MetLink - Noctilucent clouds - 29 Jan


From: "Jim Jamieson" <psa@sserc.org.uk>
Subject: Noctilucent clouds
Date: Mon, 31 Jan 2000 09:43:55 -0000
 
There was a display of these unusual clouds on Saturday evening.  I first
noticed luminosity in the north western sky at 30 minutes after sunset.  The
sky was otherwise fairly clear.  A few tropospheric clouds were silhouetted
against the twilit sky, dark because they were no longer in sunlight.  There
were at that time two luminous objects in the sky.  One was compact and
brilliant.  The other formed a more extensive, wispy sheet.
 
The condition for a definite sighting of a noctilucent cloud, so far as I am
aware, is that the sun should be at least 6 degrees below the horizon.  At
the latitude of East Lothian, where I live, the sun's rate of descent is
about 9 degrees an hour at that time of the day.  At the time of first
sighting, the sun would have had an elevation of -4.5 degrees.  10 minutes
later, the condition was met.  The compact luminous body continued to be. It
was thus a noctilucent cloud situated at the top of the stratosphere at an
altitude of about 85 km.  However the other luminous cloud had disappeared,
showing that it was much lower in the stratosphere, being a nacreous type of
cloud.  The noctilucent cloud continued to be visible until 90 minutes after
sunset.  By this time the sun's elevation was below -13 degrees, stars were
out and the evening had entered the phase called astronomical twilight.
 
Where was the noctilucent cloud situated?  Its maximum elevation had been
about 10 degrees, showing that its nearest approach was about 400 km, or
just to the north west of the Isle of Lewis.  How far south might the cloud
have been sighted?  Up to about 10 degrees of latitude, or 1000 km.  Thus
the cloud could have been seen from anywhere in the British Isles, from the
south coast appearing as a luminous band along the NW horizon.
 
It is quite amazing to realise that you can look at an object sited on the
north western edge of Britain from its south eastern corner!  I would be
interested to hear from anyone else who saw this sight.
 
Jim Jamieson
Senior Associate
SSERC
St Mary's Land
23 Holyrood Road
Edinburgh
EH8 8AE