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From the ‘observing tips’: ‘Look west 30 to 60 minutes after sunset when the sun has dipped below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.’

May 31, 2019: A huge blue cloud of frosted meteor smoke is pinwheeling around the Arctic Circle. NASA’s AIM spacecraft spotted its formation on May 20th, and it has since circled the North Pole one and a half times, expanding in size more than 200-fold.

“These are noctilucent clouds,” says Cora Randall of the AIM science team at the University of Colorado. “And they are going strong.”

Noctilucent clouds (NLCs) in May are nothing unusual. They form every year around this time when the first wisps of summertime water vapor rise to the top of Earth’s atmosphere. Molecules of H₂O adhere to specks of meteor smoke, forming ice crystals 80 km above Earth’s surface. When sunbeams hit those crystals, they glow electric-blue.

But these NLCs are different. They’re unusually strong and congregated in a coherent spinning mass, instead of spreading as usual all across the polar cap.

“This…