May 27, 2021: Something unusual is happening at the top of Earth’s atmosphere. Noctilucent clouds (NLCs) are forming, and people are seeing them from the ground even though it is only May. Andy Stables sends this photo from the Isle of Skye, Scotland, taken May 26th:

The electric-blue ripples “were clearly visible to the unaided eye,” says Stables. “This is the earliest I have ever seen them here in Scotland.”

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space about 83 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. The season for bright naked-eye NLCs typically stretches from June through August.

This year NLCs are getting an early start. We’ve already received multiple reports of sightings in Europe from latitudes as low as 50 degrees, and according to NASA’s AIM spacecraft the clouds are rapidly intensifying. In only 4 days since the clouds were first spotted, their coverage of the Arctic has multiplied 10-fold:

The reason may be extra water in the mesosphere: NASA satellite data show that 2021 is one of the wettest years since 2007. NLCs have more H₂O to work with–hence the early start and rapid growth.

In recent years, summertime noctilucent clouds have spilled as far south as Los Angeles and Las Vegas, setting records for low-latitude sightings. 2021 is shaping up to be such a year. Pro tip for northern sky watchers: Look west 30+ minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud