When sunlight hits the earth's atmosphere, blue light is most easily separated from other colors, diffused into the air, and then reflected. The light of other colors has strong penetrating ability, and shines on the earth through the atmosphere, so we can only see the blue light in the sun when we look at the sky.

At sunset, sunlight travels far in the atmosphere. Except for red light, all other colors of light can't travel that far, and they all disappear before they reach our eyes. Only red light runs farthest and can reach our eyes, so we see that the color of the sky turns red at sunset.

The air quality term "blue sky" is a common expression to evaluate air quality. 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".

Refer to the above? People's Network-Popular Science Knowledge Handwritten Newspaper

Refer to the above? Baidu encyclopedia-blue sky