Note the sharp contrast between the bright, highly structured
noctilucent clouds reflecting sunlight from far below the horizon,
and the dark tropospheric clouds in the foreground.
Images courtesy John McConnell.
Noctilucent clouds, mysterious
luminous formations, have been widely observed over Northern Ireland in
the course of the last two or three weeks. There were particularly
prominent displays on the nights of June 15/16th and June 17/18th. More
displays may follow during the next month or so.
These noctilucent "night-shining" clouds (or NLCs) are believed to form
by sunlight reflecting off ice crystals high in the atmosphere, and
usually appear in the northwest to northeast. They often appear shortly
after sunset or shortly before sunrise, lying relatively low in the
northern sky for a few weeks to a month or more around the summer
solstice at our latitude. The clouds have a silvery, electric-blue
colour and take various forms from feathery streamers to waves.
NLCs occur at very high altitudes, in excess of 50 miles high, in the
region above the stratosphere known as the mesosphere and at first sight
they look like cirrus clouds. However, they are far higher than any
normal clouds and appear to be self-luminous, but in fact shine by
reflected sunlight when the Sun is a few degrees below the observer's
horizon. Any visible lower clouds appear dark and silhouetted against
the NLCs because they are not illuminated by the Sun. Stars can often be
seen twinkling through NLCs.
NLCs appear to have been noticed first as a distinct cloud phenomenon in
1885, by the astronomer and meteorologist Thomas William Backhouse, and
were then thought to have resulted from the Krakatoa volcanic eruption
of August 1883. However, more recent studies have indicated that they
are primarily caused by meteoric particles that become covered with ice
as they fall through our atmosphere. They tend to occur at the minimum
of the solar cycle. We are currently in the deepest solar minimum for a
One of the mysteries of NLCs is why they were not reported earlier.
However, in the climate archive of the Armagh Observatory, Dr Thomas
Romney Robinson, then the chief Armagh Astronomer, made two observations
of seeing "strange luminous clouds in the northwest, not auroral" around
10pm on 1st and 4th May 1850. Although these may have been the first
recorded sightings of noctilucent clouds it is still a puzzle why no-one
appears to have seen them before 1850.
On April 25th last, the NASA Aeronomy of Ice in the Mesosphere
spacecraft celebrated its second birthday in orbit. The spacecraft,
which was placed into a polar orbit at 550 kilometres altitude, was
launched to determine what factors are responsible for the formation of
the enigmatic NLCs. The formation of NLCs may be a result of climate
change. Recently, AIM's mission has been extended to enable further
study of these clouds. Be on the alert for more possible displays in the
FOR FURTHER INFORMATION PLEASE CONTACT: John McFarland at the Armagh
Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX:
Possible Observations of Noctilucent Clouds by Thomas Romney Robinson
Noctilucent Clouds, July 2006
Timelapse Movie of Noctilucent Clouds, July 2006 by Robert Cobain
Noctilucent Cloud Photos by Martin McKenna
Video on YouTube
NLCs from Space
Last Revised: 2010 March 9th