The color of the sky (that is, the color of the atmosphere) is actually the composite color around the blue in the spectrum, which we call "blue". If there is no atmosphere, the sun we see is a very dazzling fireball in the dark space background, and astronauts in the space station can also see this scene, because sunlight loses most of the blue region of the spectrum through the atmosphere, and the sun we often see through the atmosphere is yellow. When blue light and yellow light are mixed together, it is white light (sunlight that has not passed through the atmosphere).
The extended data "blue sky" is also a popular saying of air quality evaluation. The environmental protection department in China should monitor the inhalable particulate matter, sulfur dioxide, nitrogen dioxide and other pollutants in the air. Summarize and calculate the real-time monitoring data to get the air pollution index (API) of the day, so as to judge the level of air quality.
The air pollution index used in China is divided into five grades, and the API value is less than or equal to 50, indicating excellent air quality; API value greater than 50 and less than or equal to 100 indicates good air quality. If the air pollution index of that day is below 100, it is called "blue sky". In other words, although the sky doesn't look blue, even if it rains and the air quality is good, it can still be regarded as "blue sky".
Therefore, the "blue sky" based on the air pollution index is not a blue sky in the sense of the senses. It is reported that two citizens insist on taking a photo every day to record the number of blue sky, and the number of photos displayed is 180 days. In the same period, the number of "blue sky" days officially released by Beijing was 285 days, with a difference of 100 days.
Baidu Encyclopedia-Blue Sky (Atmospheric Physics Terminology)
Baidu Encyclopedia-Sky (Chinese Vocabulary)