Last night (9/10 july) I was lucky to witness a rather good NLC display from The Netherlands. Around 2:10 UTC an area within the display definitely showed a lacunosus structure (like in cirrus clouds). This was clearly a type V (net-like structure) of NLC:
A few questions: How often do lacunusos occur in NLC? How do they form?
For who ísn't familiar with the term lacunosus: it means 'with holes'.
I observed this NLC from SW Scotland with Bill Ward and we also noticed this structure, but, only in short-lived small isolated patches. It was not as extensive as the example in your photograph. I think Bill took some photographs of it.
Hi Menno, I was glad to see your comments about lacunosus forms. NLC can be very complex but some of the standard meteorological terminology is more suited to describing what is observed than the usual NLC descriptors. Myself and Tom have been discussing this at length. I also caught a prominent lacunosus form on the night of the 26-27 June. The image is on the main web page for that date. I think, though, that the V descriptor should be used when sets of billows cross and give a distinctive "net" appearence and lacunosus should be used when the holes are rounded. I would go as far as to propose a new NLC descriptor to be used in this case. Simply type L, since it is quite distinct and I belive may be formed through a different mechanism (I don't know if that avoids or causes more confusion!). It is interesting to watch how the lacunosus develops as it is akin to fall streak holes and could show regions where particle size has grown sufficiently large for the crystals to drop out of the main layer and evaporate. Cheers, Bill.
On 13th of June 2006 we had a very exciting display in Austria. I took some images with „lacunosus“-structures. First the holes were very small, but within some minutes a large part of the cloud evaporated – the hole had the shape of a heart and at my webpages I call it the „hart in the NLC-sky“. Some structures in the cloud look like „virga“ – is that what you, Bill, call „fall streaks“? Maybe the particle size has grown large enough for the crystals to drop out of the cloud. I add an image showing the lacunosus-structures and the first sign of the NLC-hart.
The picture from last year's display which startled me with the sharpness of the holes has vanished into cyberspace somewhere. I will try and locate it! Looking back through hundreds of my pictures I found just one other display which had the characteristics I would suggest deem the special designation Type L. The distinct holes can be seen to the right hand side of the brightest element.
As a comparison here's a cirrus version of the same thing (I think, this is where the physics gets interesting...)
I hope these pictures show what I tried to describe last year. Type V is a rectilinear form of "net", that is straight elements crossing. My proposed Type L are rounded hole elements and not necessarily "net" like.
I have absolutely no doubt that this is a distinct formation within NLC which should be acknowledged and is telling us something more about what is happening up there.
I'd like to hear either via the forum, forum private message or direct email if you spot any of these. There are interesting possibilities here. thanks, Bill.
PS The NLC picture was taken at 01.37UT, 13th July 2006. PPS I've just re-read my original posting and a version of the display from last year is on the main web page archives. If I find my original I'll post a bigger version which shows the holes more clearly.
Hi Olaf, I've been off for a while and am just starting to catch up. Your video looks VERY interesting and I've only had one other report of this type. What I have also noticed is that during the brighter displays I've seen this year is that some elements can move to form what looks like the holes but I don't think they are exactly the same formation process as genuine "Type L" I think the mechanics are different but that's why it's interesting! It looks like this particular formation is indeed on the rare side. It'll be interesting to see if more develop over the coming years. cheers, Bill.
The difference I see is that the holes in the two pictures (above) I posted were generated spontaneously and not by other forms merging to give the appearance of holes or a net.
This shouts that they form differently! I've done quite a bit of poking around to find out more myself with not that much success. It doesn't seem to matter where one goes the explanations are all a bit iffy. Heavy duty sums that still rely on an ambiguous set of initial conditions.
Which reminded me of a lecture I went to a long time ago where the the lecturer said that physics was brilliant and that it could be applied to everything and anything as long as it could be assumed to be a perfect sphere... That sums it up nicely!