Saharan dust

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Satellite image: Saharan dust outbreak over the Canaries and Azores

The Sahara dust is the dry dust of the Sahara that is suspended from the blowing wind and as aerosol large distances in the earth's atmosphere can travel.

Global importance

The Sahara desert is one of the most important sources for the distribution of mineral dust particles. This dust is transported across the Atlantic to the Amazon Basin , the Caribbean and the northeastern coast of America, and in the southwestern direction across the Guinea coast , and across the Mediterranean to Europe and the Middle East.

Today it is assumed, for example, that the nutrient-poor rainforest of the Amazon is primarily supplied from the Sahara. In addition, it is assumed that the Sahara aerosols play an important role as condensation nuclei over the central Atlantic and thus form a factor in the formation of hurricanes and the course of a hurricane season.

Emergence

Summit of the Hörndlwand on February 21, 2004. An extreme foehn weather with wind peaks of up to 150 km / h in connection with a severe storm in North Africa makes for a sky in the Alps that is colored red by desert sand.

The Sahara is not a desert because it is too poor in nutrients, but too dry. Where the Central Sahara is today, a huge freshwater lake stretched until the end of the Ice Ages (the Würm Ice Age ended around 10,000 years before today). This extremely fertile area, including humus soil , literally turned to dust when North Africa got into the dry Kalmenzone . In addition, large amounts of rock were then processed into the finest particles due to severe weathering of the unprotected surface.

The factors for the formation of the Saharan dust are the high ground temperatures and the resulting thermal turbulence . Through this the dust is whirled up and remains even at heights of up to 5000 m. A cooling of the ground causes the formation of a layer of air that does not allow dust particles (particle size on average between 1 μm and 74 μm) to fall to the ground. If no wind takes these particles with them, they can remain in the air for up to 6 months.

Distribution of nutrients

Around 500 million tons of dust are produced in the Sahara every year. Due to the proportion of nutrients such as calcium and magnesium, the Saharan dust plays a role in supplying the forests. For example, 40 million tons of dust reach the Amazon rainforests every year. But Saharan dust also makes an important contribution to atmospheric fertilization on the Iberian Peninsula .

Meteorological-climatological aspects and measurements

Sirocco wind1.jpg Sirocco from Libya.jpg
Scheme and satellite image of a Scirocco / Chamsin location: left Mediterranean low and southern currents from the Sahara air to Europe, right broader southwest high-altitude currents.
Saharan dust over Biscay and the Celtic Sea

The fine dust of the Sahara is primarily and continuously transported to South America by the global trade wind drift of the tropics, but mainly in winter. On the Guinean coast this dust-laden northeast trade wind is called Harmattan ( Harmatta , 'haze time'), over the Canary Islands it is called Calima .

Saharan dust events in Europe are caused by strong south / south-west currents in front of Mediterranean lows ( Scirocco and relatives), especially when powerful cold air advances bring storms to North Africa on their back, and are also transported over the Alps by high-altitude currents and southern foehn . The wind that transports the dust to Southeastern Europe and the Middle East is the Chamsin of the Eastern Sahara.

The dust reaches heights of several kilometers and generally needs between two days and a week before it reaches the Alpine region. Such phenomena occur in Central Europe - like the weather-effective Mediterranean lows - increasingly in spring (March to June) and in autumn (October and November), a total of around 10 to 30 times a year, striking events with a more spectacular view of the sky only a few times a year . They usually only last a few hours (almost half of all events), but at least a quarter lasts at least a full day, which amounts to around 200–650 hours of immission per year on the main Alpine ridge. The average amount of dust is around 1  µg per m³ and h. Overall, the Saharan dust contributes a quarter of the total aerosol mass concentration in the high mountains (measuring point Jungfrau) on an annual average, while it no longer has a significant share in Hamburg, for example.

The iron-rich dust particles can leave clearly rust-brown deposits when it rains, which is why the term “ blood rain ” has historically been used for such events. In the 19th century, a ring of dust in the atmosphere around the whole earth was suspected as a source, only with investigations of a powerful dust storm 9–12. March 1901 G. Hellman and W. Meinardus were able to clarify the causes. Today the area of ​​origin can be determined very precisely by means of trajectory analyzes (back calculation of the flow courses).

MeteoSwiss has been running continuous measurements of dust precipitation at the high alpine research station Jungfraujoch since 2001 , which means that a meaningful time series is now available. The dust can also be measured with LIDAR instruments, where the single scattering albedo is registered with a laser. For this purpose, MeteoSwiss takes measurements in the aerological station in Payerne .

The Austrian Meteorological Institute ( ZAMG ) calculates the occurrence of Saharan dust and blood rain for Europe on the basis of the WRF Chem model and also provides a 3-day forecast online.

See also

Sahara dust deposits on the snow above the Simplon Pass

Web links

Commons : Saharan Dust  - Collection of images, videos and audio files

Forecast:

  • Saharan dust . 3-day forecast Europe ( WRF / Chem ), on the website of the Austrian Meteorological Service (ZAMG): Environment: Air quality

Individual evidence

  1. ↑ Lothar Schütz: Sahara dust transport over the North Atlantic Ocean-Model calculations and measurements. In: Christer Morales (ed.): Saharan Dust. Mobilization, transport, deposition. Papers and Recommendations (= SCOPE. 14). Wiley, Chichester et al. 1979, ISBN 0-471-99680-7 , pp. 267-277, ( PDF digital version (PDF; 1.03 MB)) . Retrieved August 6, 2015.
  2. ^ Oscar E. Romero, Carina B. Lange, Robert Swap, Gerold Wefer : Eolian-transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns. In: Journal of Geophysical Research. Series C: Oceans. Vol. 104, No. C2, pp. 3211-3222, doi: 10.1029 / 1998JC900070 .
  3. ↑ a b c Georg Feulner: The great book of the climate. Komet, Cologne 2010, ISBN 978-3-89836-866-7 .
  4. ↑ Jason P. Dunion, Christopher S. Velden: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity . American Meteorological Society. P. 13, 2004. Retrieved April 18, 2011.
  5. ↑ see Red Sky over the Alps , Abendblatt.de, February 23, 2004.
  6. ↑ Andrew S. Goudie, Nicholas J. Middleton: Saharan dust storms: nature and consequences. In: Earth Science Reviews. Vol. 56, No. 1/4, 2001, pp. 179-204, here p. 190, doi: 10.1016 / S0012-8252 (01) 00067-8 .
  7. ↑ Jean Dubief: Review of the North African climate with Particular emphasis on the production of eolian dust in the Sahel zone and in the Sahara. In: Christer Morales (ed.): Saharan Dust. Mobilization, transport, deposition. Papers and Recommendations (= SCOPE. 14). Wiley, Chichester et al. 1979, ISBN 0-471-99680-7 , pp. 27-48, ( PDF digitized (PDF; 1.98 MB)) . Retrieved August 6, 2015.
  8. ↑ a b c Robert Guderian: Handbook of environmental changes and ecotoxicology. Volume 1B: Atmosphere Aerosol / Multiphase Chemistry Dispersion and Deposition of Trace Substances Effects on Radiation and Climate. Springer-Verlag, 2013, ISBN 978-3-642-57096-4 , p. 14 f ( limited preview in the Google book search).
  9. ↑ a b c d Friedrich Löhle: Visual observations from the meteorological point of view. Verlag von Julius Springer, Berlin, 1941, p. 47 ff (reprint Springer-Verlag, 2013, ISBN 978-3-642-99248-3 ; limited preview in the Google book search).
  10. ↑ a b c d e f Weblink Saharan dust events. Meteoswiss, section climatology of Saharan dust events (accessed May 13, 2016).
  11. ↑ The importance of aerosol particles in the climate system using the example of Saharan dust. Global Atmosphere Watch, GAW letter from the German Weather Service, Meteorological Observatory Hohenpeißenberg, January 2001 (pdf, dwd.de).
  12. ↑ Hellman, G. and W. Meinardus: The great dust fall from March 9 to 12, 1901 in North Africa, Central and Central Europe. In: Abh. Königl. Preuss. Met. Inst. 2, 1901, pp. 1-93.
  13. ↑ Weblink Saharan Dust Events. Meteo-Switzerland, section Proof of Saharan dust events at the Jungfraujoch (accessed May 13, 2016).
  14. ↑ Weblink Saharan Dust Events. Meteoschweiz, section On the trail of Saharan dust with laser beams (accessed May 13, 2016);
    M. Collaud Coen, E. Weingartner, D. Schaub, C. Hueglin, C. Corrigan, S. Henning, M. Schwikowski, U. Baltensperger: Saharan dust events at the Jungfraujoch: Detection by wavelength dependence of the single scattering albedo and first climatology analysis. In: Atmos. Chem. Phys. 4, 2004, pp. 2465-2480 ( pdf , atmos-chem-phys.net).
  15. ↑ Sahara dust brings "Blood Rain" in the Upper Austrian News from April 16, 2018, accessed on April 24, 2018.