Frequently Asked Questions regarding NLC include:
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
remains a mystery. 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.
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.
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.
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.
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.
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.
The popular astronomy magazines, such as, 'Sky &
Telescope', 'Astronomy'
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. Of general interest to students of upper-atmosphere phenomena is Neil Bone's book 'The Aurora Sun-Earth Interactions' (Second Edition); Pub Wiley-Praxis, 1996, ISBN 0-471-96024-1. The second edition of this book contains a very informative chapter on NLC (updated and expanded from the book's 1991 edition).
Papers and Book by Wilfried Schröder:
Study on Noctilucent Clouds...Acata Geodetica and Geophysica, 2004
Dirunal Variation of Noctilucent Clouds, Idöjaras 2000
Noctilucent Clouds, (History, development, observations) Bremen: 2003
Last updated on 06/06/05