Noctilucent clouds (NLC) are thin clouds that form at heights around 82km near the mesopause during the summer months. At this time of the year temperatures at this height are very low; at or below ~130K ( -143 deg C!). The formation process is still open to some debate due to the extreme conditions at these altitudes. The particles that make up NLC appear to be very small, of the order of a few hundred nano meters (nm). For comparison the wavelength of visible light is between 400-700 nm!
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 observed from they pretty much look the same! This demostrates the remarkable uniformity of the conditions in which NLC form. It is also a graphic demonstration of the required viewing angle. The observer, the sun and the NLC all need to be in the correct relative position.
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. Meteoric dust has also been suggested as a source of condensation nuclei for NLC formation but but the jury is still out. However they are a good candidate. Cosmic rays could be responsible for spontaneous nucleation. Anyone who has marvled at a cloud chamber will have seen this effect. It is remarkable to see! Yet another suggestion may be that they are a consequence of mankinds activity on the atmosphere and thus a visible impact of climate change. This is the one that seems to be attracting most attention. However....
There are many of uncertanties with NLC's. What seems to be almost accepted as fact in the recent press (popular astronomy magazines and even newspapers) is that NLC are occuring earlier and over a much wider latitude range. This, I personally feel, might be nothing more than a bit of mild climate change related hysteria. Apart from some visual observations that are reported from time to time, which cannot be categorically substantiated, there seems little evidence for NLC actually occuring earlier; that is in early May or towards the end of August; with increasing regularity. They do happen but it may be nothing more than natural variability. In recent years NLC have been spotted regularly during the last week or so of May through to mid August. (This can be seen from observations posted on the NLC Net web page detailed later.)
The formation of NLC is governed by temperature, humidity and significant seasonal changes to the winds that blow at these altitudes. The best displays tend to be seen in July in the Northern hemisphere when the region around the mesopause is at it's "wettest" but it must be remembered that this is a relative expression. This part of the atmosphere has extremely low humidity even during the "wet" months. How water is transported here is open to debate and feeds into the anthropogenic angle. The pressure at the mesopause is only around 0.01mB (That's about one one hundred thousandth of the pressure at sea level!) and at the mesopause it is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. With the added influence of the very strong radiative cooling from carbon dioxide, the mesosphere is the coldest region on Earth.
Some articles state that NLC are moving further south, well perhaps. 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). This is still EXACTLY the case at present! It seems that when NLC are reported from the mid USA or southern Europe (around the 45 degree mark...) it is deemed a major event. Whilst certainly being unusual these occurences have been noted over 40 years ago. This may be a case of short memories combined with that natural variability....
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 visually in brightness is quite difficult to do quantitatively! Brighter may be an analogue for "denser". This is an interesting possibility as that would show a change in local conditions in the region of NLC formation. Using data from satellites observing in the ultra violet, evidence may be emerging of this.It should be cautioned that the AIM satellite UV images are of VERY small particles of a particular size. Larger visible particles might behave differently.
The question is whether the recent reports about NLC's represent a real, long term, statisticaly sound change? With modern digital 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 whether NLC might be changing in a significant way and whether these changes represent an anthropogenic impact.
Current visual methods, those generally employed by observers on the ground rely on a subjective scheme of brightness estimation. Over long time scales this can show up trends but cannot be used for quantitative analysis because there is no way to determine an absolute calibration.
It might make for good press but when looking at such a variable natural phenomena one should always be skeptical about interpretations based on small changes over short time scales. It does pay, however, to keep an open mind!
Even with the current batch of Earth observing satellites and for all the observing difficulties, monitoring by networks of largely amateur observers is still absolutely essential to ensure that we have good long term records. It will take a considerable time before we can say with any certainty whether any claimed changes are real or not. It could take decades but that's where good regular amateur observations come to the fore!
So get out there and enjoy these strange and beautiful ethereal clouds of the summer twilight....
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.
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 be utilised to image NLC over a wide range of wavelengths. From near UV extending into the infra red. Due to their small size NLC particles scatter short wavelengths much more efficiently. As the imaging wavelength increases the NLC become "transparent" and seem to disappear. This allows an estimation of particle size to be made as Rayleigh scattering weakens.
In narrowband imaging the NLC appears quite different to their usual look. This video from July 2011 is probably the first time NLC have been observed in this way. It shows a much more ragged look, very similar to some types of cirrus clouds.
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. (I'll get these on to youtube sometime....!)
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.
Video's from 2011 season are on YouTube. One of the best NLC's I've seen was on July 2/3 2011. The video is here.
The standard text book on NLC is Noctilucent Clouds by M.Gadsden and W. Schroder published by Springer Verlag, 1989. For those interested in all aspects of NLC observing and theory then this is the key book!
Here are a couple of scientific papers about NLC published about a decade after the book. These are now quite old however but they introduce some of the climate change topics mentioned previously. 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.
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 to the issues of observing NLC and describing the various forms NLC take. With digital imaging the sections on photographing NLC are now dated technology wise but still represent a good starting point. The best thing to do is a few experiments to see what works best for your own camera system.
A few years ago I organised the NLC 2006 conference 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....
Way out....?, it certainly was out there, ....somewhere!
Since this meeting (6 years ago!) the website and forum have gone from strength to strength in no small part to the efforts of Tom. The site now receives several 100000's hit during the season and contributions from all levels of observer from beginner to professional scientists. It is the best NLC observing site in the world!
One can now easily do a search online. There are many sites with fantastic images and videos.
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. Whether amateur or professional, beginner or experienced 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.
For lively chat and the latest from observers join the NLC Observers Forum
As part of the continuous and ongoing development of the NLC Observers Network it is now on twitter. search #BillWnlc or #nlcnet