Observing Noctilucent Clouds
by Dr. Colin Steel
What are Noctilucent Clouds ?
The terms nocti and lucent are derived from the Latin where they (very loosely) translate to night and luminous. Thus NLC are clouds which shine at night. Their actual composition is not known but there are several theories concerning their origin.
Why should astronomers be interested in clouds ?
There is a fundamental difference between NLC and any other type of clouds. Most clouds exist within about 10 km of the Earth's surface; however, NLC exist at a height of 82 km. They are thus not part of the normal weather system but appear to be more connected with astronomical phenomena.
How long have they been known ?
NLC were observed in the year 1884 but there has been a steady rise in their frequency ever since the mid nineteen sixties.
What do they look like ?
NLC look fundamentally unlike the familiar tropospheric type clouds which appear dark at night. NLC shine because they are high enough up to be illuminated. They appear pearly white in colour and often appear very delicate in texture.
When can they be seen ?
The normal noctilucent cloud season lasts during June and July. However, NLC displays in late May and early August cannot be ruled out. For the southerm hemisphere, the season is displaced by six months.
And from where ?
The normal latitude zone for observing NLC displays from is latitude 50 to 65 with 55 to 60 being particularly favoured. Thus the British Isles are a location from which to see NLC with Scotland being most favoured. North of about latitude 60, it does not get sufficuently dark during the middle of the season. NLC displays can also be seen from the corresponding southern latitudes (although there is not much land there) with the season being December and January rather than June and July.
How can they be observed ?
They can be observed in several different ways. High-power telescopes are of limited use, but visual and photographic observations are very useful.
I wish to observe them visually. How do I do this ?
The best way to observe visually is to make drawings at intervals e.g. every fifteen minutes. On the drawing, the extent of the display in azimuth and altitude should be noted e.g. azimuth 330 to 025, altitude 2 to 20. The brightness of the display (and its various parts) should be noted on a scale 1 (faintest) to 4 (brightest) as should the type of NLC (A = featureless, B = linear, C = cross-hatched, D = 'whorly'). Any other features worthy of note should be added.
I want to photograph them. How do I do this ?
This is relatively straightforward. What you need is a camera where the exposure can be controlled and a relatively stable platform. A wide-angle lens is optional but it does help in that normally a display can be photographed in one frame. Typical details are 200 ASA film, f/2, 10 second exposure. With a faster/slower film, the exposure can be decreased/increased.
When should I take the photographs ?
If you are interested in 'pretty pictures', the photographs may be taken at any time. However, with a very little discipline, photographs may be obtained that are no less 'pretty' but that are also scientifically, very useful. If picutres are taken within a second or so of each quarter-hour e.g. on the hour, quarter-past, half-past and quarter-to, and another observer, elsewhere, has done the same then triangulation can be carried out.
What can be learned from triangulation ?
From a single photograph of an NLC display, only the direction from the observer to the clouds can be inferred. However, from two such photographs, the exact position of the NLC display in three-dimensional space can be worked out. Thus it can be inferred which point on the Earth's surface they are above and the height can also be found out.
Why is the height of NLC so important ?
It is thought that noctilucent clouds exist at a temparature minumum in the Earth's atmosphere. If the noctilucent cloud height were to change, it would indicate a change in the temperature structure of the atmosphere. Thus, the height of NLC could be a possible tracer of global warming and other effects.
Who should I send my reports to ?
In the UK, NLC are dealt with by the aurora section of the British Astronomical Association and in particular by the deputy director, Dr. David Gavine, 29 Coillesdene Crescent, Joppa, Edinburgh, EH15 2JJ.
Other NLC Web Sites
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Maintained by Michael Oates
19 January, 2005