"Graham P Davis" wrote in message
-jade...
On Tue, 25 Oct 2016 18:32:39 -0000
"Bernard Burton" wrote:

On the 10th October 2016 a sheet of stratiform cloud was producing
fall-streak holes over southern UK. Howerver, holes developed in
certain parts of the sheet only. An analysis of the available data
indicates that the cloud was at a height near 25000 ft, and at a
temperature of -31 C, and its unusually cold temperature holds the
key to the unusual nature of this cloud.

http://www.woksat.info/wwp/cloud-161010-final.pdf


Just had a quick glance through [I'll take a longer look tomorrow] but
the idea that -31C is unusually low for water droplets to occur in
clouds surprised me a bit. Nearly fifty years ago, I read that a
research aircraft had found water droplets at a temperature of -60C.

Just as a switch, I've also seen ice-crystal cloud (Ci Spi) seem to turn
into water-droplet cloud but that's another story.

--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
Web-site: http://www.scarlet-jade.com/
There are more fools than knaves in the world, else the knaves would
not have enough to live upon. [Samuel Butler]




Thanks Graham. Incidentally, I did not intend to give the view that -31C is
unusually low for water droplets in clouds - I am sure it is not. I think my
thrust was that the phenomenon of fall-streak holes was, in my experience,
rare at that sort of temperature. All the research into spontaneous
nucleation (Mason and others) points to almost instaneous nucliation at -41C
at surface pressure, and this falls to -35C or so higher in the troposphere.
It is also dependant on droplet size, the temperature rising for increasing
droplet size. Water clouds undoubtably do exist at temperatures below -30C,
but it is possible that they are time limited before nucliation takes place.
Supercooled water at -60C requires an exceptionally pure atmosphere,
virtually devoid of ice nuclei.

--
Bernard Burton

Satellite images and weather data for Wokingham at:
www.woksat.info/wwp.html



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