Noctilucent clouds (popularly referred to by the abbreviation “NLC”) are high atmosphere clouds which occur over summertime at mid latitude locations. They form at very high altitudes – around 82 km above sea level – and are, thus, a quite separate phenomena from normal weather or tropospheric cloud. They appear as thin streaks of “cloud”, often a pearly-blue colour, reminiscent of “mares-tail” cirrus cloud formations.
NLCs can be seen from around mid May to early August during the darkest part of a summer’s night when the Sun is between 6 – 16 degrees below the horizon. Typically, they will occupy the northern horizon, along the twilight arch, extending to an altit e of 10 – 15 degrees. Over the NLC “season” the bright star Capella dominates this part of the sky and serves as a good marker for the NLC observer.
Observations of NLC remain of great value to professional scientists studying upper-atmosphere phenomena. Useful observations are very easy to make and require no special equipment.
The following information lists the important details you should include in your report…
LOCATION: Give the latitude and longitude of the place observations were made. Alternatively, give the name of the nearest town or city.
DATE: Use the “double-date” convention as used in reporting aurorae. That is, “June 21-22″ would refer to the night of the 21st and the early hours of the 22nd.
TIME: Try to use universal time (UT) even though British Summer Time (BST) will be in civil use for UK observers. Remember, UT = GMT = BST – 1 hour.
The following features and details should be recorded at 15 minute intervals (i.e on the hour, quarter past, half past and so on)…
AZIMUTHS If you see NLC measure the left (western) and right hand (eastern) 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. Azimuths can en be gauged by using a clenched fist, held at arms length, as a measure of 10 degrees.
ELEVATION 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.
BRIGHTNESS NLC brightness is measured on a three point scale with 1 = faint; 2 = moderate; 3 = very bright.
NLC forms are classified into 5 easily identified structures. Any combination of the following is possible…
Type 1: Veil – A simple structureless sheet, sometimes as background to other forms.
Type 2: Bands – Lines or streaks, parallel or crossing at small angles.
Type 3: Waves – Fine herring-bone structure like the sand ripples on a beach at low tide. Very characteristic of NLC.
Type 4: Whirls – Large-scale looped or twisted structures.
Type 5: Amorphous – Isolated patches of NLC with no definite structure.
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 f2.8 will normally suffice. However, it is lways 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! 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.