The high temperature has left the "group chat", when can we feel the cool and refreshing autumn weather?

This summer, most parts of my country experienced the hottest summer since 1961, with the regional high temperature intensity reaching the highest level since meteorological records began. The Central Meteorological Observatory issued high temperature warnings for 35 consecutive days, breaking the historical record; 40°C+ has become the standard in Sichuan and Chongqing, with the highest temperature extreme comparable to Turpan; the entire Yangtze River basin has experienced the most severe drought since 1961, with a rare "main flood season drought"...

This year's high temperatures are not only explosive but also very long-lasting. The Sichuan Basin, Jianghan, Jianghuai, Jiangnan, South China, and the Turpan Basin in Xinjiang have accumulated more than 20 days of high temperatures, among which the eastern part of Sichuan, the central and western parts of Chongqing, Hunan, Jiangxi, the northwestern part of Fujian, Zhejiang, and Turpan in Xinjiang have accumulated more than 30 days of high temperatures.

(Photo source: veer photo gallery)

Fortunately, the dog days of summer have ended, and many places have also withdrawn from the high temperature "group chat". According to the forecast of the Central Meteorological Observatory, starting from August 31, the continuous high temperature weather process in southern China will end, the summer heat will subside, and the cool and refreshing autumn weather is just around the corner.

From a climatic perspective, when will autumn arrive in most parts of my country?

In fact, there are standards for the beginning of autumn. It is not scientific to judge it based on physical sensation alone. In terms of meteorology, the standard for the beginning of autumn is that after the beginning of autumn (around August 7), the average daily temperature is below 22°C for 5 consecutive days. The first day of these 5 days is designated as the beginning of autumn. Generally speaking, the earliest places in my country to enter autumn are Heilongjiang and northern Xinjiang, where autumn begins in mid-August. Beijing only begins to feel autumn in early September. As for southern regions such as Jiangxi and Hunan, autumn does not officially begin until October.

(Photo source: veer photo gallery)

Why do we feel the cool and crisp weather when autumn comes?

This is because autumn's unique weather systems cause the skies to be higher and clearer.

As the transition season from summer to winter, autumn is the season when solar radiation and atmospheric circulation enter seasonal adjustments, subtropical high pressure begins to retreat southward, and warm and humid air currents weaken. As the cold air strengthens in early September, the lower atmosphere in most parts of northern my country is controlled by the continental cold high pressure.

(Photo source: veer photo gallery)

As for the upper atmosphere, the movement speed of the high-altitude weather system lags behind that of the ground system, so the high altitude in the northern region is still under the control of the subtropical high pressure. Under the control of the double high pressure of "warm above and cold below", the prevailing downdraft greatly suppresses the possibility of precipitation, and clear weather takes the opportunity to frequently "steal the spotlight", and it is very easy to see the blue sky.

In addition, human breathing depends on the pressure difference between the air pressure in the lungs and the outside atmospheric pressure through the contraction and expansion of the respiratory muscles. If the outside air pressure becomes low, breathing will feel laborious and "stuffy". During this season, the lower atmosphere is controlled by cold and high pressure, which will give people a "refreshing" feeling.

At the same time, the days are getting shorter and shorter, and the solar radiation received by the ground is significantly less than in summer. At night, due to the less cloud cover in the sky, the radiation cooling from the ground to the atmosphere is more obvious. As a result, the solar heat absorbed by the ground during the day is not enough to make up for the heat emitted at night, so the ground temperature gradually drops. It gradually drops from more than 30℃ in summer to 20℃ to 26℃, which is a more comfortable temperature for the body.

Not only has the temperature dropped, but in early autumn, the Mongolian high pressure controls my country's northern region. However, since the center value of the high pressure is relatively low at this time and the pressure gradient is not large, the northerly wind generated is not strong. Therefore, the wind that appears is mostly a pleasant cool breeze, and the sweat on the body evaporates quickly, giving people a "dry" feeling.

After the beginning of autumn, the warm and humid air that caused the hot and humid weather in midsummer has moved southward and gradually withdrawn from the mainland, while the cold air gathered in Siberia and Mongolia is "eager to try", invading southwards and "harassing" the original warm and humid air. In the places occupied by cold air, in addition to the lower temperature, the water vapor in the air is also greatly reduced. In addition, the cold high pressure "prevents" the water vapor on the ground from rising. The air humidity in early autumn generally does not exceed 60%. Under this humidity condition, the human body feels relatively dry.

(Photo source: veer photo gallery)

Finally, the high blue sky is also one of the reasons for the feeling of clear and refreshing autumn. After the concentrated "washing" of the summer rainy season, the particles in the air have a significant sedimentation effect, and the dust and impurity particles in the atmosphere are greatly reduced, which greatly improves the transparency of the atmosphere. In autumn, the air is under the control of high-pressure sinking airflow, which also inhibits the upward movement of pollutants. Therefore, the atmosphere in autumn is the purest and the air is the freshest among the four seasons.

The real autumn is coming, prepare your mood and steps!

Produced by: Science Popularization China

Author: Liu Jia (China Meteorological Publicity and Science Popularization Center, China Meteorological News Agency)

Producer: China Science Expo