A mysterious, glowing cloud loomed over San Francisco last week. (National Weather Service San Francisco/National Weather Service San Francisco)

It wasn’t a bird. It wasn’t a plane. Truth be told, nobody knew exactly what it was that loomed over San Francisco last week. It took nearly 24 hours before anyone could say for sure what was behind the mysterious, vaporous cloud that had captured Californians’ attention.

Hundreds tweeted out pictures of what they saw around 5:40 p.m. on Dec. 19 — a diaphanous trail that seemed to be generating its own light. The glowing cloud resembled what normally accompanies a rocket launch.

Indeed, United Launch Alliance was set to fire its hydrogen-fueled Delta IV Heavy into space, carrying a top-secret spy satellite for the National Reconnaissance Office. The only problem? The launch of the 1.6 million-pound vessel from Vandenberg Air Force Base was scrubbed nine minutes before the scheduled liftoff.

Queue the alien theories.

The National Weather Service’s Bay Area office posted a similar photo on Twitter, inviting followers to chime in with their guesses. Some speculated that it might be earthborn space junk reentering the atmosphere and burning up. A few reported hearing a noise. And then, finally, dash-cam video emerged, tracing the strange blue loop-de-loop cloud back to a meteor.

The American Meteor Society received nearly 180 reports of the intense fireball, clustered in Northern California but stretching from Nevada to Oregon. Brighter than the full moon, the meteor burned a vibrant green as it disintegrated over the open waters 35 miles west of the Golden Gate Bridge.

The cloud that lingered was equally special. Known as a noctilucent cloud, its appearance marks an extremely rare sighting in the Lower 48. Noctilucent clouds form in the highest levels of the atmosphere. Most clouds appear white or gray, but noctilucent clouds shine a ghostly blue long after the sun has set, because they are 50 miles high. As a result, noctilucent clouds can catch sunlight even after the sun drops below the horizon. Noctilucent clouds appear almost exclusively around the poles, where a cooperative sun angle, plenty of twilight and extreme low temperatures promote noctilucent development. Still, they’re rare.

At low latitudes, noctilucent clouds are practically unheard of — the atmosphere gets a little bit of help. On Aug. 4, 2014, a SpaceX launch from Cape Canaveral greeted early risers in Orlando with a stunning azure display.

However, that might be changing. There is research to suggest that outbreaks of noctilucent clouds may be related to climate change. As humans continue to emit methane, more of the greenhouse gas makes its way into the upper atmosphere. There, it becomes oxidized to form water molecules, adding moisture to foster noctilucent development where it otherwise wouldn’t be. In just the past century, atmospheric methane concentrations have doubled. That, plus a cooling of the mesosphere hinted at by climate models, means the beauty of these clouds may mask a hidden warning.

An outbreak of noctilucent clouds has been noted in the past two months over Antarctica.