May 24, 2019: NASA’s AIM spacecraft has spotted wispy patches of electric-blue drifting over the Arctic Ocean. This marks the beginning of the 2019 season for noctilucent clouds (NLCs). The clouds are boxed in this polar image recorded by AIM’s CIPS instrument:
These first detections from space are mostly small and faint. They won’t remain faint for long, however. Previous data from AIM have shown that NLCs are like a great “geophysical light bulb.” They turn on every year in late spring, reaching almost full intensity over a period of 10 days. This means observers on the ground should soon begin to see them.
NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the planet’s surface. The clouds are very cold and filled with tiny ice crystals. When sunbeams hit those crystals, they glow electric-blue.
This is what a fully-realized noctilucent cloud looks like, photographed over the Lille Vildmose Wild Life Park in Denmark by Pernille Fjeldgaard Jensen on July 6, 2018:
Early-season NLCs are always found at high-latitudes–e.g., Canada, the British isles, Siberia and Scandinavia. To people in those areas, we offer the following 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.
Spaceweather.com 
