Possible Observations of Noctilucent Clouds by Thomas Romney Robinson

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 th