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Interesting Images

Ghostly Blue Clouds

It looks like a ring of blue fire in the sky. But, in fact, that swirl of sapphire over the North Pole and Greenland is actually ice — that, and a bit of pulverized meteor dust.

They're called "noctilucent clouds," because they only appear after sunset. Blue and wispy, these moonlighting cirrus strands form high in the atmosphere in spring and summer, when the upper atmosphere starts to cool as the lower atmosphere warms. There, ice crystals hovering about 50 miles (80 kilometers) over Earth glom onto little particles of dust from smashed-up meteorites and other windblown sources, then condense into smoky ribbons of cloud. (The same phenomenon has been spotted on Mars.)

These are the highest clouds in the sky,and form so high up that they glow icy blue even after the sun appears to have passed below the horizon at ground level. Usually, they're only spotted at high latitudes in warm months; the ring of clouds above, shown in a composite image taken by NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft — a satellite that measures how much sunlight is being reflected into space by high-atmosphere clouds — appeared over Greenland on June 12, 2019.