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PREFACE TO THE 1939 EDITION1
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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
Meteorologique), as forecasts based upon astrological data.
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One year later Luke Howard published 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
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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.
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In this brief historical note, much use has been made of Louis Besson's very interesting work: Aperf:U historique sur
Ia Classification des Nuages, Memorial de l'Office National Meteorologique de France, No. 2, Paris 1923.
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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.
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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 employee
a l'Observatoire
Meteorologique 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".
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 Pallia-Cirrus) and the layer of Fractostratus or of
Fractocumulus (under the name of Pallia-Cumulus).
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
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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
Bart, 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
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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 Zurich 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
Quenisset 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 Meteorologique 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.
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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 1 000 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 SubCommission.
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The work was carried out largely at Paris in the course of 1930 by Messrs. Süring,
Bergeron, and Wehrle. 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.
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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.
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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.
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(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.
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(3) CLOUD DIARY - This section, which has been inserted at the suggestion of Dr.
Bergeron, has been taken from the papers prepared by the Suring 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
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.
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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.
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(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.
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(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.
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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.
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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 Stormer'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.
E. DELCAMBRE,
President of the International
Commission for the Study of Clouds.
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PREFACE TO THE 1956 EDITION
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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.
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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.
Mezin, J. Mondain and H. Weickmann.
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An Editing Committee, consisting of M. Mezin (President), R. Beaufils
(Secretary), R. Beaulieu, Bessemoulin and M. Bonnet, prepared documents between
sessions.
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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.
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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).
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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).
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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.
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The Commission for Synoptic Meteorology recommended (Recommendation
49, 'CSM-1) that the Complete Atlas should consist of only two volumes (Volume I
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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.
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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 "mothercloud", 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.
13
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Part Ill 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.
W. BLEEKER,
President of the
Commission for
Synoptic
Meteorology
531
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534
A. VIAUT,
President of the Committee
for the Study of Clouds
and Hydrometeors
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De Bilt, Paris, 4 April 1956.
15
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PREFACE TO THE 1 9 7 5 EDITION OF VOLUME I
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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 1 000
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
16
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session.
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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 hydrometeors have been introduced in the new version given in Recommendation 41 (CSM-V), with
certain drafting amendments.
17
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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.
666
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D. A. DAVIES
668
Secretary-General
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729
FOREWORD TO THE 1987 EDITION OF VOLUME II
With this new, thoroughly revised edition of Volume II of the International
Cloud Atlas a key publication is once again made available for professional
meteorologists as well as for a wide circle of interested amateurs. For meteorologists
this is a fundamental handbook, for others a source of acquaintance with the
spectacular world of clouds.
The present internationally adopted system of cloud classification is the result of
work which started in the nineteenth century. The first studies on the topic were
published by J. B. Lamarck (1802) and L. Howard (1803). The first attempt to use
photography for cloud classification was made by H. Hildebrandsson ( 1879), in
Uppsala, who prepared a cloud atlas of 16 photographs. The further development of
this work, following the recommendation of an International Meteorological Conference
which took place in Munich in 1891, resulted in the publication in 1896 of the first
International Atlas, containing 28 coloured plates accompanied by definitions and
descriptions of clouds and instructions on cloud observations in three languages
(French, German, English). The first International Atlas, which was then adopted in
almost all countries, was a great step forward in making internationally comparable
cloud
observations. This Atlas was reprinted in 1910, without
substantial
amendments. The subject of further refinement of cloud classification still remained to
the fore, however, during the following decades. As a result the International Atlas of
Clouds and Study of the Sky, Volume I, General Atlas was published in 1932 by the
International Commission for the Study of Clouds. A modified edition of the same
work appeared in 1939, under the title lnternational Atlas of Clouds and of Types
of Skies, Volume I, General Atlas. The latter contained 174 plates: 101 cloud
photographs taken from the ground and 22 from aeroplanes, and 51 photographs of
types of sky. From those photographs. 31 were printed in two colours (grey and blue)
to distinguish between the blue of the sky and the shadows of' the clouds. Each plate
was accompanied by explanatory notes and a schematic drawing on the same scale as
the photograph, showing the essential characteristics of the type of cloud.
When the World Meteorological Organization (WMO) came into being in 1951 in
place of the non-governmental International Meteorological Organization, the First
Meteorological Congress noted the need for a new International Cloud Atlas and
referred the task to the Commission for Synoptic Meteorology. Within a relatively short
time very substantial work was accomplished and the new Atlas was published in
1956 in two volumes: Volume I contained a descriptive and explanatory text on the
whole range of hydrometeors (including clouds), lithometeors, photometeors and
electrometeors; Volume II contained a collection of 224 plates (123 in black and white
and 101 in colour) of photographs of clouds and of certain meteors. Each photograph
in Volume II was accompanied by an explanatory text, to enable the pictures in
Volume II to be understood without the detailed technical definitions and
descriptions contained in Volume I.
The 1956 edition of Volume II has not been reprinted or revised until the
preparation of the present edition. A revised version of Volume l, however, was
published in 1975 under the title Manual on the Observation of Clouds and Other
Meteors. ln the meantime there have been substantial advances in techniques of
cloud photography and a growing requirement for more photographs taken at locations
outside Europe.
ln 1981 a WMO Informal Planning Meeting on Volume II of the International Cloud
Atlas drew up a plan for the preparation of a new edition. It recommended the deletion
of 26 black-and-white plates and eight in colour, and their replacement by 41 new
colour plates selected from a large number of photographs received from various
countries. The section containing illustrations of certain meteors was also expanded by
the addition of nine more plates. The legends for the new plates selected by the
Informal Planning Meeting were edited by the chairman of the meeting, Mr. R. L. Holle,
18 (temporary page number to facilitate discussions)
19
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of the U.S. National Oceanic and Atmospheric Administration, and those for the new
plates in the section on meteors by Mr. C. S. Broomfield, of the U.K.
Meteorological Office.
Later it became apparent that many of the original photographs of the 1956
edition had deteriorated with time to an extent excluding the possibility of their inclusion
in the new edition. Moreover, it was felt that the geographical distribution of the
photographs was still somewhat restricted and that the balance between the various
sections could be improved. With the approval of the president of the Commission for
Basic Systems, it was therefore decided to revise the Atlas extensively, bearing in
mind the urgent requirement for the new edition, and Mr. Holle kindly agreed to
undertake this complex task, including the soliciting at short notice of new photographs
from specialists. The final editorial work was carried out by the WMO Secretariat. The
result of the work, the present Volume II of the International Cloud Atlas, contains 196
pages of photographs, 161 in colour and 35 in black and white. Each illustration is
accompanied by an explanatory text.
The excellent work of the consultants and the authorization willingly given by all
contributors for publication of photographs in both the original volume and this new
edition are gratefully acknowledged. Particular thanks are due to the printer, whose
painstaking work permitted much of the original material to be conserved and blended
harmoniously with the new contributions.
It is felt that this new edition of the Atlas, besides being a most valuable reference
work for meteorologists and those working in aviation, in agriculture and at sea, will
also be a fascinating addition to the amateur's bookshelf.
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(G.O.P.
Obasi)
Secretary-General
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762
PREFACE TO THE PRESENT EDITION
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[To be provided prior to publication]
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INTRODUCTORY NOTE
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Certain parts of this publication constitute Annex I to the Technical
Regulations and have the legal status of standard practices and procedures.
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The sections, paragraphs and sub-paragraphs having the status of an Annex to
the Technical Regulations, except for the footnotes, are numbered in bold type. In
Chapter III. 2 only the definitions printed in italics have this status.
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22
785
PART I –METEOROLOGICAL METEORS: DEFINITION AND GENERAL CLASSIFICATION
786
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23
787
I.1 - METEOROLOGICAL DEFINITION OF A METEOR
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790
In meteorology, a phenomenon observed in the atmosphere or on the surface of the Earth is
known as a meteor. It may be a form of precipitation, a suspension, or a deposit of liquid or solid
particles. It is generally visible to human observers, but in the case of thunder it is audible.
791
792
793
794
795
Sometimes names of meteors are also applied to related concepts. For instance, the word "snow"
may refer to a type of hydrometeor (ensemble of falling particles), to snow cover (ensemble of
particles lying on the ground), or to the substance snow (as in "snow blown from mountains",
snowball). The constituent particles of snow in these three cases are either snow crystals or
snowflakes.
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797
Also, certain meteors are named based on their constituent particles; for example, the
hydrometeor “snow grains" is an ensemble of falling snow grains.
798
I.2 - GENERAL CLASSIFICATION OF METEORS
799
800
801
The nature of meteorological meteors is quite varied. However, by considering their constituent
particles or the physical processes surrounding their occurrence, meteors are classified into four
groups, namely hydrometeors, lithometeors, photometeors and electrometeors.
802
803
I.2.1
Hydrometeors
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807
A hydrometeor consists of liquid or solid water particles. Hydrometeors may fall through the
atmosphere or be suspended in it, may be blown by the wind from the Earth's surface, or may be
deposited on other objects. Snow or water on the ground is, by convention, not considered a
hydrometeor.
808
We describe the following five types of hydrometeors:
Suspended particles
 clouds,

fog ("fog" and "mist")

ice fog
809
Falling particles (precipitation)

From clouds: rain, drizzle, snow, snow grains, snow pellets,, hail and ice pellets.

From clear air: diamond dust
Precipitation can reach the Earth’s surface or completely evaporate while falling (virga). When
the falling particles reach the observer, it is usually easy to identify the type of precipitation.
This identification can then assist in identifying the cloud type especially at night.
Hydrometeors consisting of falling particles may occur as fairly uniform precipitation
(intermittent or continuous) or as showers. Showers are characterized by their abrupt
beginning and end, and by rapid and sometimes strong variation in the precipitation intensity.
Drops and solid particles falling in a shower are generally larger than those falling in nonshowery precipitation. Showers only fall from convective clouds (Cumulonimbus and Cumulus);
uniform (non-showery) precipitation usually falls from stratiform clouds (mainly Altostratus and
Nimbostratus). From this, the hydrometeor type can be useful to identify cloud genera,
23 (temporary page number to facilitate discussions)
24
especially at night.
Particles raised by the wind from the Earth's surface
 drifting snow

blowing snow

spray
These are confined to near the surface.
810
Deposit of particles that can occur either as

drops of water (deposits of fog droplets or dew)

a collection of ice particles, usually individually distinguishable although they are often
partially merged (white dew, hoar frost and rime)

smooth uniform layers of ice in which no pellet structure can be distinguished (glaze)
811
Spouts

when seen at sea, spouts appear as a cloud column or cone made up of water droplets
raised from the sea surface, and are considered a hydrometeor.

Spouts occurring on land are typically made up of solid, non-aqueous particles. In this case
they cannot strictly be classified as hydrometeors. However, the presence of the cloud
column or cone is a basic criterion and therefore spouts are classified in this Atlas within
the group of hydrometeors.
812
Hydrometeor
Suspended particles
Phenomena

clouds,

fog ("fog" and "mist")

ice fog
Falling particles
(precipitation)

from clouds: rain, drizzle, snow, snow grains, snow pellets, hail and ice pellets.

from clear air: diamond dust
Particles raised by the
wind from the Earth's
surface

drifting snow

blowing snow

spray

drops of water (deposits of fog droplets or dew)

a collection of ice particles, usually individually distinguishable although they are
often partially merged (white dew, hoar frost and rime)

smooth uniform layers of ice in which no pellet structure can be distinguished
(glaze)

funnel cloud:
o water spout over water
o tornado over land
Deposit of particles that
can occur either as
Spouts
813
814
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25
816
817
Cloud forms (genera) associated with each hydrometeor type.
Possible
Drizzle
Snow
Possible
showers
Usual
showers
Possible
showers
Possible
showers
Possible
showers
Possible
showers
Possible
Possible
Possible Possible Possible
Snow grains
Possible
Snow pellets
Possible
Diamond dust
Possible
Hail (large and
small)
Ice pellets
No cloud
Usual
Cumulonimbus
Stratocumulus
Rain
Cumulus
Nimbostratus
Possible
Falling
hydrometeor
Stratus
Altostratus
Cloud Genera associated with Falling hydrometers (precipitation)
Possible
showers
Possible
Possible
818
819
I.2.2 Lithometeors
Lithometeors
A lithometeor consists of an ensemble of particles, most of which are solid and non-aqueous. The
particles are either suspended in the air or lifted by the wind from the ground.

Suspended particles are haze, dust haze and smoke. They consist of very small dust particles,
sea-salt particles, or combustion products (e.g. from forest fires).

Particles lifted by the wind are drifting and blowing dust or sand, dust storm or sandstorm,
and dust whirl or sand whirl. When well developed, a dust whirl is known as a dust devil.
820
821
I.2.3.
822
Photometeors
Optical phenomenon produced by the reflection, refraction, diffraction or interference of light
from the sun or the moon.

On or inside clouds: halo phenomena, coronae, irisations and glory;

On or inside some hydrometeors or lithometeors: halo phenomena, coronae, glory, rainbow,
Bishop's ring and crepuscular rays;

In clear air: mirage, shimmer, scintillation, green flash and twilight colours.
823
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26
824
825
I.2.4 Electrometeors
826
Photometeors
An electrometeor is a visible or audible manifestation of atmospheric electricity. Electrometeors
can be:


related to discontinuous electrical discharges (lightning, thunder)
more or less continuous (Saint Elmo's fire, polar aurora).
827
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PART II- CLOUDS
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II.1 – INTRODUCTION AND PRINCIPLES OF CLOUD CLASSIFICATION
II.1.1
Definition of a cloud
A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both,
suspended in the atmosphere and usually not touching the ground. It may also include larger
particles of liquid water or ice as well as non-aqueous liquid or solid particles such as those
present in fumes, smoke or dust.
II.1.2
Appearance of clouds
Appearance is best described by dimensions, shape, structure, texture, luminance and colour of
the cloud. These factors are considered for each of the characteristic cloud forms.
II.1.2.1
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LUMINANCE
Light reflected, scattered and transmitted by its constituent particles determines the luminance of
a cloud. This light comes, for the most part, direct from the luminary (sun or moon or stars) or
from the sky; it may also come from the surface of the earth, being particularly strong when ice
fields or snowfields reflect sunlight or moonlight.
The luminance of a cloud may be modified by intervening haze. When haze is present between the
observer and the cloud, it may, depending on its thickness and the direction of the incident light,
either diminish or increase the luminance of the cloud. Haze also diminishes the contrast that
reveals the shape, structure and texture of the cloud. Luminance may also be modified by optical
phenomena such as haloes, rainbows, coronae, glories, etc. ("Photometeors" in Part III of this
Atlas).
During daytime, the luminance of the clouds is sufficiently high to make them easily observable.
On a moonlit night, clouds are visible when the moon is more than a quarter full. In its darker
phases, the moon is not bright enough to reveal clouds far from it, especially when the clouds are
thin. On a moonless night, clouds are generally invisible; sometimes, however, their presence may
be deduced from the obscuration of stars (noting that stars near the horizon can be obscured due
to haze), of polar aurora, of zodiacal light etc.
Clouds are visible at night in areas with sufficiently strong artificial lighting. Over large cities,
clouds may be revealed by direct illumination from below. An artificially illuminated layer of clouds
may provide a bright background against which lower cloud fragments stand out.
1
When a slightly opaque cloud is illuminated from behind, its luminance is at a maximum in the
direction of the luminary. It decreases away from the luminary; the thinner the cloud, the more
rapid the decrease. Clouds of greater optical thickness (measure of the degree to which the cloud
prevents light from passing through) show only a slight decrease in luminance with distance from
the luminary. Still greater thickness and opacity make it impossible even to determine the position
of the luminary. When the sun or moon is behind a dense isolated cloud, the latter shows a
brilliantly illuminated border, and luminous streaks alternating with shadowed bands may be seen
around it.
29
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
As the optical thickness of a cloud layer often varies from one portion of the layer to another; the
luminary may therefore be perceptible through one part of the cloud but not through another.
The varying optical thickness and the luminance of the cloud layer, especially at short angular
distances from sun or moon, may change considerably with time owing to the movement of the
cloud.
In the case of a uniformly and sufficiently opaque cloud layer, the luminary may be perceptible
when it is not too far from the zenith, but may be completely masked when close to the horizon.
Sufficiently opaque cloud layers sometimes show a maximum luminance at the zenith when the
sun or moon is at low elevation.
The light reflected from a cloud to the observer is at a maximum when the cloud is opposite to the
luminary. The luminance is stronger the greater the denseness of the cloud and its thickness in the
line of sight. When sufficiently dense and deep, the cloud reveals shades of grey showing a more
or less clear relief; the more tangential the direction of illumination, the more extensive the range
of shades.
Finally, appreciable differences in luminance exist between clouds composed of water droplets
and clouds composed of ice crystals. Ice crystal clouds are usually more transparent than water
droplet clouds owing to their thinness and to the sparseness of the ice particles. Certain ice crystal
clouds, however, occur in thick patches and, can have a high concentration of ice particles. When
these clouds are illuminated from behind, they show marked shading. They are, however,
brilliantly white in reflected light.
II.1.2.2
COLOUR
Since light of all wavelengths is almost equally strongly diffused by clouds, the colour of
the clouds depends primarily on that of the incident light. Haze between observer and cloud may
modify cloud colours; e.g., it tends to make distant clouds look yellow, orange or red. Cloud
colours are also influenced by special luminous phenomena ("Photometeors" in Part III of this
Atlas).
When the sun is sufficiently high above the horizon, clouds or portions of clouds that chiefly
diffuse light from the sun are white or grey. Parts that receive light mainly from the blue-sky are
bluish grey. When the illumination by the sun and the sky is extremely weak, the clouds tend to
take the colour of the surface below them.
When the sun approaches the horizon, its colour may change from yellow through orange
to red; the sky in the vicinity of the sun and the clouds show a corresponding coloration. The blue
of the sky and the colour of the underlying surface may still influence the colours of the clouds.
Cloud colours also vary with the height of the cloud and its relative position with regard to
observer and sun.
When the sun is close to the horizon, high clouds may still look almost white whilst low clouds
show a strong orange or red coloration. These differences in colour make it possible to obtain an
idea of the relative altitudes of the clouds; noting that clouds at the same level appear less red
when seen towards the sun than when viewed away from it.
When the sun is just above or on the horizon, it may redden the under surface of a cloud; when
this surface is corrugated, its coloration is distributed in bands alternately lighter (yellowish or
reddish tint) and darker (other tints), which make the relief more apparent.
30
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
When the sun is just below the horizon, the lowest clouds, in the shadow of the earth,
are grey; clouds at the middle levels are rose coloured and those very high may be whitish.
At night, the luminance of clouds is usually too weak for colour vision; all perceptible
clouds appear black to grey, except those illuminated by the moon, which present a whitish
appearance. Special illumination (fires, lights of large cities, polar aurora, etc.) may sometimes give
a more or less marked colouring to certain clouds.
II.1.3
Principles of cloud classification
Clouds continuously evolve and appear in an infinite variety of forms. However, there is a limited
number of characteristic forms frequently observed all over the world, into which clouds are
broadly grouped into a classification scheme. The scheme uses genera, species and varieties. This
is similar to the systems used in classification of plants or animals, and similarly uses Latin names.
There are some intermediate or transitional forms of clouds, although observed fairly frequently,
that are not described in the classification scheme. The transitional forms are of little interest;
they are less stable and in appearance are not very different from the definitions of the
characteristic forms.
There are also clouds, rarely or occasionally observed and usually only in certain parts of the
world, that are not included in the present classification scheme. Special clouds are described in
Chapter II.6.
II.1.3.1
GENERA
The classification of clouds has ten main groups, called genera. Each observed cloud is a member
of one, and only one, genus.
II.1.3.2
SPECIES
Most of the genera are subdivided into species, based on the shape of the clouds or their internal
structure. A cloud, observed in the sky and identified as a specific genus, may bear the name of
only one species.
II.1.3.3
VARIETIES
Varieties are different arrangements of the visible elements of clouds and varying degrees of
transparency.
A variety may be common to several genera and a cloud may show characteristics of more than
one variety. When this is the case all the observed varieties are included in the name of the cloud.
II.1.3.4
SUPPLEMENTARY 'FEATURES AND ACCESSORY CLOUDS
A cloud may show supplementary features attached to it, or may be accompanied by accessory
clouds, sometimes partly merged with its main body. "Supplementary features" and "accessory
clouds" may occur at any level of the cloud, or above or below it.
31
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
One or more supplementary features or accessory clouds may be observed simultaneously with
the same cloud.
II.1.3.5
MOTHER-CLOUDS and SPECIAL CLOUDS
Clouds may form in clear air. They may also form or grow from other clouds, called "motherclouds”. Depending on the change, one of two suffixes may be used.
a) “genitus” suffix. A part of a cloud may develop and more or less pronounced extensions may
form. These extensions, whether attached to the mother-cloud or not, may become clouds
of a genus different from that of the mother-cloud. They are then given the name of the
appropriate genus, followed by the name of the genus of the mother-cloud with the
addition of the suffix "genitus" (e.g. Cirrus altocumulogenitus, Stratocumulus
cumulogenitus).
b) “mutatus” suffix: The whole or a large part of a cloud may undergo complete internal
transformation, changing from one genus into another. The new cloud is given the name
of the appropriate genus, followed by the name of the genus of the mother-cloud with the
addition of the suffix "mutatus" (e.g. Cirrus cirrostratomutatus, Stratus
stratocumulomutatus). The internal transformation should not be confused with changes
in appearance resulting from the relative movement of clouds and observer.
1026
1027
1028
In addition, there are special cases where clouds may form or grow as a consequence of
certain, often localised, generating factors. These may be either natural, or the result of
human activity. Several cases of "Special clouds" can be distinguished:
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
a) "flammagenitus": Clouds may develop as a consequence of convection initiated by heat
from forest fires, wild-fires or from volcanic eruption activity. Clouds that are clearly
observed to have originated as a consequence of localised natural heat sources, such as
forest fires, wild-fires or volcanic activity and which, at least in part, consist of water drops,
will be given the name relevant to the genus followed, if appropriate, by the species,
variety and any supplementary features, and finally by the Special Cloud name
"flammagenitus", (e.g. Cumulus congestus flammagenitus, Cumulonimbus calvus
flammagenitus). [Note: cumulus flammagenitus is also known by the unofficial, common
name, 'pyrocumulus'].
b) "homogenitus": Clouds may also develop as a consequence of human activity. Examples
are aircraft condensation trails (Contrails), or clouds resulting from industry, such as
cumuliform clouds generated by rising thermals above power station cooling towers.
Clouds that are clearly observed to have originated specifically as a consequence of human
activity will be given the name of the appropriate genus, followed by the Special Cloud
name "homogenitus". For example: Cumulus cloud formed above industrial plants will be
known as Cumulus (and, if appropriate, the species, variety and any supplementary
features) followed by the Special Cloud name homogenitus; e.g. Cumulus mediocris
homogenitus.
c) Aircraft condensation trails (Contrails) that have persisted for at least 10 minutes will be
given the name of the genus, Cirrus, followed only by the Special Cloud name
"homogenitus", i.e. a Contrail will be known only as Cirrus homogenitus. Because new, or
recently formed aircraft condensation trails may undergo a fairly rapid state of change and
may display a variety of transient shapes, no species, varieties or supplementary features
will be applied to the name.
32
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
d) "homomutatus": Persistent contrails (Cirrus homogenitus) may be observed, over a period
of time and under the influence of strong upper winds, to grow and spread out over a
larger portion of sky, and undergo internal transformation such that the cloud eventually
takes on the appearance of more natural cirri-form clouds. In this instance, the resulting
clouds will be given the name of the appropriate genus (e.g. cirrus, cirrocumulus, or
cirrostratus) followed by any appropriate species, variety and supplementary features,
followed by the Special Cloud name "homomutatus", (e.g. cirrus floccus homomutatus,
cirrus fibratus homomutatus).
e) "cataractagenitus": Clouds may develop locally in the vicinity of large waterfalls as a
consequence of water broken up into spray by the falls. The downdraft caused by the
falling water is compensated by locally ascending motion of the air. These Special Clouds
will be given the name of the appropriate genus, followed by any appropriate species,
variety and supplementary feature, and followed by the Special Cloud name
"cataractagenitus" (e.g. Cumulus mediocris cataractagenitus, Stratus fractus
cataractagenitus).
f)
"silvagenitus": Clouds may develop locally over forests as a result of increased humidity
due to evaporation and evapotranspiration from the tree canopy. These Special Clouds
will be given the name of the appropriate genus, followed by any appropriate species,
variety and supplementary feature, and followed by the Special Cloud name "silvagenitus"
(e.g. Stratus fractus silvagenitus).
33
1080
1081
1082
1083
1084
II.1.4
Table of classification of clouds
[Editorial Note: New classifications in this table are denoted by yellow highlighting]
GENERA
SPECIES
VARIETIES
Supplementary
Features and
Accessory Clouds
(listed by frequency of observation)
Cirrus
Cirrocumulus
Cirrostratus
Altocumulus
fibratus
uncinus
spissatus
castellanus
floccus
stratiformis
lenticularis
castellanus
floccus
fibratus
nebulosus
stratiformis
lenticularis
castella nus
floccus
volutus
Altostratus
-
Nimbostratus
-
Stratocumulus
stratiformis
lenticularis
castellanus
floccus
volutus
Stratus
nebulosus
fractus
Cumulus
humilis
mediocris
congestus
fractus
intortus
radiatus
vertebratus
duplicatus
mamma
fluctus
undulatus
lacunosus
virga
mamma
cavum
duplicatus
undulatus
translucidus
perlucidus
opacus
duplicatus
undulatus
radiatus
lacunosus
translucidus
opacus
duplicatus
undulatus
radiatus
MOTHER-CLOUDS and SPECIAL CLOUDS
(most commonly occurring mother-clouds
are listed in the same order as genera)
GENITUS
Cirrocumulus
Altocumulus
Cumulonimbus
Homo
_
-
Cirrocumulus
Cumulonimbus
MUTATUS
Cirrostratus
Homo
Cirrus
Cirrostratus
Altocumulus
Homo
Cirrus
Cirrocumulus
Altostratus
Homo
Cirrocumulus
Altostratus
Nimbostratus
Stratocumulus
virga
mamma
cavum
fluctus
asperitas
Cumulus
Cumulonimbus
virga
praecipitatio
pannus
mamma
Altocumulus
Cumulonimbus
Cirrostratus
Nimbostratus
praecipitatio
virga
pannus
Cumulus
Cumulonimbus
Altocumulus
Altostratus
Stratocumulus
translucidus
perlucidus
opacus
duplicatus
undulatus
radiatus
lacunosus
opacus
translucidus
undulatus
virga
mamma
praecipitatio
fluctus
asperitas
cavum
Altostratus
Nimbostratus
Cumulus
Cumulonimbus
Altocumulus
Nimbostratus
Stratus
praecipitatio
fluctus
Stratocumulus
radiatus
virga
praecipitatio
pileus
velum
arcus
pannus
fluctus
tuba
Nimbostratus
Cumulus
Cumulonimbus
Homo
Silva
Cataracta
Altocumulus
Stratocumulus
Flamma
Homo
Cataracta
-
Stratocumulus
Stratus
34
Cumulonimbus
1085
1086
calvus
capillatus
-
praecipitatio
virga
pannus
incus
mamma
pileus
velum
arcus
murus
cauda
flumen
tuba
Altocumulus
Altostratus
Nimbostratus
Stratocumulus
Cumulus
Flamma
Homo
Cumulus
35
1087
II.1.5
1088
1089
Table of Abbreviations and Symbols of Clouds
1090
GENERA
Designations
SPECIES
Abbreviations
Symbols
Designations
Abbreviations
Cirrus
Ci
fibratus
fib
Cirrocumulus
Cc
uncinus
unc
Cirrostratus
Cs
Altocumulus
Ac
Altostratus
As
Nimbostratus
Ns
spissatus
spi
castellanus
cas
floccus
flo
stratiformis
str
nebulosus
neb
lenticularis
len
fractus
fra
humills
hum
Stratocumulus
Sc
mediocris
med
Stratus
St
congestus
con
Cumulus
Cu
volutus
vol
Cumulonimbus
Cb
calvus
cal
capillatus
cap
SUPPLEMENTARY FEATURES and ACCESSORY
CLOUDS
VARIETIES
Designations
intortus
Abbreviations
Abbreviations
in
vertebratus
ve
undulatus
un
radiatus
ra
lacunosus
Ia
duplicatus
du
translucidus
tr
perlucidus
pe
opacus
op
Designations
incus
mamma
virga
cavum
fluctus
asperitas
praecipitatio
arcus
murus
tuba
pileus
velum
pannus
cauda
flumen
Abbreviations
inc
mam
vir
cav
flu
asp
pra
arc
mur
tub
pil
vel
pan
cau
flm
MOTHER-CLOUDS and SPECIAL CLOUDS
GENITUS
Designations
MUTATUS
Abbreviations
Designations
Abbreviations
36
cirrocumulogenitus
altocumulogenitus
altostratogenitus
nimbostratogenitus
stratocumulogenitus
cumulogenitus
cumulonimbogenitus
flammagenitus
homogenitus
silvagenitus
cataractagenitus
1091
ccgen
acgen
asgen
nsgen
scgen
cugen
cbgen
flgen
hogen
sigen
cagen
cirromutatus
cirrocumulomutatus
cirrostratomutatus
altocumulomutatus
altostratomutatus
nimbostratomutatus
stratocumulomutatus
stratomutatus
cumulomutatus
homomutatus
cimut
ccmut
csmut
acmut
asmut
nsmut
scmut
stmut
cumut
homut
37
1092
II.2- DEFINITIONS OF CLOUDS
1093
II.2.1
1094
Some useful concepts
1095
II.2.1.1
1096
HEIGHT, ALTITUDE, VERTICAL EXTENT
HEIGHT: Vertical
distance from the point of observation (which may be on a hill or a mountain) to the
point being measured.
Surface observations - height above ground level of the cloud base; aircraft
observers – height above sea level of the cloud base.
HEIGHT OF CLOUD BASE:
ALTITUDE:
Vertical distance from mean sea level to the point being measured
VERTICAL EXTENT: Vertical
distance from the cloud base to its top
1097
1098
II.2.1.2
1099
LEVELS
1100
1101
1102
1103
1104
Clouds 1 are generally encountered over a range of altitudes varying from sea level to the top of
the troposphere (the tropopause). The troposphere has been vertically divided into three levels:
high , middle and low. Each level is defined by the range of levels at which clouds of certain
genera occur most frequently.The levels overlap and their limits vary with latitude.
1105
1106
Table #: Approximate heights, and genera in each, of the levels
Levels
High
Genera
Polar Regions
Temperate Regions
Tropical Regions
Cirrus
3 – 8 km
5 – 13 km
6 –18 km
Cirrocumulus
(10 000 – 25 000 ft)
(16 500 – 45 000 ft)
(20 000 – 60 000 ft)
Altocumulus
2 – 4km
2 – 7 km
2 – 8 km
Altostratus
(6 500 – 13 000 ft)
(6 500 – 23 000 ft)
(6 500 – 25 000 ft)
From the Earth's
surface to 2 km
From the Earth's surface
to 2 km
From the Earth's
surface to 2 km
(0 – 6 500ft)
(0 – 6 500ft)
(0 – 6 500ft)
Cirrostratus
Middle
Nimbostratus
Low
Stratus
Stratocumulus
Cumulus
Cumulonimbus
1107
1108
1109
1110
1111
1112
1113
Most clouds are confined within their level with a few notable exceptions:
(a) Altostratus is usually found in the middle level, but it often extends higher;
(b) Nimbostratus is almost invariably found in the middle level, but it usually extends into
the other two levels;
(c) Cumulus and Cumulonimbus usually have their bases in the low level, but their vertical
extent is often so great that their tops may reach into the middle and high levels.
38
1114
1115
1116
1117
1118
When the height of a particular cloud is known, the concept of levels may be of some help to the
observer in identifying this cloud. The genus can be determined by making a choice from among
the genera normally encountered in the level corresponding to its height.
1119
II.2.2
1120
Definitions of clouds
1121
1122
1123
[Editorial Comment: the definitions of this section from the previous edition of the ICA have been
moved to the tables below with very little change. New definitions are highlighted in yellow in
their respective table]
1124
11.2.2.1
1125
GENERA
1126
1127
1128
1129
1130
Consideration of the most typical forms of clouds leads to the recognition of ten genera. The
definitions of the genera given below do not cover all possible aspects, but are limited to a
description of the main types and of the essential characteristics necessary to distinguish a given
genus from genera having a somewhat similar appearance.
Cirrus
Cirrocumulus
Cirrostratus
Altocumulus
Altostratus
Nimbostratus
Detached clouds in the form of white, delicate filaments or white or mostly
white patches or narrow bands. These clouds have a fibrous (hair-like)
appearance, or a silky sheen, or both.
Thin, white patch, sheet or layer of cloud without shading, composed of
very small elements in the form of grains, ripples, etc., merged or separate,
and more or less regularly arranged; most of the elements have an apparent
width of less than one degree.
Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance,
totally or partly covering the sky, and generally producing halo phenomena.
White or grey, or both white and grey, patch, sheet or layer of cloud,
generally with shading, composed of laminae (a layer or layers), rounded
masses, rolls, etc., which are sometimes partly fibrous or diffuse and which
may or may not be merged; most of the regularly arranged small elements
usually have an apparent width of between one and five degrees.
Greyish or bluish cloud sheet or layer of striated (grooves or channels in
cloud formations, arranged parallel to the flow of the air), fibrous or
uniform appearance, totally or partly covering the sky, and having parts thin
enough to reveal the sun at least vaguely, as through ground glass or
frosted glass. Altostratus does not show halo phenomena.
Grey cloud layer, often dark, the appearance of which is made diffuse by
more or less continuously falling rain or snow, which in most cases reaches
the ground. It is thick enough throughout to blot out the sun.
Low, ragged clouds frequently occur below the layer, with which they may
39
or may not merge.
Stratocumulus
Stratus
Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud
which almost always has dark parts an arrangement of shapes closely fitted
together in a repeated pattern without gaps or overlapping (referred to as
tessellations), rounded masses, rolls, etc., which are non-fibrous (except for
virga) and which may or may not be merged; most of the regularly arranged
small elements have an apparent width of more than five degrees.
Generally grey cloud layer with a fairly uniform base, which may give drizzle,
snow or snow grains. When the sun is visible through the cloud, its outline is
clearly discernible. Stratus does not produce halo phenomena except,
possibly, at very low temperatures.
Sometimes Stratus appears in the form of ragged patches.
Cumulus
Detached clouds, generally dense and with sharp outlines, developing
vertically in the form of rising mounds, domes or towers, of which the
bulging upper part often resembles a cauliflower. The sunlit parts of these
clouds are mostly brilliant white; their base is relatively dark and nearly
horizontal.
Sometimes Cumulus is ragged.
Cumulonimbus
Heavy and dense cloud, with a considerable vertical extent, in the form of a
mountain or huge towers. At least part of its upper portion is usually
smooth, or fibrous or striated, and nearly always flattened; this part often
spreads out in the shape of an anvil or vast plume.
Under the base of this cloud which is often very dark, there are frequently
low ragged clouds either merged with it or not, and precipitation sometimes
in the form of virga.
1131
1132
II.2.2.2
1133
SPECIES
1134
1135
1136
1137
1138
1139
1140
Most cloud genera are subdivided into species due to peculiarities in the shape of clouds and
differences in their internal structure.
A cloud belonging to a certain genus, may bear the name of one species only; this means that the
species are mutually exclusive, On the other hand, certain species may be common to several
genera. When none of the definitions of the species are relevant to a genus, no species is
indicated.
Fibratus
Detached clouds or a thin cloud veil, consisting of nearly straight or more or
less irregularly curved filaments which do not terminate in hooks or tufts.
This term applies mainly to Cirrus and Cirrostratus
Uncinus
Cirrus often shaped like a comma, terminating at the top in a hook, or in a tuft,
40
the upper part of which is not in the form of a rounded protuberance.
Spissatus
Cirrus of sufficient optical thickness appearing greyish when viewed towards
the sun.
Castellanus
Clouds which present, in at least some portion of their upper part, cumuliform
protuberances in the form of turrets or towers (crenelated), some of which are
taller than they are wide, and are connected by a common base and seem to
be arranged in lines. The castellanus character is especially evident when the
clouds are seen from the side.
This term applies to Cirrus, Cirrocumulus, Altocumulus and Stratocumulus.
Floccus
A species in which each cloud unit is a small tuft with a cumuliform
appearance, the lower part of which is more or less ragged and often
accompanied by virga.
This term applies to Cirrus, Cirrocumulus and Altocumulus.
Stratiformis
Clouds spread out in an extensive horizontal sheet or layer.
This term applies to Altocumulus, Stratocumulus and, occasionally, to
Cirrocumulus.
Nebulosus
A cloud like a nebulous or ill-defined veil or layer of clouds showing no distinct
details. This term applies mainly to Cirrostratus and Stratus.
Lenticularis
Clouds having the shape of lenses or almonds, often very elongated and
usually with well defined outlines; they occasionally show irisations. Such
clouds appear most often in cloud formations of orographic origin, but may
also occur in regions without marked orography.
This term applies mainly to Cirrocumulus, Altocumulus and Stratocumulus.
Volutus
A long, typically low, horizontal, detached, tube-shaped cloud mass, often
appearing to roll slowly about a horizontal axis. The roll cloud, volutus, is a
soliton, not attached to other clouds and is an example of an undular bore.
This species applies mostly to Stratocumulus and rarely Altocumulus.
Fractus
Clouds in the form of irregular shreds, which have a clearly ragged appearance.
This term applies only to Stratus and Cumulus.
Humilis
Cumulus clouds of only a slight vertical extent; they generally appear flattened
Mediocris
Cumulus clouds of moderate vertical extent, the tops of which show fairly
small protuberances.
Congestus
Cumulus clouds which are markedly sprouting and are often of great vertical
extent; their bulging upper part frequently resembles a cauliflower.
41
Calvus
Cumulonimbus in which at least some protuberances of the upper part are
beginning to lose their cumuliform outlines but in which no cirriform parts can
be distinguished. Protuberances and sproutings tend to form a whitish mass,
with more or less vertical striations (Grooves or channels in cloud formations,
arranged parallel to the flow of air and therefore depicting the airflow).
Capillatus
Cumulonimbus characterized by the presence, mostly in its upper portion, of
distinct cirriform parts of clearly fibrous or striated structure, frequently having
the form of an anvil, a plume or a vast, more or less disorderly mass of hair.
Cumulonimbus capillatus is usually accompanied by a shower or by a
thunderstorm, often with squalls and sometimes with hail; it frequently
produces very well-defined virga.
1141
1142
1143
1144
TABLE OF SPECIES AND THE GENERA WITH WHICH THEY MOST FREQUENTLY
OCCUR
1145
1146
1147
1148
[Editorial Note: The following table from the 1975 edition did not map across well
from the original pdf file to the word equivalent. The version of the table proposed
for the new edition contains the same information, but simply has a different
appearance]
Genera
Ci
Cc
Cs
Ac
As
Ns
Sc
St
Cu
Cb
Species
fibratus (fib)
uncinus (unc)
spissatus (spi)
castellanus (cas)
floccus (flo)
stratiformis (str)
●
●
●
●
●
●
●
●
●
volutus (vol)
fractus (fra)
humilis (hum)
mediocris (med)
congestus (con)
calvus (cal)
capillatus (cap)
1149
1150
●
●
●
●
nebulosus (neb)
lenticularus (len)
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
42
1151
11.2.2.3
1152
VARIETIES
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
Varieties are the arrangements of the macroscopic elements and the degree of transparency of the
genera.
 A given cloud may bear the names of different varieties, which means that varieties are not
mutually exclusive.
 The exceptions to this are the varieties translucidus and opacus both of which are mutually
exclusive.
 On the other hand, certain varieties may be present in several genera.
 The fact that a number of varieties has been established does not imply that a specific cloud
must necessarily receive the name of one or more of those varieties
Intortus
Vertebratus
Cirrus, the filaments of which are very irregularly curved and often
seemingly entangled in an erratic and unpredictable (known as
capricious) manner.
Clouds, the elements of which are arranged in a manner suggestive of
vertebrae, ribs, or a fish skeleton.
This term applies mainly to Cirrus.
Arrangement of macroscopic elements
Undulatus
Clouds in patches, sheets or layers, showing undulations.
These undulations may be observed in fairly uniform cloud layers or in
clouds composed of elements, separate or merged. Sometimes a
double system of undulations is in evidence.
This term applies mainly to Cirrocumulus, Cirrostratus, Altocumulus,
Altostratus, Stratocumulus and Stratus.
Radiatus
Clouds showing broad parallel bands or arranged in parallel bands,
which, owing to the effect of perspective, seem to converge towards a
point on the horizon or, when the bands cross the whole sky, towards
two opposite points on the horizon, called "radiation point(s)".
This term applies mainly to Cirrus, Altocumulus, Altostratus,
Stratocumulus and Cumulus.
Lacunosus
Cloud patches, sheets or layers, usually rather thin, marked by more or
less regularly distributed round holes, many of them with fringed
edges. Cloud elements and clear spaces are often arranged in a
manner suggesting a net or a honeycomb.
This term applies mainly to Cirrocumulus and Altocumulus; it may also
apply, though very rarely, to Stratocumulus.
Duplicatus
Cloud patches, sheets or layers, at slightly different levels in two or
more layers, sometimes partly merged.
This term applies mainly to Cirrus, Cirrostratus, Altocumulus,
43
Altostratus and Stratocumulus.
Degree of transparency
Translucidus
Clouds in an extensive patch, sheet or layer, the greater part of which
is sufficiently translucent to reveal the position of the sun or moon.
This term applies to Altocumulus, Altostratus, Stratocumulus and
Stratus.
Perlucidus
Opacus
An extensive cloud patch, sheet or layer, with distinct but sometimes
very small spaces between the elements. The spaces allow the sun,
the moon, the blue of the sky or over-lying clouds to be seen. It may
also be observed in combination with the varieties translucidus or
opacus. This term applies to Altocumulus and Stratocumulus.
An extensive cloud patch, sheet or layer, the greater part of which is
sufficiently opaque to mask completely the sun or moon.
This term applies to Altocumulus, Altostratus, Stratocumulus and
Stratus.
1163
1164
1165
1166
1167
1168
1169
1170
1171
VARIETIES AND THE GENERA WITH WHICH THEY MOST FREQUENTLY OCCUR
[Editorial Note: The following table from the 1975 edition did not map across well from the
original pdf file to the word equivalent. The version of the table proposed for the new
edition contains the same information, but simply has a different appearance]
Genera
Ci
Cc
Cs
Ac
As
●
●
●
●
Ns
Sc
St
●
●
●
●
●
Cu
Varieties
intortus (in)
●
vertebratus (ve)
●
Undulatus (un)
radiatus (ra)
●
●
lacunosus (la)
●
●
●
●
●
translucidus (tr)
●
●
●
perlucidus (pe)
●
opacus (op)
●
dublicatus (du)
●
●
1172
1173
II.2.2.4
1174
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
●
●
●
●
●
●
Cb
44
Clouds sometimes have supplementary features attached to them or may be accompanied by
other usually smaller clouds, known as accessory clouds which are separate from their main body
or partly merged with it.
A given cloud may present simultaneously one or more supplementary features or accessory
clouds, which means that supplementary features and accessory clouds are not mutually exclusive.
Supplementary features
1175
1176
1177
1178
1179
1180
Incus
The upper portion of a Cumulonimbus spread out in the shape of an
anvil with a smooth, fibrous or striated appearance
Mamma
Hanging protuberances, like udders, on the under surface of a cloud.
Occurs mostly with Cirrus, Cirrocumulus, Altocumulus, Altostratus,
Stratocumulus and Cumulonimbus.
Virga
Vertical or inclined trails of precipitation (fallstreaks) attached to the
under surface of a cloud which· do not reach the Earth's surface.
Occurs mostly with Cirrocumulus, Altocumulus, Altostratus,
Nimbostratus, Stratocumulus, Cumulus and Cumulonimbus
Praecipitatio
Precipitation (rain, drizzle, snow, ice pellets, hail, etc.) falling from a
cloud and reaching the Earth's surface
Mostly encountered with Altostratus, Nimbostratus, Stratocumulus,
Stratus, Cumulus and Cumulonimbus.
Arcus
A dense, horizontal roll with more or less tattered edges, situated on
the lower front part of certain clouds and having, when extensive, the
appearance of a dark, menacing arch.
Occurs with Cumulonimbus and, less often, with Cumulus.
Tuba
Cloud column or inverted cloud cone, protruding from a cloud base; it
constitutes the cloudy manifestation of a more or less intense vortex.
Asperitas
Well-defined, wave-like structures in the underside of the cloud; more
chaotic and with less horizontal organization than the variety undulatus.
Asperitas is characterized 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 the cloud can lead to dramatic
visual effects.
Occurs mostly with Stratocumulus and Altocumulus.
Fluctus
A relatively short-lived wave formation, usually on the top surface of
the cloud, in the form of curls, or breaking waves (Kelvin-Helmholtz
waves).
Occurs mostly with Cirrus, Altocumulus, Stratocumulus, Stratus and
occasionally Cumulus.
Cavum
A well-defined generally circular (sometimes linear) hole in a thin layer
of supercooled water droplet cloud. Virga or wisps of cirrus typically fall
from the central part of the hole, which generally grows larger with
time. Cavum is typically a circular feature when viewed from directly
beneath, but may appear oval shaped when viewed from a distance.
When resulting directly from the interaction of an aircraft with the
cloud, it is generally linear (in the form of a dissipation trail). Virga
typically falls from the progressively widening dissipation trail.
45
Accessory clouds
Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.
1181
1182
1183
Murus
A localized, persistent, and often abrupt lowering of cloud from the
base of a cumulonimbus from which tuba (spouts) sometimes form.
Usually associated with a supercell or severe multi-cell storm; typically
develop in the rain-free portion of a Cumulonimbus and indicate an
area of strong updraft. Murus showing significant rotation and vertical
motion may result in the formation of tuba (spouts). Commonly known
as a 'wall cloud'
Cauda
A horizontal, tail shaped cloud (not a funnel) at low levels extending
from the main precipitation region of a supercell Cumulonimbus to the
murus (wall cloud). They are typically attached to the wall cloud and the
base of both is typically at the same height.
Cloud motion is away from the precipitation area and towards the
murus, with rapid upward motion often observed near the junction of
the tail and wall clouds. Commonly known as a 'tail cloud'.
Pileus
An accessory cloud of small horizontal extent, in the form of a cap or
hood above the top or attached to the upper part of a cumuliform cloud
that often penetrates it. Several pileus may fairly often be observed in
superposition.
Occurs principally with Cumulus and Cumulonimbus.
Velum
An accessory cloud veil of great horizontal extent, close above or
attached to the upper part of one or several cumuliform clouds which
often pierce it.
Occurs principally with Cumulus and Cumulonimbus.
Pannus
Ragged shreds sometimes constituting a continuous layer, situated
below another cloud and sometimes attached to it.
Occurs mostly with Altostratus, Nimbostratus, Cumulus and
Cumulonimbus
Flumen
Bands of low clouds associated with a supercell severe convective
storm, arranged parallel to the low-level winds and moving into or
towards the supercell.
These accessory clouds form on an inflow band into a supercell storm
along the pseudo-warm front. The cloud elements move towards the
updraft into the supercell, the base being at about the same height as
the updraft base. Note that Flumen are not attached to the Murus wall
cloud and the cloud base is higher than the wall cloud.
One particular type of inflow band cloud is the, so called, 'Beaver's Tail'.
This is distinguished by a relatively broad, flat appearance suggestive of
a beaver's tail.
46
1185
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS AND THE GENERA WITH
WHICH THEY MOST FREQUENTLY OCCUR
1186
1187
1188
1189
[Editorial Note: The following table from the 1975 edition did not map across well from
the original pdf file to the word equivalent. The version of the table proposed for the new
edition contains the same information (except for the inclusion of the new classifications) ,
but simply has a different appearance]
1184
Genera
Ci
Cc
Cs
Ac
As
Ns
Sc
St
Cu
Cb
Supplementary
Features and
Accessory Clouds
●
incus (inc)
●
●
●
virga (vir)
●
●
●
cavum (cav)
●
●
●
●
●
mamma (mam)
fluctus (flu)
asperitas (asp)
praecipitatio (pra)
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
arcus (arc)
●
●
●
murus (mur)
tuba (tub)
●
●
pileus (pil)
●
●
velum (vel)
●
●
●
●
pannus (pan)
1190
1191
●
●
cauda (cau)
●
flumen (flm)
●
47
1192
II.3 - DESCRIPTIONS OF CLOUDS
1193
II.3.1
1194
Cirrus (Ci)
1195
(Howard 1803)
1196
1197
II.3.1.1
1198
DEFINITION
1199
1200
1201
Detached clouds in the form of white, delicate filaments or white or mostly white patches or
narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both.
1202
1203
1204
II.3.1.2
1205
SPECIES
1206
1207
Cirrus fibratus (Ci fib) - BESSON 1921, CCH 1953
1208
1209
1210
1211
Nearly straight or more or less irregularly curved white filaments, which are always fine
and do not terminate in hooks or tufts. The filaments are, for the most part, distinct from
one another.
1212
1213
Cirrus uncinus (Ci unc) - MAZE 1889
1214
1215
1216
Cirrus without grey parts, often shaped like a comma, terminating at the top in a hook, or in a
tuft the upper part of which is not in the form of a protuberance.
1217
1218
Cirrus spissatus (Ci spi) - CCH 1953
1219
1220
1221
1222
Cirrus in patches, sufficiently dense to appear greyish when viewed towards the sun; it may
also veil the sun, obscure its outline or even hide it. Cirrus spissatus often originates from the
upper part of a Cumulonimbus.
1223
1224
Cirrus castellanus (Ci cas) - CCH 1953
1225
1226
1227
1228
1229
Fairly dense Cirrus in the form of small rounded and fibrous turrets or masses rising from a
common base, and sometimes having a crenelated (castle battlement) appearance. The
apparent width of the turret like protuberances may be smaller or greater than 1° when
observed at an angle of more than 30° above the horizon; distinct to Cirrocumulus castellanus
48
1230
1231
where the width is less than 1°. A width of 1° is approximately the width of the little finger at
arm’s length.
1232
1233
Cirrus floccus (Ci flo) - VINCENT 1903, CEN 1930
1234
1235
1236
1237
1238
1239
Cirrus in the form of more or less isolated, small, rounded tufts, often with trails. The
apparent width of the tufts may be smaller or greater than one degree when observed at
an angle of more than 30 degrees above the horizon; distinct to Cirrocumulus castellanus
where the width is less than 1°. A width of 1° is approximately the width of the little finger at
arm’s length.
1240
1241
1242
II.3.1.3
1243
1244
VARIETIES
1245
1246
Cirrus intortus (Ci in) - CCH 1953
1247
1248
Cirrus, the filaments of which are very irregularly curved and often seemingly
entangled in an irregular manner.
49
1249
1250
Cirrus radiatus (Ci ra) - CEN 1926
1251
1252
1253
1254
Cirrus arranged in parallel bands which, owing to the effect of perspective; appear to
converge towards one point or towards two opposite points of the horizon. These
bands are often partly composed of Cirrocumulus or Cirrostratus.
1255
1256
Cirrus vertebratus (Ci ve) - MAZE 1889, OSTHOFF
1905
1257
1258
Cirrus, the elements of which are arranged in a manner suggestive of vertebrae, ribs, or a fish skeleton.
1259
1260
Cirrus duplicatus (Ci du) - MAZE
1889
1261
1262
1263
Cirrus arranged in superposed layers at slightly different levels, sometimes merged in
places. Most Cirrus fibratus and Cirrus uncinus belong to this variety.
1264
II.3.1.4
1265
1266
1267
SUPPLEMENTARY
CLOUDS
FEATURES
AND
ACCESSORY
1268
1269
1270
1271
1272
Cirrus, usually of the species spissatus, sometimes shows mamma.
1273
CLOUDS FROM WHICH CIRRUS MAY FORM
II.3.1.5
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
Cirrus often evolves from:
 virga of Cirrocumulus (Ci cirrocumulogenitus); or


virga of Altocumulus (Ci altocumulogenitus), or
the upper part of a Cumulonimbus (Ci cumulonimbogenitus)
Cirrus clouds may also form as a result of the transformation of non-uniform
Cirrostratus by evaporation of its thinner parts (Ci cirrostratomutatus).
II.3.1.6
1285
1286
MAIN DIFFERENCES BETWEEN CIRRUS AND SIMILAR CLOUDS OF OTHER GENERA
1287
1288
1289
1290
1291
1292
Cirrus is distinguished from Cirrocumulus by:
 its mainly fibrous or silky appearance;
 the absence of small cloud elements in the form of grains, ripples, etc;
 the absence of a corona or irisation;
 possible shading in thick Cirrus and no shading in Cirrocumulus;
50
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324

the absence of virga
Cirrus is distinguished from Cirrostratus by:
 its discontinuous structure or, if they are in patches or bands, by their small horizontal extent
or the narrowness of their continuous parts;
 sometimes being thick enough to mask the sun or moon, Cirrostratus is always thin enough to
disk of the sun or moon;
 the absence of undulations;
 possible mamma, usually in the species spissatus
 the absence of a corona;
 only having, at best, a partial circular halo;
Cirrus near the horizon may be difficult to distinguish from Cirrostratus, owing to the
effect of perspective.
Cirrus is distinguished from Altocumulus by:
 its mainly fibrous or silky appearance;
 the absence of cloud elements in the form of laminae, rolls, etc;
 the usual absence of any shading and only partial shading in the species spissatus;
 the absence of undulations;
 the absence of virga;
 the absence of a corona
Thick Cirrus is distinguished from Altostratus patches by:
 smaller horizontal extent;
 mostly white appearance;
 the absence of precipitation or virga;
 the absence of undulations
II.3.1. 7
1325
PHYSICAL CONSTITUTION
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
Cirrus is composed almost exclusively of ice crystals. These crystals are in general very small;
together with their sparseness this accounts for the transparency of most Cirrus clouds.
Dense Cirrus patches or Cirrus in tufts may contain ice crystals large enough to acquire an appreciable
terminal velocity. Trails of considerable vertical extent fall from the base of Cirrus.
Infrequently, the ice crystals in the trails melt into small water droplets; the trails are then greyish, in
contrast with their usual white appearance. A rainbow may form.
The trails curve irregularly or slant as a result of wind shear and of the variation in size of the constituent
particles; consequently, Cirrus filaments near the horizon do not appear parallel to it.
Halo phenomena may occur; circular halos almost never show a complete ring, due to the narrowness
of the Cirrus clouds.
51
1340
1341
1342
II.3.1.8
EXPLANATORY REMARKS
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
Cirrus tufts with rounded tops often form in clear air. Fibrous trails may appear under the tufts;
the tops then gradually lose their roundness. Subsequently, the tufts may disappear completely;
the clouds are then in the form of filaments (species fibratus or uncinis).
1369
II.3.2
1370
Cirrocumulus (Cc) (HOWARD 1803; RENOU 1855)
1371
II.3.2.1
1372
DEFINITION
1373
1374
1375
1376
Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the
form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the
elements have an apparent width of less than one degree.
Cirrus in the form of filaments may develop also from dense Cirrus patches, from Altocumulus
castellanus and floccus and, occasionally at very low temperatures, from Cumulus congestus.
Cirrus may also form from aircraft condensation trails (contrails) that have persisted for at least 10
minutes (Ci homogenitus). No species, varieties or supplementary features are identified with Cirrus
homogenitus as new or recently formed aircraft condensation trails usually undergo a fairly rapid
state of change and may display a variety of transient shapes.
Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong
upper winds, take on the form of species and/or varieties of Cirrus. When this occurs, the Cirrus is
identified by relevant species and varieties followed by homomutatus.
Colour:
At all times of day, Cirrus not too close to the horizon is white, whiter than any other cloud in the same
part of the sky [Fig 26].
With the sun on the horizon it is whitish, while lower clouds may be tinted yellow or orange [Fig 27].
When the sun sinks below the horizon, Cirrus high in the sky is yellow, then pink, red and finally grey.
The colour sequence is reversed at dawn.
Cirrus near the horizon often takes a yellowish or orange tint [Fig 28] owing to the great thickness of
air traversed by the light in passing from the cloud to the observer. These tints are less conspicuous in
clouds in the low and middle levels.
1377
1378
II.3.2.2
1379
SPECIES
1380
Cirrocumulus stratiformis (Cc str) - CCH 1953
1381
1382
Cirrocumulus in the form of a relatively extensive sheet or layer, sometimes showing breaks.
52
1383
Cirrocumulus lenticularis (Cc len) - LEY 1894, CEN 1930
1384
1385
1386
1387
Lens or almond shaped patches of Cirrocumulus; often very elongated and usually with well-defined
outlines. The patches are more or less isolated, mostly smooth and are very white throughout.
Irisation is sometimes observed in these clouds.
1388
Cirrocumulus castellanus (Cc cas) - CCH 1953
1389
1390
1391
1392
1393
Cirrocumulus where some elements are vertically developed in the form of small turrets, rising from a
common horizontal base. The apparent width of the turrets is always less than one degree, when
observed at an angle of more than 30 degrees above the horizon. Castellanus develops due to
instability at that level.
1394
Cirrocumulus floccus (Cc flo) - VINCENT 1903, CCH 1953
1395
1396
1397
1398
1399
Very small cumuliform tufts, the lower parts of which are more or less ragged. The apparent width of
each tuft is always less than one degree, when observed at an angle of more than 30 degrees above the
horizon. Floccus develops due to instability at that level. Cirrocumulus fioccus sometimes evolves form
Cirrocumulus castellanus the base of the latter having dissipated.
1400
II.3.2.3
1401
VARIETIES
1402
1403
Cirrocumulus undulatus (Cc un) - CLAYTON 1896, CCH 1953
1404
Cirrocumulus showing one or two systems of undulations.
1405
1406
Cirrocumulus lacunosus (Cc Ia) - CCH 1953
1407
1408
1409
1410
Cirrocumulus in a patch, sheet or layer, showing small more or less regularly distributed round holes,
many of them with fringed edges. Cloud elements and clear spaces are often arranged like a net or a
honeycomb.
1411
II.3.2.4
1412
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
1413
1414
1415
1416
1417
1418
Small virga may be present, particularly falling from Cirrocumulus castellanus and floccus.
Cirrocumulus occasionally shows mamma.
Cavum is rarely observed and only when the Cirrocumulus consists of strongly supercooled water
droplets.
1419
II.3.2.5
53
1420
CLOUDS FROM WHICH CIRROCUMULUS MAY FORM
1421
1422
1423
1424
1425
1426
1427
1428
1429
Cirrocumulus often forms from the:
 transformation of Cirrus (Cc cirromutatus); or
 transformation of Cirrostratus (Cc cirrostratomutatus); or
 decrease in size of the elements of a patch, sheet or layer of Altocumulus (Cc
altocumulomutatus);
Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong
upper winds, transform into Cirrocumulus (Cc homomutatus).
1430
II.3.2.6
1431
MAIN DIFFERENCES BETWEEN CIRROCUMULUS AND SIMILAR CLOUDS OF OTHER GENERA
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
Cirrocumulus is distinguished from Cirrus by:
 having ripples or grains; any fibrous, silky or smooth portions do not collectively constitute its
greater part;
 possible virga;
 no shading;
 no partial circular halo or any other halo phenomena
Cirrocumulus is distinguished from Cirrostratus by:
 having ripples or grains; any fibrous, silky or smooth portions do not collectively constitute its
greater part;
1443
1444
1445
1446



usually being thin enough to show the disk of the sun or moon but sometimes thicker so that
only the position of the sun or moon is revealed;
possible irisation;
no halo phenomena;
1447
1448
1449
1450
1451
1452


possible virga;
possible mamma
1453
1454
1455
Cirrocumulus differs from Altocumulus by:
 most of its elements being very small (an apparent width of less than one° when observed at
an angle of more than 30° above the horizon);


not having any shading;
no halo (uncommon in Altocumulus and usually only a partial halo when of circular nature)
1456
II.3.2. 7
1457
PHYSICAL CONSTITUTION
1458
1459
1460
1461
Cirrocumulus is composed almost exclusively of ice crystals; strongly supercooled water droplets may
occur but are usually rapidly replaced by ice crystals.
A corona or irisation may sometimes be observed.
54
1462
1463
II.3.2.8
EXPLANATORY REMARKS
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
Lens or almonds shaped Cirrocumulus elements may be produced by local orographic lifting of a layer
of moist air.
In middle and high latitudes, Cirrocumulus is usually observed with Cirrus and/or Cirrostratus.
In low latitudes, Cirrocumulus is less often accompanied by Cirrus or Cirrostratus.
A cloud is not identified as Cirrocumulus if it consists of a patch of incompletely developed small
elements, such as often observed on the edges of a patch or sheet of Altocumulus or those sometimes
present in separate patches at the same level as Altocumulus.
In case of doubt, a cloud should be identified as Cirrocumulus only when it has evolved from, or is
obviously connected with Cirrus or Cirrostratus.
1474
1475
1476
1477
II.3.3
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
DEFINITION
Cirrostratus (Cs) (HOWARD 1803; RENOU 1855)
II.3.3.1
Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly
covering the sky, often producing halo phenomena.
II.3.3.2
SPECIES
Cirrostratus fibratus (Cs fib) - BESSON 1921, CCH 1953
A fibrous veil of Cirrostratus in which thin striations can be observed. Cirrostratus fibratus may develop
from Cirrus fibratus or less likely, Cirrus spissatus.
Cirrostratus nebulosus (Cs neb) - CLAYDEN 1905
A nebulous veil of Cirrostratus with no distinct detail. Sometimes the veil is so light that it is barely visible;
it may also be relatively dense and easily visible.
II.3.3.3
VARIETIES
Cirrostratus duplicatus (Cs du) - MAZE 1889, DE QUERVAIN 1908, CCH 1953
Cirrostratus arranged in superposed sheets or layers, at slightly different levels, sometimes partly merged.
Cirrostratus undulatus (Cs un) - CCH 1953
Cirrostratus showing undulations.
II.3.3.4
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
None
II.3.3.5
CLOUDS FROM WHICH CIRROSTRATUS MAY FORM
55
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
Cirrostratus may form by:
● merging of elements of Cirrus (Cs cirromutatus);
● merging of elements of Cirrocumulus (Cs cirrocumulomutatus);
● ice crystals falling from Cirrocumulus (Cs cirrocumulogenitus);
● the thinning of Altostratus (Cs altostratomutatus);
● the spreading out of the anvil of a Cumulonimbus (Cs cumulonimbogenitus)
Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong upper
winds, transform into Cirrostratus (Cs homomutatus). Occasionally, multiple persistent contrails (Ci
homogenitus) may spread and give almost complete sky coverage of Cirrostratus.
MAIN DIFFERENCES BETWEEN CIRROSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA
Cirrostratus is distinguished from Cirrus by:
● being in the form of a veil, usually of great horizontal extent; when seen in patches it is usually
forming or dissipating;
● having no distinct details such as filaments and undulations when of the species nebulosus;
● always being thin enough to show the disk of the sun or moon;
● displaying numerous halo phenomena and when of the circular form, the halos are usually
complete circles;
● possibile corona;
● no mamma.
Cirrostratus is distinguished from Cirrocumulus by:
● the absence of very fine elements in the form of grains;
● possibility of being partly shaded;
● no irisation;
● halo phenomena;
● no mamma
● no virga;
Cirrostratus is distinguished from Altocumulus by:
● the absence of rounded masses or turrets;
● never being completely shaded;
● no irisation;
● no mamma;
● no virga;
Cirrostratus is distinguished from Altostratus by:
● being thin enough to show the disk of the sun or moon, except when the sun or moon is low on
the horizon; Altostratus translucidus will reveal the position of the sun or moon; Altostratus
opacus will obscure the sun or moon;
● being partly shaded or having no shading; Altostratus translucidus is partly shaded; Altostratus
opacus is shaded throughout;
● usual presence of halo phenomena;
● no mamma,
● no precipitation;
● no virga
56
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
Cirrostratus can be confused with very thin Stratus that, at angular distances of less than 45° from the
sun, may appear very white. Cirrostratus:
● is whitish throughout;
● may have a fibrous appearance;
● often displays halo phenomena; Stratus only displays halo phenomena on rare occasions when
it consist of small ice particles
Cirrostratus can be confused for a veil of haze and is difficult to discern through haze. Haze is opalescent or
has a dirty yellowish to brownish colour. Cirrostratus is greyish in areas if there is partial shading; else it is
white throughout.
Il.3 .3.7
PHYSICAL CONSTITUTION
Cirrostratus is a cloud of moderate vertical extent and is composed of a relatively sparse number of mainly
small ice crystals. These factors combine to account for the transparency of this cloud through which the
disk (outline) of the sun is visible, at least when the sun is not too close to the horizon.
Sometimes Cirrostratus ice crystals are large enough to acquire an appreciable terminal velocity and trailing
filaments form. This gives these Cirrostratus clouds a fibrous appearance.
Halo phenomena are often observed in thin Cirrostratus; sometimes the veil of Cirrostratus is so thin that a
halo provides the only indication of its presence.
Il.3.3.8
EXPLANATORY REMARKS
Cirrostratus, not completely covering the sky usually has an irregular border fringed with Cirrus; or may
infrequently be straight edged and clear cut
Cirrostratus is never thick enough to prevent objects on the ground from casting shadows, at least when
the sun is high above the horizon. When the sun is low (less than about 30°) the relatively longer light
path through a Cirrostratus veil may reduce the light intensity so much that shadows do not exist.
The remarks about the colours of Cirrus are, to a great extent, also valid for Cirrostratus.
II.3.4
1596
1597
Altocumulus (Ac) (RENOU 1870)
1598
1599
II.3.4.1
1600
DEFINITION
1601
1602
1603
1604
1605
1606
1607
White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed
of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or
may not be merged; most of the regularly arranged small elements usually have an apparent width
between one and five degrees.
57
1608
II.3.4.2
1609
SPECIES
1610
1611
Altocumulus stratiformis (Ac str) - CCH 1953
1612
1613
An extensive sheet or layer of separate or merged elements. This is by far the most frequent species.
1614
Altocumulus lenticularis (Ac len) - LEY 1894, CEN 1930
1615
1616
1617
1618
1619
A patch in the shape of a lens or almond, often very elongated and usually with well defined outlines.
The patch can be small elements, closely grouped together; or one generally smooth unit with
pronounced shadings.
Irisation is occasionally visible.
1620
Altocumulus castellanus (Ac cas) - CCH 1953
1621
1622
1623
1624
1625
1626
Cumuliform turrets rising vertically from cloud elements connected by a common horizontal base. The
turrets:
 seem to be arranged in lines;
 give the cloud a crenelated (castle battlement) appearance;
 are sometimes taller than they are wide;
 are especially evident when the cloud is seen from the side.
1627
1628
1629
Altocumulus castellanus is an indicator of instability at that level; when it acquires considerable
vertical extent, it is classified as:
 Cumulus congestus altocumulogenitus if it is strongly sprouting or is of great vertical extent;
1630
1631
1632

Cumulonimbus altocumulogenitus if part of its upper portion is smooth, fibrous or striated, or
if the cloud produces lightning, thunder or showers of hail.
1633
Altocumulus floccus (Ac flo) - VINCENT 1903
1634
1635
1636
1637
1638
Small tufts of cumuliform appearance; the lower parts of the tufts are generally ragged and often
accompanied by fibrous trails (ice crystal virga). Altocumulus floccus is an indication of instability at
that level. Altocumulus floccus sometimes forms as a result of the dissipation of the base of
Altocumulus castellanus.
1639
Altocumulus volutus (Ac vol) - ___ 2016
1640
1641
A long, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal
axis. It usually occurs as a single line and seldom extends from horizon to horizon.
1642
1643
1644
1645
This species of Altocumulus is rare.
II.3.4.3
VARIETIES
1646
1647
Altocumulus translucidus (Ac tr) - CEN 1930
58
1648
1649
1650
A patch, sheet or layer of Altocumulus, the greater part is sufficiently translucent to reveal the
position of the sun or moon. This variety often occurs in the species stratiformis and lenticularis.
1651
Altocumulus perlucidus (Ac pe) - CCH 1953
1652
1653
1654
1655
A patch, sheet or layer of Altocumulus where the spaces between the elements allow the sun, the
moon, the blue of the sky or higher clouds to be seen. This variety often occurs in the species
stratiformis.
1656
Altocumulus opacus (Ac op) - CEN 1930
1657
1658
1659
1660
1661
1662
A patch, sheet or layer of Altocumulus, the greater part is sufficiently opaque to mask completely the
sun or moon. Most often, the base of this variety of Altocumulus is even and its apparent subdivision
into merged elements results from the irregularity of its upper surface. The base is sometimes
uneven and the elements then stand out in true relief, particularly with a low sun. The variety opacus
occurs fairly often in the species stratiformis.
1663
Altocumulus duplicatus (Ac du) - MAZE 1889, DE QUERVAIN 1908, CEN 1926
1664
1665
1666
Altocumulus of two or more superposed patches, sheets or layers, close together and sometimes
partly merged. This variety occurs in the species stratiformis and lenticularis.
1667
Altocumulus undulatus (Ac un) - CLAYTON 1896, CEN 1930
1668
1669
1670
Altocumulus composed of separate or merged elements, either elongated and broadly parallel, or
arranged in ranks and files having the appearance of two distinct systems of undulations.
1671
Altocumulus radiatus (Ac ra) - CEN 1926
1672
1673
1674
Altocumulus with approximately straight parallel bands. The bands appear to converge towards one
point, or two opposite points of the horizon.
1675
Altocumulus lacunosus (Ac la) - CCH 1953
1676
1677
1678
1679
Sheet, layer or patches of Altocumulus showing more or less regularly distributed round holes, many
of them with fringed edges. The cloud elements and clear spaces are often arranged in a net or
honeycomb manner. The details change rapidly.
1680
II.3.4.4
1681
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
1682
1683
1684
1685
Virga appears with most species of Altocumulus. Altocumulus floccus frequently dissipates, leaving
very white trails of ice crystal virga. If the Altocumulus floccus completely dissipates, the virga is
identified as Cirrus altocumulogenitus
1686
1687
1688
Mamma, cavum, fluctus and asperitas are sometimes visible in Altocumulus.
59
1689
1690
II.3.4.5
CLOUDS FROM WHICH ALTOCUMULUS MAY FORM
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
Altocumulus may form by:
 an increase in size or a thickening of at least some elements of a patch, sheet or layer of
Cirrocumulus (Ac cirrocumulomutatus);
 subdivision of a layer of Stratocumulus (Ac stratocumulomutatus);
 transformation of Altostratus (Ac altostratomutatus);
 transformation of Nimbostratus (Ac nimbostratomutatus);

spreading of the tops of Cumulus when they reach a middle level stable layer (Ac
cumulogenitus). The vertical development of the Cumulus may:
o cease at the stable layer, resulting in patches of Altocumulus spreading out from the
top of the Cumulus;
o temporarily stop at the stable layer and then resume their growth in places or
throughout, resulting in Altocumulus on the sides of the Cumulus;
Altocumulus may also be observed on or near the sides of Cumulonimbus. This Altocumulus often
forms while the mother-cloud is still in the Cumulus stage. Nevertheless, the Altocumulus is classified
as cumulonimbogenitus, not cumulogenitus.
1709
1710
1711
II.3.4.6
MAIN DIFFERENCES BETWEEN ALTOCUMULUS AND SIMILAR CLOUDS OF OTHER GENERA
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
Altocumulus is distinguished from Cirrus by:
 the absence of a mainly fibrous or silky appearance;
 the presence of cloud elements in the form of laminae, rolls, etc;
 possible shading throughout which never occurs with Cirrus;
 usually being partly shaded as against Cirrus usually being without shading;
 the possibility of cloud elements arranged in undulations;
 possibile virga;
 the usual presence of a corona
Altocumulus sometimes produces descending trails of fibrous appearance (ice
crystal virga). The cloud is identified as Altocumulus and not as Cirrus, as long as the
cloud has a part without a fibrous appearance or a silky sheen
Altocumulus is distinguished from Cirrocumulus by:

the presence of any shading, even if most of the regularly arranged cloud elements have an
apparent width of less than 1°;

most of the regularly arranged elements having an apparent width of between 1 and 5°, even
if the cloud has no shading;
possibile halo phenomena

Altocumulus is distinguished from Altostratus by:
 the presence of thin plates, rounded masses, rolls, etc;
60
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752




possibly being thin enough to see the disk of the sun or moon; Altostratus will only reveal the
position of the sun or moon or completely mask the sun or moon;
the absence of the accessory cloud pannus;
the absence of any precipitation; Altostratus may produce rain, snow or ice pellets;
the absence of halo phenomena or irisation
Altocumulus is distinguished from Stratocumulus by:
 most of the regularly arranged elements having, when observed at an angle of more than 30°
above the horizon, an apparent width between 1 and 5°;
 the absence of any precipitation; Stratocumulus can have weak falls of rain, snow or snow
pellets
Altocumulus floccus is distinguished from Cumulus fractus, humilis and mediocris by:
 often having fibrous trails (virga) compared to the often flat base of the Cumulus;
 generally being smaller than the Cumulus
1753
1754
1755
II.3.4. 7
PHYSICAL CONSTITUTION
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
Altocumulus is invariably composed of water droplets. This is evident from the fairly low
transparency of the elements and that the elements show sharp outlines when separate. When the
temperature is very low, ice crystals may form. If the droplets then evaporate, the cloud becomes
entirely an ice cloud and its elements cease to present sharp outlines. The formation of ice crystals
may take place in all species of Altocumulus; it occurs most frequently in Altocumulus castellanus and
floccus.
1769
1770
II.3.4.8
EXPLANATORY REMARKS
A corona or irisation is often observed in thin parts of Altocumulus.
The halo phenomena parhelia or luminous pillars are sometimes seen in Altocumulus, indicating the
presence of tabular-shaped ice crystals.
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
During the initial stages of its formation, Altocumulus is frequently a fairly smooth cloud of moderate
horizontal extent. This cloud then subdivides into more or less regularly arranged small elements, in
the form of laminae or tessellations.
1783
Altostratus (As) (RENOU 1877)
Altocumulus in the shape of lenses or almonds often forms in clear air as a result of local orographic
lifting of a layer of moist air.
Altocumulus frequently occurs at different levels in the same sky and is in many instances associated
with Altostratus. In this case, the air is often hazy immediately below the sheets or layers of
Altocumulus or between the elements constituting them.
II.3.5
1784
1785
II.3.5.1
61
1786
DEFINITION
1787
1788
1789
Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly
covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground
glass. Altostratus does not show halo phenomena.
1790
1791
II.3.5.2
1792
SPECIES
1793
1794
Altostratus is not subdivided into species owing to the uniformity of its appearance and general
structure.
1795
1796
1797
II.3.5.3
1798
VARIETIES
1799
1800
Altostratus translucidus (As tr) -CEN 1926
1801
1802
Altostratus, the greater part of which is sufficiently translucent to reveal the position of the sun
or moon.
1803
1804
Altostratus opacus (As op) - BESSON 1921
1805
Altostratus, the greater part of which is sufficiently opaque to mask completely the sun or moon.
1806
1807
Altostratus duplicatus (As du) - MAZE 1889, DE QUERVAIN 1908, CEN 1926
1808
1809
Altostratus composed of two or more superposed layers, at slightly different levels, sometimes partly
merged. This variety is rarely seen in Altostratus.
1810
1811
Altostratus undulatus (As un) - CLAYTON 1896, CEN 1930
1812
Altostratus showing undulations.
1813
1814
Altostratus radiatus (As ra) - CEN 1926, CCH 1953
1815
1816
Altostratus showing broad parallel bands that appear to converge towards one point or
towards two opposite points of the horizon. This variety is rarely seen in Altostratus.
1817
1818
II.3.5.4
1819
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
1820
1821
Virga and praecipitatio may be clearly visible. Pannus clouds may be observed under Altostratus.
62
1822
1823
Altostratus may show mamma.
1824
1825
II.3.5.5
1826
CLOUDS FROM WHICH ALTOSTRATUS MAY FORM
1827
1828
Altostratus may form:
1829
1830
1831
1832
1833
1834
1835



1836
II.3.5.6
1837
MAIN DIFFERENCES BETWEEN ALTOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA

by the thickening of a veil of Cirrostratus (As cirrostratomutatus);
sometimes by the thinning of a layer of Nimbostratus (As nimbostratomutatus)
from an Altocumulus layer when widespread ice crystal virga falls from the latter (As
altocumulogenitus);
sometimes, particularly in the tropics, by the spreading out of the middle or upper part of
Cumulonimbus (As cumulonimbogenitus).
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
Altostratus breaking up into patches is distinguished from Cirrus spissatus (dense) by:




having greater horizontal extent;
being predominently grey;
the possibility of precipitation in the form of rain, snow or ice pellets;
the possibility of coronae
High Altostratus translucidus (thin) is distinguished from Cirrostratus by:



Altostratus preventing objects on the ground from casting shadows;
the possibility of the sun having a ground glass effect (blurred outline of the
sun);
the absence of any halo phenomena
Altostratus sometimes has gaps or breaks where the Altostratus could be confused for a
similar layer of Altocumulus (Image 18) or Stratocumulus. Altostratus has a more uniform
appearance.
1854
1855
1856
1857
1858
1859
1860
A low layer of Altostratus opacus (thick) is distinguished from similar Nimbostratus by:




the presence of thinner parts through which the sun is, or could be, vaguely revealed;
being a lighter shade of grey;
having a less uniform base than the Nimbostratus;
possible mamma with the Altostratus
63
1861
1862
1863
Visually distinguishing Nimbostratus from Altostratus on moonless nights is difficult, if possible at
all. When rain, snow or snow pellets reach the Earth’s surface, the cloud is identified as
Nimbostratus.
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
Altostratus translucidus (thin) is distinguished from Stratus by:


the sun having a ground glass effect (blurred outline of the sun);
always having some shading and not being white, as thin Stratus may be when observed
toward the sun
Altostratus opacus (thick) is distinguished from Stratus nebulosus opacus (thick) by:


the layer being thin enough somewhere to reveal the position of the sun;
the type and intensity of precipitation. Snow is common to both clouds but will be of a greater
intensity with Altostratus. Rain and ice pellets are unique to Altostratus; weak drizzle and
snow grains are unique to Stratus;
 possible mamma
In the absence of thin parts near the sun, precipitation or mamma, it can be difficult to distinguish
between these two clouds. A continuous watch will assist, as Stratus opacus usually does not
occur in combination with other cloud genera. Altostratus rarely forms without the presence of
high level clouds.
1883
1884
1885
II.3.5.7
1886
1887
PHYSICAL CONSTITUTION
1888
1889
Altostratus is generally a layer of great horizontal extent [several tens or hundreds of kilometres] and
fairly considerable vertical extent [several hundreds or thousands of metres].
1890
1891
Alltostratus is composed of water droplets and ice crystals. In the most complete case [Figure 1], three
superposed parts can be distinguished:
1892(a)
1893(b)
1894
1895
1896
1897
an upper part, composed wholly or mainly of ice crystals;
a middle part, composed of a mixture of ice crystals, snow crystals or snowflakes and supercooled
water droplets;
(c) a lower part, composed wholly or mainly of ordinary or supercooled water droplets or
drops
Sometimes the cloud may consist of only two parts, either:
1898
1899
1900
 an upper part like (a) and a lower part like (c) [Figure 2]; or
 an upper part like (b) and a lower part like (c) [Figure 3]
Less frequently, the entire cloud may also be like (a) [Figure 4] or like (b) [Figure 5] alone.
1901
64
1902
1903
1904
The constituent particles in the lower part of Altostratus are so numerous that the outline
of the sun or moon is always blurred. Surface observers w i l l never see halo phenomena. In
the thickest parts, the position of the sun or moon may be completely concealed.
1905
1906
1907
Raindrops or snowflakes are often present in Altostratus and below its base. When
precipitation reaches the ground, it is generally of the "continuous" type and in the form
of rain, snow or ice pellets.
1908
1909
II.3.5.8
1910
EXPLANATORY REMARKS
1911
1912
1913
1914
The base of Altostratus occasionally shows a mamma-like or ragged appearance due to rain
or snow virga. Isolated virga are clearly seen when rain, before evaporating, falls farther in
some places than in others.
1915
1916
1917
1918
1919
1920
Precipitation sometimes makes it difficult to distinguish a cloud base; particularly when uniformly
falling snow completely evaporates before reaching the ground. If, however, snow melts rapidly
into rain, an apparent base may be observed at the melting level, as the visibility through rain is
greater than through snow. This "base" is very clearly visible when the rain layer is thin, which is
the case for instance if the raindrops quickly evaporate; it may be completely obscured when the
rain layer is thick.
1921
1922
Pannus clouds may be present. They;

1923
1924
1925
1926
1927
1928
occur under the Altostratus in the lower turbulent layers when these are moistened by
evaporation from precipitation;
 show a tendency to form near the 0° C level where the cooling of the air by melting snow
increases the instability of the layer underneath;
In the initial stage of formation, pannus clouds are small, sparse and well separated. They usually
occur at a considerable distance below the under surface of the Altostratus;
1929
1930
1931
Later, with a thickening Altostratus and a lowering of its base, this distance is greatly reduced. At
the same time, the pannus clouds increase in size and number and may merge into a quasicontinuous layer.
1932
1933
II.3.6
1934
Nimbostratus (Ns) (CEN 1930)
1935
1936
II.3.6.1
1937
DEFINITION
1938
65
1939
1940
1941
Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less
continuously falling rain or snow, which in most cases reaches the ground. It is thick enough
throughout to blot out the sun.
1942
Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
1943
1944
II.3.6.2
1945
SPECIES
1946
No species are distinguished in Nimbostratus.
1947
II.3.6.3
1948
VARIETIES
1949
Nimbostratus has no varieties.
1950
1951
II.3.6.4
1952
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
1953
1954
1955
Virga and praecipitatio in the form of rain, snow or ice pellets are the supplementary features
associated with Nimbostratus.
1956
The accessory cloud pannus is frequently observed under Nimbostratus.
1957
1958
II.3.6.5
1959
CLOUDS FROM WHICH NIMBOSTRATUS MAY FORM
1960
1961
1962
1963
1964
1965
1966
1967
1968
Nimbostratus develops:





usually from thickening Altostratus (Ns altostratomutatus);
rarely from the thickening of a layer of Stratocumulus (Ns stratocumulomutatus);
rarely from the thickening of a layer of Altocumulus (Ns altocumulomutatus);
sometimes by the spreading out of Cumulonimbus (Ns cumulonimbogenitus);
very rarely, when these clouds produce rain, by the spreading out of Cumulus congestus (Ns
cumulogenitus).
1969
II.3.6.6
1970
MAIN DIFFERENCES BETWEEN NIMBOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA
1971
66
1972
Nimbostratus is distinguished from Altostratus opacus (thick) by:
1973
1974
1975
1976
1977
1978


1979
1980


generally having a darker grey colour than Altostratus;
being thick enough throughout to hide the sun or moon; only the thicker parts of Altostratus
opacus hide the sun or moon. On moonless nights there may be doubt regarding the
identification of Nimbostratus or Altostratus. The cloud is identified as Nimbostratus when
precipitation (rain, snow or ice pellets) reaches the ground;
When discernable, usually having a lower base than Altostratus;
sometimes having the appearance of being illuminated from the inside; this feature is never
found in Altostratus;
1981
1982
Nimbostratus is distinguished from Altocumulus stratiformis opacus (thick layer):
1983

by falls of rain, snow or ice pellets; no precipitation falls from Altocumulus;
1984
1985
1986

having no features and usually no discernible base; Altocumulus has clearly defined elements
and a distinct lower surface;
1987
Nimbostratus is distinguished from Stratocumulus stratiformis opacus (thick layer) by:
1988
1989

having no features and usually no discernible base; Stratocumulus has clearly defined elements
(merged or separated) and a distinct lower surface
1990
1991
1992

falls of rain, snow or ice pellets; Stratocumulus has infrequent weak falls of rain, snow or
snow pellets
1993
Nimbostratus is distinguished from Stratus nebulosus opacus (thick layer without distinct detail):
1994

falls of rain, snow or ice pellets; Stratus has weak falls of drizzle, snow or snow grains ;.
1995
1996
1997

thick Stratus usually develops in the absence of other low or middle level clouds; Nimbostratus
usually evolves from an overcast layer of Altostratus
1998
1999
When the observer is beneath a cloud having the appearance of Nimbostratus, but accompanied by
lightning, thunder or hail, the cloud is classified as Cumulonimbus.
2000
2001
II .3.6. 7
2002
PHYSICAL CONSTITUTION
2003
2004
2005
2006
2007
2008
2009
Nimbostratus generally covers a wide area and is of great vertical extent. It is composed of water
droplets (sometimes supercooled) and raindrops, of snow crystals and snowflakes, or of a mixture of
these liquid and solid particles. The high concentration of particles and the great vertical extent of the
cloud prevent direct sunlight from being observed through it. The cloud produces rain, snow or ice
pellets which, however, do not necessarily reach the ground.
67
2010
Il.3.6.8
2011
EXPLANATORY REMARKS
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
Nimbostratus usually develops from thickening Altostratus, the base of which gradually lowers.
When the cloud becomes thick enough throughout to mask the sun, it is classified as Nimbostratus.
Nimbostratus usually appears as if illuminated from inside. This is a result of the absence of small
cloud droplets in its lower parts, whereby more light penetrates from above than in the case of nonprecipitating clouds of the same depth. The small cloud droplets in the lower parts of the cloud are
swept out by precipitation or they evaporate owing to the presence of colder raindrops or
snowflakes in the cloud.
Although Nimbostratus generally has no clear under surface, an apparent base is sometimes
discernible. This "base" is situated at the level where the snow melts into rain and is due to the poorer
visibility in snow than in rain. The melting level can be seen only when it is sufficiently low and when
the precipitation is not too heavy.
The under surface of Nimbostratus is often partially or totally hidden by pannus clouds resulting
from turbulence in the layers under its base, which are moistened by partial evaporation of
precipitation. At first, these pannus clouds consist of separate units; they may later merge into a
continuous layer extending up to the Nimbostratus. When the pannus covers a large expanse of the
sky, care should be exercised in order not to confuse it with the under surface of Nimbostratus.
Although pannus clouds have a tendency to dissipate, chiefly by the coalescence of their small
particles with raindrops or snowflakes falling through them, they continue to reform. In heavy
precipitation, however, the pannus particles are swept out faster than they can be replaced and the
pannus clouds disappear.
In the tropics, particularly during short lulls in the rainfall, Nimbostratus can be seen breaking up into
several different cloud layers, which rapidly merge again. The clouds then often show a very
characteristic livid colour with variations of luminance, probably due to internal gaps.
2037
II.3.7
2038
Stratocumulus (Sc) (KAEMTZ 1841)
2039
2040
II.3.7.1
2041
DEFINITION
2042
2043
2044
2045
2046
2047
Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has
dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for
virga) and which may or may not be merged; most of the regularly arranged small elements have an
apparent width of more than five degrees.
2048
2049
II.3.7.2
2050
SPECIES
2051
2052
Stratocumulus stratiformis (Sc str) - CCH 1953
68
2053
2054
2055
Rolls or large rounded masses arranged in an extended sheet or layer. The elements are more or less
flattened. This species is the most common.
2056
2057
Stratocumulus lenticularis (Sc len) - LEY 1894, CEN 1930
2058
2059
2060
2061
2062
2063
A patch of Stratocumulus, in the shape of a lens or almond, often very elongated and usually with well
defined outlines. The patch is either composed of small elements (an apparent width greater than 5°
degrees when observed at an angle of more than 30° above the horizon), closely grouped together, or
consists of one more or less smooth and usually dark unit. Irisation is possible.
This species of Stratocumulus is fairly rare.
2064
Stratocumulus castellanus (Sc cas) - CCH 1953
2065
2066
2067
2068
2069
2070
Cumuliform turrets rising vertically from cloud elements connected by a common horizontal base. The
turrets:
 seem to be arranged in lines;
 give the cloud a crenelated (castle battlement) appearance;
 are sometimes taller than they are wide;
 are especially evident when the cloud is seen from the side.
2071
2072
2073
2074
Stratocumulus castellanus can grow to a considerable size and develop into:
 Cumulus congestus stratocumulogenitus if it is strongly sprouting or is of great vertical extent;
 Cumulonimbus stratocumulogenitus if part of its upper portion is smooth, fibrous or striated,
or if the cloud produces lightning, thunder or showers of hail.
2075
2076
Stratocumulus volutus (Sc vol) - ??? 2016
2077
2078
A long, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal
axis. They usually occur singularly but are occasionally observed in successive lines of clouds.
2079
2080
This species of Stratocumulus is rare.
2081
Stratocumulus floccus (Sc flo) - ??? 2016
2082
2083
2084
2085
2086
Small tufts of cumuliform appearance; the lower parts of the tufts are generally ragged and, at very
low temperatures, accompanied by fibrous trails (ice crystal virga). Stratocumulus floccus is an
indication of instability at that level. Stratocumulus floccus often forms as a result of the dissipation of
the base of Stratocumulus castellanus.
2087
2088
II.3.7.3
2089
VARIETIES
2090
2091
Stratocumulus translucidus (Sc tr) - CEN 1930
2092
2093
2094
A patch, sheet or layer of Stratocumulus that is nowhere very dense. The greater part of the cloud is
sufficiently translucent to reveal the position of the sun or moon; the cloud may even allow the blue of
the sky to be faintly distinguished where its elements join.
69
2095
2096
Stratocumulus perlucidus (Sc pe) - CCH 1953
2097
2098
2099
2100
A patch, sheet or layer of Stratocumulus where the spaces between the elements allow the sun, the
moon, the blue of the sky or higher clouds to be seen.
2101
Stratocumulus opacus (Sc op) - CEN 1930
2102
2103
2104
2105
2106
2107
Dense Stratocumulus composed of a continuous or nearly continuous sheet or layer of large dark rolls
or rounded masses, most of which are sufficiently opaque to hide the sun or moon. The base of
Stratocumulus opacus is sometimes even, and its apparent subdivision into merged elements results
from the irregularity of its upper surface. More often, however, the under surface is uneven, and the
elements stand out in true relief.
2108
Stratocumulus duplicatus (Sc du) - CCH 1953
2109
2110
Stratocumulus comprising two or more broadly horizontal superposed patches, sheets or layers, close
together, sometimes partly merged. This variety occurs in the species stratiformis and lenticularis.
2111
2112
Stratocumulus undulatus (Sc un) - CLAYTON 1896, CEN 1930
2113
2114
2115
2116
A layer composed of fairly large and often grey elements, arranged in a system of nearly
parallel lines. A double system of undulations is sometime visible in the form of
transverse lines that cross the main system. The elements then appear to be arranged in
“rank and file”.
2117
2118
Stratocumulus undulatus occurs in the species stratiformis.
2119
Stratocumulus radiatus (Sc ra) - CEN 1926
2120
2121
2122
2123
2124
Stratocumulus showing broad, nearly parallel bands that, owing to the effect of perspective, appear to
converge towards a point or towards opposite points of the horizon. This variety looks similar to
Cumulus arranged in files ("cloud streets"), the difference being the Cumulus is individual cells
arranged in files. Stratocumulus radiatus occurs in the species stratiformis.
2125
2126
Stratocumulus lacunosus (Sc la) - CCH 1953
2127
2128
2129
2130
Stratocumulus, in a sheet or layer or in patches, with more or less regularly distributed round holes,
many of them with fringed edges. The cloud elements and clear spaces are often arranged in a net
or honeycomb manner. The details change rapidly.
2131
II.3.7.4
2132
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
2133
2134
2135
2136
Stratocumulus may show asperitas and fluctus and at very low temperatures cavum.
Stratocumulus may also how mamma; its under surface then presents an accentuated relief in the
form of udders or inverted mounds that even appear to be on the point of detaching themselves from
70
2137
2138
2139
2140
2141
2142
2143
the cloud.
Stratocumulus mamma can be confused with Altostratus opacus with a mammilated (wrinkled)
appearance.
Virga may also occur under Stratocumulus, especially at very low surface temperatures.
The feature praecipitatio rarely occurs. When it does, the precipitation (rain, snow or snow pellets) is
only of weak intensity.
2144
2145
II.3.7.5
CLOUDS FROM WHICH STRATOCUMULUS MAY FORM
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
Stratocumulus may form:
 from Altocumulus when the small elements grow to a sufficient size (Sc altocumulomutatus);
 sometimes near the base of Altostratus, as a result of turbulence or convection in the layers
moistened by evaporating precipitation (Sc altostratogenitus)
 more often near the base of Nimbostratus, as a result of turbulence or convection in the
layers moistened by evaporating precipitation (Sc nimbostratogenitus);
 from the thinning of Nimbostratus, signifying the end of a rain event (Sc
nimbostratomutatus);
 from the of the lifting of a layer of Stratus (Sc stratomutatus);
 from the convective or turbulent transformation of Stratus, with or without change of height
(Sc stratomutatus);
 by spreading of the tops of Cumulus when they reach a stable layer (Sc cumulogenitus);
usually the mid to upper part of the Cumulus gradually widens in proximity to the stable
layer. The vertical development of Cumulus may:
o cease at the stable layer, resulting in patches of Stratocumulus spreading out from the
top of the Cumulus; often the Cumulus dissipates completely from the base upwards;
o temporarily stop at the stable layer and then resume their growth in places or
throughout, resulting in Stratocumulus on the side/s of the Cumulus;




from Cumulus (Sc cumulogenitus) when towers lean and spread or are detached and spread due
to strong wind shear;
from Cumulus (Sc cumulogenitus) when convection ceases in the late afternoon and evening
and the domed tops of the Cumulus clouds flatten;
by the spreading out of Cumulonimbus (Sc cumulonimbogenitus). Stratocumulus may be
observed on or near the sides of Cumulonimbus and often forms while the Cumulonimbus is
still in the Cumulus stage. Nevertheless, the Stratocumulus is classified as
cumulonimbogenitus, not cumulogenitus;
by the spreading out of some of the lower parts of an existing Cumulonimbus (Sc
cumulonimbogenitus).
Cumulus or Cumulonimbus towers can also pass through a pre-existing layer of Stratocumulus
formed independently of them. When this occurs:
 the Cumulus or Cumulonimbus do not widen upward towards the Stratocumulus layer;
 a thinned or even a cleared zone frequently appears in the Stratocumulus around the
cumuliform towers.
2183
I.I.3.7.6
2184
2185
MAIN DIFFERENCES BETWEEN STRATOCUMULUS AND SIMILAR CLOUDS OF
OTHER GENERA
71
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
Stratocumulus can, in extremely cold weather, produce abundant ice crystal virga from which halo
phenomena may be observed. This Stratocumulus is distinguished from Cirrostratus nebulosus by:
 showing some evidence of elements, rounded masses, rolls, etc;

greater opacity than that of Cirrostratus
Stratocumulus is distinguished from Altocumulus by;
 most of the regularly arranged elements having, when observed at an angle of more than 30°
above the horizon, an apparent width greater than 5° (3 fingers at arm’s length);

possible precipitation in the form of weak falls of rain, or snow;
An extensive layer of Stratocumulus is distinguished from Altostratus by;
 having a less uniform base;



the presence of elements, rounded masses, rolls, etc;
appearing non-fibrous, except at extremely low temperatures; Altostratus often has a fibrous
appearance;
very weak and infrequent precipitation in the form of rain, snow or snow pellets, Altostratus
can have light, or occasionally moderate, falls of rain, snow or ice pellets;
Stratocumulus stratiformis opacus (thick extensive layer) is distinguished from Nimbostratus by;
 having a less uniform base;
 the presence of elements, rounded masses, rolls, etc;
 precipitation in the form of rain, snow or snow pellets being weak in intensity and infrequent
in occurrence; Nimbostratus usually has falls of rain, possible falls of snow or ice pellets that
can be up to heavy in intensity;
 possible asperitas or mamma;
 absence of pannus, a usual feature of Nimbostratus
Stratocumulus is distinguished from Stratus by;
 the presence of elements, rounded masses, rolls, etc;
 usual absence of a ragged structure, a feature of Stratus fractus; Stratocumulus floccus has a
ragged lower part with a cumuliform tuft at the top;
 weak falls of rain, snow or snow pellets; Stratus has falls of drizzle, snow or snow grains;
 possible mamma;
 possible virga; the low base of Stratus ensures precipitation reaches the ground;
 possible corona;
Stratocumulus is distinguished from Cumulus by;
 elements usually occurring in groups or patches and generally have flat tops;
 occasionally having tops in the form of domes or turrets that rise from merged bases;




possibily being shaded throughout, Cumulus is noted for its brilliant white sunlight parts;
the absence of arcus, tuba, pileus, velum and pannus;
precipitation falling uniformly in an intermittent or continuous nature; Cumulus is noted for
precipitation falling in the form of showers;
usual absence of any form of rainbow
Stratocumulus floccus may bear a strong resemblance to Cumulus fractus frayed by a fairly strong wind or
Cumulus mediocris with irregular bases on days of fresh to strong winds. Stratocumulus floccus is
72
2234
2235
2236
2237
2238
2239
2240
distinguished from these species of Cumulus by:

usually having a very ragged lower part; if there is evidence of a partial flat base it rapidly
dissipates;
 base unusually high compared to an expected base for Cumulus; Cumulus may even be
observed at a lower base
If there is doubt as to the identification, the cloud is identified as the appropriate Cumulus species.
2241
2242
2243
I.I.3.7.7
2244
PHYSICAL CONSTITUTION
2245
2246
2247
2248
2249
2250
2251
2252
Stratocumulus is composed of water droplets, sometimes accompanied by raindrops or, more rarely,
by snow pellets, snow crystals and snowflakes. Any ice crystals present are usually too sparse to give
the cloud a fibrous appearance. During extremely cold weather, Stratocumulus may produce
abundant ice crystal virga that may be accompanied by halo phenomena. When Stratocumulus is not
very thick, a corona or irisation is sometimes observed.
2253
2254
II.3.7.8
EXPLANATORY REMARKS
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
The appearance of Stratocumulus is similar to that of Altocumulus. Owing to its generally lower
height, the elements of Stratocumulus look larger and, at times, smoother than those of Altocumulus.
Stratocumulus elements are often arranged in lines or groups, showing a single or double system of
undulations. Elements may be more or less separate; however the cloud layer is more often
continuous, sometimes with gaps. The under surface of a continuous cloud layer is often uneven and
presents a relief in the form of creases, mamma, etc.
II.3.8
2265
2266
2267
Stratus (St) (HOWARD 1803; HILDEBRANDSSON AND ABERCROMBY 1887)
2268
2269
2270
II.3.8.1
2271
DEFINITION
2272
2273
2274
2275
2276
2277
Generally grey cloud layer with a fairly uniform base, which may give drizzle, snow or snow grains.
When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not
produce halo phenomena except, possibly, at very low temperatures.
Sometimes Stratus appears in the form of ragged patches.
73
2278
II.3.8.2
2279
SPECIES
2280
2281
Stratus nebulosus (St neb) - CLAYDEN 1905, CCH 1953
2282
2283
Nebulous, grey, fairly uniform layer of Stratus. This is the most common species.
2284
Stratus fractus (St fra) - CEN 1930, CCH 1953
2285
2286
2287
Stratus occurring in the form of irregular ragged shreds the outlines of which change continuously and
often rapidly.
2288
II.3.8.3
2289
VARIETIES
2290
2291
Stratus opacus (St op) – BESSON 1921, CCH 1953
2292
2293
2294
Patch, sheet or layer of Stratus, the major part of which is so opaque that it completely masks the
sun or moon. This is the most common variety.
2295
Stratus translucidus (St tr) – CEN 1926, CCH 1953
2296
2297
2298
Patch, sheet or layer of Stratus, the major part of which is sufficiently translucent to reveal the
outline of the sun or moon.
2299
Stratus undulatus (Stun) – CLAYTON 1896, CCH 1953
2300
2301
Patch, sheet or layer of Stratus showing undulations. This variety does not occur very often.
2302
2303
II.3.8.4
2304
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
2305
2306
2307
2308
The only supplementary feature of Stratus is praecipitatio (precipitation in the form of drizzle,
snow and snow grains).
2309
2310
II.3.8.5
2311
CLOUDS FROM WHICH STRATUS MAY FORM
2312
74
2313
2314
2315
Stratus may form from the transformation of Stratocumulus (St stratocumulomutatus) when it
thickens and lowers forming a fairly uniform base; or the base stays at the same height but loses its
relief or its subdivisions.
Precipitating Stratocumulus can thicken and lower and lose its relief or subdivisions. This is
not transformation to Stratus. If the precipitating Stratocumulus is devoid of relief or
subdivisions for an extended period of time, consideration must be given to the cloud having
transformed to Nimbostratus.
2316
2317
2318
2319
2320
2321
2322
Stratus often forms from the slow lifting of a fog layer, due to warming of the Earth's surface or an
increase in wind speed.
2323
2324
2325
2326
2327
Stratus fractus of wet weather (the conditions which generally exist during precipitation and a
short time before and after) is often produced by Altostratus, Nimbostratus or Cumulonimbus (St
fra altostratogenitus, St fra nimbostratogenitus or St fra cumulonimbogenitus); it may also result
from precipitating Cumulus (St fra cumulogenitus).
2328
2329
2330
II.3.8.6
2331
MAIN DIFFERENCES BETWEEN STRATUS AND SIMILAR CLOUDS OF OTHER GENERA
2332
2333
2334
Stratus can occasionally resemble Cirrus when it is frayed by the wind into the form of coarse fibres
(Stratus fractus). The fibres:
2335
2336


are not as white (except when viewed towards the sun) as Cirrus fibres;
are not as spread out as Cirrus fibres;
2337
2338

change in appearance rapidly; Cirrus is noted for slow change.
2339
Thin Stratus (translucidus) is distinguished from Cirrostratus by:
2340

not being so completely white except when viewed towards the sun;
2341
2342

possible coronae
2343
2344
2345
2346
2347
2348
2349
2350
Stratus is distinguished from Altostratus by;



the outline of the sun not being blurred; that is the disk or partial disk of the sun is
discernible;
the absence of shading in thin Stratus when viewed toward the sun; Altostratus always has
shading;
precipitation in the form of drizzle or snow grains; Altostratus has rain and ice pellets; snow
can occur with both clouds
75
2351
2352
2353
2354
2355
2356
2357
2358
2359
Stratus nebulosus opacus (thick) closely resembles Nimbostratus and may very easily be confused
with it. It is distinguished from Nimbostratus by;




having a more clearly defined and more uniform base than Nimbostratus;
having a "dry" appearance, contrasting to the “wet” appearance of Nimbostratus;
producing only weak falls of drizzle, snow or snow grains, whereas Nimbostratus nearly
always produces rain, snow or ice pellets;
not usually being preceded by other clouds of the low and middle levels; Nimbostratus nearly
always succeeds other clouds, usually of the middle level, or develops from a pre-existing
cloud
2360
2361
2362
Stratus translucidus (thin) is distinguished from Nimbostratus by the disk of the sun being
discernible, at least through its thinnest parts; Nimbostratus masks the sun or moon throughout.
2363
2364
2365
Stratus is distinguished from Stratocumulus by there being no evidence of elements, merged or
separate.
2366
2367
2368
2369
Stratus fractus is distinguished from Cumulus fractus in that it is less white and less dense. It also
has less vertical development, since it owes its formation mainly to turbulence, not heating of the air
near the earth’s surface.
2370
2371
2372
II.3.8. 7
PHYSICAL CONSTITUTION
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
Stratus is usually composed of small water droplets. When very thin, Stratus may produce a corona
around the sun or moon. At low temperatures, Stratus may consist of small ice particles. The ice
cloud is usually thin and on rare occasions, may produce halo phenomena.
Stratus, when dense or thick, often contains drizzle droplets and sometimes snow or snow
grains. It may then have a dark or even a threatening appearance.
Stratus with a low optical thickness, usually of the variety translucidus, often shows a smoky greyish
tint like that of fog when viewed at more than 90° from the sun,
2384
2385
2386
II.3.8.8
EXPLANATORY REMARKS
2387
2388
2389
2390
2391
2392
2393
2394
2395
Stratus forms under the combined effect of cooling in the lower layers of the troposphere and
turbulence due to the wind
Over land, the cooling may be a result of nighttime cooling which is particularly marked when the
sky is clear and the wind is weak, or by advection of relatively warm air over colder ground. Over
sea, the cooling is mainly due to advection of relatively warm air over colder water surface.
Stratus is sometimes observed as more or less joined cloud fragments with varying luminance. These
76
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
Stratus fractus clouds constitute a transitory stage during the formation or the dissipation of the
more common extensive uniform Stratus layer. The transitory stage is usually very short.
2411
II.3.9
Stratus fractus clouds may also form as accessory clouds (pannus) under Altostratus, Nimbostratus,
Cumulonimbus and precipitating Cumulus. They develop as a result of turbulence in the moistened
layers under these clouds.
Stratus may develop locally in the vicinity of large waterfalls as a consequence of water broken up
into spray by the falls. The Stratus will be classified by any appropriate species, variety and
supplementary feature, followed by cataractagenitus.
Stratus may develop locally over forests as a result of increased humidity due to evaporation and
evapotranspiration from the tree canopy. The Stratus will be classified by any appropriate species,
variety and supplementary feature followed by silvagenitus.
2412
2413
Cumulus (Cu)
2414
(HOWARD 1803)
2415
2416
2417
II.3.9.1
2418
DEFINITION
2419
2420
2421
2422
2423
2424
Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising
mounds, domes or towers, of which the bulging upper part often resembles a cauliflower. The sunlit
parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal.
Sometimes Cumulus is ragged.
2425
II.3.9.2
2426
SPECIES
2427
2428
Cumulus humilis (Cu hum) - VINCENT 1907
2429
2430
2431
Cumulus characterized by only a small vertical extent and appearing generally as if flattened. Cumulus
humilis clouds never produce precipitation.
2432
Cumulus mediocris (Cu med) - CCH 1953
2433
2434
2435
Cumulus of moderate vertical extent, with small protuberances and sproutings at their tops. Cumulus
mediocris generally produce no precipitation.
2436
Cumulus congestus (Cu con) - MAZE 1889
77
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
Strongly sprouting Cumulus with generally sharp outlines and often great vertical extent. The bulging
upper part of Cumulus congestus frequently resembles a cauliflower. Cumulus congestus may produce
precipitation in the form of showers of rain, snow or snow pellets. In the tropics they often release
abundant rain in the form of showers.
Cumulus congestus sometimes resemble narrow, very high towers. The tops of these towers may
detach themselves successively from the main body of the cloud. They are then carried away by the
wind and usually rapidly disintegrate, occasionally producing virga.
Cumulus congestus usually results from the development of Cumulus mediocris or infrequently,
Altocumulus castellanus or Stratocumulus castellanus.
Cumulus congestus often develops into Cumulonimbus; this transformation is:
2447
2448

visually revealed by the smooth appearance or by the fibrous or striated texture of its upper
portion;
2449
2450

apparent when lightning, thunder or showers of hail are observed.
2451
Cumulus fractus (Cu fra) – POEY, 1863, CCH 1953
2452
2453
2454
Small Cumulus with very ragged edges and with outlines continuously undergoing changes that are
often rapid.
2455
2456
II.3.9.3
VARIETIES
2457
2458
Cumulus radiatus (Cu ra) - CCH 1953
2459
2460
2461
2462
Cumulus arranged in lines nearly parallel to the wind direction (cloud streets) and usually of the
species mediocris. Due to perspective, these lines seem to converge towards a point or towards
opposite points of the horizon.
2463
2464
II.3.9.4
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
2465
2466
2467
2468
2469
Cumulus may show: pileus, velum, virga, praecipitatio (the precipitation occurs in the form of
showers), fluctus (occasionally in the species humilis), arcus (rarely), pannus (rarely) and tuba (very
rarely).
2470
2471
2472
II.3.9.5
CLOUDS FROM WHICH CUMULUS MAY FORM
2473
2474
2475
2476
2477
2478
2479
2480
Cumulus is often preceded by hazy spots in the sky out of which the clouds develop.
Cumulus may form:
 from the development of Altocumulus castellanus (Cu congestus altocumulogenitus);
 from the development of Stratocumulus castellanus (Cu congestus stratocumulogenitus);


by the transformation of Stratocumulus (Cu stratocumulomutatus); or
by the transformation of Stratus (Cu stratomutatus); this frequently occurs in the morning over
78
land.
2481
2482
2483
2484
2485
2486
Cumulus fractus of wet weather forms under Altostratus, Nimbostratus, Cumulonimbus or
precipitating Cumulus (Cu fra altostratogenitus, nimbostratogenitus, cumulonimbogenitus or
cumulogenitus).
2487
II.3.9.6
2488
MAIN DIFFERENCES BETWEEN CUMULUS AND SIMILAR CLOUDS OF OTHER GENERA
2489
2490
2491
2492
2493
2494
Cumulus is distinguished from most Altocumulus and Stratocumulus by being detached and domeshaped. When viewed from a distance, Cumulus clouds may appear merged, owing to the effect of
perspective. Care should be taken not to confuse distant Cumulus with Altocumulus or Stratocumulus.
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
 spread and form Stratocumulus cumulogenitus or Altocumulus cumulogenitus;
 penetrate and pass through existing layers of Stratocumulus or Altocumulus; or
 merge with Altostratus or Nimbostratus.
The Cumulus clouds that are spreading, penetrating or merging continue to be identified as Cumulus as
long as they remain detached from one another or they have relatively considerable vertical extent.
2517
2518
2519
2520
2521
2522
2523
2524
 continuous and rapid changes in the outlines of Cumulus fractus
If there is doubt as to the identification, the cloud is identified as the appropriate Cumulus species.
2525
II.3.9.7
2526
PHYSICAL CONSTITUTION
Cumulus tops may:
When an extensive precipitating Cumulus is directly above, it can be confused with Altostratus or
Nimbostratus or perhaps Stratocumulus stratiformis opacus (thick layer). If the precipitation is in the
form of showers the cloud is Cumulus. A continuous watch will also assist as Altostratus, Nimbostratus
and Stratocumulus stratiformis opacus are noted for their longevity; a feature not characteristic of
Cumulus cloud.
Cumulus congestus of great vertical extent is distinguished from Cumulonimbus by:
 the edges of the bulging upper parts being sharply defined and having no fibrous or striated
texture;
 no lightning, thunder or showers of hail.
Cumulus mediocris (with irregular bases and ragged in parts due to fresh to strong winds) and
Cumulus fractus (frayed by strong winds) are distinguished from Stratocumulus floccus by:
 part of the base being flat; Stratocumulus floccus usually has a very ragged lower part; if there
is evidence of a flat base it rapidly dissipates;
 the vertical structure of the cloud leaning strongly downwind;
Cumulus fractus is distinguished from Stratus fractus by:
 generally greater vertical extent;
 usually whiter and less transparent appearance;
 possible rounded or dome shaped tops, Stratus fractus always occurs as ragged shreds.
79
2527
2528
2529
2530
2531
2532
2533
Cumulus is composed mainly of water droplets. When of great vertical extent, Cumulus may release
precipitation in the form of showers of rain, snow or snow pellets.
Ice crystals may form in those parts of a Cumulus where the temperature is well below 0° C. The ice
crystals grow at the expense of evaporating super-cooled water droplets, transforming the cloud into
Cumulonimbus. In cold weather, when the temperature in the entire cloud is well below 0° C, this
process leads to the degeneration of the cloud into diffuse trails of snow.
2534
II.3.9.8
2535
EXPLANATORY REMARKS
2536
2537
Cumulus develops:
2538
2539


in convection currents resulting from heating near the Earth’s surface;
due to cooling or advection of cold air (instability) in higher layers;
2540
2541

from lifting of air layers where vertical expansion results in cooling;
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
The characteristics of Cumulus clouds depend essentially on their vertical extent; that is the vertical
distance between their base and the stable layer that has inhibited vertical development.
The degree of stability and thickness of the stable layer determine how effective this layer is at
inhibiting vertical development. When it is very stable, for example a strong inversion, the tops of
Cumulus clouds will spread out forming Stratocumulus cumulogenitus or Altocumulus cumulogenitus .
When it is stable but not very thick, the spreading out of the tops of Cumulus clouds may be only in
parts or momentary and then some tops may penetrate it.
Comments on vertical extent.
When Cumulus have:

great vertical extent (a low base and a high stable layer) the Cumulus is of the species
congestus. The vertical extent of Cumulus in tropical regions (when not under the influence of
a trade wind inversion) is generally much greater than elsewhere;
 moderate vertical extent (base and stable layer are reasonably close together) Cumulus
mediocris tops may spread out forming either Stratocumulus or Altocumulus;
 small vertical extent (base and stable layer are very close together) the Cumulus has a flattened
appearance (Cumulus humilis). They may even spread out in their entirety into Stratocumulus
or Altocumulus.
Cumulus may dissipate during the day as the surface air temperature rises and the Cumulus base rises
until its height exceeds, sometimes considerably, that of the stable layer.
When there is no vertical extent (stable layer is lower than the level where sufficient cooling had
occurred for condensation to take place) Cumulus may be present but only if there is a mechanism
forcing air to rise to reach the level where condensation could have occurred. Orographic lift is an
example of a forcing mechanism
Diurnal variation in Cumulus activity:
2568
2569
2570
2571
2572
2573

is generally pronounced over land. On clear mornings, with the sun rapidly heating the surface
of the ground, conditions are favourable for the formation of Cumulus. This formation may
begin early, when the lapse rate is steep and the relative humidity is high; it begins late, if it
occurs at all, when the lapse rate is small and the relative humidity is low. After having reached
a maximum, usually in mid-afternoon, the Cumulus activity decreases and finally the clouds
disappear in the late afternoon or early evening.
2574

Is so small over the open oceans that its existence is sometimes doubtful. When it exists,
80
maximum Cumulus activity appears to occur in the late hours of the night.
2575
2576
2577
2578
2579
2580
2581
2582
2583

near coasts has Cumulus forming over the land by day in connection with the sea breeze and
over the sea by night in connection with the land breeze.
Comments on illumination
When a well-developed Cumulus is:
 observed opposite the sun, the diffuse reflection of the sunlight falling on the surface of the
cloud reveals the relief of the protuberances by very pronounced differences in luminance;
 illuminated from the side, Cumulus shows strongly contrasted shading;

2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
illuminated from behind, the Cumulus appears relatively dark, with an extremely brilliant
border (every cloud has a sliver lining);
 against a background of cirriform (ice) clouds and away from the horizon, Cumulus appears a
little less white than the cirriform clouds and its margins appear grey, even when the Cumulus
is directly illuminated by the sun.
Whatever the illumination of the Cumulus may be, its base is generally grey.
2600
2601
2602
2603
Cumulus may develop locally in the vicinity of large waterfalls as a consequence of water broken up
into spray by the falls. The Cumulus will be classified by any appropriate species, variety and
supplementary feature, followed by cataractagenitus (e.g. Cumulus mediocris cataractagenitus).
2604
II.3.10
2605
Cumulonimbus (Cb) (WEILBACH 1880)
Cumulus may develop in convection initiated by heat from forest fires, wild-fires or from volcanic
eruption activity. The Cumulus will be classified by any appropriate species, variety and supplementary
feature, followed by flammagenitus (e.g. Cumulus congestus flammagenitus).
Cumulus may develop as a consequence of human activity such as forming in convection initiated
above power station cooling towers. When Cumulus is clearly observed to have originated as a
consequence of human activity it will be classified by any appropriate species, variety and any
supplementary features followed by the homogenitus (e.g. Cumulus mediocris homogenitus).
2606
2607
II.3.10.1
2608
DEFINITION
2609
2610
2611
2612
2613
Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers.
At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened;
this part often spreads out in the shape of an anvil or vast plume.
2614
2615
2616
Under the base of this cloud which is often very dark, there are frequently low ragged clouds either
merged with it or not, and precipitation sometimes in the form of virga.
2617
81
2618
II.3.10.2
2619
SPECIES
2620
2621
Cumulonimbus calvus (Cb cal) - CEN 1926
2622
2623
2624
2625
2626
Cumulonimbus where the sproutings of the upper parts are indistinct and flattened and have the
appearance of a whitish mass without sharp outlines. No fibrous or striated parts are visible.
Cumulonimbus calvus usually produces precipitation; when it reaches the ground it is in the form of
showers.
2627
Cumulonimbus capillatus (Cb cap) - CEN 1926
2628
2629
2630
2631
2632
2633
2634
Cumulonimbus where the upper portion has cirriform parts of clearly fibrous or striated structure,
frequently in the shape of an anvil (Cumulonimbus capillatus incus), a plume or a vast disorderly
mass of hair. In very cold air masses the fibrous structure often extends virtually throughout the
cloud.
Cumulonimbus capillatus is usually accompanied by a shower or by a thunderstorm, often with wind
squalls and sometimes with hail; it frequently produces very distinct virga.
2635
2636
II.3.10.3
2637
2638
VARIETIES
2639
2640
2641
Cumulonimbus has no varieties.
2642
2643
II.3.10.4
2644
2645
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
Cumulonimbus may show:
 praecipitatio (showers of rain, snow, snow pellets, or hailstones);
 virga;
 pannus;
 incus;
 mamma (observed frequently on the base of the projecting portion of the anvil, less
frequently on the base of the cloud);
 pileus, velum, arcus and murus;
 cauda, flumen and tuba are rarely observed.
2657
2658
II.3.10.5
82
2659
2660
CLOUDS FROM WHICH CUMULONIMBUS MAY FORM
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
Cumulonimbus most commonly evolves from Cumulus congestus (Cb cumulogenitus or Cb
cumulomutatus) that was formed in the normal manner [see paragraphs II.3.9.7 and II.3.9.8].
Cumulonimbus also forms:
 from Altocumulus castellanus (Cb altocumulogenitus); the base of the Cumulonimbus is
unusually high
 from Stratocumulus castellanus (Cb stratocumulogenitus);
 by the transformation and development of a portion of Altostratus or Nimbostratus (Cb
altostratogenitus or Cb nimbostratogenitus).
In the majority of these cases, the transformation into Cumulonimbus passes through the Cumulus
congestus stage.
2675
2676
II.3.10.6
2677
2678
MAIN DIFFERENCES BETWEEN CUMULONIMBUS AND SIMILAR CLOUDS OF OTHER GENERA
2679
2680
2681
2682
2683
When Cumulonimbus covers a large expanse of the sky, it can easily be confused with Nimbostratus,
especially when identification is based solely on the appearance of the under surface. The character of
the precipitation may help to distinguish Cumulonimbus from Nimbostratus. If the precipitation is of
the showery type, or if it is accompanied by lightning, thunder or hail, the cloud is Cumulonimbus.
2684
2685
2686
2687
2688
2689
Certain Cumulonimbus clouds appear nearly identical with Cumulus congestus. The cloud is identified
as Cumulonimbus as soon as at least a part of its upper portion loses the sharpness of its outlines or
presents a fibrous or striated texture. If it is not possible to decide on the basis of the above criteria
whether a cloud is a Cumulonimbus or Cumulus, it is identified as Cumulonimbus if it is accompanied
by lightning, thunder or hail.
2690
2691
II.3.10.7
2692
2693
PHYSICAL CONSTITUTION
2694
2695
2696
2697
2698
2699
Cumulonimbus is composed of water droplets and, especially in its upper portion, of ice crystals. It
also contains large raindrops and, often, snowflakes, snow pellets, or hailstones. The water droplets
and raindrops may be substantially supercooled.
83
2700
II.3.10.8
2701
2702
EXPLANATORY REMARKS
2703
2704
2705
2706
2707
2708
The conditions under which Cumulonimbus clouds occur are similar to those ·which are favourable
for the development of Cumulus congestus (see paragraph II.3.9.8). The transformation of Cumulus
congestus into Cumulonimbus is due to the formation of ice particles in its upper part. The presence
of ice particles is evident when some or all of the upper part loses the sharpness of its outlines or
acquires a fibrous or striated texture.
2709
2710
2711
Cumulonimbus clouds may appear either as isolated clouds or in the form of a continuous line of
clouds resembling a very extensive wall.
2712
2713
2714
2715
In certain cases, the upper portion of Cumulonimbus clouds may be merged with Altostratus or
Nimbostratus. Cumulonimbus may also develop within the general mass of Altostratus or
Nimbostratus.
2716
2717
2718
2719
Low, ragged accessory clouds (pannus) often develop under ·Cumulonimbus; these clouds are at first
separated from one another, but they may later merge so as to form a continuous layer partially or
totally in contact with the Cumulonimbus base.
2720
2721
2722
2723
2724
2725
Cumulonimbus may be described as a "cloud factory"; it can produce thick patches or sheets of Cirrus
spissatus, Altocumulus, Altostratus or Stratocumulus by the spreading out of its upper portions and
by the dissipation of the lower parts. The spreading of the highest part usually leads to the formation
of an anvil; if the wind increases strongly with altitude, the cloud top spreads only downwind,
assuming the shape of a half anvil or in some cases of a vast plume.
2726
2727
Cumulonimbus is rare in polar regions and more frequent in temperate and tropical regions.
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
Cumulonimbus may develop from convection initiated by heat from forest fires, wild-fires or from
volcanic eruption activity. Cumulonimbus that is clearly observed to have originated as a consequence
of localised natural heat sources will be classified by any appropriate species, variety and
supplementary feature, followed by flammagenitus.