Nephomancy is divination by clouds: watching the sky, noticing what the shapes seem to say, and taking it as a message. The word is built from two Greek parts, nephele, meaning cloud, and manteia, meaning divination, the same ending we find in necromancy, geomancy and chiromancy.

It sits inside a larger family called aeromancy, the reading of the air and sky. Its cousins include reading thunder and lightning, shooting stars and strange lights overhead. The tidy -mancy names are later inventions, though. Nobody in the ancient world called their craft nephomancy, and the first recorded English use of "aeromancy", in 1753, meant the study of the atmosphere rather than prophecy.

What people did long before the word existed is a patchier story. Babylonian scribes collected thousands of omens from the heavens and the weather. Han-era Chinese writers described cloud-watching as part of a system of lucky and unlucky signs. A late-antique writer, Damascius, reports a woman who saw a man and a lion take shape in the clouds. Some of these accounts are well documented and some reach us at second or third hand, and in this blog we will try to say which is which.

There is no canonical rulebook, which is part of the charm. Clouds never repeat, so a reading is always about this sky, this afternoon, and the person looking up. Today we treat it as a game of attention and imagination: an invitation to look closely and ask what you notice. It is entertainment and reflection, not prediction, and the fun is in the looking.

The earliest famous cloud-watcher in Western literature is a comic one. In 423 BC the Athenian playwright Aristophanes staged The Clouds at the City Dionysia, the great spring drama festival. The play came third, and Aristophanes later revised it. The version that survives is that revised text.

The plot follows Strepsiades, a farmer buried in debt, who hopes to learn clever arguments that will let him dodge his creditors. He sends his son to the Thinkery, a school run by Socrates, who appears in the play dangling in a basket so that he can study the sky. Socrates tells his visitor to worship not Zeus but the Clouds, which he calls the true goddesses of thinkers.

The Clouds arrive as the chorus, singing and drifting across the stage. Socrates explains that they take whatever shape they like. If they spot a shaggy, wild-looking man, they become centaurs. If they see a greedy thief, they turn into wolves. The joke is that clouds mirror whoever is looking up at them. That, remarkably, is the very idea behind every cloud reading since.

Aristophanes was not teaching divination, and his Socrates is a caricature. Plato later suggested that the comedy helped feed the suspicion that led to the philosopher's trial. But Aristophanes captured something people have always felt: a cloud can seem to change to suit your mood, your fears and your character. When you read a sky today, you are playing a much older game than you might think.

Not everyone in ancient Greece saw clouds as omens. Around 340 BC Aristotle wrote the Meteorologica, a long study of what happens in the sky and the air: rain, wind, hail, thunder, rainbows and the clouds that bring them. His explanations seem odd now. He believed the sun's heat drew moist and dry exhalations up from the earth and sea, and that clouds formed when the moist kind cooled and gathered.

The details were mostly wrong, yet the move was bold. Clouds became something to explain, with causes you could argue about, instead of something only a priest could interpret. The word meteorology still comes from his title, and means the study of things high up.

His pupil and successor Theophrastus is linked with a short work called On Weather Signs, a practical collection of what the sky tells farmers and sailors. Its entries are the ancestors of folk sayings still in use: how the sunrise looks, how clouds gather over hills, and what a certain haze might mean for tomorrow. Scholars debate how much of the book is Theophrastus himself, and how much was gathered by others.

Here is a second, quieter way of reading the sky. A weather sign does not carry a message from anyone. It is a pattern, noticed over many years, that points to what is coming. Divination and forecasting have been neighbours ever since, and the oldest sky-watchers probably did both on the same afternoon.

The richest early library of sky omens comes from Mesopotamia. Enūma Anu Enlil, named after its opening words, is a series of around seventy tablets holding roughly seven thousand omens. It was compiled and copied over many centuries, from the Old Babylonian period onward, and it covers the Moon, the Sun, the planets and the weather.

Weather had its own section, around tablets 44 to 49 in modern editions: thunderstorms, winds, rain and the signs that came with them. The Sun tablets, around 23 to 29, include omens about how the Sun looked when it appeared among clouds of particular shapes and colours. So clouds do appear, usually as part of a larger scene in the sky.

The omens follow a formula: if this happens in the sky, then this will happen on earth. Most concern the king and the state, not the ordinary person. Specialist scholars watched the heavens and sent reports to the Assyrian kings, who weighed them when deciding about war, harvest and ritual. A cloud was a matter of politics long before it was a matter of personal fortune.

You may read online that Babylonian priests practised nephomancy as a distinct art. Historians do not quite say that. What survives is a huge tradition of celestial and weather omens, in which clouds play a part, not a separate craft of reading cloud shapes. It is a good reminder that the ancient story is richer than the slogan, and we would rather tell you the careful version.

Cloud-watching in early China is called yunqi, roughly "cloud vapours", and it was woven into a wider system of reading omens in the sky. In the Han era, which began in 206 BC, clouds, comets, haloes and rainbows were all signs that officials were expected to record and interpret.

One famous source is a set of silk manuscripts found in a tomb at Mawangdui, sealed in 168 BC. It includes a text on astronomy and weather signs illustrated with rows of small painted clouds, comets and vapours, each with a short note on its meaning. It is among the oldest surviving pictorial catalogues of its kind.

Another is a manuscript from the Dunhuang caves on the Silk Road, known as Zhan yunqi shu, which places cloud divination inside the same world of auspicious and disastrous signs. Han commentators also found hidden meanings in the clouds and thunder that appear in old poems such as the Shijing and the Chuci.

Auspicious clouds, xiangyun, left a different trail. Swirling cloud patterns became a beloved motif in Chinese art, carved on palaces and painted on silk as a sign of good fortune and heavenly favour. Scholars still debate the methods, the dates and how these traditions relate to those elsewhere. We should be careful not to assume a link with Babylon or Greece: there is no evidence for one.

Say "druid" and "clouds" in the same sentence and many people will nod. It sounds right: robed priests on a misty hill, reading the sky. The trouble is that the evidence is thinner than the legend.

In medieval Irish and Welsh writing, druids and their successors are sometimes shown reading signs in the sky, clouds among them, and predicting from what they saw. These scenes are vivid, and they are often cited as proof of an ancient practice.

But careful scholarship, such as the Oxford study Fiery Shapes, which looks at celestial portents in Ireland and Wales between 700 and 1700, urges caution. It argues that druidic cloud divination was largely a literary idea of the twelfth century, a way for medieval authors to give their heroes a mysterious, pre-Christian past, rather than a well-attested ancient custom.

That does not mean medieval people ignored the sky. They recorded comets, eclipses and strange lights, and treated them as portents. It means the cloud-reading druid is a story about the past told by people living much later.

We think that is a lovely thing to know. The legend of druids reading clouds is itself a piece of folklore, and part of why the idea still has such a hold on the imagination.

Shakespeare knew that clouds can look like anything. In Hamlet, in the third act, the prince is trying to show how easily the old courtier Polonius will agree with whatever he is told. So he points at the sky.

"Do you see yonder cloud that's almost in shape of a camel?" Hamlet asks. "By th' mass, and 'tis like a camel, indeed," says Polonius. "Methinks it is like a weasel," says Hamlet. "It is backed like a weasel." "Or like a whale?" "Very like a whale."

The joke is about flattery, since Polonius will see whatever the prince wants him to see. But it only works because the audience already knew that a cloud can be a camel, a weasel or a whale, depending on who is looking. Cloud-gazing was clearly familiar enough in Elizabethan London to be a gag.

Shakespeare returns to the idea more gently in Antony and Cleopatra, where Antony speaks to Eros of the sky: "Sometime we see a cloud that's dragonish; a vapour sometime like a bear or lion, a tower'd citadel, a pendent rock." He goes on to say that the shapes dissolve into the air as we watch. In Antony's mouth, a cloud is a picture of how a life can lose its outline.

Both scenes are useful for anyone who reads clouds today. The first reminds you that an image can say more about the viewer than about the sky. The second reminds you that the image will not stay. Perhaps that is why it works so well as a moment of reflection.

Leonardo da Vinci, writing notes for painters around the turn of the sixteenth century, offered an odd tip for finding ideas. Look at a wall splashed with stains, he said, or at stones of mixed colours, and let your mind work. You might see landscapes with mountains and rivers, battles, strange faces, or costumes. In the same passage he lists the ashes of a fire, clouds and mud among the things that can set the imagination moving.

He understood that the mind meets confusion halfway. A muddle of shapes gives it nothing to resist, so it makes something up. He compared it to the way you can hear any word you like in the sound of bells. Leonardo did not call this magic. He called it a way to wake up invention, though some of his readers have always heard a hint of something more mystical.

The idea travelled. In the 1780s the English landscape painter Alexander Cozens published A New Method of Assisting the Invention in Drawing Original Compositions of Landscape. His students began with accidental blots of ink, made quickly and without thinking, then looked for scenery in them and drew it out. Cozens had in effect turned Leonardo's wall into a teaching exercise.

Cloud reading belongs to the same family. A cloud is an accidental blot on the largest wall there is. The shape belongs to no one, so anyone may find something in it, and what they find tends to be about them. We find that a gentle comfort: you do not need to be an expert to start, only willing to look for a moment longer than usual.

Before 1802, clouds had no agreed names. People spoke of mackerel skies and mares' tails, and a cloud was whatever you said it was. Then a London pharmacist and Quaker named Luke Howard, who loved to watch the sky, proposed a system that made sense of them.

Howard was born in 1772 and ran a chemical business, but he spent his spare hours recording the weather. In December 1802 he read a paper called On the Modification of Clouds to a small scientific club in London, the Askesian Society, and it was published the following year. He argued that clouds, which looked wild and random, fell into a handful of basic forms that change from one to another in a regular way.

He chose Latin for his names because it was the common language of scholars across Europe. Cirrus means a curl or lock of hair, for the high, wispy kind. Cumulus is a heap. Stratus is a layer or sheet. Nimbus is a rain cloud. He also described blends, such as cirrostratus and cumulonimbus, and those compound names are still in use.

Howard lived to be ninety-one, and he became something of a celebrity in the world of science. His scheme has been adjusted and extended, but its heart is intact. Every time someone says that a cumulus is passing or a nimbus is approaching, they are speaking his language.

Johann Wolfgang von Goethe is remembered for Faust, but he spent decades studying nature, from plants and rocks to colour and weather. When he came across Luke Howard's classification of clouds, he reacted like someone who had been handed a missing piece.

Around 1820 he wrote a set of poems in Howard's honour. There are verses for each of the cloud forms, and one for Howard himself, whom Goethe thanked for finding a way to hold the shifting shapes in the mind. The idea that a poet would write an ode to a weather classification may sound unlikely, but Goethe thought the two belonged together.

What drew him, it seems, was the combination of precision and wonder. Howard's names did not make clouds smaller. They gave people words for what they were already seeing, and words make it easier to look closely. Goethe, who distrusted purely abstract science, liked a system that stayed close to what the eye could see.

The two men also corresponded, and Howard's reputation on the European continent grew with Goethe's praise.

For cloud readers the lesson is old and still useful: naming is not the enemy of imagination. To say "cirrus" is not to stop seeing a horse's mane. It is to notice that the mane has a name, and that you can look at it again.

John Constable, born in Suffolk in 1776, believed that a landscape was only as good as its sky. In the summers of 1821 and 1822, living near Hampstead Heath on the northern edge of London, he made dozens of small oil studies of clouds, working fast outdoors. He called it "skying".

On the back of many of the studies he wrote the date, the time of day, and the direction and strength of the wind. They are as much weather records as paintings. He was trying to understand how a cloud actually looks and moves, not to paint an idealised heavenly backdrop.

In a letter of October 1821 to his friend John Fisher, he declared that the sky is the "key note", the "standard of scale" and the chief "organ of sentiment" in a landscape. A painter who neglects it, he said, throws away one of the greatest aids to his art. That was a bold thing to say in a period when skies were often a formula.

It is no accident that his studies come from the same era in which Luke Howard's names for clouds were spreading. People were learning to look at the sky properly, and to look at it as it really was.

Each of his sketches took only minutes, because the sky would not wait. Anyone who has tried to photograph a cloud and found it had changed before the shutter clicked will know the feeling.

Almost everyone has seen a face in a cloud. The tendency to find meaningful shapes in random patterns is called pareidolia, from the Greek para, beside, and eidolon, image. The German psychiatrist Karl Kahlbaum used the term in 1866, and it has been a topic of psychology ever since.

Researchers offer several explanations. One is that our visual system is tuned for faces from the first days of life, because noticing a face matters enormously to a baby. Another is that the brain runs two processes at once: one that automatically spots face-like patterns, such as two dots above a line, and another that actively searches for faces when something is ambiguous. Carl Sagan suggested an evolutionary story: spotting a face, or a predator's eyes, in the undergrowth could save your life, so a few false alarms were a fair price.

Interestingly, studies suggest that being good at recognising real faces does not predict who sees faces in clouds. The two skills seem to be separate.

Ambiguous images also invite us to read ourselves into them. Hermann Rorschach used that idea in 1921, when he published his inkblot test. Whether such tests measure anything reliably is debated, and we make no clinical claims, but the basic observation holds: when the picture is vague, the viewer fills in the detail.

That is what makes cloud reading work as a game of reflection. The sky does not know you. What you see in it, and what you feel about what you see, comes from you, which is why two people can look at the same cloud and find a dragon and a dove.

In the 1920s the American photographer Alfred Stieglitz, already famous for his city scenes and portraits, turned his camera upward. At his family's summer place at Lake George in New York he photographed clouds, and nothing else, for years.

His early sky pictures were grouped under musical titles, and a 1922 set was called Music: A Sequence of Ten Cloud Photographs. By the middle of the decade he was calling them Equivalents. He meant that each picture was an equivalent of an experience or an emotion, not a picture of a thing. People who saw the photographs could feel something without being told what they were looking at.

The pictures leave out the horizon. There is no ground, no tree, no person to give scale, only light and form in the frame. That made them feel abstract, and it let viewers turn them and see them from any side. Stieglitz wanted to show that the power of a photograph did not depend on a famous subject or a pretty scene.

The series ran, on and off, into the 1930s. It was a bold claim at a time when many doubted that photography could be art at all. If a photograph of nothing but a cloud could express the photographer's inner life, then the camera was more than a recording machine.

It is also an unusually good description of what a cloud reading is. The sky provides the shapes, and the person looking provides the feeling. A cloud is an equivalent, and the viewer decides what it is equivalent to.

Luke Howard gave clouds their names, but a global language needed global agreement. In 1887 the Swedish meteorologist Hugo Hildebrandsson and the British meteorologist Ralph Abercromby proposed a system with ten basic cloud types, based on Howard's forms and on years of observation around the world.

The first International Cloud Atlas followed in 1896, with photographs and colour plates, so that observers everywhere could match what they saw to the same pictures and words. A sailor in the Pacific and a station keeper in Norway could now report the same cloud in the same terms.

Those basic types, now called genera, still number ten: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus. Add species, varieties and supplementary features, and the vocabulary grows much richer.

The atlas is now maintained by the World Meteorological Organization, and in 2017 a new digital edition was published. It added several new features, among them one called asperitas, a name for the rolling, wave-like undersides that can make a sky look like a stormy sea seen from below. It was a rare moment when a new cloud name entered the official book, and the story of how it got there is the next stop on our tour.

For centuries clouds have been blamed for ruining picnics. In the early 2000s the British writer Gavin Pretor-Pinney decided they deserved better. In 2004 he founded the Cloud Appreciation Society, and in 2006 he published The Cloudspotter's Guide, a witty, learned book that became a bestseller.

The society's idea is simple: clouds are fascinating, and looking up is good for you. Members around the world send in photographs, argue about identification and share their favourites. It now counts many thousands of members.

Its best-known campaign concerned a roiling, wave-like cloud formation that members kept photographing and that had no official name. They called it asperitas, from the Latin for roughness, and lobbied the World Meteorological Organization to recognise it. In 2017 the WMO added asperitas to its International Cloud Atlas, the first new addition of its kind for decades.

Pretor-Pinney also argues that cloud-watching is a gentle kind of mindfulness: a reason to stop, tip your head back and pay attention to something that costs nothing. It is an argument we have a lot of sympathy for.

Nephomancy adds a playful layer on top. First look closely at what is really there, as the cloud lovers do, then ask what it makes you wonder. The looking comes first.