Saharan dust turns Athens orange

On the afternoon of 23 April 2024 a yellow-orange haze settled over Athens. Photographs of the Acropolis under a copper sky went around the world.N1N4 The haze was mineral dust from the Sahara, carried roughly 1,000 km across the Mediterranean in less than two days. Visibility fell and the authorities warned people with breathing problems to limit their time outdoors.N2

Greek meteorologists compared it with the dust storm of 21–22 March 2018, which hit Crete hardest and set records there.N3S3 The skies cleared on 24 April as the wind turned and pushed the dust north-east, over the Aegean, western Türkiye and the Black Sea.N1

The same CAMS map on four days. Darker colours mean more dust in the column of air; above 1, the sun is noticeably dimmed.

Spring dust storms in the eastern Mediterranean usually ride ahead of a low that moves east along the North African coast. Its warm southerly winds raise dust over the desert and carry it north over the sea. This event followed that pattern closely.

An upper-level trough reached down from Scandinavia to Sardinia and Tunisia. On its eastern side a strong south-westerly flow ran from Libya to Greece. Near the surface a low deepened to 999 hPa between Libya and southern Italy. Between that low and higher pressure over the eastern Mediterranean, the wind about 1.5 km up reached roughly 30 m s⁻¹ in a narrow band from eastern Libya across Crete into the Aegean: the dust's highway on the maps above. The same flow carried warm Saharan air north, so the dust arrived with unusually warm air.

The March 2018 Crete event had a similar setup: a jet streak over North Africa and a low forming over the Gulf of Sirte.S3

At the CAMS grid point nearest Athens, the daily dust optical depth reached 0.98 on 23 April. The April mean there is 0.093 (2009–2025), and in nine years out of ten the whole month averages below 0.15. A smaller pulse had passed six days earlier, on 17 April (0.45).

Satellite records show that intense dust episodes over the central and eastern Mediterranean are most common in spring, so the season was typical.S2 The strength was not: on the anomaly map, the whole of Greece and the Aegean lies far beyond the top of the colour scale.

Desert dust is not harmless. A study of 13 southern European cities found that dust raised daily PM10 on about 15 % of days, and that each extra 10 µg m⁻³ of desert PM10 went with about 0.65 % more deaths over the next two days, much as for other particles.S1 After the March 2018 event in Crete, emergency calls tripled and hospital visits for breathing problems rose about 3.5 times; flights were cancelled and solar power output fell.S4

In the CAMS reanalysis, surface PM2.5 near Athens peaked a day or two after the dust in the column: 57 µg m⁻³ on 24 April and 71 on 25 April, almost five times the WHO 24-hour guideline.G1 On 23 April much of the dust was still aloft. Station measurements would show how closely the air at street level followed this timing.

The same southerly winds also fanned early-season wildfires in southern Greece.N1

NASA's MERRA-2 is built independently of CAMS and measures the dust differently: the mass of dust in the column rather than how much it dims the sunlight. Both put the core of the plume over Greece and the Aegean on 23 April, which makes the picture more trustworthy than either alone.