How a dust storm turned the sun over Beijing a ‘Mars-like’ blue
- PM10 particles from cyclone over Mongolia are responsible for the rare phenomenon, caused by the scattering of light
- Concentrations of the particulate pollutant had risen to more than 999 micrograms per cubic metre by noon on Wednesday
The Beijing-based non-profit Institute of Public and Environmental Affairs (IPE) said the dust storm was likely to have originated from a cyclone that swept across south-central Mongolia and Inner Mongolia on Tuesday.
IPE data showed PM10 concentrations reached 9,355mcg per cubic metre at Inner Mongolia’s Xilingol League, one of the autonomous region’s three administrative districts.
“A dry winter and a rapidly warming spring leave bare land at the source of dust storms. Strong winds and an unstable atmosphere also contribute to the formation of dust storms.”
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Hazy spring in Beijing as lingering smog and multiple sandstorms shroud Chinese capital
The eerie sight on Wednesday prompted comparisons with science fiction on Chinese social media.
“As another sandstorm strikes Beijing this morning, we find a blue sun hanging above. It feels like we live in the movie Blade Runner 2049,” one resident said on Weibo.
To explain the event, one astronomer reached far beyond the Earth’s atmosphere to describe observations of similar light distortions on Mars.
“The rare phenomenon was similar to what happened on Mars. The physics behind it is the scattering of light,” said Zheng Yongchun, a science communication specialist with the Chinese Academy of Sciences’ National Astronomical Observatories.
Particles in the air can scatter light waves and change their paths, but will have different effects depending on the wavelength.
In sandstorms, a phenomenon called Mie scattering applies. If the particle’s diameter is close to or greater than the light’s wavelength, most of the light will be scattered forward.
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The dust particles scatter red and yellow waves forward, while the blue-violet end of the spectrum is almost unaffected because of its short wavelengths.
As a result, the red in sunlight is scattered while the blue is observed directly, making the sun appear blue and giving the sky a red hue.
A more familiar scattering effect applies when the air particles are much smaller than the light waves. In Rayleigh scattering, blue light is scattered 5.5 times more than red light.
On a clear sunny day, sunlight reaches our eyes after passing through the Earth’s atmosphere, where tiny air molecules lead to the Rayleigh scattering effect. Blue light is scattered across the whole sky, while the sun appears red.
