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Some photos of polar stratospheric clouds from Lappland. In Finland the PSCs' usually come without irisdescent colours. They occur in winter when there is a deep low pressure over Scandinavia. Thus forecasting their appearance is rather easy. These clouds also are called as mother-of-pearl and nacreous clouds, both names refererring to the irisdescent color in molluscs shells.
The photos above were taken on two occasions, the ones with purple glow on 30 December 1999 and the one with white clouds on 28 November 1999.