It’s a wild time of year to look at clouds in Oklahoma, and Keith Cook can attest to that. But the clouds he spotted over the weekend from near Enid weren’t associated with any supercell thunderstorms. Instead, they were clouds at the edge of space.

Cook stumbled upon the rare noctilucent cloud. They form 50 miles up, in a region of the atmosphere called the mesosphere. All of our weather-producing clouds live far lower — generally less than 50,000 feet — in the troposphere.

Sightings have been pouring in from the western half of the nation. A shimmering display of the diaphanous clouds treated Californians to a show Sunday night. Brad Charboneau, a meteorologist at the National Weather Service office in Eureka, Calif., witnessed them firsthand.

“They were unusually bright,” said Charboneau, who was driving when he spotted them. “They really stood out, because it was well after sunset. It was about 9:40 p.m.”

Sunset had occurred an hour earlier, so Charboneau knew he encountered something special.

“I thought about it for a little bit, and then it dawned on me what they were,” he said. “I pulled over really quick and started taking photos.”

At first glance, noctilucent clouds may resemble the high, wispy cirrus clouds that we often see over the Lower 48. They resemble pale brushstrokes against a blue sky, and often are bathed in an amber hue as daylight dwindles. But noctilucent clouds are different.

“They were kind of a bright, light blue,” Charboneau said. “They were brilliant.” described a “huge outbreak” of the clouds over the weekend with sightings in nine states, in addition to Oklahoma, and parts of Europe.

Because they soar so high, noctilucent clouds catch sunlight well after dark. It’s the same reason the sun sets several minutes later at the top of a skyscraper, compared with the bottom. If you pretend a noctilucent cloud is anchored atop a 50-mile-high building, it’s easy to see why they shine a brilliant blue up to two hours after sunset or before sunrise.

It’s thought that the clouds are instigated by meteor smoke. Any time you see a shooting star, the instigating space pebble deposits a thin veil of fine dust behind it as it burns up. These tiny particulates may serve as “nuclei” around which ice can form.

The source of water that high remains a mystery, however. After all, ordinary clouds are made of condensed water vapor or ice crystals, but noctilucent clouds form at heights thought to be nearly devoid of moisture. Temperatures at that level dip below minus-190 degrees, and the colder the air gets, the less moisture it can hold.

Noctilucent clouds are most common near the poles. They’re rarely seen below 50 degrees latitude, but they’ve been venturing outside of their usual bounds much more frequently in the past decade. A potential reason? Climate change.

It’s not just carbon dioxide that humans pump into the atmosphere. It’s methane, too. James Russell, principal investigator of a NASA mission tasked with studying these electric-blue clouds, warns that the increase in noctilucent episodes could be a “canary in a coal mine” for climate change.

"When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor,” Russell said in a 2012 NASA interview. In other words, more methane means more moisture high up. “This extra water vapor is then available to grow ice crystals for [noctilucent clouds].”

The rise in noctilucent cloud sightings has been known for decades. In a 2001 paper published in Advances in Space Research, researcher Gary Thomas of the University of Colorado notes “a strong upward trend” in their frequency between 1964 and 1986. Indeed, their beauty may be a hidden climate warning.

In the meantime, those across the Lower 48 might be treated to more noctilucent sightings. And while they’re tough to predict, looking upward an hour after sunset is worth a try.

“Unfortunately we don’t have any way of really predicting them,” Charboneau said. “Those clouds occur from 250,000 feet up, so we don’t have tools available to forecast them. But I know I’ll be looking for them myself tonight. It’s certainly worth keeping an eye out.”