Noctilucent clouds (NLC) are thin clouds which form at heights around 83km near to the mesopause during the summer months when temperatures are at their lowest (~130K). The formation process is still open to some debate due to the extreme conditions at these altitudes.
NLC are extremely thin and tenuous, reflecting only a tiny amount of the light that falls on them hence they are only seen under particular viewing conditions.
They are generally seen for a few weeks either side of the summer solstice primarily at latitudes between 50 and 60 degrees when the sun is between 6 and 16 degrees below the horizon, this corresponding to the midnight hours. Under these conditions the NLC are sufficiently high to still be illuminated directly by sunlight.
It is interesting to notice that no matter at what latitude NLC are photgraphed from they pretty much look the same! This demostrates the remarkable uniformity in the conditions in which NLC form. It is also a graphic demonstration of the viewing angle of NLC.
NLC are thought to be an essentially a polar phenomena with the clouds being blown away from the polar regions. Having a polar phenomena over central Europe is an intruiging thought!
NLC observations only go back to the 1880's. There were suggestions that they may have been caused by large volcanic eruptions pushing dust and water vapour high into the stratosphere which was in turn transported to the mesosphere. However no clear link has been established and an alternative suggestion MAY be that they are a consequence of mankinds activity on the atmosphere and thus a visible impact of climate change.
What seems to be almost accepted as fact is that NLC are occuring earlier and over a much wider latitude range. This, I feel, is nothing more than a bit of climate change hysteria. Apart from a few observations, which largely cannot be categorically substantiated, there seems little evidence for NLC actually occuring "earlier", that is towards the start of May.
The related issue which is receiving a lot of press is the geographical extent to which NLC are being seen. Headlines state that NLC are moving further south, well maybe...! As far back as 1967 B. Fogle in "Recent Advances in the Research of Noctilucent Clouds", Journal of the Royal Astronomical Society of Canada, Vol 61, pages 64ff, states that NLC can be seen "between 45 to 80 degrees latitude, best around 60 degrees (the list on page 66). Now it seems when NLC are reported from the mid USA or southern Europe (around the 45 degree mark...) it is a significant event. It may well be, but it's a 40 plus year old story now.
The question is whether the frequency of more recent reports represent a real change. With modern cameras, instant communication and a greater awareness of the climate it might just be that people are noticing what has always been there. With this new awareness it will still take considerable time to determine if NLC might be changing in a significant way and whether these changes represent an anthropogenic impact.
Yet another claim which is made regarding NLC is that they are getting "brighter". NLC by their nature are microscopic size particles if ice. Measuring how such a variable phenomena is changing in brightness is quite difficult to do quantitatively!
Just because those with vested interests keep on repeating these myths doesn't actually mean they're true!
Even with the current batch of Earth observing satellites, monitoring by networks of largely amateur observers is still needed to ensure that we have good long term records. It will take a considerable time before we can say with any certainty whether these presumed changes are real or not.
Another extensive NLC display seen over Paisley/Glasgow, easily visible to the casual observer on the night of the 30th June/1st July 1995.
Noctilucent cloud viewed from Kilbirnie on the night of 3/4 July 2003.
NLC are a common sight in the northern hemispheres' summer night sky but what about the southern hemisphere?
Although there have been many thousands of reports from the north there have only been a handful from the south. There have been a few organised programs to observe NLC in the southern hemisphere but due to a lack of populated land masses at the appropriate southern latitudes there are simply not enough observers available to make observations comprable to the northern hemisphere. Technology certainly helps (remote camera's and LIDAR etc) however there doesn't seem to be a substitute for a large number obsevers to compensate for weather issues.
Consequently there is data extending back to the late 1800's for the north and only the 1960's for the south!.
NLC make an excellent target for photography and with modern digital camera's it is simplicity itself to produce quite beautiful pictures.
However using digital video and CCD techniques detailed analysis can be conducted. To achive the highest contrast and best resolution monochrome narrowband methods have to be employed. This allows exquisit detail to be brought out.
CCD camera's can also be utilised to image NLC over a wide range of wavelengths. From near UV extending into the infra red
NLC can be seen to slowly drift across the sky however it is only with time lapse video imaging that thier dynamic nature can be truly appreciated.
Some results from the 2006 NLC season.
This is a time lapse video showing the NLC blowing out from the north between 0100 and 0200UT on the night of the 2/3 June 2006. (NOTE this is a 23Mb file)
A short time laspse from 14/15 June 2006. 0152 to 0202. (approx 7 Mb)
Here are 3 video clips of the bright and extensive NLC of the night of 30 June to 1 July 2006.
30 June, video 1. CCTV camera with 12mm f1.2 lens. 23.30-02.30 UT approx. (101Mb file)
30 June, video 2. Sony handcam with 5 second interval record. 00.00-01.15 UT. (64Mb file)
30 June video 3. Sony handycam with wide field adapter. 01.20-01.50 UT approx. (35Mb file)
A brief animation from July 2-3, 2008. 22:53UT-23:05UT.
For those who want to read a more about the physics of NLC here are two papers from scientific journals. The first is a general non technical review and the second is a more detailed look.
Are NLC truly a miners canary for global change. (EOS, 2003).
NLC as Possible Indicators of Global Change in the Mesosphere. (Advances in Space Research, 2001).
These are a good start with plenty of references.
A few years ago I organised the NLC 2006 meeting under the auspices of the Scottish Astronomers Group and in conjunction with the Mills Observatory, Dundee. This was an excellent gathering of NLC observers and had contributions from all of the world including almost all of Europe, the US and even as far a field as Kazakhstan! The report from the NLC forum Administrator, Tom McEwan revealed startling statistics for the NLC Observers web site and NLC Observers forum with hits totaling over 100000 coming from over 100 countries. These numbers clearly show there is a huge global audience who are interested in these mysterious clouds.
This is me, on the right, presenting results on behalf of the Kazakhstan team.
All being well there will be further meetings, probably to be held in Glasgow, stay tuned....
The International Association of Geomagnetism and Aeronomy (IAGA) published a guide "Observing Noctilucent Clouds". This was printed in limited numbers in 1995 and has been unavailable for some time. Recently, however the IAGA recently made an electronic version freely available. The can be obtained from http://www.iugg.org/IAGA/iaga_pages/pubs_prods/pubs.htm#ONC
It is the current definitive guide describing the various forms NLC take and issues of observing NLC. With recent (and rapid) advances in digital imaging technology the sections on photographing NLC are a bit dated technology wise but still represent a good starting point.
Whatever the reasons for their occurence, NLC are a beautiful and mysterious sight in the summer night sky!
With near real time and universal electronic communication available to almost everyone, NLC watching is an interesting pursuit which just might be telling us something about our home planet. Signs we may well ignore at our peril! Join in by going to the URL listed below and taking part in the NLC Observers forum. Amateur or professional everyone is welcome and encouraged to submit whatever observations/data and thoughts you may have!. It is the place to be on the net for NLC observers!
If you would like to join in with a truly international group of observers them then visit the NLC Observers Homepage.