Noctilucent cloud over Britain & Western Europe, 2022
2023 October 10
Noctilucent cloud forms in the mesosphere at an altitude of about 83km and appears annually in northern latitudes between the end of May and the beginning of August. A number of observers, situated at widely dispersed locations in the UK and Western Europe, report their sightings to the British Astronomical Association. This paper is an analysis of these observations for the summer months of 2022, together with background information about conditions in the mesosphere which contribute to the formation of polar mesospheric ice: the cause of ground-based sightings of noctilucent cloud.
Introduction
Noctilucent cloud (NLC) is the highest cloud recorded in Earth’s atmosphere, at a height of around 83km. It was first recorded in 1885 by Thomas Backhouse and has been seen since then between the end of May and beginning of August in the northern hemisphere. The frequency of sightings has increased gradually over the years; this may be accounted for, in part, by an increase in interest as well as the number and spread of observers and locations.
The increasing number of reports of noctilucent cloud encouraged NASA to launch its Aeronomy of Ice in the Mesosphere (AIM) satellite in 2007, with a view to monitoring ice forming in the polar mesosphere in both northern and southern hemispheres. The images produced by the Cloud Imaging and Particle Size (CIPS) instrument on board the AIM satellite are useful to ground-based observers, as they provide the earliest indications of ice nucleation. Following the first positive detection by CIPS of ice in the mesosphere, ground-based sightings of NLC are usually made within a few days.
The critical factors in the production of ice in the mesosphere are the temperature and water-vapour content of the atmosphere at that height. Summer buoyancy waves produce marked adiabatic cooling, allowing the mesosphere to reach the critical temperature for ice nucleation, which is around 150K (–123°C). Water vapour in the upper atmosphere gradually increases towards May, but the amount of water in the mesosphere is not as critical as a low enough temperature in the formation of ice. NASA’s Microwave Limb Sounder (MLS), carried on the Aura satellite, measures temperature and water-vapour content in the mesosphere, and these measurements can explain changes in the pattern of ice formation throughout the season.
The 2022 NLC season
Prof Cora Randall, an atmospheric scientist at the University of Colorado Boulder, has been the principal investigator of the CIPS data since 2007. She kindly regularly updates the author about ice in the mesosphere and factors such as temperature and water-vapour content, which influence the appearance of ice. On 2022 May 22, she said that ice had not been detected by CIPS, but she had been informed that the Ozone Mapping and Profile Suite (OMPS) instrument installed in Suomi NPP and NOAA-20 satellites may have detected a faint cluster of clouds in the Canadian Arctic on May 21. She also indicated that MLS data were showing that polar mesopause water vapour was rather higher than normal, but temperature was lower.1
The first traces of ice in the mesosphere appeared on May 22 and grew to a disthtinct patch over Canada on May 23. The ice was at a very high latitude but by May 25 had grown and, because of the differential rotation of the Earth and the upper atmosphere, was approaching northern Scandinavia, making sightings of NLC in Europe a possibility. The first confirmed sightings of NLC were on the night of May 26/27 by an observer in the Isle of Skye and the author in Dundee. Only one more sighting was received for May, that being on the final night of May 31/32, from Germany.
By Jun 1, the CIPS image showed that ice in the mesosphere had become more southerly, although still showing eccentric distribution round the pole. The southerly extension was situated due north of the UK and the night of Jun 1/2 produced 16 sightings, the most southerly being from Chelmsford. The CIPS images from early June showed ice forming around the pole and with that it was likely that more reports of NLC would follow. Increasing numbers of NLC sightings were received as June progressed, with particularly high numbers on Jun 20/21 (26) and Jun 22/23 (30). The most southerly report received was from a location in Hungary at 46.8°N.
An interesting item reported on Spaceweather.com, Jul 3, mentioned a sharp increase in NLC at the end of June. This comment seems to have been based on data from CIPS which showed a spike in late June but only at 70°N. Whatever the cause of this surge in NLC frequency noted by CIPS, it appears that nothing out of the ordinary was noted by observers in the UK or Europe. The overall number of NLC sightings during July was significantly less than that in June with only two nights, Jul 4/5 & 5/6, returning double figures. From these dates, numbers remained low until the end of July, although the most southerly sighting was reported from a latitude of 46.8°N, by the same observer who supplied the most southerly sighting in June.
Reports continued to be received into August, with an unexpected burst of activity on Aug 10/11 for which the most southerly report came from an observer at a latitude of 50.7°N – unusually low for this time of the NLC season. The final NLC observations were received for the night of Aug 11/12.
From reports by numerous observers and from personal observations, the author felt that the entire 2022 NLC season was rather muted in terms of NLC brightness and fewer displays reached higher elevations. Both of these factors probably contributed to fewer sightings from more southerly locations being reported, as cloud cover during the season was probably no worse than average.
Towards the end of July, the author contacted Prof Randall to express his feelings about the muted nature of the season. She replied by sending a plot showing frequencies for previous years, with that of 2022 highlighted in red (Figure 1). She commented, ‘You’ll note that at 58–60 degrees the frequencies this year are indeed less, on average, than in previous years (with the exception of a spike just before solstice which we’re investigating as possibly caused by a rocket launch…very speculative still)’.2
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