Polar Stratospheric Cloud Alert
On the evening of Tuesday 26th January, southern and eastern England plus The Netherlands experienced spectacular colourful sunsets and prolonged twilights. Very likely they were visible over an even wider area.

The top left image shows a similar sunset witnessed at Deventer in The Netherlands by atmospheric optics expert Peter Paul Hattinga Verschure. He comments �The sun glowed up in bright orange and red colours, shining through a thin layer of high cloud�.�

At top right is the twilight scene of vivid reds imaged by Pete Glastonbury at Devizes in Wiltshire, England.

The third image is by Rob Brocklehurst taken in Dorset, England and also shows some of the post-sunset red and crimson hues.

Another Netherlands image by Peter Paul Hattinga Verschure is below and shows the scene 25 minutes after sunset.

Peter Paul comments �About half an hour after sunset, a sun-lightened cloud bank remained near the westerly horizon, while all other clouds and contrails were darkened. Obviously the far clouds were in a higher atmospheric region. So I guess that was a bank of Polar Stratospheric Clouds. The more because temperatures at 22 kilometres altitude were below -80 degrees Celsius. This should be then PSC of type I. The real nacreous clouds of Type II are never seen from our flat country.�

Polar Stratospheric Clouds, PSCs, occur at heights of 15-25 km (9-16 mile) and far above our ordinary tropospheric clouds. To form in the arid and low density conditions of the lower stratosphere they need temperatures of minus 78�C or colder depending on their composition. Type I PSCs are nitrogen acid and water compounds. Type 2, the brightly shining nacreous clouds, are ice and they need slightly lower temperatures.

A large part of Europe had similar spectacular sunsets and twilights in February 2008 caused by very low stratospheric temperatures and Type I PSCs (see the thumbnail links below).

Volcanic aerosol and dust give similar sunsets and we should not rule out these possibilities at this stage.

We could be seeing the start of a series of spectacular sunsets. Look out for twilights and sunrise/sets over the next few days.


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