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Preface to the 1939 edition 1

The first published2 classification of clouds only dates back to the beginning of the 19th century  and was the work of Lamarck (1802). This celebrated naturalist did not set out to classify all possible clouds; he confined himself to distinguishing certain forms which seemed to him to be the manifestation of general causes which it would be useful to recognize. But this work, in spite of its real value, did not make any impression even in France and his nomenclature does not seem to have been used by anyone. Perhaps this was due to his choice of somewhat peculiar French names which would not readily be adopted in other countries or perhaps the paper was discredited through appearing in the same publication (Annuaire Météorologique), as forecasts based upon astrological data. 

One year later Luke Howardpublished in England a cloud classification which, in striking contrast, achieved very great success and which is the basis of the existing classification. Whereas Lamarck contented himself with defining and naming a certain number of interesting forms, Howard set out to establish a complete classification covering all possible cases. He distinguished three simple, fundamental classes — Cirrus, Cumulus, Stratus — from which all others were derived by transition or association. This conception is in some respects incorrect. If Cirrus and Cumulus are entitled to occupy a privileged position in the classification, the first representing the purest type of cloud formed of ice crystals in the high regions of the atmosphere, and the second being pre-eminently a cloud of liquid particles in the lower regions, what Howard calls Stratus does not constitute a type of the same order as the preceding two. It is not defined in terms relating to the physical state of its elements, and it may be found at any altitude. From a practical point of view, however, Howard arrived at much the same results as Lamarck. Four of Lamarck’s five principal types appear under different names in Howard’s nomenclature. It is remarkable that these two men, of such different scientific culture and never having come in contact with each other, should have arrived independently at such compatible results.

In 1840, the German meteorologist Kaemtz added Stratocumulus to Howard's forms, giving a precise definition which is in agreement with modern usage.

Renou, Director of the observatories at Parc Saint-Maur and Montsouris gave in his “Instructions météorologiques” (1855) a classification of clouds to which may be ascribed the definite origin of several names in the present nomenclature: Cirro-Cumulus, Cirro-Stratus, Alto-Cumulus and Alto-Stratus. He was the first to introduce the two latter types into the “Bulletin de l’Observatoire de Montsouris” and his example was soon followed by the observatory at Upsala. He thus introduced clouds of medium height between low clouds and those of the Cirrus family and began the development of the idea which resulted in the adoption of height as a criterion, established later by Hildebrandsson. To him is also due the definite distinction at different levels between detached and continuous clouds.

In 1863, Poey, who took observations at Havana, made known some original ideas which did not perhaps receive as much respect as they deserved, firstly because good and bad were so closely associated in them and also because he set out to create a classification of all types, without any reference at all to the main outlines which since Howard had been slowly but surely emerging from  the successive attempts in Europe. It should, however, be remembered that credit is due to him for defining Fracto-Cumulus, some radiatus varieties (under the name of Fracto-) and mammatus varieties (under the name of Globo-). In particular, he described very clearly the central sky in a depression by distinguishing the two layers, one above the other: the sheet of Altostratus (under the name of Pallio-Cirrus) and the layer of Fractostratus or of Fractocumulus (under the name of Pallio-Cumulus).

In 1879, Hildebrandsson, Director of Upsala Observatory, was the first to use photography in the study and classification of cloud forms. In his work entitled: “Sur la classification des nuages employée à l’Observatoire  Météorologique d’Upsala”, he included an atlas of 16 photographs. The classification adopted was that of Howard with a few modifications. These changes concerned especially Nimbus, which was not assigned to every rainy cloud complex (notably not to Cumulo­ Nimbus), but only to the dark, lower layer of a rainy sky, Stratus which was assigned to fog raised from the ground and remaining  at some distance  from  the Earth  and Cumulo-Stratus  which, following Kaemtz’s example, was assigned to heavy, heaped up masses of Cumulus; from Kaemtz, Hildebrandsson  also adopted Strato-Cumulus. In his first work, Hildebrandsson kept closely to his desire to adhere to Howard's plan, but at the same time took later works into consideration.

A little later, Weilbach and Ritter proposed classifications too greatly divergent from Howard’s (which in the main had already been generally accepted) to have any chance of success - as happened later in the case of those of Maze, Clayton and Clement Ley. Credit is, however, due to these authors for interesting definitions of species (subdivisions of large genera) or of varieties (particular aspects to be observed at different heights) and to Weilbach for the introduction of Cumulo-Nimbus or thunder cloud, clearly distinguished from Cumulus even when “compositus”.

Finally in 1887, Hildebrandsson and Abercromby published a classification of clouds in which they endeavoured to reconcile existing customs and, while keeping to Howard’s scheme, to effect an inclusion of later acquisitions, notably those due to Renou (introduction of Alto-Cumulus and of Alto-Stratus, distinction at each stage between the detached and continuous forms) and to Weilbach (introduction of Cumulo-Nimbus, the allocation of Cumulus and thundery clouds to a distinct family). Abercromby had previously made two journeys round the world (thus giving a fine example of scientific probity) in order to assure himself that the cloud forms were the same in all parts — a fact that is, however, only true as a first approximation. One of the principal characteristics of this classification is the importance attached to height as a criterion, since in the opinion of the authors the foremost application of cloud observations was the determination of the direction of the wind at different altitudes. They grouped the clouds in four levels, the mean heights of which they fixed provisionally from measurements made in Sweden. The international classification was the direct offspring of Hildebrandsson’s and Abercromby’s classification without any great modification.

The International Meteorological Conference, held at Munich in 1891, expressly recommended these authors' classification and gave its sanction to the appointment of a special committee entrusted with its final consideration and publication with illustrations in atlas form.  This committee met at Upsala in August 1894 and proceeded to choose the illustrations to be reproduced. With this object in view, an exhibition of more than three hundred cloud photographs or sketches had been arranged. The publication commission, consisting of Hildebrandsson, Riggenbach and Teisserenc de Bort, had to contend with great technical and more particularly financial difficulties. In the end, Teisserenc de Bort took upon himself the sole responsibility for the production of the atlas which appeared in 1896. This work contained 28 coloured plates accompanied by a text in three languages (French, German, English) giving definitions and descriptions of the clouds together with instructions on how to observe them. 

The classification laid down in the International Atlas soon became official and came into almost general use in all countries. Nearly all meteorologists  who  subsequently  published cloud studies adopted this nomenclature; but frequently it was found to be lacking in details; thus a number of meteorologists — notably Clayden and Vincent — were led to create new species or varieties without interfering with the primary forms.

Thus, thanks to a sustained effort, initiated by Howard, continued by Renou and then by Hildebrandsson and the International Meteorological Committee, an end was made to the confusion which had reigned for nearly a century in one of the most important domains of meteorology. The first International Atlas constituted a great advance by making cloud observations throughout the world truly comparable with one another.

The 1910 reprint, which contained only slight modifications, had been exhausted for many years when the International Commission for the Study of Clouds was created in London in 1921. The President, Sir Napier Shaw, started the revision of the classification by bringing forward for discussion a memoir in which he gave his own personal ideas and appealed to all members to make suggestions; the enquiry thus set on foot grew so rapidly that in 1925, Sir Napier Shaw’s successor thought it necessary to concentrate all the activities of the Commission upon the problem of the revision of the International  Atlas.

This task had become necessary for several reasons. First of all, there was a very practical reason: it was becoming urgent that the observers should be supplied with new atlases lest the quality of the observations should degenerate and differences of interpretation reappear. But, in addition to this practical reason, there were also deeper ones: the work of 1896, remarkable as it had been at the time, was evidently not perfect. From the single, but essential point of view of the standardization of observations, the experience of thirty years had revealed several gaps and instances of lack of precision, which had led to incompatible traditions in different nations as regards certain points. Moreover, meteorology had developed considerably, especially since aviation had become general. When Teisserenc de Bort and Hildebrandsson published the first atlas, the principal problem they had in mind was the general circulation; they considered the clouds above all as aerial floats, capable of revealing upper currents, and were intent upon producing a classification which would make the different types of clouds correspond with heights determined as exactly as possible. But since that period, meteorologists had become more and more interested in clouds as such. The multiplication of cloud observations and the extended data included in synoptic messages —­ which received due recognition in the new international code, Copenhagen 1929 — made possible direct synoptic studies of their distribution and paved the way for the idea of “sky” and “cloud system”, the value of which had been clearly demonstrated by the International Cloud Week, organised by the Commission for the Study of Clouds in 1923.

Observations from aeroplanes made familiar cloud aspects which were previously unknown, made them known more intimately and more completely; finally, new theories normally based upon the hydrodynamical and thermodynamical interpretation of soundings determined their physical signification and their role in disturbances. These new and very interesting points of view had to be definitely recognised.

When the Commission for the Study of Clouds met in Paris in 1926 to take account of the results of the vast enquiry which it had inaugurated and to lay the foundations of a new atlas, it found itself confronted by an abundance of literature and very diverse suggestions. Very wisely, it adopted the principle that it should only touch with extreme caution a classification which had stood the test of years and been received as it were with unanimous agreement by our predecessors. It decided to make only such modifications as were necessary to dissipate misunderstandings and further the uniformity of observations, at the same time, however, laying less stress upon the importance of height as a basis of classification.

While recognizing the necessity of paving the way for a secondary classification, it took care not to attempt its completion nor to subdivide excessively the main categories henceforth called “genera”; it made a rule only to introduce “species” which were generally accepted by all, leaving the way clear for progressive additions in future. Having thus given witness to a prudent, conservative spirit and placed the work of 1896 in a secure position, the Commission for the Study of Clouds proceeded, on the other hand, to give practical satisfaction to the new spirit. Having from the outset considered it premature to attempt a cloud classification based upon physical properties — reserving the study of that question until after a new International Cloud Year (conceived in connection with and to be realized at the same time as the Polar Year 1932-1933) — it adhered to this attitude and refused to rely upon any theory, however attractive it might appear.  Nevertheless, it decided to put on record information which had been acquired by observation in the sky or on the charts. Thus it was resolved to include: 

(1) A chapter on the observation of clouds from aircraft for which the well-known work of Mr. C.K.M. Douglas, aviator as well as meteorologist, was largely drawn upon;

(2) A classification of “types of skies”, based upon the cloud structures in depressions, as emerging from the work of the Norwegian and French schools; in order to mark the importance of this innovation, the title of the Atlas was altered to “International Atlas of Clouds and of Types of Skies”.

The Commission for the Study of Clouds met a second time in Zürich in September 1926 to make definite arrangements for the projected atlas. Meanwhile, an imposing collection of photographs of clouds, skies and aerial views — borrowed mainly from the collections of Messrs. Cave, Clarke and Quénisset and of the Fundacio Concepcio Rabel — had been assembled to provide abundant illustrations for the atlas.

In order that the Commission's project should be subjected to the widest criticism before the final atlas was undertaken, the Director of the French Office National Météorologique decided to issue at the expense of his Office the Commission’s project in the form of a “Provisional Atlas”. The wide distribution of this atlas answered the purpose perfectly; remarks and suggestions flowed in from all parts of the world. These numerous documents were examined at Barcelona in 1929 by the Commission and all the suggestions were examined and classified with great care. The illustration of the Atlas was also carefully reviewed and the Commission’s task in this respect was facilitated to an extraordinary extent by the magnificent exhibition of cloud photographs arranged by the Fundacio Concepcio Rabel at the time of the meeting.

The Commission for the Study of Clouds met again at Copenhagen in September 1929 at the time of the meeting of the Conference of Directors. The suggestions received since the meeting at Barcelona were considered and the final scheme agreed, except for a few details. It was proposed that an extract of the Complete  Atlas for the use of the observers should be published quickly in order to facilitate the application of the new International Code, in which observations of the types of skies figured largely.

The question of publication could be approached in exceptionally favourable circumstances, thanks to the truly magnificent gift of a Catalonian Maecenas, Rafael Patxot, to whom cloud science was already indebted for the interesting work of the Fundacio Concepcio Rabel; this generous contribution made it possible to print 1000 free copies of the Complete Atlas and to offer it, as also the abridged edition, for sale at a very low price. A Sub-Commission was appointed, with Professor Süring as President, to prepare a programme for the Cloud Year and to study the physical processes of cloud formation and evolution with a view to compiling eventually an appendix to the General Atlas. Two other appendices were suggested, one on tropical clouds, the other on special local formations and the preparation of these two parts was delegated to Dr. Braak and Dr. Bergeron respectively. The Conference of Directors approved the Commission’s propositions in their entirety and delegated its powers, as far as the production of the atlas was concerned, to a special Sub-Commission.

The work was carried out largely at Paris in the course of 1930 by Messrs. Süring, Bergeron, and Wehrlé. The German and English translations were prepared by Dr. Keil, Mr. Cave and the Meteorological Office, London. The abridged edition finally appeared in 1930, just before the new code came into force. Another year was required to finish the illustrations of the Complete Atlas and the chapters not included in the abridged edition. Meanwhile, the Süring Sub-Commission had held meetings at Brussels (December 1930) and at Frankfurt (December 1931), and it seemed opportune to incorporate in the Complete Atlas a part of the work relating to the observation of clouds and hydrometeors.

The book now appearing bears the sub-title: “I. General Atlas” (the second and following volumes will consist of the appendices to be published later) and consists of a text and a collection of 174 plates.

The text is divided into five sections:

(1) CLOUDS — The amended text of the old Atlas. The principal modifications are:

a) the definition of Cirrocumulus which is more restricted than formerly;

b) the distinction between Cumulus and Cumulonimbus; the latter being characterised by ice crystals in its summit or by showers;

c) the distinction between Altocumulus and Stratocumulus;

d) the introduction of Nimbostratus (low Altostratus) in order to avoid confusion (due to the equivocal definition of Nimbus) between the low, rainy layer resulting from the downward extension of Altostratus and the very low, closely packed clouds (Fractostratus or Fractocumulus of bad weather) which often form beneath the Altostratus or the above-mentioned low layer.

The commentaries to the definitions have been considerably enlarged in the form of “Explanatory remarks”, written from a very practical standpoint with special reference to the needs of observers and stressing the distinctions between kindred forms. In some cases, species have been introduced but as previously stated this secondary classification is intentionally limited to cases on which there is unanimous agreement; it is moreover considerably simplified by the addition of a certain number of varieties common to different levels. In order to mark the fact that the names of the clouds have become symbols, the etymology of which should not be unduly stressed, they have in all cases been written as a single word.

(2) CODE — The second part consists of a practical and detailed commentary for the use of observers, with explanatory remarks concerning the general arrangement and hints how to avoid confusion in the specifications of the new code of low, middle and high clouds; it would perhaps be more appropriate to call it a code of the types of skies, since the arrangement of cloud masses in the sky plays an essential role in it and it has been conceived in such a fashion that all types of sky classified in the fifth part can be represented by the combination of three figures.

It was thought best to abstain from all “synoptic” considerations in the text, the observer being supposed to ignore the general situation; nevertheless, it is not desirable that he should be deprived entirely of the real help to be derived from connecting the type of sky with the evolution of disturbances. There will therefore be found at the end of this section a diagram showing where the different lower, middle and upper skies specified in the code are situated relative to a disturbance.

(3) CLOUD DIARY  This section, which has been inserted at the suggestion of Dr. Bergeron, has been taken from the papers prepared by the Süring Sub-Commission for the Cloud Year. It includes a model table for noting cloud observations and detailed instructions of how entries should be made in it. These are supplemented by precise descriptions of different hydrometeors or weather phenomena, a subject which has given rise to divergent national traditions and in which there was need of amendment and unification.

(4) OBSERVATION OF CLOUDS FROM AIRCRAFT — As the classification of clouds is based upon their appearance as seen from the ground, it seemed useful to add a note on their appearance from the point of view of the observer in an aircraft, inasmuch as the more complete knowledge which he may acquire from the fact that he can get near to and on top of them (at least, in the case of lower and middle clouds) makes it possible to simplify the classification considerably by including only the really essential distinctions in structure. The increase in the number of meteorological flights especially in connection with temperature soundings, necessitated the inclusion of this chapter.

(5) TYPES OF SKIES — The enumeration of the genera or even of the species of clouds in the sky at a given moment does not suffice to characterize the type of sky, that is to say, to specify precisely the sector of the disturbance affecting the place of observation and, in consequence, it does not indicate the general character of the “weather”. What really characterizes the type of sky is the aggregate of individual clouds and their organization. A special classification of skies is therefore needed which, while being in accordance with the experience of qualified observers, shall also be consistent with the nature of the physical processes and the structure of disturbances. In addition, such classification facilitates the identification of cloud genera and in certain cases (especially in thundery conditions) it compensates, at least in parts, for vagueness.

Collection of plates — The total number of plates is 174(101 photographs taken from the ground, 22 from aeroplanes and 51 for types of sky), 31 of which are in two colours. Two colours are used where there is occasion to distinguish the blue of the sky from the shadows of the clouds. Most of these are included in the abridged edition, which is intended for the use of the general mass of observers who need detailed guidance. Each plate is accompanied by explanatory notes and a schematic representation on the same scale as the photograph, setting out its essential characteristics.

Thanks to the generosity of Mr. Cave, who has done so much for cloud science, the appendix dealing with tropical clouds edited by Dr. Braak, constituting Volume II of the complete work, has already appeared in French in connection with the requirements of the Polar Year. It is hoped that the appendix dealing with special clouds, constituting Volume III will appear soon. This will include, in particular, Professor Störmer’s beautiful photographs of stratosphere clouds. Finally, it is also hoped that the results of the Cloud Year will enable the Süring Sub-Commission to prepare a fourth volume dealing with the physical processes involved in the formation of clouds, which will be epoch-making in the history of meteorology.


President of the International

Commission for the Study of Clouds.

1 The preface to the 1939 edition was nearly identical with that of the 1932 edition. The modifications introduced into the text of the 1939 edition consisted of revised abbreviations of cloud forms, changes in the clouds code and changes in the symbols and descriptions of various weather phenomena.

2 In this brief historical note, much use has been made of Louis Besson’s very interesting work: Aperçu historique sur la Classification des Nuages, Mémorial de l’Office National Météorologique de France, No. 2, Paris 1923.

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