The meaning and characteristics of vernal equinox
At the vernal equinox, the direct point of the sun is on the equator, and then the direct point of the sun continues to move northward, so the vernal equinox is also called the "ascending equinox". In ancient times, it was also called "Japan-China", "Day and Night" and "Mid-spring Moon".
The vernal equinox is an important solar term, which is of great significance in astronomy: the northern and southern hemispheres divide the day and night equally. In climate, it also has obvious characteristics. Since then, the direct position of the sun has continued to move from the equator to the northern hemisphere. The day in the northern hemisphere began to be longer than the night, and the day in the southern hemisphere began to be shorter than the night.
During the spring equinox, China has entered a bright spring except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China. At this time, the climate is mild, the rain is abundant and the sun is shining, and the wintering crops in most parts of China enter the spring growth stage. [ 1]
The significance of the vernal equinox, first, refers to the equal division of day and night, each of which is 12 hours; Second, the vernal equinox is in the middle of spring (from beginning of spring to Changchun), and the spring is equally divided.
The traditional method of dividing the four seasons in our country takes the "four stands" in the 24 solar terms as the starting point of the four seasons and the dichotomy and dichotomy as the midpoint. For example, in spring, beginning of spring is the starting point, the vernal equinox is the midpoint, and the long summer is the end point.
The division of the four seasons in the west takes "dichotomy and dichotomy" as the starting point of the four seasons, such as spring as the starting point and summer solstice as the end point. Western countries are located at higher latitudes, far away from the intersection of Huangchi and Huangchi, and take "equal division to" as the starting point of the four seasons, which actually reflects the local climate better than "Li Si". The four seasons divided by "dichotomy" and "dichotomy" in the west are one and a half months later than those divided by "Li Si" in China.
Sunlight rays
On the vernal equinox, the sun shines almost directly on the equator of the earth, and the whole world is almost as long as day and night. After the vernal equinox, the direct point of the sun began to enter the northern hemisphere, and the days became longer and the nights became shorter in all parts of the northern hemisphere. The vernal equinox is an important solar term, which not only has astronomical significance: the northern and southern hemispheres are equally divided day and night, but also has obvious characteristics in climate. During the spring equinox, China has entered a beautiful spring scenery except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China. On the vast land, willows are green, yingfei grass is long, wheat is jointing and rape is fragrant.
Air pressure condition
During 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. When the long wave trough moves eastward, there will be continuous rainy and cold weather due to the intersection of cold and warm air masses. [4]
Temperature and precipitation
During the vernal equinox, the daily average temperature in all parts of China rose steadily to above 0℃ except the alpine mountain area and the area north of 45 north latitude. At this time, the cold has passed and the temperature has risen rapidly, especially in North China and Huanghuai Plain. The daily average temperature rises to above 10℃ almost at the same time as the rainy areas along the Yangtze River and the south of the Yangtze River, entering a bright spring. On the vast land, on the bank of willow green, the grass grows in Fei Ying, the wheat is jointing, the rape blossoms are fragrant, the peach blossoms are red, and Li Bai is coming in spring. South China is a scene of late spring. According to the climate law, the precipitation in Jiangnan increased rapidly at this time, and it entered the period of "peach blossom flooding" in spring; In the northeast, north and northwest regions where "spring rain is as expensive as oil", precipitation is still very little, so resisting the threat of spring drought is the main problem of agricultural production.