The eerie scene that was created in the previous days in the country with the transport of African dust dominated the discussions of the days.
The phenomenon aroused the interest of the scientific community, which hastened to explain what it was that “painted” the sky red and gave this effect.
In the wake of the above, the director of EMY, Thodoris Kolidas, brought back his related post from last month, in which he tried to explain why we have more dust in recent years in our area and what is the source of the dust.
Why is more dust transported?
“Dust transport to the central Mediterranean is characterized by episodes lasting from 2 to 4 days. Central Algeria is the most frequent source area for suspended particles, as well as other possible sources are the Hoggar Massif and Tibesti mountains in northern Chad,” Kolydas explains.
Dust transported from Africa’s Sahara Desert blankets the atmosphere of Attica, Athens, on April 23, 2024
“The world’s largest source of dust is in the Bodily Basin in Chad, in the area between the ever-shrinking Lake Chad (today one-twentieth of its 1960 size) and the Sahara. The cavity releases 1.270 million tons of dust a year, ten times more than when measurements began in 1947.
“Overall across the Sahara and the Sahel at its southern border, dust volumes have increased 4 to 6 times since the 1960s. The most affected countries are Niger, Chad, northern Nigeria, Burkina Faso and Mauritania.
»During the spring season (March-April), the Sharav cyclones moving eastward along the North African coastline transport dust into the eastern Mediterranean. A typical sandstorm that has a range of 200 kilometers carries 20 to 30 million tons of dust, sometimes up to 100 million. The dust that circulates in the atmosphere worldwide reaches 2 to 3 billion tons a year.”
The “orange” veil
The sky was “painted” in the colors of orange and red not only in the capital but in many cities of the province, with the atmosphere being particularly suffocating.
Dr. Vassilis Amoiridis, Director of Research at the IAADET of the National Observatory of Athens, referred to one of the most serious episodes of African dust transport ever recorded and explained why the sky took on this characteristic reddish color.
Dust transported from Africa’s Sahara Desert blankets the atmosphere of Attica, Athens, on April 23, 2024
As he said, “sunlight in its visible spectrum, i.e. in the region where the human eye can recognize electromagnetic radiation, spans the region approximately between 400 and 700nm. The shortest wavelengths are in the blue, while the longest are in the red.
As visible light passes through the Earth’s atmosphere, it is scattered by atmospheric molecules and suspended particles. Atmospheric molecules (which are much shorter than the wavelengths of visible light), scatter the radiation inversely proportional to the wavelength (indeed to the fourth power!).
This means that blue light is scattered much more strongly, while red much less. The phenomenon was studied in the 19th century by Lord Rayleigh and is named in his honor “Rayleigh scattering”.
Rayleigh scattering is responsible for both the blue sky light and the red light we see at sunrise and sunset.
Dust transported from Africa’s Sahara Desert blankets the atmosphere of Attica, Athens, on April 23, 2024
Blue light comes from intense scattering of short wavelengths (actually purple is more scattered, however this is not visible to the human eye). At sunrise or sunset, the solar disk is low on the horizon and the radiation has a longer path through the atmosphere to reach the dawn or sunset observer respectively.
As a result of the longer optical path, blue light is scattered more efficiently, leaving only yellow and red.
In addition to Rayleigh scattering, the atmosphere is illuminated by Mie scattering from airborne particles. In particular, the red left over from Rayleigh scattering at a sunset is more scattered when we have enough suspended particles in the atmosphere.
For this reason we observe spectacular sunsets with a bright red illumination of the sky when we have an eruption of a volcano, or transport of desert dust in the atmosphere.
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