Why is the sunny weather in autumn called "crisp in autumn"?

Autumn is not as gray as winter and spring, nor as dark clouds cover the sun as summer, but a blue sky with a few white clouds occasionally. The temperature is neither too hot nor too cold, which is very pleasant. So people often praise it with "crisp autumn".

After autumn, in the northern hemisphere, around the summer solstice (June 2 1 in the Gregorian calendar), the sun shines directly on the ground and gradually becomes oblique, and the time of day gradually becomes shorter and the time of night gradually becomes longer. In autumn, the heat absorbed by the ground is gradually less than that in summer, and the water evaporated from the ground into the air is also reduced, so there are fewer clouds in the air. Even if there are clouds, it is a faint layer with good visibility, forming a scene of blue sky and Wan Li.

In addition, after the autumn, the warm and humid air flow that caused the sultry weather in midsummer has moved southward and withdrawn from the mainland, while the cold air gathered in Siberia and Mongolia has invaded most provinces in China, driving away the original warm and humid air flow. In the place occupied by cold air, in addition to the temperature drop, the water vapor in the air also decreases, and the water vapor on the ground rises less, and the sky becomes sunny and partially cloudy. As a result, Wan Li is clear in the daytime and starry at night, which makes people feel that the autumn sky is especially Gao Shuang.

Due to the blue sky in Wan Li during the day and cloudless in Wan Li at night, the heat on the ground can dissipate freely. In addition, after autumn, the days are short and the nights are long, and the solar heat absorbed during the day is not enough to make up for the heat emitted at night, so the ground temperature gradually decreases, but the range is not large, so it is not cold at first and the temperature is pleasant. At the same time, people sweat less after autumn, even if they sweat a little, because of the lack of water in the air, the dry and cool air makes the sweat on people evaporate quickly, and the clothes worn on them have a cool feeling, thus giving people a feeling of "refreshing".