Nacreous Clouds, Sweden ~ Imaged by Roel Wijtmans ( photosynthesis in nature ) in February 2011 and a reminder that in the north the nacreous cloud season of a needed cold stratosphere and sometimes high surface winds to loft upwards water vapour is on us again.   All images ©Roel Wijtmans, shown with permission
The same scene scene later on and after the sun was well set and the sky had darkened. The nacreous clouds now glow like unworldly lamps. Yet they shine only by reflected and diffracted light from the sun still shining at their stratospheric 15 - 35 km altitude.

Later on their true nature is better revealed.

The brightly iridescent nacreous clouds (Type II Polar Stratospheric Clouds) are embedded by less bright and colourful Type I PSCs.

Type II (nacreous) clouds are made of tiny ice crystals at temperatures of -85C and below. Type I clouds can form at slightly higher temperature (-78C) and are nastier stuff. They are solid and liquid specks of nitric acid hydrates and nitric/sulfuric acid solutions. Their surfaces are catalysts for conversion of chlorine species to more active free radicals that can then destroy stratospheric ozone.

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Two images from earlier in the afternoon (the sequence of four was taken over 2-3 hours).

At right the clouds against a still bright sky might be mistaken for ordinary tropospheric iridescent clouds. Look for distinguishing clues like slow movement relative to lower and darker clouds, longevity and brightening as the sky darkens.