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]

On Tuesday, 25 October 2016 21:44:56 UTC+1, Graham P Davis wrote:
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/


Observation.
The cloud consisted of 2 okta of a thin sheet of what looked at first sight to be high altocumulus stratiformis undulatus translucidus, but had a rather odd dull appearance, with a complete absence of the brightness normally associated with a water cloud. The colour was an almost uniform grey, with an absence of any shadowing. This cloud sheet had an edge to the SW, and seemed to be extensive beyond this edge.

So who of us is allowed wild guesses experts ot inept?
For me if the ice was on point of formation the crystals would have been stratified diferently to a set formation. 25 thousand is about the height of crystalisation isn't it?
Would the optic been of a polarised sheet?

If so you were seeing a snow cloud forming.
But you should know that?

many of these patches have a similar orientation and shape, aligned roughly W-E.
Interestingly, what can also be seen are a number of cloud holes, most evident over the English Channel SE of the Isle of Wight. These have the appearance of fall-streak holes that can form when there is a thin layer of super-cooled water cloud, indicating an atmospheric layer saturated with respect to water, and super-saturated with respect to ice.

Surely as ice forms it falls straight away isn't this what ammatus is all about?they are the nest thing to downdraughts are they not?
Of the sort that throws aircraft around?

"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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