Noctilucent cloud over Britain & Western Europe, 2018

2018 Aug 8/9, 02:27 UT at Tarbatness. Photographed by Denis Buczynski
2018 Aug 8/9, 02:27 UT at Tarbatness. Photographed by Denis Buczynski

A single report of NLC was received for May 22/23, but ice in the mesosphere as indicated by the Cloud Imaging & Particle Size (CIPS) instrument of the Aeronomy of Ice in the Mesosphere (AIM) satellite did not appear until May 27. Following that date, there were remarkably few sightings reported until the night of June 18/19, when there was a sudden rise from single figures to 22.

Images from the AIM satellite were not available in 2017, but it was possible in 2018 to compare sighting numbers and locations against the CIPS images during that year. It is interesting to note that sightings until Jun 18/19 were sparse, despite the CIPS images showing a considerable amount of ice in the mesosphere from May 31. Cloud cover over Europe and the UK was no worse than usual, and was probably less than average from about mid-June until after the first week in July, when conditions deteriorated. There is no obvious reason for the small number of observations reported during this period.

Clearer conditions from mid-June may well account for the rise in numbers of observations from Jun 18/19 until Jun 25/26; this proved to be the most productive time of the 2018 season. July conditions at the author’s location were poor and this was reflected over much of Europe, resulting in relatively modest numbers of sightings with the exception of Jul 2/3, 5/6, 7/8 & Jul 29/30.

This continued into August, with the most prolific evening being that of Aug 6/7. Despite the poor sky conditions later in the season, it should be noted that there were eight nights in August when NLC sightings were reported. After 2009, which was an exceptional year, 2018 had the greatest number of nights of NLC sightings in August since 2007.

Analysis of observations

It is not a very frequent occurrence to find NLC at an elevation of 90° (overhead) or at an even greater southerly elevation. This occurred several times during the 2018 season, notably on Jun 18/19 & 24/25 from Scotland and Jul 5/6 from two locations in England, when NLC was seen as far south as Selsey. The nature of NLC illumination is such that, if the cloud is present overhead, it will only reflect light from the Sun when it is at a shallow angle below the horizon. A sighting of NLC overhead is therefore more likely to occur relatively early in the evening after the Sun has set, or in the hour or so before sunrise.

The correlation between the indicated latitude of ice in the mesosphere as seen by CIPS and the locations of actual sightings is not always clear, but more recent changes to the algorithm which produces the CIPS imagery have brought these in line on more frequent occasions. The polar ‘daisies’ overlaid on maps are produced over a 24-hour period and are a combination of a number of orbital strips taken over that time. Ice in the mesosphere seen over a particular geographic area may therefore not be seen as NLC, because of light and timing differences. Individual strips are also shown on the AIM website and these should be examined to see if any correlate with a particular location and time when NLC is observed.

Towards the end of the 2018 season, the author had an enquiry from Prof Cora Randall, Principal Investigator of the CIPS experiment, as to whether our UK and European observers had been reporting NLC more frequently that year. It seems that observers in North America had been doing so but, from the figures for 2018, it could be seen that this was not so for our observers. This, however, was not true for the author who recorded a total of 31 displays; a significantly greater number than for any year since 2006. As is often the case, this may reflect local sky conditions during this particular year.

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