Altocumulus clouds float above Washington skies in September. Characterized by rolled layers and globular masses, they often signal that afternoon thunderstorms are on the way. (Salwan Georges/The Washington Post)

The sky has brought nothing but trouble lately — hurricanes, tornadoes, torrential rain. For good reason, people look up with wary eyes. On ordinary days, a pleasant wisp of white or a menacing thunderhead serve a more pedestrian preoccupation, a pragmatic barometer for how to dress or where to go.

Then there are those who gaze skyward for contemplation, even inspiration — especially when that azure canopy is tinted by white.

Whether they're streaking from horizon to horizon or floating in whiffs and puffs, clouds play large in our lives. They saturate our speech: Think of cloud nine. Our songs: Cue Joni Mitchell. Our literature, our art, even our geology is filled with clouds. What are some of the highest mountains in Colorado named? Mount Nimbus, Stratus, Cirrus & Cumulus.

As Yeats wrote, "I know that I shall meet my fate somewhere among the clouds above."

For climatologists, though, clouds are scientifically important as well as deeply challenging.

"Clouds are hard to model," NASA associate research scientist Kate Marvel said. "They're the result of water vapor or ice crystals coalescing around microscopic bits of dust, particles of smoke and sea salt. . . . So they have a dual effect on climate. They trap the heat from the planet and spit it back down, making things warmer, but they also block sunlight, which is a cooling effect."

All of which frustrates climatologists trying to understand the effect of clouds on the planet.

"Even small changes to the distribution of clouds with rising temperatures could substantially diminish or enhance global warming," said David Romps, a professor of Earth and planetary science at the University of California at Berkeley.

Stratocumulus clouds ride low in the sky. (Salwan Georges/The Washington Post)

In July 2016, the journal Nature published research showing cloud cover had increased over Earth's polar regions, accelerating greenhouse gas concentrations.

"Right now, clouds are more air conditioner than heater," Romps noted, "but that will change as the planet warms."

Some scientists have even theorized certain distinctive cloud formations can be harbingers of imminent earthquakes. A Chinese engineer claims to have successfully predicted more than 30 temblors.

Psychologically speaking, clouds also have both positive and negative impact. Overcast weather turns us inward and helps us focus, the experts say. Sunny weather, by contrast, slows cognition.

Researchers in Australia tested their theory with an experiment several years ago. They showed — for the first time in a real-life setting — weather-induced moods can significantly affect memory. On rainy, cloudy days, which caused a gloomy mood, the ability to recall objects was three times greater than on sunny days, despite all the positive vibes they triggered.

To artists and writers, clouds have been everything from eye-catching backdrops in paintings to humorous metaphors in literature — even objects of philosophical inquiry. Where would religion be without clouds? The Bible's many references start at the very beginning with Genesis 9:13. ("I have set my rainbow in the clouds," God says, "and it will be the sign of the covenant between me and the Earth.")

When 17th century French thinker Rene Descartes wasn't busy becoming the father of modern philosophy ("I think, therefore I am,") he was looking up at the sky and musing confidently on his scientific acumen.

"We imagine [clouds] so lofty that even the poets and painters depict them as the throne of God," he wrote. "This makes me hope that if I explain their nature . . . one will easily believe that it is possible in the same way to find the causes of everything wonderful on the Earth."

More than 400 years later, Dutch visual artist Berndnaut Smilde creates his own fleeting clouds indoors. He uses a complicated mix of smoke and water vapor, then captures the results on photographs before the clouds disintegrate back into nothingness.

“Nimbus PowerStation” was photographed in an abandoned building in East Perth, Australia. (Courtesy of Berndnaut Smilde/Courtesy of Berndnaut Smilde)

“Nimbus Green Room” shows the beaux-arts War Memorial Veterans Building in San Francisco. (Courtesy of Berndnaut Smilde/Courtesy of Berndnaut Smilde)

Smilde has conjured clouds in a Turkish bath in Istanbul, at a deserted mining company in Belgium, even in a centuries-old London dungeon.

"You need a certain space and want it to float," he explained. "The search for the ideal cloud can take a long time."

Clouds have often come with numbers attached, the most prevalent being cloud nine. It played a starring role in The Temptations' 1968 album of the same name, with its eponymous Grammy-winning song. It then resurfaced 19 years later as an album title for former Beatle George Harrison.

The phrase, indicating a state of euphoria, may have originated from the early cloud classification system in which cloud nine was the tallest formation. Then again, cloud seven carried you into bliss, according to the 1960 American Dictionary of Slang. You definitely didn't want to end up on cloud eight — it meant you were drunk, per a 1930s book about the criminal "underworld."

Gavin Pretor-Pinney sees cloud-watching as a leisure activity. On a lark in 2005, the Londoner founded the Cloud Appreciation Society. Its membership today tops 43,500 with "cloudspotters" ringing the globe from the Congo to Canada, Iceland to Iran, China to Chile. The United Kingdom is home to more than half the organization's members. The United States is second with nearly 10,000.

Many regularly send their best cloud photos to Pretor-Pinney, who uses them for his "cloud of the day" and "cloud of the month" emails. In fact, interest has grown so much that the Cloud Appreciation Society is now a full-time business. Pretor-Pinney, an art designer by training, is leading "sky holidays" to particularly cloudy parts of the world. The first trip was to Canada in February to see the Northern Lights.

"I've always felt the sky to be the most dynamic, most poetic part of nature," Pretor-Pinney said. "So it's always struck me as a shame that it's gotten bad press."

The society even helped identify a new class of cloud, a turbulent formation named asperitas that was officially added this March to the 121-year-old International Cloud Atlas.

The group, of course, has nothing good to say about lovers of cloudlessness — including beachgoers, most prominently. They call the worship of monotonous cloudlessness "blue sky thinking."

Pretor-Pinney rejects such a limited view of the heavens. "Cloudspotting is a conscious invitation to daydream, a sensitivity to your surroundings," he said. "It's a kind of sky geekiness, which is beautiful."

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These fair-weather clouds go by the name cumulus humilis. (Salwan Georges/The Washington Post)