It is cool.

Atmospheric molecules and suspended particles in the sky themselves do not emit light, but when they are irradiated by sunlight, they will become light sources of scattered light. In the spectrum of sunlight, shorter wavelengths such as violet, blue and cyan are most easily scattered by atmospheric molecules and particles. Especially in autumn, the direct position of the sun has moved to the southern hemisphere, and the sunlight shining in the northern hemisphere is oblique. The distance of the sun's rays passing through the atmosphere gradually becomes longer, especially below 8 kilometers. The light scattered by molecules and particles in the lower atmosphere, mainly blue, also increases obviously, so the sky appears blue.

Feature expression

In autumn, the warm and humid airflow in the south gradually withdrew to the south, and the dry and cold air in the north invaded the south step by step. The temperature drops, the humidity drops, and the atmosphere becomes stable. On the one hand, it makes the weather clear easily, with clear skies in Wan Li during the day and starry skies at night, giving people a sense of "sky high"; On the other hand, in autumn, the temperature is low and the humidity is low, so people sweat less. Even if they sweat, they are easy to volatilize, and the clothes they wear will not stick to their skin when they are wet. For those who have just survived the hot and muggy weather, they will feel "cool", so people use "the sky is crisp" to praise autumn.