POLAR STRATOSPHERIC CLOUDS: The most beautiful clouds on Earth just made an appearance over Norway. "I've been on the lookout for polar stratospheric clouds (PSCs) all winter," reports Fredrik Broms from Kvaløya. "This morning, I caught a split-second glimpse of some really colourful PSCs, and within an hour another gap in the storm clouds revealed a bigger display."

https://imgur.com/6kGpuO4

"Some of the clouds were very colourful, including an absolutely stunning greenish cloud surrounded by purple," says Broms.

Polar stratospheric clouds are very rare. Normally the stratosphere has no clouds at all. PSCs form when the temperature in the Arctic stratosphere drops to a staggeringly-low -85 C. Then, and only then, can widely-spaced water molecules begin to coalesce into tiny ice crystals -- the "stuff" of PSCs. High-altitude sunlight shining through the crystals creates iridescent colors more intense and pure than any other cloud on Earth.

In a typical Arctic winter, PSCs appear two or three times at most.

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