Les is essentially correct. Cloud droplets can exist in a supercooled state
in the atmosphere, that is as liquid water at temperatures down to -40C.
However, the saturation vapour pressure over water is greater than that over
ice at the same temperature, with a maximum difference near -12C. This means
that a cloud layer consisting of supercooled water droplets, by definition a
layer with 100 % relative humidity wrt water, will
have a relative humidity of 110% wet ice at -10C, and 121% at -20C. Thus,
should ice crystals start to form, either spontaneously, or due to some
contaminant in the layer, possibly introduced from the exhaust of an
aircraft, or just due to the turbulent disruption of the layer by an
aircraft, or by orographic influences, the ice crystals will grow very
rapidly, locally reducing the RH in the layer towards 100 % wrt ice, well
below 100% wrt water, causing the remaining water to evaporate, and the ice
crystals to precipitate out of the layer. The result is as seen in the
photos, a hole in the water cloud in which a patch of ice crystal virga
hangs. The effect is known as the Bergeron process, and is an important
production mechanism for rainfall/snowfall.
--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.woksat.info/wwp.html

"Ken Cook" wrote in message
...
Hi, All,
I have put three pics on the ultra slow (sorry-Orange’s fault) website
(pictures page) given by a friend of mine of a cloud “hole” he
photographed within a mile of Stamford, Lincolnshire, on Sunday 19th
October 0935Z. They are in sequence and pic 3 was taken from almost
underneath the hole about 20 minutes later.

http://copley.website.orange.co.uk

Clouds are not my strong point but my theory is that the hole was
formed by a jet passing through, exhaust evaporating the Sc sheet and
the saturated air mixing with the air below forming the trail.
Am I even close? Any ideas?
Ken
Copley, nr Barnard Castle, Teesdale

http://copley.website.orange.co.uk