Nacreous Clouds

Southern Norway had vivid nacreous clouds on 22nd December 2014.

Here are images by Morten Kleiva and Steinar Midtskogen . There is also a time lapse video by Steinar.

Nacreous clouds are 15 – 25 km high in the stratosphere, far above ordinary tropospheric clouds. To form they need very low stratospheric temperatures of less than minus 85 Celsius and so are only seen in winter at high latitudes like Scandinavia, Iceland and northern Canada. But, exceptionally, they can occur slightly more south and might – very rarely – be seen over England. Look for them after sunset and before sunrise.

They are composed of very small ice crystals and these diffract the high altitude sunlight to form their vivid iridescent colours. The necessary water is lofted up into stratosphere by gravity waves induced by mountains and high tropospheric winds.

'Ordinary' tropospheric iridescent clouds are often mistaken for stratospheric nacreous. There is actually no comparison for nacreous colours are intense, far brighter and more saturated than those of iridescent clouds.
A time-lapse video by Steinar Midtskogen.

The clouds remain visible during the brief Norwegian winter day.
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The clouds were visible before dawn on the 22nd - as here. They were visible again after sunset as in Morten's topmost image.

See the video below for their appearance in daylight.