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November-December 2017

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Introducing Asperitas: The Newest Cloud in the Sky

The top of the cold air pool in a thunderstorm outflow is a prime location for asperitas waves to form, as above Hiawatha, Iowa in 2006.

The top of the cold air pool in a thunderstorm outflow is a prime location for asperitas waves to form, as above Hiawatha, Iowa in 2006.

There is a birthday coming up. On March 23, 2018, World Meteorological Day, asperitas will be one year old. On that date in 2017, the World Meteorological Organization (WMO) “gave birth” to a brand new cloud designation—asperitas—by including it, along with 11 siblings, in the first revision of the International Cloud Atlas (ICA) in several decades. The methodical workings of a United Nations agency meant that the gestation period was long—almost a decade—and marked the first such addition since cirrus intortus was added to the pantheon of approved cloud names in 1951. The ICA, originally several hefty, faux-leather bound volumes, was updated, expanded and, most importantly, made available online to professionals and weather enthusiasts everywhere ( While the atmosphere is essentially chaotic, avoiding chaos in the naming and reporting of clouds requires creating a global standard for all to use.

How the asperitas came to be designated as a distinct supplementary cloud feature is a story of persistence and a triumph for citizen scientists, and of crowd sourcing aided by digital phone cameras and the power of the Internet. It also required the tenacity and perseverance of an Englishman who, while having no formal training in meteorology, has done much to bring the joy of cloudspotting to a myriad of weather lovers worldwide.

Appreciating Clouds

Gavin Pretor-Pinney, a native of West London, always had a love of science. Entering Oxford University initially with a double major in physics and philosophy, he ultimately received his BS degree in psychology and philosophy. Then followed a Master in Arts from Central Saint Martin's College of Art and Design. A polymath by nature, his eclectic ventures included co-founding The Idler magazine (“literature for loafers”), which championed eschewing the daily rat race to let the mind breathe and actually think. Not being all that idle himself, Pretor-Pinney also maintained a career in advertising, all the while mastering the ukulele and authoring a popular book encouraging others to strum along.

But the impetus for his most popular book, The Cloudspotter's Guide: The Science, History, and Culture of Clouds, was an epiphany of sorts—involving clouds. Finally taking some of his own advice to kick back from the frantic everyday life and make room in the brain for new ideas, he spent seven sabbatical months in Rome just “living and learning.” He admired the fierce convective cloudscapes that boil up over central Italy in summer, but also the variety of portrayals of clouds in the art of the city's museums. “Clouds so often were portrayed as the sofas of saints,” he noted.

Upon his return to London, clouds kept entering his thoughts, not so much the physics of dry and moist adiabatic expansion and the adventures of cloud condensation and ice nuclei, but their cultural and psychological significance. He concluded that clouds had been getting bad rap. As he expounded in his 2013 TED Talk (, most people only notice clouds when they get in the way of the sun. The word cloud is weighed down with negative connotations such as a “cloud on the horizon,” “a cloudy future,” or maligning dreamers as those with their “head in a cloud.”

Asperitas clouds wave above western Norway’s Lofoten Islands in this image from July, 2006, one of many submitted to the Cloud Appreciation Society.

Asperitas clouds wave above western Norway’s Lofoten Islands in this image from July, 2006, one of many submitted to the Cloud Appreciation Society.

Pretor-Pinney resolved that clouds needed a defender. The science of nephology—the study of clouds—dates as far back as the sixth century BCE Greek philosopher Thales of Miletus. Yet beyond, and perhaps underlying, the scientific study of clouds is the sheer indolent pleasure of cloudspotting. Simply appreciating the performance of the celestial mists in the theater of the sky is good for the soul. Such time “wasted” can clear the mind of the detritus of hectic modern living, perhaps making space for surprising or pleasant inspiration. Clouds are the expressions on the face of the atmosphere—daily performance art that is free for the taking. One merely has to look up to enjoy that silver lining. No degree required.

Now, readers of Weatherwise love clouds—the good, the bad, and the ugly—and especially the beautiful. But not everyone reads Weatherwise. And alas, far too few of our fellow citizens ever even bother to simply look up.

Unbeknownst to Pretor-Pinney, he had an ally on the other side of the Atlantic. Jack Borden, a Boston TV reporter, was on assignment one sunny day in the early 1980s and noticed a gorgeous display of cumulus clouds overhead. He then stopped 20 random pedestrians, placed his hand above their eyes like a visor and asked them to describe the sky. Typically they replied it was “sunny.” “What about the clouds?” he queried. Not one could describe the panorama of boiling cumulus above their heads. Thus began his “crusade to make people more sky aware” through his organization For Spacious Skies ( Rather than promoting time wasting by idly staring out windows, participating teachers found students, encouraged to become more aware of their natural surroundings, had enhanced their educational motivation.

Asperitas clouds have been reported around the world. This specimen was captured over Tasmania.

Asperitas clouds have been reported around the world. This specimen was captured over Tasmania.

Determined to do his part to rescue clouds from their bad PR, Pretor-Pinney accepted an invitation in late 2004 to address a literary festival in Cornwall in the southwest of England. Concerned that attendees would not be intrigued by a lecture about something as “banal” as clouds, he employed a bit of marketing savvy, entitling it “The Inaugural Lecture of The Cloud Appreciation Society.” This was a prescient move. To his amazement, at the lecture's conclusion, many approached the podium requesting membership in the society. A few months crash-studying books on “how to make a Web site” resulted in the Internet launch of New members, abetted by the proliferation of cell phone cameras, enthusiastically submitted a rain of cloud photographs, both beautiful and astonishing. Sufficiently quirky and differentiated from an endless parade of cat videos, Yahoo UK and Ireland awarded it the “Most Weird and Wonderful Website of 2005.” Things quickly went viral. Today over 43,000 fellow cloud appreciators from 119 countries join together in admiring and documenting the fortuitous formations above them. Some even find their photos being enjoyed by thousands when selected for the “Cloud a Day” email. Then, in one of the twists that make life in our digital age so fascinating, the simple aesthetic enjoyment of clouds by everyday folks led to a modest, but meaningful, scientific breakthrough.

What Is This Cloud Called?

The pictures kept streaming in. Cumulus by the hundreds, of course; some nice lenticulars and the always inspiring crepuscular rays; clouds in the shape of hearts or bunny rabbits. Then on June 20, 2006, Jane Wiggins, a resident of Cedar Rapids, Iowa, snapped and submitted a picture of an interesting display, but she wondered: OK, exactly what is that cloud? It just didn't resemble anything in the International Cloud Atlas. It was wild and wavy, and looked like a storm-tossed sea as viewed from below. At a loss, Pretor-Pinney dubbed it the “Jacques Cousteau cloud,” thinking of the famous underwater environmentalist. A few months later there arrived another one, and then more and more from around the world. The cloud was distinctive and clearly not one of the “officially” designated cloud types. Knowing the proper name of a flower, mineral, tree, or bird greatly enhances ones appreciation and interest in the subject at hand, and the same is true with clouds. As stated by the eminent Harvard biologist E. O. Wilson, “The beginning of wisdom is to get things by their right names.” A name was needed.

Pretor-Pinney's cousin, a Latin teacher, recalled verses by the ancient poet Virgil who described stormy seas using the verb aspero, to roughen, the past participle being asperatus. A name had been found.

Applied on the CAS Website whenever another such weird cloud picture was submitted, the name asperatus stuck. The press soon picked up on “a cloud without a name getting named,” because the meteorological profession had not. Not resting there, Pretor-Pinney inquired of the Royal Meteorological Society as to what it might take to get the cloud officially recognized. The Society agreed the cloud seemed distinctive enough to warrant a separate appellation, but “one should not hold one's breath,” as such matters were the purview of the august, but glacially moving, WMO.

The sky above Erm in The Netherlands displayed the chaotic undulations characteristic of asperitas.

The sky above Erm in The Netherlands displayed the chaotic undulations characteristic of asperitas.

In 2008, an approach to the WMO did not generate much encouragement, as such matters required a revision of the nomenclature and therefore of the ICA itself, which had remained essentially fixed since the 1970s. But times were changing. The WMO realized that alternative cloud atlases were appearing on the Internet, threatening the global standardization of cloud classifications. In order to maintain international consistency and uniformity, as well as make the ICA widely available via a simple click of a mouse, a new, and totally online, version was planned. This opened the door for this “orphan” cloud, plus 11 others that were commonly recognized, but which were never officially sanctioned.

How Do You Name a Cloud?

If you have a baby, perhaps you consult a book of popular names. Not so with clouds. In fact, the creation of a taxonomy of clouds had been attempted numerous times from the sixteenth century onward, never with lasting success—until the evening of December 20, 1802.

Luke Howard, of 7 Bruce Grove, Tottenham, London, a 30-year-old pharmaceutical chemical manufacturer and avid amateur meteorologist, nervously addressed an audience of perhaps 50 people attending a regular meeting of the Askesian Society in a basement meeting hall on Lombard Street. His topic, “On the Modification (Classification) of Clouds,” seemed an odd one for a group used to being entertained by dynamic scientific expositions such as some new wonder in electromagnetism. And how could one ever hope to bring order to the apparently infinite number of shifting and miasmic shapes in the sky? Even the Old Testament had asked, “Can anyone understand the spreading of the clouds?” Howard, who also prepared more than adequate pencil and watercolor sketches of his subjects, intoned:

If clouds were the mere result of the condensation of vapour in the masses of atmosphere which they occupy, if their variations were produced by the movements of the atmosphere alone, then indeed might the study of them be deemed a useless pursuit of shadows, an attempt to describe forms which, being the sport of winds, must be ever varying, and therefore not to be defined. But the case is not so with clouds.

Howard saw order in the disarray. Rather than futilely trying to describe each individual member of the army of clouds, he grouped them into three basic categories. Cirrus were described as “parallel, flexuous, or diverging fibres, extensible in any or all directions.” Cumulus were “convex or conical heaps, increasing upward from a horizontal base.” Stratus were “a widely extended, continuous, horizontal sheet, increasing from below.” These basic elements were further combined into four types: the cirro-cumulus, cirro-stratus, cumulostratus, and cumulo-cirro-stratus or nimbus (later to be called cumulonimbus). Richard Hamblyn, author of The Invention of Clouds, proclaims that by the end of the lecture, the unknown chemist “… gave language to nature's most ineffable and prodigal forms….” He had a forged “the language of the skies.”

Asperitas clouds are not new to the sky, and photographers have long been puzzled by their pictures, such as this shot from Minneapolis, Minnesota in 1985 taken by the author.

Asperitas clouds are not new to the sky, and photographers have long been puzzled by their pictures, such as this shot from Minneapolis, Minnesota in 1985 taken by the author.

Today, the names cumulus, cirrus, and stratus are part of the English language—except they are Latin. In fact, in the ICA nomenclature, all cloud names are in Latin. The scheme follows the general principles of the naming of animals and plants developed by 18th century Swedish botanist and zoologist Carl Linnaeus. The Linnaean system of naming imposed a needed discipline upon science. Latin was a universal (if not almost dead) language. A tree, a frog, or a cloud would be called the same name by a true scientist in any culture. In an historical footnote, at virtually the same time as Howard devised his scheme, a French scientist, H. B. Lamarck, published a nomenclature system in which he tied to lump clouds into broad families such as En forme de voile (hazy clouds), En balayeurs (broom like clouds), and the like. But with Napoleon on the rampage in Europe, using the French language was a clear detriment. Howard's system caught on, and he became an instant scientific celebrity, being elected Fellow of the Royal Society (1821) and joining the British (now Royal) Meteorological Society when it was founded in 1850.

Meteorology by its nature is an international science. The cold winds screaming down the fjords of Norway today will bring a freeze to gardens in Belgium tomorrow. Of necessity, nations banded together to advance the field as the 19th century ended. The 1896 International Meteorological Congress in Paris basically incorporated Howard's ideas into the first edition of the International Cloud Atlas. It was replete with hand colored (and very low resolution) photographs of clouds, here finally given official names and descriptions. Weather observers, as well as poets and painters, could look at a boiling, puffy cloud and know it was a cumulus. Lamarck's efforts did have one lasting impact, as his insight to separate the clouds by their altitude (the familiar high, middle, and low) was incorporated into the 1896 ICA taxonomy.

Asperitas and More “New” Clouds

While climate change is having a plethora of impacts upon our environment, these “new” clouds are not among them. They have been hiding in plain sight for eons. The dinosaurs gazed upon them, perhaps as that pesky asteroid was inbound to Chicxulub, heralding their extinction some 65 million years ago. From the time of the earliest humans through the residents of Cedar Rapids in 2006, those who spotted them could only be baffled by how weird the clouds looked. Now they can call their friends and say, “Hey, check out the cool asperitas!”

But wait—weren't these clouds first called asperatus? And didn't the WMO name them asperitas? Yes and yes. Turns out, it's a Latin thing. The Linnaean cloud classification is first by genus (the ten familiar Cirrus through Cumulonimbus, etc.). These can then be modified by a species (Altocumulus lenticularis). Then can be added one or more varieties (opacus, radiatus, etc.) Finally can be appended accessory clouds and supplementary features, which is where asperitas fits in. A supplementary cloud name must be a noun. The nominative for aspero (for those who remember any Latin) is asperitas. So the past participle, asperatus, used as a placeholder until an official designation was made, has now been retired. The ancient Romans did not leave behind any YouTube recordings of their spoken language, but it is generally assumed the accent would have been placed on the second syllable (though Americans gravitate to placing it on the third).

Asperitas is now defined by the WMO in the International Cloud Atlas in the following way:

Well-defined, wave-like structures in the underside of the cloud; more chaotic and with less horizontal organization than the variety undulatus. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects. Occurs mostly with Stratocumulus and Altocumulus.

Since the WMO was going to update the ICA, it decided to tidy up the cloud nomenclature that had emerged over the past few decades. A new species, volutus, is an old friend to thunderstorm watchers. It can include the roll clouds often marking the leading edge of the cold outflow. Five additional supplementary clouds were added. The wonder-inducing Kelvin-Helmholtz wave clouds now are termed fluctus. The otherworldly hole-punch or fallstreak clouds have become cavum. Severe storm chasers can now impress their friends by flaunting their knowledge of the internationally approved vocabulary when spotting a murus (wall cloud), one perhaps with a distinct flumen (beaver's tail) or cauda (another type of tail cloud) adorning a supercell.


The roll cloud, often found above the gust front of a thunderstorm outflow, is now officially termed Volutus.

The curly waves of the tongue-twisting Kelvin-Helmholtz Instability clouds have been “rebranded” in the new International Cloud Atlas as Fluctus.

The “hole punch cloud,” often created by aircraft penetrating supercooled altocumulus clouds triggering cirrus fall streaks, now has a formal name, Cavum.

The “hole punch cloud,” often created by aircraft penetrating supercooled altocumulus clouds triggering cirrus fall streaks, now has a formal name, Cavum.

The WMO was on a roll, so it also gave official names to more specialty clouds. Clouds that form from the spray of waterfalls are cataractagenitus. The wisps of clouds that sometimes rise above forests due to extreme evapotranspiration are silvagenitus. If those forests were to burn, the massive release of heat and moisture can form a pyrocumulus, now officially designed flammagenitus. A jet plane high overhead, spewing a long-lasting contrail, is parent to homogenitus, and any cloud that is modified by human activity is termed homomutatus.

Wisps of clouds can be found rising above forests releasing vast amounts of water vapor via evapotranspiration, creating a specialty cloud type named Silvagenitus.

The location of Victoria Falls in Zimbabwe is no secret due to the puffs of clouds rising over the surrounding plains in a formation now termed Cataractagenitus.

A Victory for Citizen Science

Sadly, our scholarly dissertation upon cloud names now draws to a close. But we must first note that it was just “ordinary folks” and not scholars who “discovered” asperitas. (I will admit to taking a nice picture of one of those “weird clouds” in 1985 and was quite flummoxed as to what to call it.) With the digital phone camera revolution, everyone is now a photographer, able to create and disseminate high quality images at the click of a button. The Cloud Appreciation Society essentially crowd-sourced the evidence that was submitted to the WMO. (To date some 7,000 candidate asperitas photos have been collected by members.) This windfall allowed faculty and students at the Department of Meteorology at the University of Reading (England) to conduct detailed analyses of the conditions under which the asperitas were observed by matching up the photos with concurrent meteorological data. Current thinking suggests that asperitas' bizarre motions are associated with gravity wave oscillations being trapped within an elevated temperature inversion layer while traveling horizontal distances of tens of kilometers, thus distorting pre-existing stratiform clouds. The tops of thunderstorm outflow cold pools and shallow cold frontal inversion layers aloft seem to be prime locales.

The static photos shown here are impressive. However, when seen in time lapse video, such as that from storm chaser Mike Olbinski (, one fully appreciates the real strangeness of these “new clouds on the block.” You can see more examples at

Asperitas can be a scary looking cloud, having a certain disquieting apocalyptical appearance. One could easily imagine a giant mothership UFO descending from them at the start of the Great Alien Invasion. But if they arrive, you can calm your friends' jitters by cheerfully informing them that they are just viewing a marvelous specimen of asperitas.

If you have an iPhone, try the Cloud Appreciation Society's CloudSpotter app. Think you see an asperitas? Click it and submit it. Volunteers at CAS will let you know if you have caught one. You can even earn “points” for your photo as well as for shots of other iconic clouds. Maybe one day while cloudspotting a strange formation, you may ask, “What the heck is that cloud?” What to do? Shoot it. Send it in. Who knows, there may just be something “new” under the sun floating overhead waiting to be named. We are all creatures living in the ocean of air that is our atmosphere, and it is only right to have proper names for all the denizens of the sky. So, do remember to look up and spend a little more time with your head in the clouds. 


WALTER LYONS, Certified Consulting Meteorologist, is a past president of the American Meteorological Society, and Member 0024 of the Cloud Appreciation Society. His career path has included establishing lightning detection networks and investigating sprites, elves, halos, and gigantic jets high above thunderstorms. He is president of the educational website, 


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