A volutus, or roll cloud, photographed in Szprotawa, Poland. This type of cloud was just added as a new species in the 2017 International Cloud Atlas. 
 
 A dramatically turbulent cloud formation called asperitas and the bizarrely cylindrical roll cloud are among the newest additions to the international handbook for cloud identification. 
 
 The World Meteorological Organization has released a new, digitized version of its "International Cloud Atlas," the global reference book for meteorologists and skywatchers alike. It's the first update for the atlas since 1987 and the first version to be fully web-based. The release also marks a red-letter day for amateur enthusiasts in the Cloud Appreciation Society, who get the satisfaction of seeing the asperitas cloud that they discovered become an official scientific category. 
 
 "It is a new classification of cloud, with a chaotic, turbulent appearance, that was proposed by the Cloud Appreciation Society back in 2008, based on photographs sent to us from members all around the world," the organization posted on its blog. "It is a classic example of citizen science, in which observations by the general public, enabled by the technology of smartphones and the Internet, have influenced the development of this most official of classification systems." [Gallery: The Craziest Clouds Ever] 
 
 The cloud atlas' classification system is not for the faint of heart. There are 10 basic "genera" of clouds, most of the names of which would be familiar to schoolchildren: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus and cumulonimbus. 
 
 Within each genera are species, which describe the internal structure of the shape of clouds. A lenticularis cloud, for example, is a flattened pancake shape that looks a bit like a giant UFO. Each type of cloud can only have one species, but the same species types are present in multiple different genera of clouds. 
 
 An image of a photometeor halo included in the new International Cloud Atlas. Halos are caused by light reflecting or refracting off tiny ice crystals suspended in the atmosphere, according to the Atlas. Credit: WMO International Cloud Atlas/Claudia Hinz, DMD 
 
 Next, the classification system includes varieties of clouds, which describe the clouds' transparency and arrangement. A single cloud can have multiple varieties, except in the case of opacus (opaque) clouds varieties and translucidus (translucent) varieties, which are mutually exclusive: One doesn't let any sun through, and the other does. [See Photos of a Various Cloud Types] 
 
 Thus, a standard cloud identification might look something like "altocumulus stratiformis translucidus perlucidus undulates" — an altocumulus cloud arranged in a thin sheet (stratiformis) that is translucent to sunlight, has small gaps allowing blue sky to show through (perlucidus) and which has wavy features (undulates). 
 
 The cloud atlas update doesn't alter any of the basic classifications for clouds, but it does add several new twists. The roll cloud, known as the volutus, is defined as a new species of cloud. These horizontal, cylindrical clouds form where descending cold air pushes warm, moist air upward. Under the right wind conditions, the atmospheric turbulence condenses into a tubular cloud that seems to roll across the sky. 
 
 An undulating asperitas cloud, photographed at Shorewell Park in Tasmania, Australia, at 7:48 a.m. local time on Feb. 20, 2004. Credit: WMO International Cloud Atlas/ © Gary McArthur 
 
 The atlas also adds five new "supplementary features" to its classification system. Supplementary features are unusual parts of, or attachments to, clouds. The wave-like undulations of the undersides of clouds, dubbed "asperitas," are among these new features. Another is "cavum," known colloquially as a hole-punch cloud because the cloud has a large circular gap through which the sky can be seen. A "cauda" is a horizontal feature that extends like a tail from a cloud. "Fluctus" features are wave formations that look like something a cartoon surfer would ride; they're commonly known as Kelvin-Helmholtz waves and are caused by wind shear. Finally, "murus" features, or wall clouds, are familiar to any storm chaser for their role in the formation of tornadoes. 
 
 The other main changes to the atlas are the addition of a new accessory cloud, or a cloud that accompanies another, larger cloud, and the establishment of five new special clouds, which describe unusual cloud formation circumstances. The new accessory cloud type is called "flumen" and describes a low cloud associated with severe supercell storms. 
 
 The five new special clouds are: cataractagenitus, describing clouds that develop from the spray of large waterfalls; flammagenitus, describing clouds formed under the influence of wildfires; homogenitus, describing clouds formed by human activities, such as airplane contrails; silvagenitus, describing clouds formed under the influence of moisture from respiring trees; and homomutatus, describing clouds originally made by humans that gradually transform into more natural-looking forms, like a contrail that eventually spreads in the wind. 
 
 The cloud atlas is available online and will be official unveiled today (March 23) for World Meteorological Day. 
 
 Original article on Live Science. ||||| 00:50 New Cloud Types Added Asperitas and murus are just two of the names you'll see among several new classifications added to an updated cloud reference released this week by the World Meteorological Organization. Meteorologist Kait Parker has the details. 
 
 At a Glance Several new cloud classifications have been added by the World Meteorological Organization. 
 
 The new classifications are the first in 30 years for the International Cloud Atlas. 
 
 Asperitas and murus are just two of the names you'll see among several new classifications added to an updated cloud reference released this week by the World Meteorological Organization (WMO) . 
 
 This update is the first in 30 years for the International Cloud Atlas , which the WMO calls "the global reference for observing and identifying clouds." 
 
 A new cloud species has been added to the atlas called volutus, more commonly known as a roll cloud by meteorologists. Cloud species are subdivisions of the 10 basic cloud "genera," the WMO says . 
 
 <img class="styles__noscript__2rw2y" src="https://s.w-x.co/roll-large.jpg" srcset="https://s.w-x.co/roll-large.jpg 400w, https://s.w-x.co/roll-large.jpg 800w" > A roll/volutus cloud in Sterling, Virginia, during September 2013. (National Weather Service Baltimore/Washington) 
 
 Roll/volutus clouds are a relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thunderstorm (or occasionally a cold front), they are completely detached from the base of the cumulonimbus cloud. Volutus is a Latin term for rolled, which perfectly matches their appearance. 
 
 (MORE: How a Roll Cloud Forms ) 
 
 Five new supplementary features have also been added to the updated cloud atlas. Those new names include asperitas, cavum, cauda, fluctus, and murus, all of which are currently known as other more common names by sky watchers. 
 
 Asperitas, Latin for wave-like and roughness , has been added to the atlas thanks to the work of the Cloud Appreciation Society . 
 
 <img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/asperatus_oh-1.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/asperatus_oh-1.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/asperatus_oh-1.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Asperitas Clouds Cincinnati, Ohio, on August 3, 2015. (Ron Steele) 
 
 “Asperitas was first identified with the help of citizen science, enabled by modern technology. When Cloud Appreciation Society members send us photographs of dramatic skies from around the world, it is possible to spot patterns. This is how the proposal for a new classification came about, and we are delighted the WMO has chosen to include it in their definitive reference work for cloud classification,” said Gavin Pretor-Pinney, founder of the Cloud Appreciation Society. 
 
 (MORE: The Formation of Undulatus Asperatus ) 
 
 The other four new supplementary features now in the atlas also have more commonly known names among meteorologists. 
 
 Murus and cauda are a wall cloud and tail cloud, respectively. They are features of cumulonimbus clouds, which are associated with thunderstorms. Wall clouds are a lowering, rotating cloud base and sometimes the location where tornadoes can develop. A tail cloud extends horizontally away from the wall cloud and illustrates air feeding into the storm . 
 
 <img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/wall-tail-cloud.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/wall-tail-cloud.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/wall-tail-cloud.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Wall Cloud with a Tail Cloud NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL) 
 
 Hole-punch, or fallstreak, clouds have been given the supplementary feature name Cavum, while Fluctus has been added to describe Kelvin-Helmholtz wave clouds. 
 
 (MORE: Hole-Punch Clouds | Kelvin-Helmholtz Clouds ) 
 
 <img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/wave-clouds.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/wave-clouds.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/wave-clouds.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > Kelvin-Helmholtz Wave Clouds Breckenridge, Colorado, in late October 2015. (Instagram/danielbannach) 
 
 Clouds that formed or grew from localized factors have also been added to the atlas. Among the five new "special clouds" is homogenitus, which is a contrail created by airplanes. 
 
 The International Cloud Atlas was first published in the 19th century and was last updated in 1987. This new 2017 version will primarily be accessible through the internet , but it could be published in print later. 
 
 “The International Cloud Atlas is the single most authoritative and comprehensive reference for identifying clouds. Its reputation is legendary among cloud enthusiasts and it serves as an essential training tool for professionals working in meteorological services, and in sectors such as aviation and shipping,” said WMO Secretary-General Petteri Taalas. 
 
 MORE ON WEATHER.COM: Mammatus Clouds ||||| The World Meteorological Organization (WMO) has released its new, long-awaited, digitized International Cloud Atlas – the global reference for observing and identifying clouds, which are an essential part of weather, the climate system and the water cycle. It was released for the World Meteorological Day on 23rd March. 
 
 The new Atlas combines 19th century traditions with 21st century technology. It contains hundreds of images submitted by meteorologists, photographers and cloud lovers from around the globe. 
 
 It includes new classifications, including volutus, a roll cloud; clouds from human activities such as the contrail, a vapour trail sometimes produced by airplanes; and asperitas, a dramatic undulated cloud which captured the public imagination. It also features meteorological phenomena like rainbows, halos, snow devils and hailstones. 
 
 “The International Cloud Atlas is the single most authoritative and comprehensive reference for identifying clouds. Its reputation is legendary among cloud enthusiasts and it serves as an essential training tool for professionals working in meteorological services, and in sectors such as aviation and shipping,” said WMO Secretary-General Petteri Taalas. 
 
 “Throughout the centuries, few natural phenomena have inspired as much scientific thought and artistic reflection as clouds,” said Mr Taalas. “More than two millennia ago, Aristotle studied clouds and wrote a treatise addressing their role in the hydrological cycle. And today, scientists understand that clouds play a vital role in the Earth’s energy balance, climate and weather,” said Mr Taalas in a message for World Meteorological Day on 23 March. 
 
 “If we want to forecast weather we have to understand clouds. If we want to model the climate system we have to understand clouds. And if we want to predict the availability of water resources, we have to understand clouds,” said Mr Taalas. 
 
 Understanding Clouds is the theme of this year’s World Meteorological Day, which celebrates the anniversary of the convention establishing WMO in 1950 and showcases the essential contribution of National Meteorological and Hydrological Services to the safety and wellbeing of society. 
 
 The themes chosen for World Meteorological Day reflect topical weather, climate or water-related issues. Understanding Clouds was chosen to coincide with the launch of the digital version of the International Cloud Atlas, after an exhaustive revision process and a public call for images from all over the world. 
 
 19th Century Traditions 
 
 The International Cloud Atlas was first published in the late 19th century. It contains a detailed manual of standards and numerous plates of photographs of clouds and certain other weather phenomena. It was last updated in 1987 – before the Internet era. For the first time, the 2017 edition will primarily be a web-based portal, allowing for much richer content and presentation. It may be published in hard copy at a later date. 
 
 “This is THE world reference for observing and classifying clouds and other weather phenomena. The Atlas contains pictures, definitions, and explanations that are accepted and used by all WMO’s 191 Member countries and territories,” said Bertrand Calpini, President of WMO’s Commission for Instruments and Methods of Observation (CIMO), which oversaw the revision process. 
 
 “This new edition brings together for the first time all types of measurements, including very high-tech surface-based, in situ and space observations and remote sensing, thus giving to the human observer a revolutionary tool to understand clouds,” said Mr Calpini. 
 
 “The International Cloud Atlas task team had to choose from thousands of images from meteorologists, cloud lovers and photographers around the world,” said Chi-ming Shun, Director of the Hong Kong Observatory, which is hosting the web portal. “We selected the best of the best. We feel honoured to be involved in such a visible product of WMO. We applaud WMO for updating the International Cloud Atlas which provides a unique platform for engaging the public in better understanding clouds, weather and climate.” 
 
 Classification of Clouds 
 
 The present international system of Latin-based cloud classification dates back to 1803, when amateur meteorologist Luke Howard wrote The Essay on the Modifications of Clouds. 
 
 There are ten basic cloud “genera,” which are defined according to where in the sky they form and their approximate appearance. The new International Cloud Atlas has made no additions to these 10 genera. 
 
 High-level clouds typically have a base above about 5 000 metres (16 500 feet); middle-level clouds have a base that is usually between 2 000 and 7 000 m (6 500 to 23 000 feet); and low-level clouds usually have their base at a maximum of 2 000 m (6 500 feet). 
 
 Most cloud names contain Latin prefixes and suffixes which, when combined, give an indication of the cloud’s character. These include: 
 
 Alto: mid-level (though Latin for high) 
 
 Cirrus/cirro: feathers, wispy 
 
 Cumulus/cumulo: heaped up/puffy 
 
 Nimbus/nimbo: rain-bearing 
 
 Stratus/strato: flat/layered and smooth 
 
 The 10 genera are subdivided into “species,” which describe shape and internal structure, and “varieties,” which describe the transparency and arrangement of the clouds. In total there are about 100 combinations. 
 
 The new International Cloud Atlas has added a new species: volutus or roll cloud (from the Latin volutus which means rolled), which occurs within the genera Altocumulus and Stratocumulus. It describes a long, typically low, horizontal tube shaped cloud mass that often appeals to roll about a horizontal axis. 
 
 Five new supplementary features have been added: asperitas, cavum, cauda (often known as tail cloud), fluctus (widely known as Kelvin-Helmholz wave) and murus (known as wall cloud). 
 
 Best known of these is asperitas (from the Latin meaning roughness), Photographs of the dramatic, wave-like cloud captured the popular imagination around the world. The Cloud Appreciation Society argued for a new classification to be used to describe clouds with this appearance. The Atlas includes the winning photograph from a Cloud Appreciations Society competition on Asperitas. 
 
 
 
 “Asperitas was first identified with the help of citizen science, enabled by modern technology. When Cloud Appreciation Society members send us photographs of dramatic skies from around the world, it is possible to spot patterns. This is how the proposal for a new classification came about, and we are delighted the WMO has chosen to include it in their definitive reference work for cloud classification,” said Gavin Pretor-Pinney, founder of the Cloud Appreciation Society. 
 
 A new accessory cloud, flumen, has been included. Commonly known as “beaver’s tail,” it is associated with a supercell severe convective storm. 
 
 The International Cloud Atlas also proposes five new “special clouds:” cataractagenitus, flammagenitus, homogenitus, silvagenitus and homomutatus. The suffix genitus indicates localized factors that led to cloud formation or growth, while mutatus is added when these caused the cloud to change from a different form. These special clouds are influenced by large waterfalls, localized heat from wildfires, saturation of air above forests and humans. Thus, a common example of homogenitus is contrails, sometimes seen after aircraft. 
 
 The World Meteorological Day ceremony will be webcast live to allow cloud enthusiasts around the world to have their first view of the new International Cloud Atlas and the new insight it provides into the beauty and power of nature. 
 
 Details of World Meteorological Day are available here 
 
 The message from WMO Secretary-General is available here 
 
 The International Cloud Atlas is available at https://cloudatlas.wmo.int 
 
 The new classifications are available here 
 
 The World Meteorological Organization is the United Nations System’s authoritative voice on Weather, Climate and Water 
 
 WMO website: public.wmo.int 
 
 For more information, please contact: 
 
 Clare Nullis, Press Officer, Communications and Public Affairs, Tel: +41 22 730 8478 or +41 79 709 13 97 (cell), e‑mail: cnullis@wmo.int. ||||| Only Meteor Type 
 
 Clouds Special Clouds and other Features Hydrometeors Lithometeors Photometeors Electrometeors Others 
 
 Clouds 
 
 Select Cloud Genus Cirrus(Ci) Cirrocumulus(Cc) Cirrostratus(Cs) Altocumulus(Ac) Altostratus(As) Nimbostratus(Ns) Stratocumulus(Sc) Stratus(St) Cumulus(Cu) Cumulonimbus(Cb) 
 
 Calvus Capillatus Castellanus Congestus Fibratus Floccus Fractus Humilis Lenticularis Mediocris Nebulosus Spissatus Stratiformis Uncinus Volutus 
 
 Duplicatus Intortus Lacunosus Opacus Perlucidus Radiatus Translucidus Undulatus Vertebratus 
 
 Arcus Asperitas Cauda Cavum Fluctus Flumen Incus Mamma Murus Pannus Pileus Praecipitatio Tuba Velum Virga 
 
 Altocumulus Altostratus Cataractagenitus Cirrocumulus Cumulonimbus Cumulus Flamma Homo Nimbostratus Silvagenitus Stratocumulus 
 
 Altocumulus Altostratus Cirrocumulus Cirrostratus Cirrus Cumulus Homomutatus Nimbostratus Stratocumulus Stratus 
 
 Special Clouds and other Features 
 
 Stratospheric Clouds : Polar Stratospheric Clouds (Nacreous Clouds) - Type I Stratospheric Clouds : Polar Stratospheric Clouds (Nacreous Clouds) - Type II Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Unknown Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type I (Veils) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IIa (Bands) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IIb (Bands) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IIIa (Billows) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IIIb (Billows) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IVa (Whirls) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IVb (Whirls) Mesospheric Clouds : Polar Mesospheric Clouds (Noctilucent Clouds) - Type IVc (Whirls) Mesospheric Clouds : Complex Structures - O Mesospheric Clouds : Complex Structures - S Mesospheric Clouds : Complex Structures - P Mesospheric Clouds : Complex Structures - V Aircraft condensation trails (Contrails) Asperitas Banner cloud Cirrus clouds formed from the spreading out of contrails Clouds formed above large waterfalls (e.g. Victoria or Niagara Falls) Clouds formed over forests Aircraft dissipation trails (Distrails or 'canal clouds') Fallstreak holes / Hole-punch clouds Fumulus (e.g. Cumulus formed above power stations) Horseshoe vortex Kelvin-Helmholtz waves Levanter (i.e. cloud that forms from the Rock of Gibralter) Pile d'assiettes (multi- layered lenticular clouds) Pyrocumulus (e.g. cumulus clouds formed above forest/wild fires, volcanoes) Roll cloud (e.g. the Morning Glory) Rotor cloud The Table Cloth (cloud formed over Table Mountain, South Africa) Anvil dome Anvil rollover / mushroom Back-sheared anvil Barber pole Bear's cage Beaver's Tail Clear slot Collar cloud Cumuliform anvil Feeder (inflow) bands Inflow stinger Knuckles Overshooting top Rain foot Rain-free base Supercell Tail cloud Turkey Tower Wall cloud Spouts - Funnel cloud Spouts - Tornado Spouts - Landspout Spouts - Waterspout Spouts - Gustnado 
 
 Meteors other than Clouds 
 
 Advection dew Advection fog Advection hoar frost Arctic sea smoke/frost smoke/steam fog Blowing snow Clear ice Deposit of fog droplets Dew proper Diamond dust Drifting snow Drizzle Fern frost/Window frost/Ice flowers Fog Frost flowers Glaze Hail Hard rime Hill fog/Upslope fog Hoar frost proper Ice fog Ice pellets Mist Needle ice/frost pillars/frost column Radiation fog Rain Sea fog Shallow fog/ground fog Small hail Snow Snow drift Snow grains Snow pellets Snow rollers Snowflakes/snow crystals Soft rime Spout Spray Supercooled drizzle Supercooled rain White dew 
 
 Blowing dust or sand Drift dust or sand Dust haze Dust storm or sandstorm Dust whirl or sand whirl (dust devil) Haze Smoke Wall of dust or sand 
 
 22-degree Halos 46-degree Halos Anthelion Anti-crepuscular rays Antiselene Bishop's ring Brocken spectre Circumhorizon Arcs Circumscribed halo Circumzenithal Arcs Cloud shadows Contrail shadows Corona: lunar corona Corona: solar corona Crepuscular rays Fata Morgana Fog bow Glory Green flash Halo: lunar halo Halo: solar halo Irrisation on clouds Light pillar Lowitz arcs Mirage Mirage: inferior Mirage: mock mirage Mirage: superior Novaya Zemlya effect Paranthelia Parantiselenae Paraselenae Parhelia Parhelic circle Rainbow Rainbow: Primary bow Rainbow: red Rainbow: reflected bows Rainbow: reflection bows Rainbow: secondary bow Rainbow: spray bow Rainbow: supernumerary bows Rainbow: twinned bow Scintillation Shimmer Subsun Sun/moon pillars Sundogs Supralateral arc Tangent Arcs Terrestrial light pillars Twilight colours Twilight colours: Alpengluhen Twilight colours: purple light Twilight colours: twilight arch Twilight rays Twilight shadows 
 
 Anvil crawler Anvil zits Blue jets Elves Polar aurora Red sprites Saint Elmo's fire Thunderstorm - Lightning Thunderstorm - Thunder 
 
 Climate Classification 
 
 Select Main Climate Equatorial (A) Arid (B) Warm Temperate (C) Snow (D) Polar (E) 
 
 Select Climate Precipitation Desert (W) Steppe (S) Fully Humid (f) Summer Dry (s) Winter Dry (w) Monsoonal (m) 
 
 Select Climate Temperature Hot Arid (h) Cold Arid (k) Hot Summer (a) Warm Summer (b) Cold Summer (c) Extremely Continental (d) Polar Frost (F) Polar Tundra (T) 
 
 CH Code 
 
 Select CH Code / 0 1 2 3 4 5 6 7 8 9 
 
 CM Code 
 
 Select CM Code / 0 1 2 3 4 5 6 7 8 9 
 
 CL Code 
 
 Select CL Code / 0 1 2 3 4 5 6 7 8 9 
 

– Look up in the sky: It's a bird, it's a plane, it's a … volutus, or is that an asperitas? If you're not sure what kind of cloud you're seeing above your head, consult the World Meteorological Organization's International Cloud Atlas, newly updated just in time for World Meteorological Day on Thursday. The Weather Channel reports that a bunch of new classifications have been added to the atlas (the first fully web-based version) for the first time in three decades, including the volutus "roll cloud" and aforementioned asperitas, as well as human-created cloud types like contrails (the condensation trails left in the wake of airplanes). There's even a new "accessory" cloud known as the flumen (aka "beaver's tail"), which sometimes follows along with a convective storm, a WMO press release notes. The science behind the atlas's cloud classifications, which Live Science calls "not for the faint of heart," is a sophisticated one. The 10 cornerstone clouds, referred to as "genera," are the ones you've likely heard of—stratus and cumulus, among others. Then the classification gets more granular with "species," which break down the genera clouds further based on shapes, and from there to "varieties," which indicate transparency and arrangement. The recent additions fall under the species category; there's also a new "supplementary features" section of "unusual parts"—such as the aforementioned asperitas, a wavy look to a cloud's underside—and five "special" clouds, including the contrail and clouds formed by waterfall mist. "Few natural phenomena" are as inspiring as clouds, WMO Secretary-General Petteri Taalas notes. (Shifting cloud patterns are worrisome.)