Noctilucent cloud over Britain and Western Europe, 2015–2016

2017 July 21

Ken Kennedy

The first reliable historical report of noctilucent cloud (NLC) was by Thomas Backhouse on 1885 June 01. A previous report by Thomas Robinson at Armagh has been proposed as an earlier sighting but this was on 1850 May 01, which is outside the now accepted period during which NLC is likely to be seen. Very few reports in more recent times have been before the second week in May or later than the third week in August.
Since the 1880s, NLC has been reported with increasing frequency, which may be partly accounted for by the fact that observer numbers have increased over the years and these observers are in widely spread locations. Even with this rider it is generally agreed that the frequency of NLC has increased in real terms since the early sightings in the late 19th century. An association with the solar cycle has been suggested on a number of occasions, and data collected by the author from the work of previous observers and from that gathered by him since 2006 tend to confirm that NLC frequency is greatest at or near solar minimum and least near solar maximum. During a prolonged spell of solar inactivity between 2006 and 2010, NLC frequency reached a maximum, but the solar cycle (24) which followed that minimum, and which is now in decline, has been the least active since solar cycle 14 which began in 1902 February and ended in 1913 August.
Following a number of years when solar activity was at a very low level and NLC frequency was correspondingly high, there was a distinct reduction in its frequency with the commencement of solar cycle 24 in 2010. The question has arisen over the past few years as to the frequency of NLC following this rather poor solar maximum and it is interesting to note that NLC frequency stayed at much the same level through the years of increasing solar activity from 2010, including the year of maximum in 2014. This paper looks at NLC frequency and distribution of observations during the first two years of solar cycle 24’s decline from maximum.
A summary of observations received for each night on which NLC was observed in 2015 and 2016 is shown in Tables 2 and 3. A small number of sightings submitted by observers in the USA and Canada has been included for completeness.
The NLC forms used in Tables 2 and 3 are described below: (continued…)

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