I have just received this message which I thought might be of interest

Cheers, Alastair.

Space Weather News for June 3, 2010
http://spaceweather.com

Every summer since the late 19th century, Earth's polar skies have lit
up with gossamer, electric-blue clouds, twisting and rippling in the
twilight sky. They're called noctilucent ("night-shining") because
they can be seen after dark. The origin of the clouds, which hover at
the very top of Earth's atmosphere in close proximity to space itself,
is uncertain. They have been linked to causes as varied as meteoroids,
climate change, and the icy exhaust of the space shuttle.

News flash: The 2010 noctilucent cloud season has just begun in the
northern hemisphere, with sightings over Russia, Scotland and
Denmark. Although noctilucent clouds were once a phenomenon of high
latitudes only, in recent years they have been sighted in the United
States as far south as Colorado and Utah. Visit http://spaceweather.com
for photos and observing tips.

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On Thu, 03 Jun 2010 03:44:32 -0700, Alastair wrote:

snip
Although noctilucent clouds were once a phenomenon of high latitudes
only, in recent years they have been sighted in the United States as far
south as Colorado and Utah.
snip

That bit caught my eye. If that report is accurate, what has caused this?

--
Bewdley, Worcs. 90m asl.