I have posted some pictures of this cloud formation ,observed in SE London
~12-13Z in -
http://www.ukweatherworld.co.uk/foru...sts=16&start=1
(Registration required)
Ian aptly terms it 'Vaulted heaven Cloud' as it does indeed resemble gothic
vaulting in a cathedral.Pictures do not convey the 3D appearance of the
cloud.Much more detail , particularly of small scale waves and lamination
are apparent in the high resolution originals.
After a little research it appears documentation of 'Reticular' formations
are rare-so I post some remarks here.

The formation was part of an As/Ac cloud sheet over SE England that had
formed on a humidity/temperature inversion which caused Anaprop on weather
radar during the morning.The actual area of reticulation was of the order of
a few 10's of kms in dimension and moved SW to NE across the London area.It
was a few kms ahead of a band of rain on a short wave trough feature moving
in the same direction.Report suggest much of the rain falling from the
leading edge of the band was not reaching the ground.
In appearance the reticulation was of light holes or indentations in the
cloud sheet with a net or weblike skein of darker cloud surrounding
them.Also apparent were many fine scale laminations and waves.Embedded in
the area was a linear or partial arc cloud lowering behind which was a vault
from which some virga appeared to be falling.Immediately behind that was an
area of highly laminated and striated cloud.It was not possible to tell
whether this feature was moving with or through the cloud sheet.
The Herstmonceux 12Z sounding may be representative of the atmospheric
profile at the time (a perhaps debatable assumption)-
http://weather.uwyo.edu/upperair/ima...3882.skewt.gif
It shows mostly relatively dry air to 750 hPa , a saturated cloud layer from
750-800 hPa and then nearly saturated air to ~300hPa, with, perhaps another
layer cloud at ~575 hPa.Winds are light and variable below the cloud layer
then veer and strengthen from SE to SSW at ~500hPa.
Data from the Chilbolton Lidar ,some distance to the SW-
http://www.met.rdg.ac.uk/radar/cgi-b...?date=20060506
show the cloud sheet at ~1Km height ahead of the approaching rain
band.Interestingly ppn from a layer of higher cloud is falling into this.
It seems reports of reticular cloud are rare in the meteorological
literature.Scorer's 'Clouds of the World' mentions them .The most detailed
reports I have found are -

Katharine M. Kanak Jerry M. Straka
Monthly Weather Review: Vol. 130, No. 2, pp. 416-421.An Unusual Reticular
Cloud Formation

and
The Mysteries of Mammatus Clouds by Schultz and Coauthors (in press JAS)
available at-
http://www.cimms.ou.edu/~schultz/papers/mammatus.pdf
(~60Mb file!)
The atmospheric profile from Oklahoma in the MWR article resembles the
Herstmonceux sounding though the synoptic situation is completely different
(the reticulation was observed on the underside of a thunderstorm outflow
anvil )
A number of different explanations for reticulation are proposed by the
various authors centering on the mixing of dry air into the cloud either by
transport from below or subsidence from above due to weak convection .Here
the presence of wind shear and wave activity at the base of the As cloud
sheet might lift dry air into the cloud. Another speculation is that ppn
falling into the cloud sheet from the higher cloud could promote weak
convection, subsidence and fall out.
The cloud lowering and vault observed resembles a wave feature or even
'hydraulic jump' ,but I won't speculate beyond that.

Comments welcome, particularly from those with a better knowledge of cloud
physics than mine,
regards,
David



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