Dust satelite
Photo: NASA


“Did you notice that the sky between the clouds over Stockholm had turned white that day? Mineral dust from northern Africa had come all the way to Sweden, says Matthias Tesche, a post-doctoral scientist at ITM, as he inches forward to point at his computer screen where there are colourful clusters representing dust clouds.


In contrast to small air molecules that are responsible for the familiar blue sky, mineral dust particles are much larger in size and can scatter light at every wavelength visible to the human eye. This results in a sky with a milky white appearance even in the absence of clouds. 

Tracking desert dust with LIDAR 

The scientists detected the dust particles from Sahara with the help of a LIDAR (LIght Detection And Ranging). When tracking dust clouds, LIDARs are the only instruments that can provide information on the vertical distribution of particles in the atmosphere. LIDAR is the same as RADAR only with light. RADARs emit electromagnetic waves that are scattered when they meet a target and then the instruments measure the amount of that which is scattered. The time between the emission of the light pulse and the detection of the back-scattered photons allows us to calculate the distance between the instrument and the particles, Matthias Tesche explains. Besides information on distance, scientists use the LIDAR data to derive information about particle concentration and shape. This way they get clues on the type and likely origin of the particles they observe. ITM, in collaboration with the Department of Meteorology (MISU) at Stockholm University, has been running a LIDAR on the roof of the Arrhenius Laboratory since August 2010.

According to Matthias Tesche, a special set of conditions was at play that helped edge the Saharan dust cloud towards Stockholm. A strong high pressure system over Eastern Europe caused hot African air to move northwards over the Mediterranean. This flow pattern brought extremely high temperatures to central Europe and enabled the transport of the Saharan dust from northern Africa to Scandinavia, he explains. He continues: Normally, clouds and precipitation remove mineral dust from the atmosphere during transport but their incidental absence during the high-pressure conditions allowed for the dust to make it all the way up here.

As we pour over a computer-generated graph that plots out the distribution of Saharan dust as a function of distance from the ground (height) over time, Matthias Tesche explains that he first noted the presence of dust clouds over Stockholm on Saturday 21 June while enjoying a bowl of fresh strawberries at Överjärna gård, near central Stockholm. It turned out that the LIDAR had detected the first traces of dust early that morning already at about three kilometres above the ground. Later that day, the instrument picked up additional layers with lower dust concentrations at four kilometres above ground. By 22 June the dust clouds were gone. A change in the atmospheric flow caused the event to end in the morning of the next day, says Matthias Tesche and points to the empty space on the graph where the dust clouds had once been.

The last time Saharan dust visited the Nordic countries was in April 2011. The dust event of 2011 was strong enough to be observed from space, says Matthias Tesche. He acknowledges that Saharan dust particles, which hover above Spain, Italy, or Greece almost every year, rarely make it past the Alps as they are usually removed from the air long before they reach higher latitudes. 

Desert dust out of reach for human lungs 

What about the lungs of all the excited holiday-makers who enjoyed the warm sun outdoors on 21 June? In terms of the health effects of desert dust on people, Matthias Tesche offered some reassuring news. Dust particles are too big to be deposited in the lungs. They don't pose a threat to human health, if kept to tolerable levels, which should be the case after long-range transport. Also, most of the time dust clouds move in elevated atmospheric layers and are out of reach for the people at the surface, he says. 

To most of the Stockholm residents the Saharan dust's short-lived stay left no traces behind other than a memory of a white sky on Midsummer Day.

Text: Stella Papadopoulou