Summary: Possible Observations of Noctilucent Clouds by Thomas Romney Robinson
C. J. Butler
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
Noctilucent Clouds are cirrus-like high altitude (~82km) clouds normally visible only at night during
summer months from latitudes 50-65 degrees north.They are believed to be composed of ice crystals
which reflect the light of the Sun shining from below the observer's horizon.
Noctilucent Clouds (NLC) were first reported by Backhouse (1885) and Jesse (1890) from
observations made in 1884-5, following the huge volcanic eruption of Krakatoa in 1883. This led to a
widespread belief that they were caused by volcanic ejecta. Later studies have suggested that they are
more likely to be composed of extra-terrestrial particles which, as they descend through the
atmosphere to the vicinity of the mesopause, become coated with water ice (WMO Report, 1970).
Figure 1. Noctilucent Clouds photographed at Maghaberry, Co Down, (lat 54º30') at 01:25 UT on 12 June
1995 by John McConnell.
NLC are known to be highly variable in frequency with a preference to occur near sunspot minimum.
Suggestions that long term trends in the frequency of occurrence may be related to human influence
on global climate change have been questioned by von Zahn (2003) who has shown that such trends
are heavily influenced by observational selection effects. As von Zahn has pointed out, the single
most significant quantifiable characteristic of NLC, namely their altitude, has not changed over the
120 years they have been observed. Nevertheless, in the overall study of climate change, due to their
high altitude, it is of interest to know if indeed long term changes have occurred.