In the morning of April 11, Mt. Eyjafjallajökull, a volcano which is covered by a glacier, erupted in the southwest of Iceland. Its cloud of ashes rises up to altitudes of 10 – 12 kms and has been shifted towards Central Europe by a northerly airstream since Thursday (animation).
The ash particles are slowly sinking downwards in the air, obstructing aviation in many places. In the atmosphere they dim the light (photos C. Hinz 1-2-3) and make Bishop´s Ring visible (photo P. Krämer), which is caused by light refraction on the aerosoles.
In high levels of the atmosphere, the particles act as additional nuclei for condensation, on which humidity (which under normal circumstances is not sufficient for cloud formation) freezes and forms ice crystals generating so-called “Invisible Cirrus Clouds”. Size and/or density of the ice crystals is in most cases not high enough to make the clouds visible, but their existence can be proved by the formation of faint halos such as sun pillars (photo Ina Rendtel), sundogs (photo Reinhard Nitze), or the 22°-halo (photo Brigitte Rauch).
There are still doubts regarding the appearance of the colourful twilight effects known from the eruption of Mt. Sarychev. Measurements with a Lidar effected by the Hohenpeissenberg Meteorological Observatory have shown that most of the aerosoles are at altitudes between 3.000 and 7.000 meters. A heavy rainshower should be enough to wash them out of the atmosphere and make the air clean again. An elevated concentration of sulphuric acid, which after the eruption of Mt. Sarychev formed several layers at different altitudes and caused beautiful purple light and afterglow effects, has not been measured at all. Probably the SO2 ejected by Mt. Eyjafjallajökull is chemically combined to water at the moment when the ash cloud is formed. The explosions, however, are generated by the contact of lava with ice, and every time a part of the glacier falls into the lava, there is plenty of water provided for such a reaction.
Authors: Claudia Hinz, Peter Krämer and Wolfgang Hamburg
In the last days of May there was an expansive dust layer above central Europe that caused a lot of interesting atmospheric phenomena. The story begun on 27th of May, when an ochre colored aureole surrounded the sun, and the sunset was pale yellow what repeated the next morning. There was a 7-8 degrees wide, dark greyish band by the horizon (image by Ágnes Kiricsi, in Vecsés, Hungary) this made the dawn to be late and the sunset to be too early. On the next days Bishop’s ring was observed through Central Europe – in the Czech Republic, in certain parts of Germany, and in Hungary. (This Bishop’s ring looks the same as it was produced by volcanic particles.)
In the Alps the dust strongly reduced the transparency of air (to about 3-4 kms). The photo was taken by Bertram Radelow in Davos, Switzerland. The situaution was the same in the northern and eastern parts of the Alps.
The last two days of May passed away with the attenuation of the dust layer, dust fell out – both dry and wet way. At my hometown the dry version occured and a thin layer of fine yellowish powder subsided on the plants and the parking cars. In Germany there was a reddish-ochre coloured muddy rain falling that caught attention. The same thing happened at some places in Hungary too where rainshowers washed out the dust leaving muddy traces on everything.
In the meanwhile the dust „cloud” could even reach southern Scandinavia, where it also produced „colorless” sunset. The photo was taken by Andras Uhrin in Stavanger, Norway.
The origin of the dust is in the deserts of Africa. Late spring and early summer there are proper conditions in the Sahara and Sahel region for the fine dust to lift up high in the atmosphere where hot, dry winds transport it towards Central Europe, causing the same phenomena almost each year. There were „blood rains” in Europe caused by this Saharan dust over the centuries, like the ’Blutregen’ in 1901 in Germany, when the dust mass that fell out was 1-4 grams / square meter! The micron sized particles are mixed of fine sand and fine mica pellets.
Posted by Noli
Very large coronae were observed in Southern Finland at the beginning of May 2006. Their radius was about 45° and they had a bluish disk and a brown ring. The sky was very clear and quite dark outside the corona. Also, very beautiful red sunsets were seen on those days.
The phenomena were visible on many days between May 2-11 and were possibly produced by smoke carried from Eastern Europe.
In the above corona image taken on 11th May a small segment of another photo has been added to show the size of 22° halo for comparison.
The sunset was photographed on 4th May.
Large coronae can be seen during very warm weather periods when the sky is usually hazy. This was the first large corona seen in Finland since the Bishop’s rings of 1991-1992. Smaller coronae (radius of about 5°) with a blue disk and brown ring have sometimes been observed.
[Text & Report: Jari Piikki, Juva Finland]
This corona was photographed in December 2000 while camping at the El Tatio geyser field at 4300 meters elevation in the Chilean Andes. The corona radius is approximatly 50°. Blue inner disk extends to about 35° from the sun. In between the blue and red, a green band was seen visually, but it was not reproduced on film. Altocumulus clouds were developing from the stuff that made the corona. The corona may be related to the dust that is released in large amounts in the air from some mines in the area.
Cyan saturation was increased in the photo to make the inner blue disk better visible. Otherwise the photo is as scanned from the slide.
Posted by Marko Riikonen
After the 1991 Philippinian Pinatubo eruption, a large diffraction corona around the sun became visible worldwide. It was seen for many months in 1992 and 1993. This corona phenomenon is known as a “Bishops”” Ring”. It appears when sunlight shines through stratosperic layers of volcanic dust. The pattern of light and color is the same as that of the common corona in cloud droplets, consisting of a blueish white aureole directly around the sun, surrounded by a reddish to brownish diffuse and broad ring. The radius, however, is large and comparable to that of the common halo, being typically about 25 degs. This photograph was taken by Peter-Paul Hattinga Verschure, this one with super wide-angle lens and his second with a 20mm, from Deventer in The Netherlands on 29th March 1992. It was a spring day with very transparant air conditions, the best circumstances to see this phenomenon.