[BAA-ebulletin 00894] Nacreous clouds

[Rick]

[BAA-ebulletin 00894] Nacreous clouds
(c) 2016 British Astronomical Association    http://www.britastro.org/

Nacreous clouds were seen from Cambridge this evening.  This type of cloud is fundamental to the creation of the ozone hole over Antarctica and is rarely seen from the UK.  The clouds form over the polar regions at heights of around 10 to 20km.  They are usually visible during civil twilight - the denser cloud has beautiful pearly coloured margins and is quite bright, but there is also a diffuse component that appears like stratocumulus cloud, but is much higher.

This winter has seen an unusually cold Arctic stratosphere, and over the last couple of days the "polar vortex" with the coldest air has become very elongated and has reached down to the UK.  There is a small chance that the clouds might again be visible before sunrise on February 2, but after that the area of cold air moves away from the UK.

Jonathan Shanklin
BAS Fellow
British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, England

[Rick]

[BAA-ebulletin 00895] Nacreous clouds (2)
(c) 2016 British Astronomical Association    http://www.britastro.org/

Responses to my previous email show that the clouds were observed across the UK on the evening of February 1.  The latest forecasts now suggest that the conditions that lead to their formation will persist over the UK for several days, perhaps until the evening of February 4.  Do watch out for them - it could be many years before you get another chance!

Jonathan Shanklin
BAS Fellow
British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, England

[Rick]

[BAA-ebulletin 00903] Nacreous clouds - again!
(c) 2016 British Astronomical Association    http://www.britastro.org/

The upper atmospheric conditions that gave rise to the display of stratospheric clouds at the beginning of February look set to return at the end of the month. Last time whilst some of the clouds showed the "nacreous" or "mother-of-pearl" forms, the majority appeared like thin stratocumulus, a normal form of quiet weather cloud.  The difference however was that the stratospheric clouds remained relatively bright long after normal stratocumulus would have turned dark at sunset.  The clouds might be seen from northern Scotland from February 27, and later from southern England until perhaps March 1.  The current forecast weather conditions favour Scotland seeing the clouds before sunrise on February 28, with southern England seeing them after sunset the same day.  The rest of the time it may be too cloudy with lower level clouds.

There are plenty of pictures of the clouds on the web, eg http://www.bbc.co.uk/news/in-pictures-35462395 though virtually all these show the spectacularly coloured form, rather than the generally colourless diffuse form.

Jonathan Shanklin
BAS Fellow
British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, England