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The previous edition of the International Cloud Atlas, which appeared in 1956, consisted of two volumes: Volume I, containing a descriptive and explanatory text, and Volume II, containing a set of plates intended to illustrate the text. The present publication is a new edition of Volume I designed to replace the original edition. The preface describes the circumstances which led to the decision to publish a new edition and pays tribute to the numerous meteorologists who have devoted part of their time and efforts to the preparation of this new, and greatly improved, version of the text of the Atlas.
At its fourth session (Wiesbaden, 1966), the Commission for Synoptic Meteorology (CSM) examined the replies received from Members to an inquiry on the visibility criteria used for reporting mist and fog and also on the question of whether mist and fog should be considered as one and the same hydrometeor. It is pointed out in this connexion that, in the 1956 edition of the International Cloud Atlas, these phenomena were treated as two distinct hydrometeors.
CSM considered that mist and fog were produced by the same processes and that they should be regarded as one and the same hydrometeor, on the understanding, however, that the terms "fog" and "mist" might continue to be used to denote different intensities of the phenomenon, the term "mist" being synonymous with a slight fog, and the visibility limit of 1000 metres, used hitherto, being maintained as a criterion of intensity.
At the same session, CSM also examined a proposal to revise the definitions and descriptions of hydrometeors contained in the 1956 edition of the International Cloud Atlas. The reason for the proposal was that there had been important advances in the physics of hydrometeors since CSM had recommended the use of the descriptions.
The Commission agreed that such a review was needed, particularly as regards hydrometeors appearing in polar and mountainous areas. For this purpose it decided to set up a Working Group on Description of Hydrometeors (Resolution 8 (CSM-IV)) composed of the following members: L. Dufour (Belgium), chairman of the group and representative of the Commission for Aerology, G. A. Gensler (Switzerland), E. Hesstvedt (Norway), H. D. Parry (United States of America), B. V. Ramanamurthy (India) and A. Rouaud (France).
The group recommended, in the first place, that a cloud should be classified as a hydrometeor, which would call for a change in the definition of "cloud", which did not refer to such a classification. The definition of "meteor" also had to be changed, because the latter had hitherto been defined as "a phenomenon other than a cloud ...". The group also reviewed all the definitions and descriptions of hydrometeors other than clouds.
Following the recommendations made by the Working Group on Description of Hydrometeors set up by Resolution 8 (CSM-IV), CSM recommended, at its fifth session (Geneva, 1970), that the revised definitions and descriptions of hydrometeors other than clouds should be adopted and that Volume I of the International Cloud Atlas should be amended accordingly (Recommendation 41 (CSM–V)). At its twenty-second session, the Executive Committee approved this recommendation and requested the Secretary-General of WMO to arrange for the publication of the revised text (Resolution 14 (EC–XXII)).
In view of the fact that the 1956 edition of Volume I of the International Cloud Atlas was out of print and that the principle adopted by Sixth Congress to the effect that any publication constituting an annex to the Technical Regulations — which was partially true of Volume I of the Atlas — should be transformed into a Manual, the Advisory Working Group of the Commission for Basic Systems (CBS-formerly the Commission for Synoptic Meteorology (CSM)) considered, at its second session (Geneva, 1971), that a preliminary draft of a new edition of the volume should be prepared, containing, in principle, only such texts as had the legal status of provisions of the Technical Regulations, including, of course, the revised definitions adopted by CSM at its fifth session.
In accordance with the above-mentioned decision of the CBS Advisory Working Group, an expert (Mr. A. Durget, France) was invited to revise Volume I of the International Cloud Atlas. At its sixth session (Belgrade, 1974), CBS recommended that the draft revised text should be pub­lished to replace the 1956 edition and that the Secretary-General should be requested to arrange for the publication of an appropriate amendment to the Abridged Atlas to bring it into line with the new edition of Volume I of the International Cloud Atlas (Recommendation 18 (CBS–VI)). At its twenty-sixth session, the Executive Committee approved this recommendation and requested the Secretary-General to implement it (Resolution 3 (EC–XXVI)).
During his work on the revision of the Atlas, the expert decided that it was virtually impos­sible, and certainly not desirable, to exclude from the Atlas those parts of the 1956 edition which did not have the legal status of the provisions of the Technical Regulations. Some of those parts could not, in fact, be separated from the passages to which they referred and which were retained as being provisions of the Technical Regulations without their deletion seriously affecting the clarity and consistency of the work. Other parts not having the status of Technical Regulations and whose inclusion in the Atlas was not strictly speaking indispensable deserved none the less to be maintained in view of their great value for users of the Atlas. Part IV, however, of the 1956 edition of Volume I — entitled "Journal of clouds and meteors" — had not been included in the new edition since it was not of international interest.
Since the two volumes of the International Cloud Atlas are well known under this title, the same title has been retained for the present edition. But for reasons of consistency with other WMO publications constituting annexes to the Technical Regulations and accordingly described as "Manuals", this publication also bears the subtitle "Manual on the observation of clouds and other meteors".
Moreover, those parts of the book having the legal status of Technical Regulations are distinguished from the rest by a different type of print. Similarly, a system of paragraph numbering similar to that used in the Technical Regulations has been employed.
As a consequence of the adoption of the new definition of "cloud", which is now regarded as a hydrometeor, and having regard to the corresponding change in the definition of "meteor", it was thought necessary to re-arrange the layout of the Atlas completely, by changing the order of the parts and chapters so as to give the definitions of "meteor" and "hydrometeor" before dealing in detail with clouds and other meteors.
The present edition of Volume I thus consists of three parts. The first, in addition to the new definition of "meteor", contains a general classification of meteors into hydrometeors (including clouds), lithometeors, photometeors and electrometeors, as well as definitions of each of these four groups of phenomena. These various texts have been taken from Chapter I of Part II of the 1956 edition of Volume I, taking into account, in their wording, the new concepts set forth in Recommendation 41 (CSM–V) approved by Resolution 14 (EC–XXII).
Part II deals exclusively with clouds. It recapitulates, with a few essentially drafting changes, the various chapters dealing with clouds in the 1956 edition of Volume I: definition of a cloud and classification of cloud; definitions of the genera, species and varieties, etc., of clouds; descriptions of clouds; orographic influences; clouds as seen from aircraft; special clouds; observation of clouds from the Earth's surface; the coding of clouds in the codes CL, CM and CH and the corresponding symbols.
Part III deals with meteors other than clouds. It consists of three chapters: classification of, and symbols for, meteors other than clouds; definitions and description of meteors other than clouds; and observation of these meteors from the Earth's surface. The texts relating to hydro­ meteors have been introduced in the new version given in Recommendation 41 (CSM–V), with certain drafting amendments.
The three appendices to the 1956 edition of Volume I, namely: Appendix I — Etymology of Latin names of clouds, Appendix II — Historical bibliography of cloud classification, and Appendix III — Bibliography of cloud nomenclature, have been maintained unchanged in the present edition of the Atlas. The alphabetical index of words and expressions has also been maintained with appropriate updating.
On behalf of the World Meteorological Organization, I should like here to express my gratitude to Mr. L. Dufour and to the members of his working group, as well as to Mr. A. Durget, for their valuable contributions to the preparation of this edition of the volume.
D. A. DAVIES