defined by climatology. However, after the vernal equinox and solar terms, the climate is mild, most areas in the south are full of rain and sunny, and the wintering crops enter the spring growth stage. There is an agricultural proverb in Central China, "Wheat starts at the vernal equinox, and every moment is worth a thousand dollars". The temperature in most parts of the south continues to rise, but it often doesn't rain as fast as the vernal equinox. In late March, the average temperature was 13℃ ~ 15℃ in north China and 15℃ ~ 16℃ in south China. Most of the plateau has been melted by snow and ice, and the average temperature is about 5℃ ~ 10℃. This is a good season for sowing rice, corn and other crops, and afforestation is also particularly suitable. However, before and after the vernal equinox, there is often a strong cold air intrusion in South China, and the temperature drops obviously, and the lowest can be below 5℃. Sometimes a small stream of cold air will invade continuously, resulting in low temperature and rainy weather for several days, which is extremely harmful to agricultural production. According to this feature, we should make full use of the weather forecast, grasp the cold tail and warm head, and sow in time.

At this time, in the northeast, north and northwest regions where "spring rain is as expensive as oil", precipitation is still not much, and resisting the threat of spring drought is the key to agricultural production.

After the vernal equinox, the daily average temperature in all parts of China rose to above 0℃ steadily, except for the mountainous areas in winter and the area north of 45 degrees north latitude. The cold has passed, and the temperature has risen rapidly, especially in North China and Huanghuai Plain. The daily average temperature almost rises to above 10℃ with the rainy areas along the Yangtze River and the south of the Yangtze River, and it has entered a bright spring. On the vast land, the willows are green, the grass is long and the warblers are flying, the wheat is jointing, the rape blossoms are fragrant, and the flowers are pink, Li Bai and Huang. South China presents a scene of late spring. Judging from the climate law, the precipitation in the south of the Yangtze River suddenly increased at this time, and it entered the period of "peach blossom flooding" in spring.