How is the crisp autumn air formed? What's the scientific truth here?

A leaf falls and knows the autumn of the world. The cool autumn is moving forward, and the curtain of hot summer is slowly pulled down behind. From then on, the sky in Wan Li was blue, and the clouds and winds blew, cooling first and then cooling, and a picture of "crisp autumn" across Shenchuan began to splash ink and spit.

There is no biting cold in winter, I don't like the noise of spring, and I don't like the exposure and rain in summer. This season of autumn, like a fresh and ethereal fairy, like a modest and elegant gentleman, makes people intoxicated in endless tranquility and ease.

As early as the Tang Dynasty, the great poet Du Fu once said in "Cui Shi Dongshan Cottage": "Love you, Yushan Cottage is quiet and crisp in autumn." From then on, the word "crisp autumn" has become a representative word to describe the autumn weather through the circulation and polishing of time and space. It describes the clear sky and cool weather in Wan Li in autumn. There are four seasons in a year. Why only autumn has such distinctive features? What is the weather principle behind it?

The word "autumn crisp" can be divided into two parts. Let's start with "crisp autumn". The autumn sky always makes people feel deep and noble. The main reasons are as follows:

Wang Wei wrote in "Mountain in Autumn Night" that "the mountain is empty after rain and stands in late autumn", which describes the quiet autumn mountain after rain, which is an excellent description of autumn visibility. Generally speaking, the atmospheric visibility in autumn can generally reach 10 km to 30 km, which often brings the surprise of "seeing the south mountain leisurely".

After the baptism of the rainy season in summer, the settling effect of particulate matter in the air is obvious, and the dust impurity particles in the atmosphere are greatly reduced, which greatly improves the transparency of the atmosphere. Rayleigh law shows that the size of particles is related to the wavelength of scattered light waves. In autumn, there are less coarse particles such as dust in the atmosphere, which reduces the scattering ability of the sky to long-wavelength light, so the blue-violet light with short wavelength accounts for a large proportion in the sky, and the sky will be high and the clouds will be light.

Second, because the warm air flow in the south is weakened after the summer, the pressure in the mainland is changing from a low-value system to a high-value system. However, the southward withdrawal of the upper subtropical high is often slower than the southward movement of the ground high-value system, so the upper and lower altitudes are often controlled by the high pressure, which makes the whole atmosphere sink, so there is less rain and more sunny days, and the sky is densely covered with high clouds.

Third, with the transition season from summer to winter, the situation of solar radiation and atmospheric circulation entered seasonal adjustment, the subtropical high began to withdraw southward, and the warm and humid airflow weakened. For the upper atmosphere, because the moving speed of the upper air weather system lags behind that of the ground system, the upper air in the northern region is basically under the control of the subtropical high. Under the double high pressure control of "warm up and cold down", the prevailing downdraft greatly inhibits the possibility of precipitation, so it is easy to see the clear weather in Wan Li.

Let's talk about "cool breeze". There are two main reasons for the fresh air and cool weather in autumn:

First, due to the blue sky in Wan Li during the day and cloudless in Wan Li at night, the heat on the ground can be dissipated freely. In addition, after autumn, the days become shorter and the nights become longer. The heat absorbed by the sun during the day is less than the heat emitted at night, so the ground temperature gradually decreases. However, because the temperature did not drop much, it was not cold at first and the temperature was pleasant.

Second, people's breathing depends on the air pressure in the lungs and the external air pressure to generate a pressure difference through the expansion and contraction of the respiratory muscles. If the outside air pressure becomes low, people will feel tired and "stuffy" when breathing. Under the control of high pressure in autumn, you will naturally feel refreshed.

From this point of view, "crisp autumn air" is a scene under the influence of multiple factors such as air pressure, temperature, humidity and visibility.

Although the "crisp autumn" weather makes people feel very comfortable, everyone should pay more attention to sun protection, hydration and "covering up in spring and freezing in autumn".

Because the air is dry after autumn, we should pay more attention to hydrating in our daily life to prevent dry skin and lack of water. Although the days are shorter and the nights are longer after autumn, the average sunshine hours and the intensity of ultraviolet rays have not decreased due to the lack of "protection" of clouds, so we should continue to take sun protection measures. In addition, we must follow the tradition of "covering spring and freezing autumn". "Frozen autumn" means not to put on too many clothes too early during the season transition from summer to winter. Proper cool stimulation is helpful to exercise cold tolerance. In the environment where the temperature is gradually decreasing, after a certain period of exercise, it can promote the metabolism of the body, increase heat production and improve the adaptability to low temperature in winter. By the same token, when the season begins to change, the temperature is still unstable and the summer heat has not subsided. Add too many clothes too early. Once the temperature rises, you will sweat and catch a cold.