The International Cloud Atlas

The International Cloud Atlas



11 June 2015

The World Meteorological Organization is advancing with a major project to revise and update the International Cloud Atlas. The aim is to produce a user-friendly, digital-based product which is an authoritative, comprehensive and up-to-date source of reference and is also interesting and accessible to a wide audience.

Progress in updating the International Cloud Atlas was presented during the World Meteorological Congress, as were proposals to recognize new clouds, including those produced from human activities like aviation.

“I see the Cloud Atlas as the business card of WMO,” said Bertrand Calpini, President of the Commission for Instruments and Methods of Observations, which is overseeing the project.

The International Cloud Atlas was developed as a standardized reference document for operational observation systems, which underpin weather forecasts and longer-term climate predictions. It is an important training tool for meteorologists, as well as for those working in aviation and at sea.

It has also been very popular with amateur enthusiasts and cloud spotters.

“The target audience is still primarily professional observers, but it also of wider interest to the general public,” said George Anderson of the Met Office and Royal Meteorological Society in the United Kingdom.

The roots of the International Cloud Atlas date back to the 19th century. The International Meteorological Conference published the first International Cloud Atlas containing 28 coloured pictures in 1896.

The existing Cloud Atlas has two volumes and was originally published in 1956. Volume I is a detailed technical manual of standards. Volume II contains around 220 plates of photographs of clouds and certain other meteors (such as precipitation types, haze, rainbows, and lightning) . Each photograph is accompanied by explanatory text to enable the pictures in Volume II to be understood without the detailed technical definitions and descriptions contained in Volume I, and has been revised and updated on several occasions, most recently in 1987 with the addition of new photographs.

“Science, technology and photography have moved on in the past 40 years, so there is a need to update the Cloud Atlas,” said Mr Anderson at a side event during Congress.

Mr Anderson outlined proposals by a WMO Task Team of international experts. It is intended to update the imagery in the atlas with the high resolution, colour imagery available today, while retaining existing images to preserve traceability. It is also planned to extend the atlas adding new photographic examples showing seasonal climatic/geographical variations in appearance.

The Task Team will invite National Meteorological Services and meteorological societies around the world to submit images and supporting data for possible inclusion. This is a time-consuming process because the emphasis is on technical accuracy and authority rather than speed, but it is hoped that a web-based Atlas will be ready in 2016.

Cloud Classification

The present international system of Latin-based cloud classification dates back to 1803, when amateur meteorologist Luc Howard wrote a book The Modifications of Clouds.  WMO currently recognizes ten cloud genera (basic classifications), which describe where in the sky they form and their approximate appearance:

These genera are subdivided into 14 species (secondary classifications), which describe shape and internal structure, and 9 varieties (tertiary classifications), which describe the transparency and arrangement of clouds.  Not all genera have all species, and not all species have all varieties, but in all there are about 100 combinations. In additional to the first three levels of classification, certain supplementary features and accessory clouds are also defined.

The international Task Team has given consideration to possible new cloud classifications, and proposes recognition of a new species Volutus (from the Latin volutus which means rolled) It also proposes some new “special clouds” like Homogenitus (from the Latin homo meaning man and genitus meaning generated or made. Its common names include Contrails (from aircraft)


Most public interest has focused on a proposal by the Cloud Appreciation Society, based in the UK, to recognize the so-called “asperatus” (after the Latin adjective meaning rough). The Task Team identified this as: "A formation made up of well-defined, wavelike structures in the underside of the cloud, more chaotic and with less horizontal organization than undulatus. It is characterised 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 cloud can lead to dramatic visual effects."

Undulatus already exists as a cloud variety, the third level of classification, which is generally associated with stratiform clouds. The Task Team has proposed that asperitas (note spelling: after the Latin noun meaning roughness) be included as a new supplementary feature.

All the proposals are subject to approval by WMO’s Executive Council.

Volume I of the Cloud Atlas is a technical manual of standards and therefore any revision will entail amending WMO’s technical regulations. Congress approved a proposal that this will be done remotely, without the need for a formal session. It also stressed the need for extra resources to finance the project and a printed publication.

A scanned version of both volumes of the Atlas is available on the WMO website here 

Direct access to Vol II here.

Hard copies of the Atlas can be purchased for SFr 40 from WMO. Details here.

Details of a photo competition by the Cloud Appreciation Society is here

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