Noctilucent Clouds, 1993 - 2016
When the light evenings of summer arrive, it can be tempting to give up looking for celestial objects and instead try studying cloud formations! In fact, some clouds form at such high altitude that they are of interest to astronomers: the noctilucent clouds. The term noctilucent comes from Latin and means luminous at night. 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 and there is a risk of mistaking cirrostratus illuminated by streetlights or moonlight for them.
The observing reports below by members of OASI illustrate the diversity of noctilucent cloud phenomena that can be seen.
Roy Gooding Observing From West Ipswich, 19 July 2016
Roy observed noctilucent clouds from his home at 03:40 UT on 19 July 2016. He captured the images below with a Canon SX700 HS compact camera mounted on a gorilla tripod on the windowsill pointing through an open window. The camera was set to automatic, giving an exposure time of 1/8 second.
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Andy Gibbs Observing From West Ipswich, 04 July 2014
Andy observed noctilucent clouds over Ipswich railway station at approximately 02:00 UT on 04 July 2014.
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Neil Morley Observing From West Ipswich, 31 August 2010
Neil observed the following display of noctilucent cloud on the evening of 31 August 2010, 19:30 - 19:45 UT.
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Mike Harlow Observing From Newbourne, 15 July 2009
Mike observed the following display of noctilucent cloud on the morning of 15 July 2009, 04:03 - 04:43 UT.
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Sue Collison Observing From Woodbridge, 14 July 2009
Sue observed a brief display of noctilucent clouds from Woodbridge around 21:30 UT on 14 July 2009. This was her first observation of the phenomenon and, like many others seeing it for the first time, she was mesmerised, describing it as beautiful and wonderful. Below are three of Sue's photographs taken with an Olympus SP550UZ camera.
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Nigel Evans Observing From North Ipswich, 13 July 2009
Mike Harlow Observing From Newbourne, 16 June 2009
Mike observed a modest display of noctilucent cloud at 22:00 UT; it lasted for approximately 25 minutes and then gradually fainted. Mike took the following images with a pocket digital camera.
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Martin Cook Observing From East Ipswich, 14 July 2006
Martin captured the following images at circa 20:15 UT using an Olympus C-765uz camera, set to automatic, with wide-angle lens.
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Pete Richards Observing From Nacton, 14 July 2006
Pete captured the following images at 21:35 UT using a Sony Cybershot DSC-P100 5.1 megapixel compact digital camera. He used the camera in automatic mode, and it selected a sensitivity equivalent to ISO320 with a one minute exposure at F2.8.
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Mike Harlow Observing From Bucklesham, 19 June 2005
Mike captured the following image using a 55 mm lens on an SLR with 200 ASA film at f/5.6 for 30 seconds.
Mike Harlow Observing From Bucklesham, 14 June 2005
Mike captured the following image using a 55 mm lens on an SLR with 200 ASA film at f/5.6 for 30 seconds.
Mike Harlow Observing From Newbourne, 18 June 1999
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, Mike saw a modest display from Newbourne (near Ipswich) 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. Mike took several 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.
Mike Harlow Observing From Newbourne, 08 July 1997
Mike observed a display of noctilucent clouds in a clear sky. At approximately 22.05 UT, he took a photograph with a 55 mm lens and another with a 135 mm lens, both at f/4 for 10 seconds 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.
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Mike Harlow Observing From Newbourne, 18 June 1997
Mike observed noctilucent cloud through gaps in 'terrestrial' cloud low on the north-west horizon. He took the following image with a 135 mm lens at f/4, 10 seconds on Kodak 400 ASA slide film at 22.35 UT.
Mike Harlow Observing From Trimley St. Mary, 01 July 1996
Mike observed noctilucent cloud from Trimley St. Mary during the period 21.20 - 22.40 UT. He took the following photograph with a four second exposure on 200 ASA film with a 135 mm lens at f/5.6.
Mike Harlow Observing From Trimley St. Mary, 28 June 1993
By chance, I walked outside on 28 June 1993, a little before 11.00pm, and saw a rather 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. As usual in such situations, I stood watching the phenomenon for some time before remembering that I possessed a camera and should be taking pictures! I gave David Payne a call to confirm that I wasn't seeing things and then took my camera on its 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 shots, one of which is shown 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.
Explanation
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:
- Veil: a structureless sheet.
- Bands: straight or slightly curved lines, parallel or crossing at shallow angles.
- Billows or waves: herringbone or ripple patterns.
- Whirls: complete or partial loops.
- Patches: small discrete units.
Visibility categories:
- Very weak: barely visible against the twilight sky.
- Clearly detected: but having low brightness.
- Clearly visible: standing out sharply against the twilight sky.
- Very bright: attracting the attention of casual observers.
- Extremely bright: noticeably illuminating objects facing it.
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.