But for most of human history, what a cloud was, physically, hardly mattered; instead, we understood clouds as psychic refuges from the mundane, grist for our imaginations, feelings fodder. Clouds both influenced our emotions and hung above us like washed-out mirrors, reflecting them. The English painter John Constable called the sky the “chief organ of sentiment” in his landscapes. And our instinct, as children, to recognize shapes in the clouds is arguably one early spark of all the higher forms of creative thinking that make us human and make us fun. Frankly, a person too dull to look up at the sky and see a parade of tortoises or a huge pair of mittens or a ghost holding a samurai sword is not a person worth lying in a meadow with. In “Hamlet,” Polonius’s despicable spinelessness is never clearer than when Hamlet gets him to enthusiastically agree that a particular cloud looks like a camel, then not a camel at all, but a weasel. Then, not a weasel but a whale. Polonius will see whatever Hamlet wants him to; he is a man completely without his own vision.

We look for meaning — portents — in the clouds as well, the more grown-up version of picking out puffy animals. “There’s a long history of people finding signs in the sky,” Pretor-Pinney told me, from Constantine seeing the cross over the Milvian Bridge to the often-belligerent protesters outside Pretor-Pinney’s talks, who are convinced that the contrails behind commercial airplanes are evidence of a toxic, secret government scheme and are outraged that Pretor-Pinney — the righteous Lorax of clouds — refuses to expose it. In short, clouds exist in a realm where the physical and metaphysical touch. “We look up for answers,” Pretor-Pinney says. And yet, we often don’t want empirical answers. There has always been a romantic impulse to protect clouds from our own stubbornly rational intellects, to keep knowledge from trampling their magic. Thoreau preferred to understand clouds as something that “stirs my blood, makes my thought flow” and not as a mass of water. “What sort of science,” he wrote, “is that which enriches the understanding but robs the imagination.”

The scientific study of clouds grew out of a collection of madly appreciating amateurs who struggled with this same tension. The field’s foundational treatise was first presented to a small scientific debating society in London one evening in 1802 by a shy Quaker pharmacist named Luke Howard. Howard, then 30, was not a professional meteorologist but a devoted cloud-spotter with a perceptive, if wandering, mind. His interest in clouds started early. His biographer, Richard Hamblyn, explains that as a young student in Oxfordshire, Howard seems to have found school magnificently boring. He couldn’t bring himself to pay attention, except to his Latin teacher, who punished daydreaming with beatings. Today Howard might covertly pull out his phone and read a link a friend shared about, say, an eccentric society in England that appreciates clouds. But poor Howard’s boredom was analog: all he could do was look out the classroom window at the actual clouds rolling by.

Howard’s intention that night in London was to bring clouds down to earth without depleting their loftiness. After years of closely observing clouds, his appreciation of them had hardened into analysis. He now insisted that, though clouds may appear to be blown around in random, ever-changing shapes, they actually take consistent forms, forms that can be distinguished from one another and whose changes correspond to changes in the atmosphere. Clouds can be used to read what Howard called “the countenance of the sky”; they are an expression of its moods, not just in a poetic way, as Constable meant, but meteorologically.

Howard’s lecture was eventually published as “On the Modifications of Clouds, and on the Principles of Their Production, Suspension and Destruction.” It stands as the ur-text of nephology, the branch of meteorology devoted to clouds. Howard divided clouds into three major types and many intermittent varieties of each, all similarly affixed with Latin names or compounds. (He had learned his Latin well.) Like Linnaeus, who used Latin to sort the fluidity of life into genera and species, Howard used his new cloud taxonomy to wrest our understanding of the world’s diversity from superstition and religion. His signature assertion that “the sky, too, belongs to the Landscape” can be read as a call for empiricism — a conviction that science can, in fact, measure out the mystical.

Nearly a century later, Howard’s work would be picked up by another energetic amateur, the Honorable Ralph Abercromby. Abercromby was the bookish great-grandson of a celebrated English war hero. He was apparently so meek and frail (“never robust, even as a boy,” one tribute read after his death) that he was forced to drop out of school and was rarely able to hold a job. He served briefly in the military but seemed completely unsuited to soldiering; deployed to Newfoundland in 1864, Abercromby began theorizing about how the fog there was produced. Later, stationed in Montreal, he scrutinized the wind. It would have been tempting for his superiors to label him “absent-minded” or “unfocused” but, in retrospect, it was just another case of a young man intensely focused on something few people considered worthy of attention — another case of a young man in love with clouds.