Wednesday, February 27, 2008

Unusually bright twilights over Western Europe

Between February 17 and February 20, 2008, large parts of Western Europe witnessed a series of unusually bright morning and evening twilights.
A few minutes after a quite normal sunset, the western skies began to burn in a strange yellow light which was bright enought to illuminate the landscape and giving a quite unreal touch to houses and trees.
Some minutes later, the yellow light in the west became surrounded by a brownish rim, turning into purple within some minutes. The yellow part of the sky slowly shrank towards the horizon, turning into orange and later into red and crimson. Some observers also reported of a dark, brownish-red light in the east which surrounded the whole horizon reaching up to 10° high.
The strange lights and colours in the sky were visible for up to about one hour after sunset. A similar “light-show” also appeared in the morning, starting with a crimson light over the eastern horizon and ending with the bright yellow light short before sunrise. The yellow illumination of the landscape could even be perceived through layers of low clouds (stratus) in some areas.
The phenomenon was reported from the British Isles, Scandinavia, The Netherlands and Germany, and there are even reports of unusual twilight observations from northern Spain.

These in some cases weird-looking twilights were probably caused by an outbreak of polar stratospheric clouds (PSC). These form at temperatures below -78°C in the stratosphere, at an altitude of about 20 – 25 km above the ground.
Soundings made at several stations showed that temperatures in the stratosphere really were unusually low over western Europe; up to -87°C (De Bilt) were measured, the lowest since measurements began in the 1980s. This makes the formation of PSC over a large area possible. Some photographs also show faint structures in the light, giving hints that they actually were caused by PSC.

Polar stratospheric clouds have never before been observed so far south. Normally, they can only be seen from Scandinavia, Canada and Alaska. Only in 1999 there was a confirmed observation of PSC from northern Germany.

Authors: Peter Krämer, Bochum & Claudia Hinz, Brannenburg

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