Observing Noctilucent Cloud
The observing procedure requires that, when NLC is present, the following features and details should be recorded at 15 minute intervals (i.e precisely on the hour, quarter past, half past and so on).
This essential information is often combined with additional details:
Type I: Veil – A simple structureless sheet, sometimes as background to other forms.
Type II: Bands – Lines or streaks, parallel or crossing at small angles.
- IIa: Bands with diffuse, blurred edges.
- IIb: Bands with sharply defined edges.
Type III: Waves – Fine herring-bone structure like the sand ripples on a beach at low tide. Very characteristic of NLC.
- IIIa: Short, straight and narrow streaks.
- IIIb: Wave-like structure with undulations.
Type IV: Whirls – Large-scale looped or twisted structures.
- IVa: Whirls with small angular radius (0.1 – 0.5 degrees).
- IVb: Simple curve(s) with angular radius of 3 – 5 degrees.
- IVc: Large-scale whirls.
- Type O: A form which does not fit into types I – IV.
- Type S: NLC with bright ‘knots’.
- Type P: Billows crossing a band.
- Type V: A net-like structure.
(The NLC structure classifications replaced a previous system based on a 1970 WMO publication. Note, the the old ‘ amorphous’ structure type is no longer in use)
If possible, measure the angle subtended by the uppermost part of the display. A simple alidade can be made from a protractor and plumb line for this purpose (a similar device can also be used for accurately measuring azimuths).
If you see NLC measure the horizontal extent of the display. This is measured in degrees with west = 270, north = 000, east = 90 and south = 180. Polaris defines the northern point of your horizon.
NLC brightness is measured on a five-point scale:
- Very weak NLC, which are barely visible against the twilight sky.
- NLC clearly detected, but having low brightness.
- NLC clearly visible, standing out sharply against the twilight sky.
- NLC very bright and attracting the attention of casual observers.
- NLC extremely bright and noticeably illuminating objects facing it.
If aurora occurs coincident with the NLC, this should be mentioned and detailed separately in your report.
It helps to periodically make notes regarding sky clarity and the presence of tropospheric cloud.
In theoretical terms, a night identified as having no NLC present from a certain location, is of equal importance to a positive sighting. Negative reports must cover the whole period of the NLC observing ‘window’, that is both the pre and post midnight spells.
Simple sketches of the NLC can be very useful. These are best made in negative form with the darker parts of the sketch corresponding to the brighter NLC.
Photographs of NLC can easily be taken with an SLR camera firmly fixed to a tripod. Using 200 ISO film (colour slide gives good results) an exposure of 3 – 6 seconds with a lens aperture setting of f2.0 will normally suffice. However, it is always best to bracket exposures; that is, deliberately under and over expose a few frames to obtain a choice of exposures. With luck, at least one should be correctly exposed! Standard (50mm)or wide angle lenses will capture most of the display. If possible, try to time exposures accurately on the hour, quarter, half past etc. Such exposures may then be compared to photographs taken at the same instant from different locations and possibly be used for height determination analysis.
Further information on NLC observing can be found in ‘Observing Noctilucent Cloud’ by Michael Gadsden and Pekka Parviainen, published by the International Association of Geomagnetism & Aeronomy. It is available, free, as a pdf download, from here and contains extensive advice and information on how to observe and report NLC.