Today (September 7), which marks the solar term of Baengno, is 'Blue Sky Day,' aimed at raising awareness about the importance of clean air.
The government held the 7th Blue Sky Day ceremony in Seoul, emphasizing the significance of improving air quality and international cooperation. The 'International Day of Clean Air for blue skies' was established as an official UN observance at the 74th UN General Assembly in December 2019, following a proposal from Korea. Since 2020, it has been celebrated annually on September 7, with this year's theme being 'Action for Climate and Clean Air.'
On Blue Sky Day, attention is drawn to the commonly described 'high and blue autumn sky.' There is a scientific reason why the autumn sky appears clearer and more vivid than in other seasons.
The blue color of the sky is due to sunlight scattering in the atmosphere. Among the various colors of light that make up sunlight, blue light, which has a shorter wavelength, scatters more strongly in all directions when it collides with atmospheric molecules like nitrogen and oxygen, compared to red light. This phenomenon is known as 'Rayleigh scattering.' The scattered blue light enters our eyes, making the sky appear blue.
In autumn, this blue light can appear even more pronounced. The atmosphere becomes drier than in summer, leading to reduced water vapor and less influence from relatively larger particles in the air. When there are many particles like water vapor or dust, light of various wavelengths scatters together, making the sky look white or hazy.
The transition from the frequent rains of summer, which wash away dust and particles from the atmosphere, to the dry autumn air also contributes to clearer skies. As haze decreases and visibility improves, distant objects become clearer, making the sky feel higher and deeper than it actually is.
However, it is not always possible to see a deep blue sky in autumn. The color of the sky and visibility can vary significantly depending on the current atmospheric conditions, including fine dust, yellow dust, humidity, and cloud cover.
* This article has been translated by AI.
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