Posted by Dr. Eddy Graham on 05 February 2016, 12:58 PM
One of the most widespread displays of nacreous clouds ever witnessed in the British Isles took place recently over 1-2 February 2016. In some places (such as Dublin, Ireland), there is no recorded evidence of any previous occurrence, right back to the 19th century, at least (Dixon 1953).
Most of central and southern Scotland, northern England (including Manchester and Leeds) and eastern Ireland witnessed the clouds over an unprecedented length of time. In Dublin, the clouds apparently remained in the sky for nearly 48hrs (again a very unusual duration). A few example photographs are shown below (courtesy Garry Quinn photography and Kieran Commins, Met Eireann).
The clouds were coincident with an moderately-intense ozone hole of 200-250 DU (Dobson Units) over north-west Europe (normal ozone is about ~360 DU) - lower values have been documented in the past, for example 165 DU in November 1999 (Graham 2002). This is to be expected in this case, for ozone is responsible for heating of the stratosphere - and so a lack of it usually means a colder stratospheric air temperature. If cold enough (below -80 to -85degC), nacreous clouds (also known as 'Mother-of-Pearl' or 'PSC: Polar Stratospheric Clouds') can form.
By Kieran Commins (Met Eireann)
The clouds involved in this display were generally considered to be Type 2 PSC, and so not directly involved in ozone depletion. However, it is possible that some Type 1 PSCs may have been present too (they deplete ozone at a rate of about 1% per day). Some photographs of suspect PSC Type 1 clouds were taken early on 3 February (please ask author if interested).
The ozone layer is, of course, vital for life on Earth. Without it, we would suffer from harmful UV radiation, which causes skin cancer and damages crops. Northern Hemisphere ozone, although nowhere nearly as low as in the Antarctic, has shown some loss in recent decades.
Here's the Northern Hemisphere ozone distribution for 2 February, courtesy of NASA Ozone Arctic Watch (http://):
As stated by Hood and Manny (2002), such holes in the Northern Hemisphere are mostly caused by dynamics in the stratosphere, and the contribution from ozone depleting PSCs is minimal.
And here's the 30hPa air temperatures (about 24km, which was the height of the clouds during the November 1999 event, as shown by Graham , courtesy of NOAA CPC (http://):
As you can see, there's a clear minimum of stratospheric air temperature below -80degC, located right over the UK/Ireland i.e. highly suitable conditions for PSC formation.
And finally, on Twitter on 2 Feb, I'd thought that I'd spotted the nacreous clouds themselves on NASA Terra Channels 7-2-1. It looks like there was a hydraulic jump generated by the very strong west wind (nearly 200kts at jetstream level) hitting the Wicklow mountain perpendicularly, side-on (see sat image below). Dörnbrack et al. (1999) state that the air temperature can drop significantly in stratospheric mountain wave crests (due to rapid uplift). In the case of 1-2 february, the air was likely cooled rapidly to saturation and a low-enough temperature for PSC formation (i.e. below -85degC).
However, closer analysis of MetOp imagery shows that this particular cloud had a cloud top temperature of only -30C to -40C. So alas, no PSC! (they are probably too thin optically to be seen on standard visible/infra-red satellite imagery, Graham 2002).
(with thanks to Bernard Burton of http://www.woksat.info/ for this final image)
Conclusion: Whilst outstandingly beautiful, a more holistic view of these clouds could easily be 'Harbingers of Doom'. I think more research is needed on the exact causes of this outstanding display.
Appendix: Nacreous clouds (such as above) form in the lower stratosphere between 20-25km. Previous sightings near/around the British Isles include the following:
- 16 Feb 1996: Northern England
- 30 November 1999: Aberdeen and E Scotland
- 19 January 2008: Belgium
- 9 December 2012: Aberdeen and Isle of Harris
- 1-2 February 2016: This event
i.e. Their incidence appears to be on the increase during recent decades, which is in line with global ozone depletion caused by human pollutants, but it may also be coincidence.
Websites on previous sightings:
Stornoway 5 February 2016
Dixon, F.E. (1953). Weather in old Dublin. Dublin Historical Record, 13(3/4), pp.94-107.
Dörnbrack, A., Leutbecher, M., Kivi, R. and Kyrö, E. (1999). Mountain‐wave‐induced record low stratospheric temperatures above northern Scandinavia.Tellus A, 51(5), pp.951-963.
Graham, Eddie. "Nacreous clouds in November 1999 and record low ozone."Weather 57, no. 3 (2002): 107-109.
Hood, L.L. and Manney, G.L. (2002). Nacreous clouds. Weather, 57(10), pp.393-394.