Monday, April 19, 2010

Influences of the Mt. Eyjafjallajökull eruption on the atmosphere

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

Friday, April 16, 2010

Double Rainbow and reflected Rainbow

When I wanted to go to work in the morning of February 24, 2010, I noticed a colourful rainbow forming during a short local rainshower. At first the weather had been fine that morning, and ist was almost calm, but then some rain clouds came up from the southwest and started to cover the sky. The sun was shining brightly at that hour (about one hour after sunrise) and made the rainbow shine in especially bright colours. So I hurried to get my camera and started to make some photographs. While I was taking pictures, I was astonished to see a third bow forming, which intersected with the secondary bow. At the first moment I thought that there was something wrong with my eyes, but I could see it also when I looked at the pictures already taken on the camera display.

I was excited, although I could not make head or tail of it – I had never seen such a thing before. Later, as I searched the world wide web for an explanation, I learned that the phenomenon must have been caused by a reflection. But the reason for the phenomenon was still not clear, as there was nothing between me and the rainbow that could have reflected any light.
Only much later I could solve the mystery: The reflection was caused by the Hallwilersee (Lake Hallwil), which was at about 3 kms behind the position where I had taken the pictures that day. The lake cannot be seen from the place where I saw the rainbow, as there is a hill between the lake and that place. So I did not take this possibility into consideration at first. The position and elevation of the sun, my position towards the lake and the distance from the lake fitted perfectly that morning to form this rare phenomenon for a few moments.

Author: Matthias Frei, Dürrenäsch, Canton Aargau, Switzerland

Friday, April 02, 2010

Bishop's ring

Last Sunday, March 28, I went up on the Magdalena Mountains (in central New Mexico) to pick up two instruments from the laboratory there, and saw what would qualify as Bishop's ring around the sun for most part of the day. The mountain ridge lies at an altitude of about 10,500 feet above sea level. The sky was very clear and dry, and there was light wind at the time.

One of the photos I took is shown to the right. It was taken with a Nikon D700 with a 24-70/2.8 AF-S lens set at 24mm focal length and f/13 aperture.

To the eye the ring had a pronounced blue aureole with brown outer ring. I think the altitude of the observation rules out low-altitude aerosols being responsible. The central blueish aureole was relatively small; it was only a couple degrees in diameter. I estimate the radius of the outer brown ring to have been about 10 to 15 degrees.

Although the atmosphere was stable and quiet at the time I saw the ring, the southwest of the USA including New Mexico has had very strong winds and dust storms over the past week. In general, the spring months see many strong windstorms in this area. I believe the ring was caused by fine dust in the upper atmosphere and not from volcanic activity somewhere.