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Visit the Noctilucent Cloud 2005-2006 Gallery / 2007 Gallery and 2008 Gallery
For observers at mid northern latitudes the summer nights remain bathed in bright twilight hampering most forms of astronomical observation. These mild summer nights are a joy for amateur observers however deep sky observing and even the aurora are washed out to insignificance due to 'nautical twilight' persisting all night long. All is not lost as Summer nights bring with them a different kind of phenomena. If you are located between latitudes 55 degrees and 65 degrees north or south of the equator then you will be graced by the presence of Noctilucent Clouds or NLC's. The NLC season typically runs from late May until early August in the northern hemisphere and December and January for southern hemisphere observers however sightings outside this 'window' have been recorded.
In recent years I have been determined to observe every NLC display that appears each Summer period in the hope of gaining more proficiency with my observations and a better understanding of their appearance, frequency and movement by gaining as much experience as I can.
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My own introduction to NLCs began seven years ago when I accidentally encountered these eerie glowing clouds at the end of my Summer comet hunting sessions in the hours before dawn. I recall feeling mesmerized by the beautiful structure and colours they contained and knew deep down that this phenomena was very special, however at the time I did not realize what they were or their true importance. I gradually began researching through various books and magazines to get some understanding of what they were and how to go about observing them, I was fortunate enough to befriend a veteran NLC observer with 30 years of NLC observing under his belt at my local astronomical society who since has developed my interest in NLCs to a high level. |
The Summer of 2005 and in particular 2006 produced the best displays I have ever seen, I never realized that nature could paint such beautiful art work in the sky. Each display was unique and the best of them covered much of the entire night sky with breathtaking colours. These were unforgettable sights!
If you have never seen a large Noctilucent cloud formation before then be prepared to be amazed. These clouds promise you a profound visual experience and once you have seen a good one then chances are that you will be hooked for the rest of your life. Warning - be prepared to miss out on alot of sleep - you won't regret it.
NLCs bare little resemblance to normal clouds which form in the troposphere, a region of the Earth's atmosphere located 14 km (9 miles) high, this low altitude results in common clouds appearing dark against the Summer sky as they reside in the Earth's shadow hidden from the Sun's glare. Noctilucent is a Latin derivative which loosely translates as 'night shinning' and shine they do. The term 'Noctilucent Clouds' (Leuchtende Nachtwolken) was probably first coined by O. Jesse who was then active at the Berlin Observatory making the first photographic measurements of clouds. When the Sun is between 6 degrees and 16 degrees below the horizon the NLCs which are located at a height of 82 km in the mesosphere are illuminated by sunlight where they become visible to ground based observers.
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Very little is known about NLCs even today, in fact the details of modern NLC science could be presented on the back of a post card. These clouds are mysterious in the extreme and observations and images are sought after by researchers making this field of astronomy and meteorology one of the few areas where amateur observers can contribute to science. The genesis of the Noctiulcent clouds are as elusive as the clouds themselves and theoretical at best however a popular and plausible belief among researchers suggests that NLCs are made up of meteoritic dust particles incased in ice crystals. These ice crystals which contain dust particles from meteoroids, comets and asteroids are so small that their size is likened to the the |
particles in cigarette smoke, they are so transparent that they only reflect 1 in 1000 of the incident light they receive from the Sun hence why their tenuous profile is never seen in the day time sky. NLCs are very thin, measurements have suggested their optical thickness is in the region of 10 - 4 nm with cloud particles in the range of 50 nm.
The mesosphere where the NLC particles reside has a cool temperature of - 150 degrees celsius. Here, there is very little water but at this temperature molecules of water will form ice around dust were the process of nucleation induces the NLCs to drop by several kilometers to the altitude in which they are normally seen. The coldest temperatures here form over the Earth's polar regions during the Summer months which explains why NLCs are only seen at this time. Some researchers suggest that the presence of NLCs are an indicator of global warming, the reasoning being that increased temperatures at lower altitudes results in decreasing temperatures in the mesosphere inducing the process of nucleation and hence NLC formation. Much further information using modern ground based observations, historical observations and science gathered from the new AIM satellite launched in 2006 will shade new light into NLC formation. For now NLCs remain as exciting and mysterious as they have always been.
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It is well known that the first observation of an NLC was made by Thomas W. Backhouse in 1885 fully two years after the violent eruption of the Volcano on Krakatoa island. A Tsunami generation event which completely destroyed the Krakatoa island and sent immense concentrations of volcanic ash into the upper atmosphere which was thought by many to be the progenitor event that led to the formation of NLC's which began to be manifest with more frequency after the event however more recent studies suggest the event was not responsible for their formation at all. Garsden and Schroder (1989) debated on why there was no sightings of NLC prior to 1885 and suggested that the eruption was significant because it alerted observers to make careful studies of the Summer twilight sky. |
It's certainly a good theory however one must find it difficult to accept that no one watched the twilight sky at all predating the eruption.
As it turns out there is at least one possible NLC sighting that predates the Krakatoa event by 35 years. Dr Christopher John Butler research astronomer at Armagh Observatory, N. Ireland who is passionate in the field of historical meteorological and atmospheric phenomena highlights observations recorded in the observatories meteorlogical records by Thomas Romney Robinson who was the third director of the observatory at the time. Robinson made a series of interesting observations between 1849 and 1852 however two of his entries in May 1850 may describe Noctilucent clouds. One the 1st of May he notes 'strange luminous clouds in NW, not auroral'. This does sound very much like NLCs even though early May falls outside the typical NLC 'window' however it is still possible as NLCs can form at Armagh's latitude within this period. (Read the historical pdf doc)
An early observer of NLCs, Robert Leslie, gives a striking account of these clouds describing them as...'weird small cloud forms, at times very regular, like ripple marks in sand, or the bones of some great fish or saurian embedded on a slab of stone'. (Ken Kennedy 2006). A marvelous description which I am sure gave birth to the 'herring bone' classification structure in modern times.
Visual observations of NLCs are fairly straight forward however you will need to remark on a number of key factors to make your observations worth while for data collection.
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Measure the full angular extend of the display in degrees. North = 000 degrees, East = 90 degrees, South = 180 degrees and west = 270 degrees. The magnitude + 2.0 naked eye star 'Polaris' defines your northern point above the horizon. Example: you seen an NLC display extending from the west to 10 degrees west of north, your azimuth will be 270 degrees - 010 degrees. It can also help to measure the full length of the display using the hand method. Hold out your hand and arms length and spread your fingers. |
1) The distance between your little finger and thumb = 20 degrees
2) The distance between your 2nd finger and thumb = 15 degrees
3) Your full fist = 10 degrees
4) Three finger tips = 5 degrees
5) Your thumb = 2 degrees
6) Little finger tip = 1 degree
Measure the angular height of the NLC display using the hand method and if it is clear to see, measure the height of the base above the horizon although this will not always be possible as the base can merge with horizon murk or clouds.
The brightness of a particular NLC display can be estimated using the official 5 point scale.
Very dim NLC present, barely brighter than the twilight background, may have the appearance of a faintly glowing white mist or fog. The fainter type 1 displays may require averted vision and are mostly missed by inexperienced NLC observers.
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Clearly visible against the background twilight but still with a relatively low surface brightness. Type 2 NLCs can manifest as a distinct blue glow against the sky as a pre - cursor to increased structure development.
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This brightness level indicates a quite obvious display standing out clearly against the background twilight. They are unmistakable.
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Bright NLC display, eye catching, very easy to see and would be noticed by casual observers.
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Extremely bright and can illuminate objects facing the clouds. Often type 5 NLCs can wash out bright stars such as Capella by several magnitudes. These will impair your dark adaption and would be seen by many with ease and are often confused with the aurora by the general public. A type 5 NLC display is unforgettable!
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Each NLC display is composed with various forms of structure of which there are four main groups, Veil (type I ), bands (type II ), Waves or Billows (type III ) and whirls (type IV ). Each class is further split into four other specific classes. Occasionally NLCs will possess a structure that does not fit into the four main groups so another four point 'unknown category' exists. See the image examples below for all the major classes. An NLC display may consist of only one class or many of the classes at the same time. With a little practice you will become proficient with matching an NLC display with its appropriate classification letters and numbers.
Veil type NLC apparitions have no structure taking the appearance of a sheet, fog or mist and sometimes seen as a background to other forms. Seen on its own in the dead of night the veil can look ghostly! Bright type 3 - 5 veils are easy however type 1 - 2 are often confused with the Summer twilight glow by inexperienced observers. Can you tell the difference?
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These are streaks or lines which may be parallel, meet and/or cross at small angles. They are further subdivided into two more categories.
Bands with blurred, soft or diffuse edges.
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Bands or streaks with sharp well defined edges.
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The definitive NLC structure, these look like sand ripples on a beach at low tide or 'herringbone' structure and often seen in conjunction with type IIa and IIb bands. See Robert Leslie's personal description above. Waves are further subdivided into two category's.
Short, straight and narrow streaks.
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Wave like structure with undulations.
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Whirls consist of looped or twisted structures and are subdivided by radius of the whirl.
Whirls with small angular radius (0.1 - 0.5* )
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Simple curves with angular radius between 3 - 5 *
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Large scale Whirls
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The complex structure falls into four main category's.
NLCs which do not fit into types I - IV
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NLCs with bright knots
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Billows crossing a Band
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Net like structure, similar to a 'chess board'.
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This concludes the main classification system for NLCs and no doubt it will be revised in years to come.
In the rare chance that an aurora display coincides with an NLC display then this must be noted. Record colour, brightness, altitude and azimuth of any aurora present including its form and position in relation to the NLC display or simply take one or two photographs.
As with all kinds of astronomical observations one must keep note of the sky conditions. Record the presence of moonlight, murk, mist, transparency, seeing and the presence of any tropospheric clouds.
If you do not own a camera then why not try making a simple sketch of the NLC forms at 15 min intervals. Note the various NLC forms, height and azimuth and any comments on colour and movement. Four sketches during a 60 min period can show the NLCs development or decline very well. Record all your observations using universal time (U.T) or local time zone.
A few tips to bare in mind..
NLCs can appear in the sky at any time during the night, they often manifest after sunset, at midnight or before dawn so keep vigilant. A regular check of the northern sky every night and many times during a night is essential if you want to spot NLC activity. It is estimated that for a given NLC window period combined with average sky conditions, a single observer may see + 12 NLC displays during a season although this will largely depend on the frequency of clear nights from a given location. Do keep in mind one thing...premature retirement - this happens when a bright NLC display appears after sunset, later particularly in late July and early August when NLCs will appear to fade and drop in height as the night draws in. To many it would seem that the show is over for the night however this is only the Sun dropping lower below the horizon. Very often in the hours before dawn the NLCs will return as the Sun begins to climb again which can reveal a display that borders on breathtaking which will be missed by many so do keep this in mind.
High level Cirrus cloud can easily be mistaken for NLC especially just after sunset when the low level Sun illuminates these delicate cloud strips with a golden or red colour. Watch these carefully as Cirrus will betray its identity by its 'normal' movement and darker profile as the Sun drops lower. NLCs tend to appear in the region of sky above the Sun however formation east and west of the sunset point is not uncommon.
Use a pair of 7X50 mm or 10X50 mm binoculars to scan the twilight sky after sunset. This method can detect NLCs that would be too faint for the naked eye. NLCs differ from tropospheric cloud as they can bare magnification. Non NLCs will have soft edges with low contrast and a dark profile. Real NLCs are unmistakable!
When a large extensive NLC display is present you will often be startled by the sudden movement of these creepy glowing clouds. Keep your eye on the bright star Capella in the north and watch the movement of the NLCs as they drift by this star. This movement is often shocking and this affect is even more impressive in binoculars. NLCs have two kinds of movement..
1) Watching carefully you will notice the NLC formation drifting to the west (left) in real time - this effect is due to the rotation of the Earth. Keep in mind that NLCs move at 400 mph.
2) NLCs will appear to either drop in height or grow in height. The mechanism responsible for this is the Sun which lurks close by under the horizon. NLC altitude will drop with a lowering Sun after dusk and rise with the climbing Sun before dawn.
Taking several images separated by 5 - 10 minutes will show this movement easily. Often the waves or bands of a type 5 display will occult Capella as they pass causing the star to fade and brighten like a cataclysmic variable star. In some cases Capella will be washed out completely by a display which can be several magnitudes brighter than this star!
One of the most striking visual aspects of the NLCs are their colour which is mostly due to the position of the Sun below the horizon producing atmospheric effects. At the horizon which often merges with the base of NLCs one can see red, orange, yellow and golden colours. Moving higher into the main body of the display the NLCs typically sport stunning white or silver colour however green is not uncommon. The top of the display may well have a striking electric blue or purple canopy. The general blue colouring is caused by absorption of incident sunlight by ozone in the Chappius bands which reside in the yellow portion of the spectra (Gadsen and Parviainen. 1995). These colours combined together can make for a beautiful art work against a canvas of stars. A short exposure with a camera will capture these very well however be careful of overexposure which will reveal an unnatural 'white' image. An underexposed image will capture the colours more accurately as after all you want to record this phenomena as accurately as can.
The use of a polarizing filter for visual and photographic observations can produce successful results. These filter out certain rays of light and select others giving a view or image with increased colour saturation and contrast. They come in two main types PL (Linear Polarizing) and PL - CIR (Circular Polarizing). The PL - CIR is recommended unless you have a manual focus camera with no beam splitter. These filters can be purchased from any good camera retailer.
NLCs are a beautiful photogenic subject and images taken of bright displays are not only satisfying but are of use to researchers. Getting an image of NLCs is easy but getting a good image may be a little more difficult. Good images of very bright displays can be captured using an ordinary off the shelf digital camera. Attach the camera to a tripod, switch your camera to night mode which will take an exposure from 1 - 3 seconds, select your highest ISO setting and set your focus to infinity using the 'mountain scene' function. Set the timer for between 2 - 10 seconds to reduce vibration and take your shot. Type 4 and 5 displays will be easily captured however the fainter types will be rather more difficult and will involve simple post image processing techniques (contrast increase, adaptive lightning, saturation increase) to 'tidy' the image into an acceptable result. If you do not have access to a tripod then improvise using a window sill, fence post or car rooftop etc...be creative.
With more expensive 'bridge' or top of the range DSLRs the process is a little different. An ISO setting between 200 - 400 combined with an exposure between 5 - 15 seconds, wide aperture and of course a timer and tripod to reduce vibrations. You can achieve delightful results if your camera has a high optical zoom function ( 5 - 10X ) or you own a telephoto lens. Zoom in close to your target area and manually focus your camera using a bright star before hand like Capella, Arcturus or a bright planet and take your exposure. Some caution is required here as the NLCs may trail along with the stars so dropping the exposure down will produce more pleasing results. These settings are meant to be a rough guideline as the NLCs themselves or the sensitivity of your camera may need very different settings. The general idea is to capture the NLC display clearly without over exposing the clouds themselves our the sky background, bracket your shots and you will find the perfect setting that is right for you.
This is all you need to capture 'pretty pictures' but if you would like your images to be of scientific value then you will have to go about things slightly differently. Always take your images on the hour and every quarter of an hour after that. Example: 23.00/ 23.15/ 23.30/ 23.45/ 00.00. The reason for this is very straight forward - two or more observers at different locations who take images at the same time will have collected enough data for triangulation which can determine NLC height.
Selecting a suitable foreground for NLC observation and photography is important. Essentially you will need a clear unobstructed horizon along the NW - NE sky sector as many displays are of very low altitude and would go missed completely. A dark location away from lights is helpful although NLCs do stand up very well to light pollution in comparison to the aurora. Select a region of the countryside with a group of trees or small buildings to give a sense of scale. To make your images more exciting try to include other astronomical and meteorological phenomena within the NLCs such as Iridium Flares, Meteors, Aurorae, lightning and bright deep sky objects such as M45, M31 or M34. If you can get a crescent Moon with Earthshine or a planet in the frame then all the better. You are only limited by your imagination.
Some observers set up a web cam or video camera on their home roof top pointing to the north all night while they go to sleep. The next day they simply go through the footage and extract individual NLC images during the night or use a number to create a short animation! Type 5 NLCs can even be filmed using digital camcorders and I have been known to capture images using the camera in my mobile phone with surprisingly good results.
Mysterious, beautiful, colourful, stunning, unpredictable and photogenic, NLCs are not to be missed so make them a part of your life on warm Summer nights. What will the next NLC season bring?
I strongly encourage everyone to report all visual NLC observations to the UKNLC Homepage run by Tom Mc Ewan.
I would also like to invite everyone interested in NLC observation, discussion and history to join the new UKNLC Forum run by Tom Mc Ewan. This unique forum is the first of its kind and just recently launched in 2007.
I would also encourage you to send your NLC images and reports to Ken Kennedy (BAA NLC & Aurora section) at the following email address: ken.kennedy42@btinternet.com
I wish you an exciting and rewarding NLC season!
Martin Mc Kenna
'Observing Noctilucent Clouds' by M. Gadsden and P. Parviainen, originally published in 1995, now available in pdf format: (below)...