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On 18 February 2008 a rare display of polar stratospheric clouds (type 1) was seen over a wide area of Britain and Europe.

Apparently this was due to the formation of frozen nitirc acid trihydrate (NAT).  The air temperature at 20 km (30 mb) was very cold (-80° and below) which allowed the clouds to form by the freezing of the NAT. 

It was *very* rare for clouds of this type to form so far south (and it is felt that this might have been the first time that this has happened at such low latitudes - they are normally only seen in polar regions).  

Pictues taken between 1752:30 and 1757 UTC from Willingdon, East Sussex are given below when the sun was between 5.8° and 6.5° below the horizon.  Sunset was 1718 UTC although the elevation of the South Downs to the west mean that from this location the sun sets about 20 minutes earlier at this time of the year.

A very interesting paper on this event was published in Weather April 2009 Vol 64 No 4 by C Hinz et al.