On Wednesday, 26 October 2016 17:01:21 UTC+1, MartinR wrote:
On Wednesday, 26 October 2016 16:18:50 UTC+1, Graham P Davis wrote:
On Wed, 26 Oct 2016 11:43:59 +0100
"Eskimo Will" wrote:



With cirrus being precipitation, should it be counted as a cloud? And
if it is a cloud, isn't snow Cirrus? ;-) The same could be said of As
since that is merely precipitation, perhaps with the Ac mother-cloud
still in existence above it. Ever since I joined the Met Office, I've
been tempted to report low-level Cirrus when I've had decaying snow
showers drifting in from the North Sea and sometimes had half the sky
covered with snow but no reportable clouds. Never had the guts to try
it though; it would have been a waste of effort in any case.

--

When I was in the UKMO old-timers who had spent a spell in Antarctica would mention diamond dust (ice fog) and considered it as cirrus on the deck. Whether the sparkly ice crystals were neutrally buoyant or fell down slowly they never said. It's true that typical mare's tails cirrus (s****atus?) seem very often to originate from something vaguely cumuliform and look like precipitation or virga.

MartinR

I reported diamond dust at Kinloss around 25 years ago. It was obviously nowhere as cold as it gets in The Arctic, getting down to -10°C with shallow mist and patches of fog on a snow surface. I wouldn't have seen it (but I might have felt it) if ATC hadn't had their searchlight trained on me in the Met enclosure doing the 2100 observation!

Its an effect that I suppose is very much like virga falling as ice crystals in cirrus, and probably happens a lot more widely than we imagine in the upper atmosphere.