Why is there no frost in the first frost?

Today is the first frost, and the last solar term in autumn has come. According to the "twenty-four solar terms", it should be "qi stagnation, dew like frost", which is the transition from autumn to winter.

Why doesn't the first frost frost?

When the messenger "first frost" comes in early winter, Shencheng looks like autumn: crisp autumn, blue sky and white clouds, colorful leaves ... Are the 24 solar terms inaccurate? In fact, the accuracy of the 24 solar terms is limited by geographical conditions. The climatic characteristics of the 24 solar terms are formed according to the characteristics of the Yellow River Basin.

In the first frost solar term, frost flowers will appear in many places in the north and gradually enter the winter; East China is autumn; The south is a transitional period from summer to autumn. The word "first frost" is closer to the climate characteristics of North China. As early as half a month ago, parts of the northeast and northwest have entered the winter. For example, Shenyang officially entered the winter as early as June 65438+ 10/0.

Generally speaking, during the first frost solar term, the characteristics of crisp autumn in Shanghai are more obvious, with an annual average temperature of 17.0℃ and an average precipitation of 25.0 mm (represented by Xujiahui Station, from 65438+1October 23rd to165438+1October). The temperature is mostly between 13 ~ 2 1℃, and the highest temperature during the day rarely exceeds 25℃, which is just in line with the weather situation this week.

In this "warm" weather, Shanghai is "first frost" without frost, so when can the "frost" in Shanghai come? Zhang, chief service officer of Shanghai Meteorological Service Center, said that although the weather is cold, there is far from frost in Shencheng at this time. According to statistics, the average first frost date in Shanghai will be165438+1late October.

Autumn is always bluer?

The ancients left many classic sayings in solar terms in the first frost, such as Heshui Zuo's "Every autumn and dusk dew becomes frost, which is somewhat condensed into yang". As the poem says, the color of the sky is always clearer in autumn, the sunset in the evening is redder and the sunset is brighter. In fact, the sky always seems bluer during the day.

Li Yan, a senior engineer in the Meteorological Service Center of China Meteorological Bureau, once said that "the air is crisp in autumn" is the result of the comprehensive effects of air pressure, temperature, humidity, visibility and other factors. After concentrated "scouring" in summer and rainy season, the settling effect of particulate matter in the air is obvious, the dust impurity particles in the atmosphere are greatly reduced, and the atmospheric transparency is greatly improved. In autumn, the air is controlled by high-pressure downward airflow, which also inhibits the upward movement of pollutants.

Therefore, the breath of autumn is the purest among the four seasons, and the air is the freshest. According to Rayleigh scattering law, due to the reduction of coarse particles such as dust and water droplets, the proportion of blue-violet light with shorter wavelength in the sky increases obviously, so the autumn sky is bluer and higher.

Although the "crisp autumn" weather makes people feel very comfortable, the days get shorter and the nights get longer after autumn. Due to the lack of "protection" of clouds, the average sunshine hours have not decreased, and the intensity of ultraviolet rays has not weakened, so we should continue to take sun protection measures. At this time, the air is relatively dry, so it is necessary to replenish water in daily life to prevent dry skin and lack of water.