Frequently Asked Questions on NLC include:
- What are Noctilucent Clouds?
- Where and when do NLC’s appear?
- Where can NLC be seen from?
- Where in the sky do they appear?
- What does NLC look like?
- Can ordinary cloud be mistaken for NLC?
- Where can I find out more about NLC?
What are Noctilucent Clouds?
Noctilucent clouds (popularly referred to by the abbreviation “NLC”) are high atmosphere cloud formations thought to be composed of small ice-coated particles; their precise nature is not fully understood. They form at very high altitudes – around 82 km above sea level – and are, thus, a quite separate phenomenon from normal weather or tropospheric cloud.
Where and when do NLC’s appear?
NLC’s are very tenuous in nature and are only visible against a twilit sky background when the clouds occupy a sunlit portion of the Earth’s atmosphere. They are never seen in daylight skies. Therefore, the best time to look for NLC is during the deep twilight’s of summer when the Sun lies between 6 – 16 degrees below the horizon. Each year, NLC incidence tends to peak just prior to and for a few weeks after the summer solstice (from each hemisphere). Normal visibility limits for northern hemisphere observers are from mid-May to Mid-August though they are occasionally reported outwith these times.
Where Can NLC Be Seen From?
The twilight conditions which render NLC visible, impose a latitude restriction on their visibility. They are, therefore, most often seen from locations which lie between Latitude 50 and 60 degrees in both hemispheres (though they are occasionally reported beyond this latitude band). Through most of June and July (from the northern hemisphere) these locations never attain true astronomical darkness and twilit skies persist all night. The latitude range takes in Northern America, Canada and Northern Europe. Very few populated land masses exist at the corresponding latitudes in the southern hemisphere.
Where In The Sky Do They Appear?
Generally NLC will be seen close to the sunward horizon, perhaps extending to around 15 – 20 degrees above the horizon, along the twilight arch. They can be more extensive (on rare occasions encroaching the equatorward half of the sky) close to dusk and dawn when the solar illumination is at its most favourable. Similarly, at local midnight, the NLC is poorly illuminated and the cloud sheet, if present, will recede closer to the horizon.
What Does NLC Look Like?
They appear as complex interwoven streaks or knots of “cloud”. Colour is generally white or a distinctive pearly-blue tone, sometimes with a golden lower edge. Structure is reminiscent of daytime cirrostratus formation. A representative selection of photographs can be viewed from the images page.
Can Ordinary Cloud Be Mistaken For NLC?
Yes. At times high cirrus cloud can appear to be brighter than the twilit background, especially when a bright moon is present or local light pollution problems are extreme. Experienced observers generally have no trouble in identifying true NLC. Binoculars can assist in correct identification; cirrus cloud tends to be nebulous when viewed through binoculars while NLC bears magnification, showing finer detail which the naked eye would otherwise be unable to resolve. Bright displays of NLC are quite unmistakable and can be an awe-inspiring sight.
Where can I Find Out More About NLC?
This website is a good starting point for finding out more about NLC. Searching the Internet will produce further information. There is also a public Forum on NLC at nlcnet.proboards.com/ where discussion, advice, images, archives, reports and early warnings can be obtained.
The popular astronomy magazines, such as, ‘Sky & Telescope’, ‘Astronomy’, The Sky at Night and ‘Astronomy Now’, occasionally include news and feature articles on NLC. ‘The Astronomer‘ magazine provides rapid publication of provisional NLC reports each summer. Various astronomical and meteorological Journals are also worth keeping an eye on for news of NLC research. Visiting a good reference library is always worthwhile. The standard reference book on NLC is ‘Noctilucent Clouds’ by M.Gadsden and W. Schroder; Pub. Springer-Verlag, 1989, ISBN 3-540-50685-3. An NLC observing manual is also currently available; ‘Observing Noctilucent Clouds’ by M. Gadsden and P. Parviainen; Pub. International Association of Geomagnetism & Aeronomy, 1995, ISBN 0-9650686-0-9 – also available as a PDF download.