Noctilucent Clouds Noctilucent CloudsNacreous and Noctilucent clouds are both phenomena of the high atmosphere, but at very different levels. While "ordinary" or Tropospheric clouds (the ones which give us our weather) are just a few miles high, Nacreous clouds are around 12 miles high and Noctilucent clouds are an incredible 50 miles up. They look totally different from each other, being "rainbow coloured" and "electric blue" respectively, and should therefore never be confused. Neither form is seen in daylight skies, which helps one to distinguish Nacreous clouds from their Iridescent cousins. These also have rainbow fringes, caused by the similar-sized droplets of which they are composed breaking up the sunlight, but are low-level phenomena usually seen quite close to the Sun which clearly resemble "ordinary" clouds. Noctilucent and Nacreous clouds on the other hand are only ever seen near twilight (at dusk or dawn), have a wispy or veil-like structure, and can be very bright.
| Both types of cloud appear to "glow" in the sky because they are still illuminated by the Sun while the tropospheric clouds around them are in shadow. The diagram on the right shows the basic geometry. As Noctilucent clouds are higher than Nacreous, they remain visible for longer - for up to 2 hours after sunset compared to the 1 hour or so for Nacreous. The increased darkness can make their appearance even more dramatic and unexpected. |  |
Named for their resemblance to mother-of-pearl, whose visual appearance is created by the organically produced crystalline substance nacre, nacreous clouds are composed of small (about 10 micro-metres across), similar-sized, ice crystals which form when the lower stratosphere cools to below -85 Celsius. These scatter incoming sunlight, producing a circular "halo" round each of them. As the degree of scattering depends on the wavelength (i.e. colour) of the light, the overall halo is in fact composed of superimposed halos of different colours: this gives them the appearance of a "mini-rainbow". If the crystals are of a very similar size the halo produced by every crystal will also be the same size, meaning they will reinforce each other to give the colouration characteristic of nacreous clouds.
Because they require very low temperatures, nacreous clouds are hardly ever seen below high latitudes: it is only on the rare occasions when the jet stream diverts the polar vortex southwards that they may be seen in the UK. One such occasion was in early February 2016, when for a few days they were glimpsed all over the country. I saw mine more-or-less by accident, when I glanced out of the window while closing the curtains! It wasn't very dramatic, but it was definitely there.
The images shown here were all taken around 5pm on 2nd February 2016, just 20mins after sunset and with the Sun only 2deg below the horizon, and so the sky is still quite bright. Despite this, one can clearly see that the "weather clouds" are in shadow while the "coloured clouds" are still illuminated: this is characteristic of nacreous (and of course noctilucent) clouds. Also, the "coloured clouds" don't really look like clouds at all, more like a plastic film or bubble floating in the sky - again, characteristic of nacreous clouds. The time sequence is left to right and then down to the second row, showing how the clouds developed over a period of a few minutes. The two images on the right are taken with a "longer lens" by the way, rather than being close-ups cropped from other images.
The dark clouds approaching from the right soon obscured their nacreous cousins unfortunately, and by the time skies had cleared the coloured veils had disappeared. I had reports from a friend that there was a "nacreous display" early in the morning of the next day but that was it from this part of the country as atmospheric conditions changed very soon afterwards, removing the frigid conditions necessary for nacreous clouds to form. Still, I was pleased to have had an opportunity to see this rare phenomenon, if only briefly.
The literal meaning of 'noctilucent' is "lit at night" or "night shining". This refers to the fact mentioned above that Noctilucent Clouds (NLCs) are only seen in the deep twilight when, because they are so high, they are still sunlit. The best time to look for NLCs is when the Sun lies between 6 - 16 degrees below the horizon. Look from west to north 60 to 90 mins after sunset, and if you see luminous blue-white tendrils spreading across the sky you've probably spotted a noctilucent cloud. Alternatively, book a trip to the ISS - they've been photographed from there as well!
NLCs are tenuous cloud-like formations, thought to be composed of small (a mere 0.3 micro-metres across) ice-coated particles and possibly seeded by the microscopic dust particles left behind when meteors burn up on entry; their precise nature remains a mystery however. Their distinctive colour is not due to the "halo producing" effect present in nacreous clouds, as the particles are too small to scatter light in this way, but to the absorption of red and yellow light by the ozone in the atmosphere at high altitudes. The characteristic wave-like structure of NLCs is thought to be caused by "gravity waves" - pressure and temperature fluctuations produced by storms at the Earth's surface propagating upwards into the mesosphere and disturbing the particles there.
Each year, NLC incidence tends to peak just prior to and for a few weeks after the summer solstice - from mid-May to mid-August for northern hemisphere observers. This is because the very low temperatures needed for ice to form so high in the atmosphere (below -123 Celsius) are only present around this time (because of the effects of the global circulation in the middle atmosphere). Also, the twilight conditions which render NLCs visible impose a latitude restriction on their visibility. They are, therefore, most often seen from locations which lie between latitude 50 and 60 degrees: through most of June and July in the northern hemisphere these locations never attain true astronomical darkness and twilit skies persist all night. There is some evidence that NLCs are being seen more & more frequently and at lower latitudes though - some theories link this to climate change.
But let's see what they really look like:-
They return in 2024
I had never seen NLCs (or perhaps just had not recognised them for what they were!) so specially looked out for them during summer 2009 as this had been a particularly good "season" elsewhere. I was rewarded on 14th July when at around 22:25BST the sky lit up with the characteristic blue-white glow. The Sun was 9deg below the horizon at this time - just right for NLCs. A patch of "normal" cloud is visible to bottom left, showing the great difference in appearance between it and the NLC: note that this cloud is dark, as it is in shadow (as illustrated by the diagram above).
The NLCs pictured show the typical characteristics of this type of phenomenon, which I show expanded here to give a better view. On the left we have the uniform white illumination of "stratus"-type cloud while on the right the rippled effect of "cirrus"-type. Note that the rippled sections are only lit on their lower edge i.e. from the direction of the setting Sun.
Although I kept a general look out for NLCs after that, nothing exactly jumped out at me until almost exactly 6 years later, on 17th July 2015 in fact, when, travelling home quite late at night from a concert, I happened to look due north and saw something bright in the sky. Given that the time was by then heading for 11pm, this was strange! On arriving home I quickly ran to a vantage point and looked north again, but by this time whatever it was had reduced in intensity quite considerably. It did still look "NLC-like" though, so I ran back to get my camera to take some shots. By the time I did so it was just past 11pm, fully 2hrs after sunset, and so the Sun would have been 12deg below the horizon - this, coupled with the fact that the cloud on this occasion was very low down on the horizon, no doubt accounted for the less impressive appearance of this sighting compared to the previous one. The pictures did seem to confirm it was an NLC though, as it had the distinctive "electric blue" colour, so I'm definitely counting it!
These pictures show just how much darker it was than on the last occasion - and that's with a 4sec exposure at ISO800 and slight enhancement afterwards! The pink glow of twilight is still visible on the horizon though, below a dark band of "normal" cloud. There's also a thinner band overlying the noctilucent cloud, emphasising the difference between the two types. The star just visible to upper-right centre in the left-hand image is Menkalinan (magnitude 1.87). It is better seen in the right-hand image, together with Pi Aurigae (mag. 4.28) just above it.
One can calculate the distance the NLC is away from its elevation above the horizon. There is another star, Psi Aurigae, visible within the left-hand side of the NLC in the original images, whose elevation is 5deg 28.4min. Using this value, and taking into account the curvature of the Earth, assuming the NLC is at a height of 50 miles its distance is about 360 miles! It was thus over the northern part of the North Sea when observed. While this might sound unlikely, images captured by the NASA AIM satellite show that it is true.
 | Indeed, 360mls is very close. I trawled through several years-worth of AIM images and found only a few where NLCs were shown just to the north of the UK at a time when they could have been observed, of which this one is perhaps the best. It is only a partial representation of the situation though, as the satellite captures just the swathe of atmosphere below it, not a "wide angle" image. Maybe more importantly, the imaging device on the satellite "sees" in the ultra-violet, not the visible, and is not particularly sensitive as it is designed to examine the bright Polar Mesospheric Clouds around the North Pole. The relatively faint NLCs at lower latitudes (such as those seen from the UK and Europe) may therefore not be detected: note that the edge of the lighter-blue area in the image cuts off somewhat sharply rather than slowly fading away, as one might have expected. It is thus probable that NLCs below the detection threshold were present farther south. And this isn't just my excuse to explain why there's little to be seen - I contacted the mission controllers to check! However, it does explain why seeing NLCs from the southern part of the UK and from Europe is relatively uncommon. |
For further information on the AIM mission, click here or to browse the images click here. A new window/tab will open in each case.
I wasn't granted another view until 4 years after that when on 21st June 2019, the Summer Solstice, on returning home at about 10:20pm I happened to glance at the northern sky and saw an extensive display of blue-white ripples so distinctive of NLCs. These persisted until about 11pm, with some wisps still visible until 11:15pm. Many pictures were taken, a few of which I am showing here.
I reported the sighting to the Aurora and Noctilucent Cloud section of the British Astronomical Association, who replied to say that many such sightings had been reported that night, from as far south in the UK as Sussex. The "Spaceweather" website reported extensive sightings on the Continent, as far south as Italy, during what was described as the most remarkable season for NLCs in decades.
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| Looking North at about 10:30pm | A close-up shot shows much detail in the wave-like structures |
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| A slightly lower point of view reveals more complex structure nearer to the horizon | |
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| A view somewhat to the West shows the great extent of the phenomenon as the NLCs begin to fade towards 11 o'clock | |
Having waited 4 years between the last two observations, the next interval was just exactly one calendar year, when on 21st June 2020, again the Summer Solstice, I looked out of an upstairs window at 11:10pm upon getting out of the bath (!) and saw just a hint of blue-white ripples low down to the north. I wasn't particularly surprised, as the "Spaceweather" website had reported sightings at low latitudes in the USA in the previous days, but it was still a thrill to see them. I quickly donned trousers and a top, grabbed my camera & tripod, and headed outside with my hair still wet.
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| A wide-angle view of the main display. The star visible to top left is Capella |
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| Panning across the display from right to left shows more of its detail |
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| And as we see a second more whispy display another star appears - Menkalian |
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| Before we finally reach the farthest extent of the display |
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| A close-up and then telephoto view of the display shows firstly the contrast between the dark "normal" clouds and the bottom-illuminated, glowing, NLCs, and then their distinctive wave-like structure | |
I continued observing until about 11:45pm but the display continued beyond that time. I again reported the sighting to the Aurora and Noctilucent Cloud section of the British Astronomical Association, who replied to say that sightings had been reported from as far south in the UK as London, which is quite unusual.
Any bets on another display at the Solstice in 2021?
Or how about just a mere fortnight? Maybe having a bath on a clear summer's night is the way to encourage NLCs, as on 5th July I again looked out of the upstairs window upon wrapping myself in a towel and, lo & behold, there they were again - but rather earlier this time as it was just 10:30pm. On went the trousers and top once more and out I ventured, camera & tripod in hand.
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| The display was shifted round to the east a little as compared to the Solstice and so I had to take up a slightly different view-point - which accounts for the different relative positions of the pylon and telegraph pole to the left. Otherwise, I adopted the same procedure as for the previous display. This then is a wide-angle view of the main display |
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| As before, panning across the display from right to left shows more of its detail. Note that the earlier time has resulted in the sky being considerably brighter and a very different colour |
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| Note also that this display has a very different structure from the one a fortnight before, with much more fine detail visible. By comparison with the appearance of the displays in earlier years, this may well be because of the different angle of illumination by the Sun, given that it is not so far below the horizon on this occasion as compared to on the solstice |
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| This fine structure is really very intricate, with many of the wave-like eddies so characteristic of NLCs. The final image shows white NLCs below the electric-blue ones, with a structure more like the Solstice display. As with the amount of detail visible, the colour is also probably an effect of the different illumination angle of clouds at different heights and/or distances. However, it is well-established that NLCs do appear in several forms - the electric-blue ones here would be described as "Type IIIb" (undulating billows) and the white ones as "Type IIa" (diffuse bands) | |
I was forced to cease observing at about 10:45pm, when my camera battery decided to die (!), but in any case the display had disappeared by 11:05pm. I once again reported the sighting to the Aurora and Noctilucent Cloud section of the British Astronomical Association, who again replied to say that sightings had been reported from as far south in the UK as London, and there were also many such reports on the Spaceweather website
All bets were off for the timing of the next display, but if anyone drew 23rd June 2021 in their local sweepstake then they would have won the money!
I was beginning to wonder whether the 2021 solstice had passed me by when I looked out of my north-facing upstairs window a couple of days later (no, not just after a bath this time!) and saw the tantalising signs of another appearance of those blue-white ripples. The "Spaceweather" website and the British Astronomical Society had both reported sightings in the previous days, but cloudy skies had prevented me from seeing anything myself. So - out with my camera & tripod to see what I could capture.
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| A wide-angle view of the central section of the main display. The star visible just to the right of the brightest area, well above the electricity pole, is Capella. I would categorise the structure as a mixture of Type IIb (bands with sharply defined edges) and Type IVc (large-scale whirls) - very different from the sightings in all earlier years |
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| The detailed structure of the clouds was very complex, and reminiscent of the visible surface of Jupiter. These are Type IVc structures. There's even a Great Blue Spot! (top left of first image). Dark wisps of "normal" clouds can just be seen overlaying their noctilucent cousins. In the right-most image, Capella is to far right and Menkalinan is on the same level, about a quarter of the way in from the lefthand side (also visible at upper centre on the left-most image) |
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| As the Sun set further below the horizon the highest level of blue colouration slowly disappeared but the tendrils to the right became more pronounced. Capella can again be seen above the electricity pole |
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| A panorama of the entire display, built from a series of images taken while panning across from left to right |
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| This device allows you to move across the full panorama, in order to better appreciate its structure. Click on the arrows to move either left or right. When you get to farthest left or right the image flips round to the other end |
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| Finally, a wide-angle view of the far right of the panorama, not seen at this scale in earlier views. Note the remarkable dark tendrils extending across the centre of the entire view and the separate section of noctilucent clouds between the main blue-white display and the tree to far right. The yellow-red colour of this section is a result of its very low elevation, descending into the dusky sunset | |
I mentioned in connection with the display in 2015 that there is a satellite in orbit round the Earth, known as AIM, which takes regular images of the high latitudes in order to detect Polar Mesospheric Clouds, the close cousins of Noctilucent Clouds. I also implied that because these images cover quite narrow swathes of the atmosphere below the satellite it is rather unusual to find one of them which includes the UK at just the time when NLCs had been observed. If you are interested to find out why this is so, click here. Otherwise, read on!
As with all scenarios where chance plays a part, occasionally you get lucky and the 2021 display was one of those times. Not "direct hit" lucky, but pretty close.
 | On the left is a small section of the data captured by AIM on all of its orbits on 24th June, showing part of the swathe taken at 09:46 GMT that morning. While not at exactly the same time as when observations were in progress in the UK, it is reasonably close and so should be indicative of the situation late on the evening of the 23rd. As can clearly be seen, there are extensive amounts of bright cloud to the east of the UK, extending south from the Shetlands through the North Sea to the near continent. It is also clear that the clouds must have extended further east, past the edge of the swathe, thus accounting for the reports received from countries bordering the Baltic. Indeed, an extract from the orbit swathe showing more of Europe (on the right) has traces of cloud much further south, extending through western Germany almost to Italy. Unfortunately, as these traces are right on the eastern edge of the swathe, it's not possible to say how extensive they were. |  |
Pretty good evidence to explain why the display was so good that evening, I think!
Unfortunately, skies were then cloudy for many days so no more sightings were glimpsed until July came round. Then, on the 4th, I peered out of my north-facing window after (yes, you guessed it!) getting out of the bath and was sure I could see something glistening low down near the horizon. Nothing for it but to quickly towel-down and head out to my usual viewing site (the local allotments!) with camera & tripod.
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| And, as it turned out, I was right! Not as bright or as high in the sky as the solstice display, but definitely there - the display might look brighter, but I used a longer exposure than previously to capture it, as you can tell from the way the stars stand out much more. The bright one to upper-left of centre is Capella again, with Menkalinan mid-left & down a bit (with Pi Aurigae just above it) and Al Anz to the right & down from Capella. The one visible to far right is Epsilon Persei - it doesn't have a proper name - and, finally, the faint one close to the top at mid-right is 53 Persei |
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| As we zoom into the central section the structure becomes clearer. Again we can see again Menkalinan, Capella and Al Anz (from left to right), with Pi Aurigae now more obvious above Menkalian. Note how the display is all well below Capella whereas that in June had Capella sitting right in the middle. This is because the clouds this time were farther away from me. I would categorise the cloud structure as Type IIa (diffuse bands) |
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| The lower altitude of the clouds as seen from my location accentuated the difference in brightness across the display caused by the more acute angle at which the last rays of the setting Sun, now well below the horizon, were illuminating them, and also brought out lovely gradations in the yellow, green & blue colours of the clouds as they gradually faded into the twilight sky. |
But the displays didn't quite stop there! Skies were clear for a couple of days after the 4th but no obvious NLCs showed themselves in the usual direction. However, on the 6th I had this feeling that the rather brighter cloud streak to the North-West might just be what I was looking out for, so off I went again with camera & tripod.
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| And this is what I saw. Definitely not the usual bright structured display but instead just a slight wisp - clearly the right sort of colour though, and so I felt sure it must be an NLC. I thus reported it to the BAA and was relieved to hear that a member in Epsom had observed something and so I wasn't imagining things! Very much the least impressive display so far though, which actually only lasted for about 15mins around 11pm. But that was my hat-trick for the year, and with some of the "season" still to go I was on the look-out for more displays. |
And yes, as the evening of Bastille Day, 14th July, came around I was in luck once more - this really is a good year! Not only did the clouds appear again but this time it was against a backdrop of "red sky at night", making the view really dramatic.
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Quite a low-altitude display again but a clear horizon allowed it to be set against the glow of sunset. The star just visible to top centre might be thought to be Capella again but in fact it is Menkalinan - Capella disappeared off the top when the image was cropped to make it comparable to the one below! |
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Although this image looks similar to the one above, it is in fact a panorama constructed by "stitching together" seven smaller images using the program Hugin rather than being a crop of a single wide-angle image. It is more obviously dissimilar in another way though - this display has a colour & structure which is substantially different from that of the display above! It really was taken on the same evening though, and just 15 minutes after the first one - note that Menkalinan has moved slightly to the right. The star now visible high above the small electricity pylon on the right is Al Anz, by the way. The images look so different because of two factors - the altitude of the Sun and the speed of tropospheric winds. As the Sun descends further over the horizon its rays strike the clouds at a more acute angle which tends to direct more of the reflected rays towards an observer, making the clouds brighter. Winds in the troposphere can easily exceed 100 mph and so can quickly disturb any high-level clouds, making the display of NLCs change quite quickly. Such changes are not always noticeable to the naked eye though as, because the clouds are so far away, the evolution of the structures can be quite subtle from moment to moment, only becoming obvious in images taken some time apart. And what about the star over the "spiky" tree to the left, you might say? Click/tap on the image to magnify the relevant area if you can't see it (and again to revert to the original image). In fact I'm pretty sure the object was not actually a star. Examination of individual frames taken at about the time I was panning across for the panorama shows that the object wasn't there at 23:17:24 but it was there at 23:17:52. I think I can rule out an aircraft as there is only one bright object in the frame despite it being an 8 sec exposure - aircraft navigation lights flash more frequently than this. This really only leaves a satellite "flare"; a sudden momentary brightening caused by sunlight reflecting off a shiny surface on the satellite. The most well-known flaring satellites were the early constellations of the Iridium communication satellites, but in principle any satellite can flare. | |
 | The possibility of it being a flare was, I felt, increased by the unexpected sighting of not one but three bright but very brief flashes in different parts of the western sky when awaiting a pass by the ISS three days after the NLC display, and indeed reports of similar phenomena on online forums. Also, close examination of an enlarged view of the bright object, as in the image to the left, shows it to be a streak whose centre is much brighter than the ends - a classic characteristic of a flare. A star, even if "smeared out" by its motion during a long time exposure, would show as a short line with sharp end-points - quite different from the object in this image. I must therefore conclude that this was a remarkable but entirely fortuitous capture. |
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Close-up views of the display show that although the cloud structure is predominantly Type IIa (diffuse bands) there are also areas showing interesting waves & ripples. The star in the left-hand image is Menkalinan, and in the right-hand we see Al-Anz (with Eta and Zeta Aurigae visible between it and the pylon) |
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And that was it for the year. No more sightings plus a certain amount of cloud to hide anything that might have been there ended my "season". I thus looked forward to the 2022 Solstice with eager anticipation!
And I was not disappointed! Right on cue, a slight glow low down in the north after midnight on June 21st persuaded me to take my camera & tripod on another excursion up to the allotments. The display was not particularly extensive or bright, but great to see.
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| A wide-angle view of the display, slightly enhanced to compensate for the lack of brightness. The brightest stars visible are our old friends - to right of centre we have Menkalian with Pi Aurigae above and Capella is at far right. |
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| A close-up view brings out the detailed structure, which I would classify as Type IIb - diffuse bands. |
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| Although it's not always easy to notice this by eye over a short period, the structure of NLC displays can change remarkably quickly. This animation was constructed from images taken just 100 seconds apart! It should be no surprise that displays change though, as they are just clouds like any other, although made up of different types of particle than "daytime" clouds, and so will be affected by winds in the same way. |
I then only had to wait a couple of days before getting my next view, which was more extensive and much brighter than the first, and also closer to the horizon - meaning it was somewhat further away.
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| This wide-angle panorama, at the same scale as for the display on the 21st but from a slightly different viewpoint, shows how much more extensive this display was than the previous one. Also, it is "as taken" rather than being enhanced, showing how much brighter it was. Once again, in the middle we have Menkalian with Pi Aurigae above and Capella further right. It is also possible to see, below and to the right of Capella, the three stars making up the triangle known as the 'Haedi', or Kids. Capella means "little [female] goat" in Latin and these stars are traditionally said to be her offspring. |
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| More detail is again shown in this "wide-angle closeup" (if that's not a contradiction in terms!). Note how far the display has moved to the left as compared to the image above and how much the overall structure has changed. This was in a period of 25 minutes. |
I noted above that the display on the 23rd was closer to the horizon than that on the 21st, implying it was further away. In fact, it was not all that much closer to the true horizon, which was of course not visible because of the line of trees and other obstructions on my apparent horizon. I thus estimated the true elevation of the displays using the known elevation of the stars visible in the images. This showed that the centre of the display was at 2.33 degrees elevation on the 21st and 2.03 degrees on the 23rd - a much smaller difference than might have been assumed from the images above.
To work out how far away the displays were, I devised a spreadsheet which uses geometry and trigonometry to calculate NLC distance from their observed elevation, taking account of the height of the observer and of their local horizon, as well as the slight variation of the Earth's radius with latitude. I found that if one assumes the NLC were 55 miles above the Earth (a reasonable mid-point estimate) then the display on the 21st had a line-of-sight distance of 530 miles, with that on the 23rd being 545 miles distant. This puts them over a point about two-thirds of the way from Orkney to Shetland, to the north of Scotland.
 | While this might be thought unlikely, in fact it is entirely plausible as can be shown by considering data taken by the AIM satellite, which I have referenced a number of times on this page (click here for further information). Its coverage of the UK is intermittent and so it is unlikely that an image will have been taken at exactly the time when NLC observations were made. However, on the left is a small section of the data captured by AIM on 27th June 2022, showing part of the swathe taken at 11:34 GMT that morning. Extensive amounts of bright cloud can be seen to the north-west of the UK, extending south-east from Iceland towards the north coast of Scotland close to the location of the Orkney & Shetland islands. This clearly shows that displays of NLC located in the position indicated by my calculations are indeed quite possible. |
And, disappointingly, that was it for the year. Somewhat unusual to see two displays a couple of days apart and then nothing, but reports from the BAA Auroras & NLC Section were also quite sparse. I may possibly have missed some displays due to patchy low cloud in just the wrong place (despite the very dry summer overall) and because I rely on looking north out of an upstairs window with a limited field of view to see what's about rather than regularly walking to my observation position (it's quite a long way!). A further confusing factor is the sometimes quite extensive skyglow in the relevant part of the sky from the town about 8 miles away. This tends to be yellow rather than the white or blue of true NLCs but can easily persuade you that there's something to be seen - while a stroll in the dark just to find that there actually isn't can be pleasant, it can also be frustrating! Ah well, here's to the Solstice 2023.
But no - NLCs around the 2023 Summer Solstice were in fact a complete no-show, as they were at all other times as well. I might have missed them of course, but they didn't make their presence felt to me I'm afraid and I'm sure the rather rainy weather around then will have decreased my chance of seeing anything in any case. Maybe 2024 will oblige?
This time it was "Ah yes!". Not quite on the solstice but in fact on Midsummer Day 2024. While contemplating the running of a bath just before 11pm BST on 24th June (baths and NLCs seem to have an intimate connection here, as you can read in my other reports!) I looked out of the (north-facing) landing window and saw some wisps of white'ish cloud together with other very dark formations. The white cloud was definitely not typical of NLC but the fact that it was white rather than black, i.e. still illuminated by the sun well after sunset, suggested that it was indeed what I was hoping for. A quick trip up to my usual "clear northwards horizon" viewing site (aka the allotments) convinced me the clouds really were NLC though, so I returned with my camera & tripod and took the following images.
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| Here's a wide-angle view of the display as I saw it on arrival at my observing site, slightly adjusted to compensate for a degree of over-exposure of the original image due to the sky being surprisingly bright. The star just visible half-way up, directly above the electricity pole (to the right of the central pylon), is Capella and that to far right is Mirfak, with Gamma Persei above and further to the right. I think you can see why I was doubtful whether the white cloud was noctilucent as it definitely doesn't have the classical NLC structure. |
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| The situation is a little clearer in this zoomed-in shot of the central section of the display, though it is still difficult to accurately classify it as there are elements of several forms present - predominantly Type II to the right, both Type IIa (bands with soft edges) and Type IIb (bands with soft edges), with Type IVb (medium-scale whirls) to the left, all overlaid onto Type I (a veil), as seen between the main display and the powerline. Capella is now more easily seen though, above the electricity pole. |
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| This image shows the great difference between the visual appearance of "ordinary" (tropospheric) clouds, which are dark as they are no longer illuminated by the Sun, and noctilucent clouds, which are still illuminated because their higher altitude enables them to catch the Sun's rays later into the night. The two types of cloud appear to be in the same position in the sky but this is a "geometric illusion". Although the NLC are much higher than tropospheric clouds they are also much further away and so the angle of view of the two types is often very similar. Because NLC are much higher than tropospheric clouds they are affected by different wind patterns so often move in different directions. Click or tap on the above image to see this in action (and again to return to the original view). The situation is complicated by the tendency of tropospheric louds to dissipate as well as move, but compare the motion of the dark cloud to the right of the image (which moves from left to right) with that of the NLC in the centre (which moves from right to left). Note that Capella makes a brief appearance at far right of the last frame! |
| Although the NLC became slightly more extensive as time passed they were beginnig to fade by 11:25pm, and cloud was continuing to come across from the west (left), so I ceased picture taking at that point. And as it turned out, that was it for the year. I thus look forward to the 2025 "season". |
