Monday, September 08, 2008

Volcanic Twilights over Europe

On August 7, the Kasatochi Volcano, situated on the Aleutian Islands near Alaska, erupted. Clouds of ashes and sulfur dioxide were ejected up to 15 km into the stratosphere.

During the following 3 weeks, the volcanic clouds spread over the whole northern hemisphere, causing widespread intense twilight colours and often also crepuscular rays. These were first reported from Northern America during mid August, but at the end of the month, these "volcanic twilights" were also observed in Europe.

In the evening of August 29, several observers reported a strange and intense yellow light around sunset, followed by a purple light. Some of them were reminded of the unusual twilights between February 17 and 20, which were caused by PSC.

On August 30, skies were clear over Germany, and so many observers could see a kind of silvery cloud stripes a few minutes before sunset. These stripes were orientated north-south and at first glance looked like cirrus or cirrostratus clouds. But during the day these clouds had not been visible at all, and when looking at them carefully, one could see that they were higher than normal high clouds. The contrails of some airplanes were obviously below these clouds, and as the contrails turned reddish in the light of the setting sun, the clouds still remained bright. So they must have been floating higher up in the air, somewhere in the
Stratosphere.

After sunset, the clouds got a more brownish-yellow hue, but turned pink only about 20 minutes after sunset. Some observers also reported intense crepuscular rays. The purple light faded about half an hour after sunset.

In the morning of August 31, the colours and cloud stripes could also been observed. In the evening, a cold front with thunderstorms approached the western parts of Germany. While even the tops of the cumulonimbus clouds were already dark, the stratospheric clouds still lay in plain sunlight. That evening, instead of the regular stripes of the day before, they looked more like irregular waves.

During the first days of September, the strange twilight colours could still be observed over southern Germany, while for the rest of the country morning and evening skies looked quite normal again.

But as there are still volcanic ashes in the stratosphere, the colours may return. So keep watching the skies before sunrise and after sunset.

Author: Peter Krämer, Bochum, Germany

1 comment:

johanis said...

hi there..
watta nice post!

wanna exchange link??