Nacreous cloud. View of a nacreous cloud illuminated by the sun after it has moved below the horizon. Nacreous clouds are a rare cloud form characterised by an iridescent appearance. They are most noticeable after sunset when they are seen against a dark background, as here. They are high-altitude clouds, typically forming at around 20-25 kilometres above the ground. They are usually only visible from areas around the poles at times when stable low pressure weather systems prevail. This cloud was photographed in Finland in January 1997.

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