This is a selection of views of the display of nacreous clouds on 30th November 1999 which was widely seen across Scotland and the far north of England. These images are from a few photographers who were kind enough to allow me to include them here. Copyright of all images remains with the original photographers.
Nacreous clouds are unmistakable because of the powerful iridescent colours which remain in the high stratospheric clouds long after sunset. They are one of the most striking atmospheric phenomena.
© George Soja & Brian Ward, Aberdeen
Cloud detail view from an image taken Aberdeen with a digital camera . Much fine structure is visible on the cloud edges. The colours are real and perhaps appear even more saturated to the naked eye than these images can convey. The brightness of the clouds tends to make digital cameras underestimate the strength of the colours.
Here a sequence of shots from Aberdeen showing the evolution of the clouds and colours with time.
Copyright © 1999 George Soja & Brian Ward, Aberdeen
The same event viewed from St Andrews taken on a conventional camera and showing the strange lenticular shapes of the various cloud edges. They were taken at about the same time as the later ones from Aberdeen.
Copyright © 1999 Richard Taranto, Virginia USA (visiting St Andrews)
The view from Edinburgh Royal observatory, images by kind permission of Jason Cowan and ROE. These show the earlier stages of the display with fine broad areas of cloud against a deepening blue sky after sunset.
The complete set of Jasons images were online for a while at Edinburgh Royal Observatory alas the original full images seem no longer to be online.
Copyright © 1999, Jason Cowan & UK ATC Royal Observatory, Edinburgh
It is worth noting that these nacreous clouds are a rare form of stratospheric cloud which occur high up in the stratosphere. They require unusually cold conditions of about -80C in the stratosphere to form and are usually associated with a strong northwesterly airflow over the UK. The spectacular colours are caused by diffraction effects in particles which are very uniform and have sizes comparable with the wavelength of light.
The clouds also provide a platform for the chemical reactions that deplete ozone and the ozone layer crippled shortly after the November display before recovering to normal levels again. There was briefly an ozone hole over Europe arising from the accelerated ozone depleting reactions which occur on the cloud particles. The trace from Brussels Royal Meteorological Observatory at Uccle shows the catastrophic effect this display had on ozone levels - driving the ozone concentration briefly down to about 200 DU well below the normal level for the time of year (grey band).
When they occur in the Arctic or Antarctic regions they are called polar stratospheric clouds and are being studied by various groups including NASAs SAGE III Ozone Loss and Validation Experiment, SOLVE.
It is the finer Type I PSC's consisting of nitric acid trihydrate which give the spectacular colourful displays. It is the very fine particle size which gives rise to the colourful diffraction effects.
Both sorts of cloud can accelerate the destruction of ozone.
The graphic of characteristics of PSCs Copyright © 1989 O.B. Toon is reproduced here by kind permission of the NASA Solve group.
There has since been another new display of nacreous clouds on 29th January 2000 visible from the south of England. And some images of this most recent display over Norfolk taken by Les Cowley are already online on his web page. One of his images looks almost exactly like a blue gas flame hanging in the sky. Related to this display, nacreous clouds were also seen for several days in late January at Kiruna, Sweden
They are rare enough that I cannot remember a display prior to 1996 that was visible from the UK and would be grateful if anyone can provide me with photographs of any past or future events. I have been sent a list of dates when nacreous clouds were visible from the UK.
The list is taken from Bulletin American Meteorological Society Vol55, No3, March 1974 which gives a listing of UK displays upto that date combined with more recent data, and other sources. The UK displays are:
|1884||December||6, 7, 8, 10, 11, 12, 13, 18|
|1885||December||28, 29, 31|
|1951||August||10||seen by experimental test flight
|1974||February||22||Weather v31, Jun76, p184|
|1987||January||6||Weather v42, Apr87, p126|
|1989||January||15 ?||? Weather, v45, Oct90, p369|
Keith Edwards kindly provided additional details of nacreous displays reported in Weather magazine since 1946 that I did not know about. The BAMS report for 10th August 1951 was not reported in Weather and is slightly suspect. And it is uncertain whether the iridescent clouds seen on 15 January 1989 were in fact nacreous clouds.
The exceptional years of 1884 and 1885 were as a result of temporary global cooling initiated by the huge explosion of Krakatoa on 26th August 1883. They are a part of the spectacular sunsets described in contemporary records of the time. I don't know if the run of displays in the 1930's have been assigned any cause.
When the original display occurred in February 1996 it took everyone by surprise and was hailed as a once in a lifetime event to see these spectacular clouds from the UK. We have now had 3 separate displays in the past five years which may be an indication that the stratosphere is getting cooler (probably as a consequence of the troposphere becoming warmer, but perhaps directly by thinning of the ozone layer).
More heat trapped in the lower levels of the atmosphere can allow the upper layers to cool. And thinning of the ozone layer can also lead to cooling of the stratosphere. Either way it makes the formation of stratospheric clouds more likely and so enhances ozone depletion. There are reports that the past winter has seen unusually cold conditions in the northern stratosphere - an observation which is consistent with the increasing frequency of nacreous cloud displays. The EU press release on the preliminary results of this winters THESEO 2000 research program show a marked Arctic ozone level decline which can affect Europe. If this trend continues then colourful nacreous displays may become more commonplace at UK latitudes and provide a publicly visible litmus indicator of climate change and ozone depletion in action.
Please report any new UK sightings of nacreous clouds.