The blood-red colored sky during twilight over Germany in mid-March and the dense, very high cirrus clouds on these days were the result of an extraordinary weather event: high concentrations of Saharan dust in the troposphere.

It is very rare for such large amounts of dust to be transported from the Sahara as far as northern Europe.

In the Spanish region of Andalusia, record values ​​of up to 3700 micrograms of fine dust per cubic meter of air were measured.

Even above the Hohenpeißenberg weather station in Bavaria, the concentration was still 170 micrograms and was therefore more than three times as high as the limit of 45 micrograms recommended by the World Health Organization as still “healthy”.

It is not unusual for dust from the Sahara to be blown as far as Central Europe by high-level winds.

The North African desert is the world's largest source of aerosol particles in the troposphere.

It is estimated that more than 1.8 billion tons of Saharan dust are released into the air every year due to wind erosion.

According to measurements by the German Weather Service on Hohenpeissenberg, part of it can be measured above the weather station on average about 30 days a year, especially in spring - but almost always only in low concentrations.

In mid-March, however, a storm southwest of the Ahaggar Mountains in Algeria and Mali kicked up very large amounts of Saharan dust, which was then blown from the Celia low-pressure system towards the Iberian Peninsula.

Another low pressure area then transported even more to Central Europe and Scandinavia, from where the dust then spread clockwise towards Eastern Europe and finally to the Black Sea.

Closed ceiling of cirrus clouds

The special thing about it was not only the large amount of fine dust in the atmosphere, but also that it had penetrated the entire troposphere.

In the higher tropospheric layers, the particles reacted with the water vapor.

The dust particles served as sublimation nuclei for the water molecules, which froze on them to form small ice crystals.

So many ice crystals formed that they formed cirrus clouds, which in turn formed a dense cloud cover in the upper troposphere.

On the weather images from the American earth observation satellite Terra and the European Sentinel-3 satellite, these cirrus clouds can be seen as a closed cloud cover, which extends in the east from the Balkans via Poland to Finland and at the same time obscures the sun over the whole of Germany, France and the Iberian Peninsula .

The temperature changes in the high troposphere triggered by the massive sublimation produce their own air movements, which meteorologists call "dust-induced baroclinic storms".

This rare weather phenomenon was only described for the first time a few years ago and since then has only been observed and measured a few times over the Gobi desert and the Sahara.

So far, one of these storms in March 2015 has been considered the strongest of these events.

At that time, the cirrus clouds initiated by desert dust stretched as a dense layer over 3500 kilometers from the eastern Mediterranean to Asia.

Although the cirrus cloud cover, also sublimated on Saharan dust, did not extend as far in March this year, it may have contained even more dust than the storm seven years ago.