Polar stratospheric clouds or PSCs, also known as nacreous clouds, from nacre, or mother of pearl, due to its iridescence, are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters 49,000–82,000 ft. Tromsø lies in Northern Norway. Most of Tromsø, including the city centre, is located on the island of Tromsøya, 350 kilometres 217 mi north of the Arctic Circle.

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