Since the International Cloud Atlas (ICA) was last updated (four decades ago in the case of Volume I; three in the case of Volume II) our understanding of some types of clouds and other meteorological meteors has advanced, and technology has fundamentally changed our world. We have witnessed the creation of the internet, email and mobile telephones with digital cameras. Yet the cloud atlas has only been available in print format.
Accurate and consistent cloud and weather observations remain critically important for weather, climate and hydrology, so ensuring that observations are globally standardized remains an important need. In the absence of on-line access to the ICA, alternative atlases began to appear on the web, and with them returned a threat to the global standardization of cloud classification, a key reason for the original development of the ICA in 1939.
Awareness of this threat was raised in discussions during the meeting of the WMO Presidents of Technical Commissions and Regional Associations in 2011 and an appeal was made for the relevant WMO technical commissions to address the situation. The President of the Commission for Instruments and Methods of Observation (CIMO) agreed to consider the matter. Later that year, the President of CIMO together with the President of the Royal Meteorological Society and key staff from the WMO Secretariat reached consensus that the ICA remained an important WMO standard, but was out of date and in urgent need of modernization. The CIMO Management Group agreed in 2013 to take responsibility for the ICA and formed a Task Team to examine the requirements in detail and the feasibility of carrying out the required work. At its sixty-sixth session, the Executive Council agreed that the ICA should remain the world’s authoritative, primary source of cloud classification, be fully comprehensive and contain the most up-to-date information, and that it was the responsibility of WMO to regularly maintain this WMO Integrated Global Observing System (WIGOS) document that is fundamental to the operation of the National Meteorological and Hydrological Services (NMHSs). The Council supported the CIMO proposal to carry out an extensive revision and update of the ICA, to make it the undisputed web-based global reference standard for the classification and reporting of clouds and meteors. The sixteenth session of CIMO (CIMO-16) reconvened the Task Team to carry out a detailed and extensive revision of both volumes of the ICA. Finally, the Seventeenth World Meteorological Congress (Cg-17) delegated the Executive Council to approve the atlas, once complete, and ensure its prompt publication. In accordance with Decision 34 of the Council’s sixty-eighth session (EC-68), the President of WMO approved the draft text of the revised Atlas on behalf of the Council in January 2017.
With this revision, the ICA has for the first time been configured as a website for electronic access. This electronic format will facilitate its use in training and teaching. The website has been designed by the Task Team and experts from the Hong Kong Observatory and the latter has built the website and operates it on behalf of WMO.
The overall structure of the text of the 1975 edition has been largely retained and can be directly navigated from main tabs on the website home page. The first tab leads to the Definition of a Meteor and the General Classification of Meteors, the second tab to Clouds, and the third to Meteors Other than Clouds. A fourth tab guides to Observing Clouds, which contains those sections from Part II of the previous edition that dealt with making and coding observations. The fifth tab provides access to an entirely new feature of the atlas, the Image Viewer, which enables to see all the images gathered for this edition of the atlas, to access metadata associated with each image, to perform filtered searches for images of particular clouds or other meteors, and for side-by-side comparison of different images. Two final tabs make up the remainder of the new ICA and comprise an entirely new Glossary of relevant terms, and a section containing Other Information, which provides access to the Appendices, the Prefaces of the previous editions and to downloadable pdf versions of the previous editions.
The existing classifications have been reviewed and all have been retained. Several new, formal cloud classifications have been introduced. These include one new species (volutus), five new supplementary features (asperitas, cauda, cavum, fluctus and murus), and one new accessory cloud (flumen). The species floccus has been formally recognized as being able to occur in association with stratocumulus. The separate section on Special Clouds has been removed, and the cloud and meteor types previously discussed within this section have been integrated into the cloud classification scheme as cataractagenitus, flammagenitus, homogenitus, silvagenitus, and homomutatus.
The text itself has been thoroughly reviewed and revised to modernize the style of language. It is more readable, and is expanded in areas where scientific understanding has increased since the previous edition; especially for Meteors Other Than Clouds where several more phenomena have been included. For example, snow devil and steam devil have been added as hydrometeors, together with details regarding types of tornadoes. The optical phenomena (photometeors) have been thoroughly expanded with illustrations of various types of halo phenomena, rainbows and mirages. Upper atmospheric electrometeors known as “sprites” and “jets”, not yet commonly known when the previous edition was published, have also now been added. Other changes include replacement of the terms “bad weather” and “other than of bad weather” with “wet weather” and “dry weather”, and of “étage” with “level”.
A great improvement is the ability to view relevant images directly within the text. Previously this required simultaneous examination of printed Volumes I and II. It is notable that only minor changes have been made to those parts of the text that constitute regulatory material. While the Task Team considered many changes, the desire for modernity gave way to the stronger need to preserve traceability of observations, and the original regulatory wording is retained in all but a few instances. A few small errors noted in the 1975 edition have been corrected and in a few places, descriptions of outdated observing techniques have been omitted.
The pictorial aids for cloud classification have been modernised, with a flow chart containing new colour illustrations provided courtesy of MeteoSwiss. These give a pictorial path to determine the correct coding for a cloud observation. A Cloud Identification Guide of basic genera is included for the benefit of amateur weather enthusiasts.
The images of Volume II of the 1987 edition have been replaced with new, high resolution, colour, digital images contributed by cloud enthusiasts from all over the world, with detailed descriptions. A much greater number of images is included. In some cases, multiple examples are included to show variations that can exist within a single classification. This will illustrate differences due to season, climatic zone, or the stage of development of the cloud. Some time-lapse or video imagery has been included for selected classifications where this helps the observer understand the stages of evolution of that cloud type. Selected cloud image examples are now essentially case studies, with associated metadata added such as synoptic analyses, satellite or radar imagery, and atmospheric soundings. Many images from the 1987 Volume II have been retained as additional metadata for newer images to enable historical traceability of observations. New images, in some cases several, are also provided for each type of hydrometeor (other than clouds), lithometeor, photometeor and electrometeor. In total, more than 600 new images are included in the web-based version of the atlas, many with supplemental metadata.
In all, the changes in this latest edition of the International Cloud Atlas are extensive, though the core content is similar. The result is a credit to the Task Team, which worked tirelessly over several years to produce this result. WMO is indebted to Dr Stephen Cohn (chair, United States of America), Mr Michael Bruhn (co-chair, Australia), Mmes Eliane Thürig-Jenzer (Switzerland), Colleen Rae (South Africa) and Marinés Campos (Argentina), and Messrs George Anderson (United Kingdom of Great Britain and Northern Ireland), Ernest Lovell (Barbados), Jim Trice (United Kingdom of Great Britain and Northern Ireland) and Tam Kwong Hung (Hong Kong, China) for their efforts, as well as to cloud expert Mr Frank Le Blancq (Jersey) for his many contributions. In particular, WMO is thankful to Hong Kong Observatory for their tremendous support in development, hosting and maintenance of the website.
I am confident that this new web-based version of the International Cloud Atlas will be well-received by one and all, professionals and amateurs alike, and will once again restore its place as the global standard for the observation and reporting of clouds and other meteors.