Orwell Astronomical Society (Ipswich)
Noctilucent Clouds, 1993 - 2022
When the light evenings of summer arrive, it can be tempting to give up looking for celestial objects and instead study cloud formations! In fact, some clouds form at such high altitude that they are of interest to astronomers: noctilucent clouds. They usually appear as complex interwoven streaks or knots, often pearly white, sometimes with a blue tinge, and may have a golden lower edge. They look similar to cirrostratus; indeed, there is a risk of mistaking cirrostratus illuminated by streetlights or moonlight for them. The term noctilucent comes from Latin and means luminous at night.
The webpages by John Murrell provide an excellent introduction to noctilucent clouds, the phenomena that they can present, and how to observe them. Also check the link from Murrell's pages to a document produced by the International Association of Geomagnetism and Aeronomy on noctilucent clouds.
Reports by members of OASI of observations of noctilucent clouds are below.
I rose early in the morning of 11 August to observe the passage of the latest Starlink constellation (launched at 02:14 UT on 10 August 2022). Although I saw the satellites exit the Earth's shadow, they were back-lit by the Sun and poorly seen. However, looking northward I saw noctilucent clouds per the video below! The main stars in the background are of Ursa Major. The pair Mizar/Alcor comes in from the left part way through. The Sun is about 9° below the horizon.
Noctilucent cloud visible just after midnight on 21 June 2022, looking NNE from Needham Market. (Note that a catenary of power lines runs above the middle of the image.)
A modest display of noctilucent clouds in the northwest.
Image taken at approximately 21:45 UT. Nikon D3200, 14 mm lens, f4.5, 1/6 sec exposure.
On 22 June 2021, the BAA Alert service advised the presence of a widespread display of noctilucent cloud. Although the sky over Suffolk was clear, I could not see any evidence of the phenomenon. Nevertheless, the event signalled that the 2021 noctilucent cloud season had arrived.
On 23 June, there were signs of NLC activity even before it was dark, on the northern horizon and extending to higher altitudes. I deployed a Sony A7S with 50 mm lens in an upstairs window providing a good vantage point for the horizon and a second A7S with fisheye lens in the front garden to capture the whole spectacle. (With the second camera, I had to accept the usual urban hazards: the neighbouring houses and the street lights).
The cameras have a resolution of approximately 4200x2800 px. For each camera, I produced two time-lapse videos: (a) resample the full image at 1620x1080 px resolution, and (b) crop a 1620x1080 px section of the full frame. For each camera, the two videos are concatenated below. Top: camera with 15 mm lens; bottom: 50 mm lens.
There was an impressive display of noctilucent clouds on the evening of 05 July 2020. I captured the following images between 21:10 and 21:30 UT. Canon 1200D with 18-55 mm standard kit lens. Exposures between 1 and 1.3 seconds, ISO 800.
The following video was taken from an upstairs window with a Sony A7S camera. Capella is the bright star at low altitude.
The following video was taken with a Brinno TLC200 Pro time-lapse camera. It shows noctilucent cloud appearing out of the bright dusk sky, then eventually fading away. Unfortunately the last part of the sequence is marred by reflections from street lights. Capella is the bright star at low altitude, Mirfak is the more obvious one at centre right.
I set up time lapse cameras and a DSLR (Sony A7S) viewing towards the north. All the cameras did an impressive job of capturing noctilucent clouds before the images became bleached out by the dawn.
The following video is from the DSLR. The bright star Capella is at altitude over 10° at the start of the sequence.
Clouds to the north, a sign of approaching dawn. But I could see a star through them and the Sun was still 10° below the horizon, meaning that they were noctilucent clouds!
The images were taken at 03:13 and 03:14 UT from location 48.9525° N, 1.05684° W, Tessy Sur Vire, northern France. Equipment: FV-5 camera app installed on a Huawei Y.635 Smartphone, aperture fixed at F/2.2, automatic exposure and focus settings. Pictures taken with the phone handheld and are slightly out of focus.
Cloud classification estimated as a combination of Type IIIb billows (closely spaced roughly parallel short streaks with an undulating wave-like structure) and Type IIa bands (streaks with diffuse edges). See the classification published by the International Association of Geomagnetism and Aeronomy (referenced above).
03:13 UT
03:14 UT
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Taken at 03:40 UT with a Canon SX700 HS compact camera mounted on a gorilla tripod. The camera was set to automatic, giving an exposure time of 1/8 second.
Noctilucent clouds over Ipswich railway station imaged at approximately 02:00 UT.
Shots taken 04:03 - 04:43 UT.
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Taken around 21:30 UT with an Olympus SP-550 Ultra Zoom camera. This was Sue's first observation of noctilucent cloud and, like many others seeing it for the first time, she was mesmerised, describing it as beautiful, wonderful.
A modest display of noctilucent cloud occurred at 22:00 UT, lasted for approximately 25 minutes, then gradually fainted. Images taken with a pocket digital camera.
Images captured circa 20:15 UT using an Olympus C-765 Ultra Zoom camera, set to automatic, with wide-angle lens.
Images taken at 21:35 UT with a Sony Cybershot DSC-P100 5.1 megapixel compact digital camera. The camera was in automatic mode and selected a sensitivity of ISO 320 and a one minute exposure at F2.8.
Image captured with an SLR using a 55 mm lens and 200 ASA film at f/5.6 with a 30 s exposure.
Image captured with an SLR using a 55 mm lens and 200 ASA film at f/5.6 with a 30 s exposure.
The observing season for noctilucent cloud in 1999 ended with just one positive sighting by OASI members. At 22.20 UT on Friday 18 June, I saw a modest display from Newbourne and observed it through to 23.10 UT. The cloud was confined to altitudes below Capella and stretched for approximately 45° in azimuth along the north to north-western horizon. I took photographs using 17 mm, 24 mm, 55 mm and 135 mm lenses on 400 ASA film. Although the cloud exhibited little structure, the photographs show some of the characteristic 'herring bone' structure on a range of scales.
Noctilucent clouds observed in a clear sky. Photographs taken at approximately 22.05 UT with a 55 mm lens and a 135 mm lens, both at f/4 for 10 s on 400 ASA Kodak slide film. Capella is visible to the lower left of the cloud edge in the 55 mm shot and stars in Perseus are visible in the 135 mm photo.
55 mm lens
135 mm lens
Noctilucent cloud visible through gaps in 'terrestrial' cloud low on the north-west horizon. Image taken at 22.35 UT with a 135 mm lens at f/4, 10 s on Kodak 400 ASA slide film.
Noctilucent cloud visible 21.20 - 22.40 UT. Photograph with a 135 mm lens at f/5, 4 s exposure on 200 ASA film.
By chance, I walked outside on 28 June 1993, a little before 11.00pm, and saw a rare and beautiful atmospheric phenomenon, noctilucent cloud. Initially I didn't know what I was looking at, and my first thought was that it was an aurora. However, I soon realised that the colour and location were wrong: an aurora is often green or pink and extends high into the sky, but the glow that I saw was a pale silvery-blue quite low in the north-west. I stood watching for some time before remembering that I possessed a camera and should be taking pictures! I called David Payne to confirm that I wasn't imagining things and then took my camera and tripod outside. By this time it was about 22:10 UT and quite dark with stars easily visible and the Moon and Jupiter low in the south-west. I took several shots of between 5 and 30 seconds exposure. The best, one of which is below, were of 10 seconds exposure with a 55 mm lens at f/4 on 400 ASA colour print film printed by Boots. The prints demonstrated beyond doubt that what I had seen really was noctilucent cloud: the wave-like structures visible in the prints were similar to those in references [1] and [2] below.
Noctilucent clouds occur at an altitude of over 80 km and therefore remain in sunlight and continue to glow long after sunset. (By comparison, most 'terrestrial' clouds occur below 10 km altitude.) The height of 80 km is significant because that is where an atmospheric temperature minimum of -110°C occurs. Noctilucent clouds are thought to be associated with the condensation of ice on particles of meteoritic dust in the atmosphere. Direct sampling by rockets has shown that when noctilucent cloud is visible there is between 100 and 1000 times more dust at a height of about 80 km than when there is no cloud. The dust particles themselves are between 0.05 - 0.5 micrometres in diameter: this is evidence of meteoritic origin, since there is no mechanism to raise such particles from the surface of the Earth to an altitude of 80 km. Photographs have revealed wind speeds of up to 650 kph associated with noctilucent clouds.
Noctilucent clouds form in the cold of the summer polar mesopause and can be seen when the Sun is between 6° and 16° below the horizon. The season for observing noctilucent clouds is from mid-May to mid-August, peaking for a few weeks on either side of the solstice. They are generally visible from latitudes of 50° - 70°: below 50° they do not form and above 70° it never becomes dark enough to see them. The UK is favourably placed for observations. Note that the only sighting of noctilucent clouds at a latitude of 45°N (lower than the usual minimum latitude) was in 1908 following the Tanguska meteorite impact - which lofted a considerable quantity of dust into the upper atmosphere.
The 1880s saw the first reports of noctilucent clouds. Serious scientific study of the phenomenon began as early as 1896 when simultaneous photographs with cameras 35 km apart showed the clouds to be at an altitude of 80 km. Reports of sightings were increasingly frequent in the late 20th Century, probably as astronomers became more skilled at recognising the phenomenon.
Amateurs are largely responsible for observations of noctilucent clouds; however, there are some academic groups interested, for example the University of Colorado. For photography, a film speed of 200 ASA is suitable with exposure times ranging from a few seconds to a few tens of seconds at stops from f2 to f5.6. A rigorous report of an observation of noctilucent cloud should record the time of observation, height of cloud, position in azimuth, structure and visibility, and should be repeated every 15 minutes. The following categories should be used in reports.
Structure categories:
Visibility categories:
References and further reading:
[1] Astronomy Now, July 1981, p31.
[2] Fred Schaaf, Sky & Telescope, 1993, 86(1), p.66 and 69.
[3] Robert K Soberman, Scientific American, June 1963.