Oslo: Sky watchers were in for a pleasant surprise when they spotted rare rainbow-coloured 'mother-of-pearl' clouds in the Norwegian skies.
Polar stratospheric clouds were seen over the Arctic circle yesterday just after sunset.
The clouds, also known as "mother of pearl", or "nacreous" are icy structures formed in the lower stratosphere when temperatures drop to around minus 85 degrees Celsius.
Unlike normal grey-white clouds which can be seen at altitudes of only 5 to 10 km, PMCs float through the stratosphere (25 km) and are fantastically colourful.
Sunlight shining through tiny ice particles - 10 micrometres - across produce the characteristic bright iridescent colours by diffraction and interference.
"Nacreous clouds outshine and have much more vivid colours than ordinary iridescent clouds, which are very much poor relations and seen frequently all over the world," atmospheric optics expert Les Cowley said.
These clouds, which are associated with the formation of ozone holes, float much higher than ordinary clouds and produce unforgettable colours during the hours around sunset, he said.