The International Commission for the Study of Clouds (C.E.N.) of the International Meteorological Organization (I.M.O.), created in 1921, was dissolved by the Extraordinary Conference of Directors (London, 1946). It was replaced by the Committee for the Study of Clouds and Hydrometeors (C.C.H.), established by the International Meteorological Committee of the International Meteorological Organization in compliance with a resolution of the Commission for Synoptic Weather Information (Resolution 16, C.S.W.I., Paris 1946). The Conference of Directors of the International Meteorological Organization instructed the C.C.H. to prepare a revised and up to date version of the International Atlas of Clouds and Types of Skies (Resolution 153, CD, Washington 1947). The decision to prepare a new atlas was inspired, on the one hand, by the exhaustion of the previous 1939 edition and, on the other hand, by new developments in our knowledge of clouds and hydrometeors, as well as by modifications in international cloud codes.
The Committee for the Study of Clouds and Hydrometeors held several sessions in which the following members participated: A. Viaut, T. Bergeron, J. Bessemoulin, W. Bleeker, C.F. Brooks, C.K.M. Douglas, L. Dufour, N.R. Hagen, B.C. Haynes, M. Mézin, J. Mondain and H. Weickmann.
An Editing Committee, consisting of M. Mézin (President), R. Beaufils (Secretary), R. Beaulieu, J. Bessemoulin and M. Bonnet, prepared documents between sessions.
In 1951, when the I.M.O. was replaced by the World Meteorological Organization (W.M.O.), the Committee for the Study of Clouds and Hydrometeors proposed to the First Congress of W.M.O. that the new edition of the Cloud Atlas should consist of four volumes and it presented a draft of Volumes I, II and III. Volumes I and III covered essentially the same ground as Volume I of the present Atlas and Volume II was a collection of photographs of clouds and meteors. Volume IV was intended to be a treatise on the physics of clouds and meteors.
The First Congress of the World Meteorological Organization decided (Resolution 18, Cg-I) to refer the draft to the Commission for Synoptic Meteorology (C.S.M.) for further study and completion. The Committee for the Study of Clouds and Hydrometeors itself became a “Working Group for the Study of Clouds and Hydrometeors” attached to the Commission for Synoptic Meteorology (Resolution 35, Cg-I).
The contents of the Atlas and plans for its publication were discussed at the Second and Third Sessions of the Executive Committee (Lausanne, 1951; Geneva, 1952). It was decided that an Abridged Atlas in one volume consisting of a condensed text and a selection of photographs, for the use of surface observers, and an Album designed to meet the limited but special needs of airborne observers, should also be prepared (Resolution 9, EC-II; Resolution 36, EC-III).
The Working Group for the Study of Clouds and Hydrometeors presented to the First Session of C.S.M. (Washington, 1953) an improved version of the original draft submitted to Congress. The improvement resulted from further study at various sessions of the Working Group and from remarks received from members of the Commission to whom copies had been distributed.
The Commission for Synoptic Meteorology recommended (Recommendation 49, CSM-I) that the Complete Atlas should consist of only two volumes (Volume I containing the text and Volume II the plates). It also formally recommended the publication of an Abridged Atlas and of an International Cloud Album for airborne observers. Finally, C.S.M. considered that a compendium on the physics of clouds and meteors, though highly desirable, should not at present form part of the Cloud Atlas.
The Working Group for the Study of Clouds and Hydrometeors was dissolved by C.S.M. However, a few individuals were requested to continue and to complete the work of the Committee for the Study of Clouds and Hydrometeors.
At its Fourth Session (Geneva, 1953), the Executive Committee adopted Recommendation 49 of C.S.M. and directed the Secretary-General to take the necessary steps, in consultation with the President of C.S.M. when required, for an early publication of the Atlas (Resolution 30, EC-IV).
The English text was then passed to Mr. E.G. Bilham for editing, in accordance with the wishes of the Commission for Synoptic Meteorology and a decision by the Executive Committee.
During the translation of the text of the Complete Atlas into French, for which Mr. J. Bessemoulin was responsible in accordance with a request by C.S.M., it became obvious that many parts required thorough revision. A special editing committee was then established consisting of the following persons: Dr. W. Bleeker, Dr. M.A. Alaka, Mr. R. Beaufils and Mr. J. Bessemoulin. This committee met several times in Geneva and in De Bilt and established the final English and French texts.
The President of the Commission for Synoptic Meteorology accepts responsibility for the changes thus made in the original text which was studied at the First Session of the Commission; these changes were necessary in order to avoid ambiguities and internal inconsistencies.
The content of the present Volume I, which is essentially descriptive and explanatory, differs materially from that of the former “General Atlas”.
The grouping of clouds into “cloud families” has been abandoned; the classification into genera has been maintained but some details in the definitions have been modified.
The species and the varieties have been extended and considerably modified. The same remark applies to the “accidental details” which have been renamed “supplementary features” and “accessory clouds”. A new concept, that of “mother-cloud”, has been introduced.
Certain “special clouds” are discussed separately; a brief description is given of the most important of these clouds, such as nacreous clouds, noctilucent clouds, etc.
The “Note on the Observation of Clouds from Aircraft” of the former General Atlas has been replaced by a chapter describing the particular appearance presented by clouds when they are observed from aircraft.
The part “Types of Skies” of the former General Atlas has been deleted. New points of view have arisen and existing ideas, particularly with regard to tropical skies, are in the course of evolution, thus making it difficult to synthesize the various existing concepts.
The chapter “Definition of Hydrometeors” of the former General Atlas has been considerably expanded. The former classification of hydrometeors has been replaced by a classification dividing meteors into four groups. The term “hydrometeors” designates the first of these groups, and applies solely to aqueous meteors. The descriptions of the hydrometeors are based mainly on those adopted at Salzburg in 1937. The other groups of meteors are “lithometeors”, “photometeors” and “electrometeors”.
The parts intended primarily for the use of observers have also been expanded. Part I contains more elaborate instructions for observing clouds and meteors. Part IV gives two models of a “Journal of clouds and meteors”. Part V contains detailed instructions and pictorial guides for the coding of clouds.
The final change consists of the addition of Appendices providing information of a general nature and an Alphabetical Index to simplify consultation of the Atlas.
Volume II is a collection of 224 plates in black and white and in colour, the object of which is to illustrate the text of Volume I. The plates consist of photographs of clouds (viewed from the Earth’s surface and from aircraft) and of certain meteors; each photograph is accompanied by an explanatory legend.
The French Meteorological Service has contributed materially to the preparation of the texts and the photographic plates and their legends. The Netherlands’ Meteorological Service also gave considerable assistance during the final stages of the preparation of the Cloud Atlas.
The undersigned who have been closely connected with the preparation and publication of the Cloud Atlas wish to thank all those who have contributed to the texts, and in particular Messrs. J. Bessemoulin and R. Beaufils of the French Meteorological Service and Dr. M.A. Alaka of the Secretariat of W.M.O., for their enthusiastic assistance during the final phase of the composition of the text. They also thank all the persons who provided photographs to illustrate the International Cloud Atlas.
President of the Commission for Synoptic Meteorology
President of the Committee for the Study of Clouds and Hydrometeors
De Bilt, Paris, 4 April 1956.