On May 17th, NASA's AIM spacecraft detected the first noctilucent clouds (NLCs) of summer. Those first electric-blue smudges were barely visible. Since then, however, the clouds have rapidly intensified. Recent images from orbit show a growing bank of NLCs pinwheeling just inside the Arctic Circle:
NLCs in May 2020
NLCs are Earth's highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the ground. The clouds form when summertime wisps of water vapor rise up to the mesosphere, allowing water to crystallize around specks of meteor smoke. Last summer, they spread as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings.

It's early in the 2020 season, so the clouds are still concentrated around the North Pole. Nevertheless, people in Europe are starting to see them. Johny Krahbichler sends this picture taken last night (May 26th) from Ängelholm, Sweden:
NLCs over Angelholm, Sweden
© Johny Krahbichler
"These night glowing clouds are pretty common during the summer here in Sweden," says Krahbichler. "But it's rare that they glow this brightly over such a large area. As soon as I saw them I ran to get my camera. The glow from the clouds ended up matching the glow of my LED strip inside!"

Noctilucent clouds have been likened to a great "geophysical light bulb" because they turn on abruptly, reaching almost full intensity over a period of ~10 days. By early June, therefore, we can expect the clouds to spread farther south with a significant increase in brightness. The circular rhythm of the pinwheel motion (caused by a 5-day planetary wave) may even allow us to start issuing predictions of latitude ranges where the clouds are most likely to appear.

Stay tuned for updates--and be alert for electric blue.