Beginning in mid-August 2020, California entered the wildfire season early. In related news, the sky in the San Francisco Bay Area on September 9, because of the continuous burning of wildfires in California, showing a doomsday-like orange-red, is the most popular among the people. , and keep reposting the photos on the Internet. Accordingly, the Emerging Technology Media Center invited experts to analyze the reasons why the sky turned orange due to forest fires. Zhu Shiwei (Professor.
Department of Physics, National Taiwan University), Li Jianxuan (Master Student, Department of Physics, National Taiwan University) photo background removing September 28, 2020 Q1. Why did the forest fires cause the blue sky to turn orange-red? There is news that it is caused by smog. Why does smog make the sky look like this? Before answering this question, we must first understand that the color of the sky as we normally see it is mainly affected by the scattering of sunlight through the atmosphere. Scattering refers to the change of direction of travel of photons after elastic collision with molecules or suspended particles in the air. Common scattering in life can be roughly divided into two types: Rayleigh scattering and Mie scattering, as shown in Figure 1. Rayleigh scattering means that when light hits a particle with a diameter much smaller than the wavelength of the light, the light will scatter in all directions, and the shorter the wavelength of the light, the greater the scattering intensity (the scattering intensity will be inversely proportional to the fourth power of the wavelength).
Mie scattering means that when light hits a particle with a diameter equal to or larger than the wavelength of light, the light will mainly scatter in the direction it originally traveled. But unlike Rayleigh scattering, Mie scattering has no fixed relationship between scattering intensity and wavelength. 1602125498-826302171-e1602128967863 Photo Credit: Zhu Shiwei, Li Jianxuan Figure 1: The relationship between Rayleigh scattering and Mie scattering particle size, light wavelength and scattering intensity (Note 1). The horizontal axis in the figure is the ratio of particle diameter to wavelength length, and the vertical axis is the scattering intensity after data normalization. The blue area is Rayleigh s