Cloud with a Silver Lining

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 century.

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 (AIM) 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 coming weeks.

FOR FURTHER INFORMATION PLEASE CONTACT: John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; jmf at sign

arm.ac.uk.

Last Revised: 2010 March 9th