Noctilucent cloud over Britain & Western Europe, 2021

2023 February 7

Ken Kennedy

A report of the Aurora & Noctilucent Cloud Section. Director: S. Brantingham.

This paper describes conditions leading to noctilucent cloud (NLC) sightings in the northern hemisphere during 2021, reporting temperature and water vapour density at 83km altitude. It analyses reports made by observers in the UK and Western Europe, indicating the frequency with which NLC was seen and the location of the most southerly sighting for each night when it was observed.

Introduction

Speculation about the timing of annual first sightings of noctilucent cloud (NLC) in the northern hemisphere has been a topic of conversation among observers for many years, but the launch of the Aeronomy of Ice in the Mesosphere (AIM) satellite in 2007 provided a more robust guide as to when NLC might first be seen. Images taken by the Cloud Imaging and Particle Size (CIPS) instrument on board AIM show the earliest signs of ice in the mesosphere and,¹ following this first detection, ground-based observers somewhere usually see NLC a few days later. Records drawn from ground-based observations since 2006 indicate that the first NLC sightings are usually towards the end of the third week in May and into the first few days of the fourth week. This is a remarkably tight time constraint, but it seems to have been consistent for a number of years.

The AIM satellite also allows interested observers to predict the start of the NLC season in the southern hemisphere. As seen on the CIPS images, ice has generally appeared in the southern mesosphere a few days before or after Nov 21. In 2019, ice was noted by the CIPS image on Nov 17, which was a few days earlier than average, but in 2020 November there was a significant delay in the appearance of ice and it was first recorded at the later date of Dec 10. This delay in the onset of ice in the southern mesosphere led to widespread speculation about whether it would presage a similar delay in the northern hemisphere in 2021. It appears that the southern polar vortex had persisted longer than usual and had prevented vertical air movement which cools the mesosphere and allows ice to form. Because of land and ocean distribution, there are differences between the conditions which allow ice to form in each hemisphere and there was no substantial reason to believe the delayed southern onset of NLC would be paralleled by a delay in the northern hemisphere.

Formation of ice and subsequent visible NLC in the mesosphere is dependent on two factors: the amount of water vapour in the mesosphere at an altitude of around 85km, and the temperature at a similar level. In early May, Prof Cora Randall said ‘At the moment, 2021 is one of the coldest years since AIM was launched 14 years ago’, and with additional information supplied by the Microwave Limb Sounder (MLS) on NASA’s Aura satellite showing water-vapour concentration to be high, a good season was predicted with a possible early start.² It can be seen from Figure 1 that the mesospheric temperature started to rise gradually after day 127 (May 7) and by day 160 (Jun 9) was at an average level.

Analysis of observations

Despite the thoughts that a delay in onset of ice in the northern hemisphere was unlikely, the author, and a number of NLC observers, looked rather anxiously at the CIPS images from mid-May, and there was general relief when ice appeared in the image of May 20 followed by the first report of NLC on May 21 from Germany. ‘We saw the first NLC from Germany on May 21st,’ reported Gerd Baumgarten of the Leibniz Institute of Atmospheric Physics. ‘The clouds appeared in the feed from one of our high-resolution cameras located in Collm (51.3°N, 13.0°E).’³ It is interesting to note the more southerly latitude of Collm for a first sighting of the season. A report was also received from an observer in Zschortau, Germany, at a latitude of 51.5°N, and contributed to Tom McEwan’s NLC website.⁴ Two images were also submitted, but the author was not convinced of the appearance of NLC on either image and has not included this report in Table 1.

Despite predictions of an early start to the season, there were only a few reports of NLC in May. However, early June saw a significant increase in sightings, especially the evening of Jun 4/5 with 25 reports. Following then, there were fewer reports until Jun 21/22 when the most southerly report of the season was received from Valencia in Spain. Another night with well-observed NLC was that of Jun 23/24.

July started relatively quietly, with small numbers of nightly observations, until Jul 6/7 when 10 reports were received. A similar number was received for the night of Jul 12/13, but of note is the fact that there was a sighting from as far south as we are likely to get from UK observers: the Isles of Scilly. July saw the night of maximum numbers of observations in 2021, with 42 reports being received for Jul 14/15. Following this outstanding night, numbers of observations tailed off and although NLC was seen on four nights in August, only six sightings were received.

Prof Cora Randall kindly sent a plot of the NLC frequency in 2021 compared with that in other years, as recorded by the AIM satellite (Figure 2). The year 2021 is indicated by the red line. Cora summarises: ‘An average or late start was followed by a steep increase in clouds so that by late June, 2021 had more clouds than in any other CIPS season. Frequencies mostly remained higher than average through the middle of the season (aside from the short dip most evident at 65°N near day 10–15), before declining to a fairly average season end.’⁵

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