PREFACE TO THE 1939 EDITION1 - World … · 18 One year later Luke Howard published in England a...

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PREFACE TO THE 1939 EDITION1 1

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The first published2 classification of clouds only dates back to the 6 beginning of the 19th century and was the work of Lamarck (1802). This 7 celebrated naturalist did not set out to classify all possible clouds; he 8 confined himself to distinguishing certain forms which seemed to him to be the 9 manifestation of general causes which it would be useful to recognize. But 10 this work, in spite of its real value, did not make any impression even in 11 France and his nomenclature does not seem to have been used by anyone. 12 Perhaps this was due to his choice of somewhat peculiar French names which 13 would not readily be adopted in other countries or perhaps the paper was 14 discredited through appearing in the same publication (Annuaire 15 Meteorologique), as forecasts based upon astrological data. 16

17 One year later Luke Howard published in England a cloud classification 18

which, in striking contrast, achieved very great success and which is the basis 19 of the existing classification. Whereas Lamarck contented himself with 20 defining and naming a certain number of interesting forms, Howard set out to 21 establish a complete classification covering all possible cases. He 22 distinguished three simple, fundamental classes-Cirrus, Cumulus, Stratus - 23 from which all others were derived by transition or association. This 24 conception is in some respects incorrect. If Cirrus and Cumulus are entitled to 25 occupy a privileged position in the classification, the first representing the 26 purest type of cloud formed of ice crystals in the high regions of the 27 atmosphere, and the second being pre-eminently a cloud of liquid particles in 28 the lower regions, what Howard calls Stratus does not constitute a type of the 29 same order as the preceding two. It is not defined in terms relating to the 30 physical state of its elements, and it may be found at any altitude. From a 31 practical point of view, however, Howard arrived at much the same results as 32 Lamarck. Four of Lamarck's five principal types appear under different names 33 in Howard's nomenclature. It is remarkable that these two men, of such 34 different scientific culture and never having come in contact with each other, 35 should have arrived independently at such compatible results. 36

37 In 1840, the German meteorologist Kaemtz added Stratocumulus to 38

Howard's forms, giving a precise definition which is in agreement with modern 39 usage. 40

41 Renou, Director of the observatories at Parc Saint-Maur and Montsouris 42

gave in his "Instructions météorologiques" (1855) a classification of clouds to 43 which may be ascribed the definite origin of several names in the present 44 nomenclature: Cirro-Cumulus, Cirro-Stratus, Alto-Cumulus and Alto-Stratus. 45 He was the first to introduce the two latter types into the "Bulletin de 46 l'Observatoire de Montsouris" and his example was soon followed by the 47 observatory at Upsala. He thus introduced clouds of medium height between 48 low clouds and those of the Cirrus family· and began the development of the 49

1 The preface to the 1939 edition was nearly identical with that of the 1932 edition. The modifications introduced

into the text of the 1939 edition consisted of revised abbreviations of cloud forms, changes in the clouds code and changes in the symbols and descriptions of various weather phenomena. 2 In this brief historical note, much use has been made of Louis Besson's very interesting work: 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 50 Hildebrandsson. To him is also due the definite distinction at different levels 51 between detached and continuous clouds. 52

53 In 1863, Poey, who took observations at Havana, made known some 54

original ideas which did not perhaps receive as much respect as they 55 deserved, firstly because good and bad were so closely associated in them 56 and also because he set out to create a classification of all types, without any 57 reference at all to the main outlines which since Howard had been slowly but 58 surely emerging from the successive attempts in Europe. It should, however, 59 be remembered that credit is due to him for defining Fracto-Cumulus, some 60 radiatus varieties (under the name of Fracto-) and mammatus varieties (under 61 the name of Globo-). In particular, he described very clearly the central sky in a 62 depression by distinguishing the two layers, one above the other: the sheet of 63 Altostratus (under the name of Pallia-Cirrus) and the layer of Fractostratus or of 64 Fractocumulus (under the name of Pallia-Cumulus). 65

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In 1879, Hildebrandsson, Director of Upsala Observatory, was the first to 67 use photography in the study and classification of cloud forms. In his work 68 entitled: "Sur la classification des nuages employee a l'Observatoire 69 Meteorologique d'Upsala", he included an atlas of 16 photographs. The 70 classification adopted was that of Howard with a few modifications. These 71 changes concerned especially Nimbus, which was not assigned to every rainy 72 cloud complex (notably not to Cumulo Nimbus), but only to the dark, lower layer 73 of a rainy sky, Stratus which was assigned to fog raised from the ground and 74 remaining at some distance from the Earth and Cumulo-Stratus which, 75 following Kaemtz's example, was assigned to heavy, heaped up masses of 76 Cumulus; from Kaemtz, Hildebrandsson also adopted Strato-Cumulus. In his 77 first work, Hildebrandsson kept closely to his desire to adhere to Howard's plan, 78 but at the same time took later works into consideration. 79

A little later, Weilbach and Ritter proposed classifications too greatly 80 divergent from Howard's (which·in the main had already been generally accepted) 81 to have any chance of success - as happened later in the case of those of Maze, 82 Clayton and Clement Ley. Credit is, however, due to these authors for interesting 83 definitions of species (subdivisions of large genera) or of varieties (particular 84 aspects to be observed at different heights) and to Weilbach for the introduction 85 of Cumulo-Nimbus or thunder cloud, clearly distinguished from Cumulus even 86 when "compositus". 87

88 Finally in 1887, Hildebrandsson and Abercromby published a classification 89

of clouds in which they endeavoured to reconcile existing customs and, while 90 keeping to Howard's scheme, to effect an inclusion of later acquisitions, notably 91 those due to Renou (introduction of Alto-Cumulus and of Alto-Stratus, distinction 92 at each stage between the detached and continuous forms) and to Weilbach 93 (introduction of Cumulo-Nimbus, the allocation of Cumulus and thundery clouds to 94 a distinct family). Abercromby had previously made two journeys round the world 95 (thus giving a fine example of scientific probity) in order to assure himself that the 96 cloud forms were the same in all parts - a fact that is, however, only true as a first 97 approximation. One of the principal characteristics of this classification is the 98 importance attached to height as a criterion, since in the opinion of the authors 99 the foremost application of cloud observations was the determination of the 100 direction of the wind at different altitudes. They grouped the clouds in four levels, 101 the mean heights of which they fixed provisionally from measurements made in 102 Sweden. The international classification was the direct offspring of 103 Hildebrandsson's and Abercromby's classification without any great modification. 104

105 The International Meteorological Conference, held at Munich in 1891, 106

expressly recommended these authors' classification and gave its sanction to 107 the appointment of a special committee entrusted with its final consideration 108 and publication with illustrations in atlas form. This committee met at 109 Upsala in August 1894 and proceeded to choose the illustrations to be 110

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reproduced. With this object in view, an exhibition of more than three 111 hundred cloud photographs or sketches had been arranged. The publication 112 commission, consisting of Hildebrandsson, Riggenbach and Teisserenc de 113 Bart, had to contend with great technical and more particularly financial 114 difficulties. In the end, Teisserenc de Bort took upon himself the sole 115 responsibility for the production of the atlas which appeared in 1896. This 116 work contained 28 coloured plates accompanied by a text in three languages 117 (French, German, English) giving definitions and descriptions of the clouds 118 together with instructions on how to observe them. 119

120 The classification laid down in the International Atlas soon became official 121

and came into almost general use in all countries. Nearly all meteorologists who 122 subsequently published cloud studies adopted this nomenclature; but 123 frequently it was found to be lacking in details; thus a number of meteorologists 124 - notably Clayden and Vincent - were led to create new species or varieties 125 without interfering with the primary forms. 126

127 Thus, thanks to a sustained effort, initiated by Howard, continued by 128

Renou and then by Hildebrandsson and the International Meteorological 129 Committee, an end was made to the confusion which had reigned for nearly 130 a century in one of the most important domains of meteorology. 131 The first International Atlas constituted a great advance by making cloud 132 observations throughout the world truly comparable with one another. 133

134 The 1910 reprint, which contained only slight modifications, had been 135

exhausted for many years when the International Commission for the Study 136 of Clouds was created in London in 1921. The President, Sir Napier Shaw, 137 started the revision of the classification by bringing forward for discussion a 138 memoir in which he gave his own personal ideas and appealed to all members 139 to make suggestions; the enquiry thus set on foot grew so rapidly that in 1925, 140 Sir Napier Shaw's successor thought it necessary to concentrate all the 141 activities of the Commission upon the problem of the revision of the 142 International Atlas. 143

144 This task had become necessary for several reasons. First of all, there 145

was a very practical reason: it was becoming urgent that the observers should 146 be supplied with new atlases lest the quality of the observations should 147 degenerate and differences of interpretation reappear. But, in addition to this 148 practical reason, there were also deeper ones: the work of 1896, remarkable 149 as it had been at the time, was evidently not perfect. From the single, but 150 essential point of view of the standardization of observations, the experience 151 of thirty years had revealed several gaps and instances of lack of precision, 152 which had led to incompatible traditions in different nations as regards certain 153 points. Moreover, meteorology had developed considerably, especially since 154 aviation had become general. When Teisserenc de Bort and Hildebrandsson 155 published the first atlas, the principal problem they had in mind was the 156 general circulation; they considered the clouds above all as aerial floats, 157 capable of revealing upper currents, and were intent upon producing a 158 classification which would make the different types of clouds correspond with 159 heights determined as exactly as possible. But since that period, 160 meteorologists had become more and more interested in clouds as such. The 161 multiplication of cloud observations and the extended data included in 162 synoptic messages which received due recognition in the new international 163 code, Copenhagen 1929 - made possible direct synoptic studies of their 164 distribution and paved the way for the idea of "sky" and "cloud system", the 165 value of which had been clearly demonstrated by the International Cloud 166 Week, organised by the Commission for the Study of Clouds in 1923. 167

168 Observations from aeroplanes made familiar cloud aspects which were 169

previously unknown, made them known more intimately and more completely; 170 finally, new theories normally based upon the hydrodynamical and 171

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thermodynamical interpretation of soundings determined their physical 172 signification and their role in disturbances. These new and very interesting 173 points of view had to be definitely recognised. 174

175 When the Commission for the Study of Clouds met in Paris in 1926 to 176

take account of the results of the vast enquiry which it had inaugurated and to 177 lay the foundations of a new atlas, it found itself confronted by an abundance 178 of literature and very diverse suggestions. Very wisely, it adopted the 179 principle that it should only touch with extreme caution a classification which 180 had stood the test of years and been received as it were with unanimous 181 agreement by our predecessors. It decided to make only such modifications 182 as were necessary to dissipate misunderstandings and further the uniformity 183 of observations, at the same time, however, laying less stress upon the 184 importance of height as a basis of classification. 185

186 While recognizing the necessity of paving the way for a secondary 187

classification, it took care not to attempt its completion nor to subdivide 188 excessively the main categories henceforth called "genera"; it made a rule 189 only to introduce "species" which were generally accepted by all, leaving the 190 way clear for progressive additions in future. Having thus given witness to a 191 prudent, conservative spirit and placed the work of 1896 in a secure position, 192 the Commission for the Study of Clouds proceeded, on the other hand, to 193 give practical satisfaction to the new spirit. Having from the outset considered 194 it premature to attempt a cloud classification based upon physical properties - 195 reserving the study of that question until after a new International Cloud Year 196 (conceived in connection with and to be realized at the same time as the Polar 197 Year 1932-1933) - it adhered to this attitude and refused to rely upon any 198 theory, however attractive it might appear. Nevertheless, it decided to put on 199 record information which had been acquired by observation in the sky or on 200 the charts. Thus it was resolved to include: 201

(1) A chapter on the observation of clouds from aircraft for which the 202 well-known work of Mr. C.K.M. Douglas, aviator as well as meteorologist, was 203 largely drawn upon; 204

(2) A classification of "types of skies", based upon the cloud structures in 205 depressions, as emerging from the work of the Norwegian and French 206 schools; in order to mark the importance of this innovation, the title of the Atlas 207 was altered to "International Atlas of Clouds and of Types of Skies". 208

209 The Commission for the Study of Clouds met a second time in Zurich in 210

September 1926 to make definite arrangements for the projected atlas. 211 Meanwhile, an imposing collection of photographs of clouds, skies and aerial 212 views - borrowed mainly from the collections of Messrs. Cave, Clarke and 213 Quenisset and of the Fundacio Concepcio Rabel - had been assembled to 214 provide abundant illustrations for the atlas. 215

216 In order that the Commission's project should be subjected to the widest 217

criticism before the final atlas was undertaken, the Director of the French 218 Office National Meteorologique decided to issue at the expense of his Office 219 the Commission's project in the form of a "Provisional Atlas". The wide 220 distribution of this atlas answered the purpose perfectly; remarks and 221 suggestions flowed in from all parts of the world. These numerous documents 222 were examined at Barcelona in 1929 by the Commission and all the 223 suggestions were examined and classified with great care. The illustration of 224 the Atlas was also carefully reviewed and the Commission's task in this 225 respect was facilitated to an extraordinary extent by the magnificent exhibition 226 of cloud photographs arranged by the Fundacio Concepcio Rabel at the time 227 of the meeting. 228

229 The Commission for the Study of Clouds met again at Copenhagen in 230

September 1929 at the time of the meeting of the Conference of Directors. 231

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The suggestions received since the meeting at Barcelona were considered 232 and the final scheme agreed, except for a few details. It was proposed that an 233 extract of the Complete Atlas for the use of the observers should be 234 published quickly in order to facilitate the application of the new International 235 Code, in which observations of the types of skies figured largely. 236

237 The question of publication could be approached in exceptionally 238

favourable circumstances, thanks to the truly magnificent gift of a Catalonian 239 Maecenas, Rafael Patxot, to whom cloud science was already indebted for the 240 interesting work of the Fundacio Concepcio Rabel; this generous contribution 241 made it possible to print 1 000 free copies of the Complete Atlas and to offer 242 it, as also the abridged edition, for sale at a very low price. A Sub-Commission 243 was appointed, with Professor Süring as President, to prepare a programme 244 for the Cloud Year and to study the physical processes of cloud formation and 245 evolution with a view to compiling eventually an appendix to the General Atlas. 246 Two other appendices were suggested, one on tropical clouds, the other on 247 special local formations and the preparation of these two parts was delegated 248 to Dr. Braak and Dr. Bergeron respectively. The Conference of Directors 249 approved the Commission's propositions in their entirety and delegated its 250 powers, as far as the production of the atlas was concerned, to a special Sub-251 Commission. 252

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The work was carried out largely at Paris in the course of 1930 by Messrs. Süring, 254 Bergeron, and Wehrle. The German and English translations were prepared by Dr. Keil, 255 Mr. Cave and the Meteorological Office, London. The abridged edition finally 256 appeared in 1930, just before the new code came into force. Another year was 257 required to finish the illustrations of the Complete Atlas and the chapters not included 258 in the abridged edition. Meanwhile, the Süring Sub-Commission had held meetings at 259 Brussels (December 1930) and at Frankfurt (December 1931), and it seemed opportune 260 to incorporate in the Complete Atlas a part of the work relating to the observation of 261 clouds and hydrometeors. 262

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The book now appearing bears the sub-title: "I. General Atlas" (the second and 264 following volumes will consist of the appendices to be published later) and consists of 265 a text and a collection of 174 plates. 266

267 The text is divided into five sections: 268

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(1) CLOUDS - The amended text of the old Atlas. The principal modifications 270 are: 271

(a) the definition of Cirrocumulus which is more restricted than formerly; 272

(b) the distinction between Cumulus and Cumulonimbus; the latter being 273 characterised by ice crystals in its summit or by showers; 274

(c) the distinction between Altocumulus and Stratocumulus; 275

(d) the introduction of Nimbostratus (low Altostratus) in order to avoid confusion 276 (due to the equivocal definition of Nimbus) between the low, rainy layer resulting from 277 the downward extension of Altostratus and the very low, closely packed clouds 278 (Fractostratus or Fractocumulus of bad weather) which often form beneath the 279 Altostratus or the above-mentioned low layer. 280

The commentaries to the definitions have been considerably enlarged in the 281 form of "Explanatory remarks", written from a very practical standpoint with special 282 reference to the needs of observers and stressing the distinctions between kindred 283 forms. In some cases, species have been introduced but as previously stated this 284 secondary classification is intentionally limited to cases on which there is unanimous 285 agreement; it is moreover considerably simplified by the addition of a certain 286 number of varieties common to different levels. In order to mark the fact that the 287 names of the clouds have become symbols, the etymology of which should not be 288 unduly stressed, they have in all cases been written as a single word. 289

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(2) CODE – The second part consists of a practical and detailed commentary 291 for the use of observers, with explanatory remarks concerning the general 292 arrangement and hints how to avoid confusion in the specifications of the new code 293 of low, middle and high clouds; it would perhaps be more appropriate to call it a code 294 of the types of skies, since the arrangement of cloud masses in the sky plays an 295 essential role in it and it has been conceived in such a fashion that all types of sky 296 classified in the fifth part can be represented by the combination of three figures. 297 298

It was thought best to abstain from all "synoptic" considerations in the text, the 299 observer being supposed to ignore the general situation; nevertheless, it is not 300 desirable that he should be deprived entirely of the real help to be derived from 301 connecting the type of sky with the evolution of disturbances. There will therefore be 302 found at the end of this section a diagram showing where the different lower, middle 303 and upper skies specified in the code are situated relative to a disturbance. 304

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(3) CLOUD DIARY - This section, which has been inserted at the suggestion of Dr. 306 Bergeron, has been taken from the papers prepared by the Suring Sub-Commission for 307 the Cloud Year. It includes a model table for noting cloud observations and detailed 308 instructions of how entries should be made in it. These are supplemented by precise 309

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descriptions of different hydrometeors or weather phenomena, a subject which has given 310 rise to divergent national traditions and in which there was need of amendment and 311 unification. 312

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(4) OBSERVATION OF CLOUDS FROM AIRCRAFT - As the classification of clouds is 314 based upon their appearance as seen from the ground, it seemed useful to add a note 315 on their appearance from the point of view of the observer in an aircraft, inasmuch as 316 the more complete knowledge which he may acquire from the fact that he can get near to 317 and on top of them (at least, in the case of lower and middle clouds) makes it possible 318 to simplify the classification considerably by including only the really essential distinctions 319 in structure. The increase in the number of meteorological flights especially in 320 connection with temperature soundings, necessitated the inclusion of this chapter. 321

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(5) TYPES OF SKIES - The enumeration of the genera or even of the species of 323 clouds in the sky at a given moment does not suffice to characterize the type of sky, that 324 is to say, to specify precisely the sector of the disturbance affecting the place of 325 observation and, in consequence, it does not indicate the general character of the 326 "weather". What really characterizes the type of sky is the aggregate of individual 327 clouds and their organization. A special classification of skies is therefore needed 328 which, while being in accordance with the experience of qualified observers, shall also 329 be consistent with the nature of the physical processes and the structure of 330 disturbances. In addition, such classification facilitates the identification of cloud genera 331 and in certain cases (especially in thundery conditions) it compensates, at least in 332 parts, for vagueness. 333

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Collection of plates - The total number of plates is 174(101 photographs taken 335 from the ground, 22 from aeroplanes and 51 for types of sky), 31 of which are in two 336 colours. Two colours are used where there is occasion to distinguish the blue of the 337 sky from the shadows of the clouds. Most of these are included in the abridged 338 edition, which is intended for the use of the general mass of observers who need 339 detailed guidance. Each plate is accompanied by explanatory notes and a 340 schematic representation on the same scale as the photograph, setting out its essential 341 characteristics. 342

343 Thanks to the generosity of Mr. Cave, who has done so much for cloud science, 344

the appendix dealing with tropical clouds edited by Dr. Braak, constituting Volume II of 345 the complete work, has already appeared in French in connection with the 346 requirements of the Polar Year. It is hoped that ·the appendix dealing with special 347 clouds, constituting Volume III will appear soon. This will include, in particular, 348 Professor Stormer's beautiful photographs of stratosphere clouds. Finally, it is also 349 hoped that the results of the Cloud Year will enable the Süring Sub-Commission to 350 prepare a fourth volume dealing with the physical processes involved in the 351 formation of clouds, which will be epoch-making in the history of meteorology. 352

353 E. DELCAMBRE, 354

President of the International 355 Commission for the Study of Clouds. 356

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PREFACE TO THE 1956 EDITION 378 379

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The International Commission for the Study of Clouds (C.E.N.) of the 382 International Meteorological Organization (I.M.O.), created in 1921, was dissolved by 383 the Extraordinary Conference of Directors (London, 1946). It was replaced by the 384 Committee for the Study of Clouds and Hydrometeors (C.C.H.), established by the 385 International Meteorological Committee of the International Meteorological 386 Organization in compliance with a resolution of the Commission for Synoptic 387 Weather Information (Resolution 16, C.S.W.I., Paris 1946). The Conference of 388 Directors of the International Meteorological Organization instructed the C.C.H. to 389 prepare a revised and up to date version of the International Atlas of Clouds and 390 Types of Skies (Resolution 153, CD, Washington 1947). The decision to prepare a 391 new atlas was inspired, on the one hand, by the exhaustion of the previous 1939 392 edition and, on the other hand, by new developments in our knowledge of clouds and 393 hydrometeors, as well as by modifications in international cloud codes. 394

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The Committee for the Study of Clouds and Hydrometeors held several sessions 396 in which the following members participated: A. Viaut, T. Bergeron, J. Bessemoulin, 397 W. Bleeker, C.F. Brooks, C.K.M. Douglas, L. Dufour, N.R. Hagen, B.C. Haynes, M. 398 Mezin, J. Mondain and H. Weickmann. 399

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An Editing Committee, consisting of M. Mezin (President), R. Beaufils 401 (Secretary), R. Beaulieu, Bessemoulin and M. Bonnet, prepared documents between 402 sessions. 403

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In 1951, when the I.M.O. was replaced by the World Meteorological Organization 405 (W.M.O.), the Committee for the Study of Clouds and Hydrometeors proposed to the 406 First Congress of W.M.O. that the new edition of the Cloud Atlas should consist of 407 four volumes and it presented a draft of Volumes I, II and III. Volumes I and III 408 covered essentially the same ground as Volume I of the present Atlas and Volume II 409 was a collection of photographs of clouds and meteors. Volume IV was intended to 410 be a treatise on the physics of clouds and meteors. · 411

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The First Congress of the World Meteorological Organization decided 413 (Resolution 18, Cg-I) to refer the draft to the Commission for Synoptic Meteorology 414 (C.S.M.) for further study and completion. The Committee for the Study of Clouds 415 and Hydrometeors itself became a "Working Group for the Study of Clouds and 416 Hydrometeors" attached to the Commission for Synoptic Meteorology (Resolution 417 35, Cg-I). 418

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The contents of the Atlas and plans for its publication were discussed at the 420 Second and Third Sessions of the Executive Committee (Lausanne, 1951; Geneva,· 421 1952). It was decided that an Abridged Atlas in one volume consisting of a 422 condensed text and a selection of photographs, for the use of surface observers, and 423 an Album designed to meet the limited but special needs of airborne observers, should 424 also be prepared (Resolution 9, EC-II; Resolution 36, EC-III). 425

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The Working Group for the Study of Clouds and Hydrometeors presented to the 427 First Session of C.S.M. (Washington, 1953) an improved version of the original draft 428 submitted to Congress. The improvement resulted from further study at various 429 sessions of the Working Group and from remarks received from members of the 430 Commission to whom copies had been distributed. 431

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The Commission for Synoptic Meteorology recommended (Recommendation 433 49, 'CSM-1) that the Complete Atlas should consist of only two volumes (Volume I 434

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containing the text and Volume II the plates). It also formally recommended the 435 publication of an Abridged Atlas and of an International Cloud Album for airborne 436 observers. Finally, C.S.M. considered that a compendium on the physics of clouds 437 and meteors, though highly desirable, should not at present form part of the Cloud 438 Atlas. 439

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The Working Group for the Study of Clouds and Hydrometeors was dissolved by 441 C.S.M. However, a few individuals were requested to continue and to complete the 442 work of the Committee for the Study of Clouds and Hydrometeors. 443

444 At its Fourth Session (Geneva, 1953), the Executive Committee adopted 445

Recommendation 49 of C.S.M. and directed the Secretary-General to take the 446 necessary steps, in consultation with the President of C.S.M. when required, for an 447 early publication of the Atlas (Resolution 30, EC-IV). 448

449 The English text was then passed to Mr. E.G. Bilham for editing, in accordance 450

with the wishes of the Commission for Synoptic Meteorology and a decision by the 451 Executive Committee. 452

453 During the translation of the text of the Complete Atlas into French, for which Mr. 454

J. Bessemoulin was responsible in accordance with a request by C.S.M., it became 455 obvious that many parts required thorough revision. A special editing committee was 456 then established consisting of the following persons: Dr. W. Bleeker, Dr. M.A. Alaka, 457 Mr. R. Beaufils and Mr. J. Bessemoulin. This committee met several times in Geneva 458 and in De Bilt and established the final English and French texts. 459

460 The President of the Commission for Synoptic Meteorology accepts responsibility 461

for the changes thus made in the original text which was studied at the First Session of 462 the Commission; these changes were necessary in order to avoid ambiguities and 463 internal inconsistencies. 464

465 The content of the present Volume I, which is essentially descriptive and 466

explanatory, differs materially from that of the former "General Atlas". 467 468 The grouping of clouds into "cloud families" has been abandoned; the 469

classification into genera has been maintained but some details in the definitions have 470 been modified. 471

472 The species and the varieties have been extended and considerably modified. 473

The same remark applies to the "accidental details" which have been renamed 474 "supplementary features" and "accessory clouds". A new concept, that of "mother-475 cloud", has been introduced. 476

477 Certain "special clouds" are discussed separately; a brief description is given of 478

the most important of these clouds, such as nacreous clouds, noctilucent clouds, 479 etc. 480

481 The "Note on the Observation of Clouds from Aircraft" of the former General Atlas 482

has been replaced by a chapter describing the particular appearance presented by 483 clouds when they are observed from aircraft. 484

485 The part "Types of Skies" of the former General Atlas has been deleted. New 486

points of view have arisen and existing ideas, particularly with regard to tropical skies, 487 are in the course of evolution, thus making it difficult to synthesize the various existing 488 concepts. 489

490 The chapter "Definition of Hydrometeors" of the former General Atlas has been 491

considerably expanded. The former classification of hydrometeors has been replaced 492 by a classification dividing meteors into four groups. The term "hydrometeors" 493 designates the first of these groups, and applies solely to aqueous meteors. The 494 descriptions of the hydrometeors are based mainly on those adopted at Salzburg in 495 1937. The other groups of meteors are "lithometeors", "photometeors" and 496 "electrometeors". 497

498 The parts intended primarily for the use of observers have also been expanded. 499

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Part Ill contains more elaborate instructions for observing clouds and meteors. Part IV 500 gives two models of a "Journal of clouds and meteors". Part V contains detailed 501 instructions and pictorial guides for the coding of clouds. 502

503 The final change consists of the addition of Appendices providing information of a 504

general nature and an Alphabetical Index to simplify consultation of the Atlas. 505 506 Volume II is a collection of 224 plates in black and white and in colour, the object 507

of which is to illustrate the text of Volume I. The plates consist of photographs of 508 clouds (viewed from the Earth's surface and from aircraft) and of certain meteors; each 509 photograph is accompanied by an explanatory legend. 510

511 The French Meteorological Service has contributed materially to the preparation 512

of the texts and the photographic plates and their legends. The Netherlands' 513 Meteorological Service also gave considerable assistance during the final stages of 514 the preparation of the Cloud Atlas. 515

516 The undersigned who have been closely connected with the preparation 517

and publication of the Cloud Atlas wish to thank all those who have contributed to the 518 texts, and in particular Messrs. J. Bessemoulin and R. Beaufils of the French 519 Meteorological Service and Dr. M.A. Alaka of the Secretariat of W.M.O., for their 520 enthusiastic assistance during the final phase of the composition of the text. They 521 also thank all the persons who provided photographs to illustrate the International 522 Cloud Atlas. 523

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W. BLEEKER, 525 President of the 526 Commission for 527

Synoptic 528 Meteorology 529

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A. VIAUT, 531 President of the Committee 532

for the Study of Clouds 533 and Hydrometeors 534

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De Bilt, Paris, 4 April 1956. 537

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PREFACE TO THE 19 75 EDITION OF VOLUME I 539

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The previous edition of the International Cloud Atlas, which appeared in 1956, consisted of 542 two volumes: Volume I, containing a descriptive and explanatory text, and Volume II, containing a 543 set of plates intended to illustrate the text. The present publication is a new edition of Volume I 544 designed to replace the original edition. The preface describes the circumstances which led to the 545 decision to publish a new edition and pays tribute to the numerous meteorologists who have 546 devoted part of their time and efforts to the preparation of this new, and greatly improved, version 547 of the text of the Atlas. 548

549 At its fourth session (Wiesbaden, 1966), the Commission for Synoptic Meteorology (CSM) 550

examined the replies received from Members to an inquiry on the visibility criteria used for 551 reporting mist and fog and also on the question of whether mist and fog should be considered as 552 one and the same hydrometeor. It is pointed out in this connexion that, in the 1956 edition of the 553 International Cloud Atlas, these phenomena were treated as two distinct hydrometeors. 554

555 CSM considered that mist and fog were produced by the same processes and that they 556

should be regarded as one and the same hydrometeor, on the understanding, however, that the 557 terms "fog" and "mist" might continue to be used to denote different intensities of the 558 phenomenon, the term "mist" being synonymous with a slight fog, and the visibility limit of 1 000 559 metres, used hitherto, being maintained as a criterion of intensity. 560

561 At the same session, CSM also examined a proposal to revise the definitions and 562

descriptions of hydrometeors contained in the 1956 edition of the International Cloud Atlas. The 563 reason for the proposal was that there had been important advances in the physics of 564 hydrometeors since CSM had recommended the use of the descriptions. 565

566 The Commission agreed that such a review was needed, particularly as regards 567

hydrometeors appearing in polar and mountainous areas. For this purpose it decided to set up a 568 Working Group on Description of Hydrometeors (Resolution 8 (CSM-IV)) composed of the 569 following members: L. Dufour (Belgium), chairman of the group and representative of the 570 Commission for Aerology, G. A. Gensler (Switzerland), E. Hesstvedt (Norway), H. D. Parry 571 (United States of America), B. V. Ramanamurthy (India) and A. Rouaud (France). 572

573 The group recommended, in the first place, that a cloud should be classified as a 574

hydrometeor, which would call for a change in the definition of "cloud", which did not refer to such 575 a classification. The definition of "meteor" also had to be changed, because the latter had hitherto 576 been defined as "a phenomenon other than a cloud ...". The group also reviewed all the 577 definitions and descriptions of hydrometeors other than clouds. 578

579 Following the recommendations made by the Working Group on Description of 580

Hydrometeors set up by Resolution 8 (CSM-IV), CSM recommended, at its fifth session (Geneva, 581 1970), that the revised definitions and descriptions of hydrometeors other than clouds should be 582 adopted and that Volume I of the International Cloud Atlas should be amended accordingly 583 (Recommendation 41 (CSM-V)). At its twenty-second session, the Executive Committee approved 584 this recommendation and requested the Secretary-General of WMO to arrange for the publication 585 of the revised text (Resolution 14 (EC-XXII)). 586

587 In view of the fact that the 1956 edition of Volume I of the International Cloud Atlas was out 588

of print and that the principle adopted by Sixth Congress to the effect that any publication 589 constituting an annex to the Technical Regulations -which was partially true of Volume I of the 590 Atlas - should be transformed into a Manual, the Advisory Working Group of the Commission for 591 Basic Systems (CBS-formerly the Commission for Synoptic Meteorology (CSM)) considered, at its 592 second session (Geneva, "1971), that a preliminary draft of a new edition of the volume should be 593 prepared, containing, in principle, only such texts as had the legal status of provisions of the 594 Technical Regulations, including, of course, the revised definitions adopted by CSM at its fifth 595

16

session. 596 597

In accordance with the above-mentioned decision of the CBS Advisory" Working Group, an 598 expert (Mr. A. Durget, France) was invited to revise Volume I of the International Cloud Atlas. At 599 its sixth session (Belgrade, 1974), CBS recommended that the draft revised text should be pub 600 lished to replace the 1956 edition and that the Secretary-General should be requested to arrange 601 for the publication of an appropriate amendment to the Abridged Atlas to bring it into line with the 602 new edition of Volume I of the International Cloud Atlas (Recommendation 18 (CBS-VI)). At its 603 twenty-sixth session, the Executive Committee approved this recommendation and requested the 604 Secretary-General to implement it (Resolution 3 (EC-XXVI)). 605

606 During his work on the revision of the Atlas, the expert decided that it was virtually impos 607

sible, and certainly not desirable, to exclude from the Atlas those parts of the 1956 edition which 608 did not have the legal status of the provisions of the Technical Regulations. Some of those parts 609 could not, in fact, be separated from the passages to which they referred and which were retained 610 as being provisions of the Technical Regulations without their deletion seriously affecting the 611 clarity and consistency of the work. Other parts not having the status of Technical Regulations 612 and whose inclusion in the Atlas was not strictly speaking indispensable deserved none the less 613 to be maintained in view of their great value for users of the Atlas. Part IV, however, of the 1956 614 edition of Volume I - entitled "Journal of clouds and meteors" - had not been included in the 615 new edition since it was not of international interest. 616

617 Since the two volumes of the International Cloud Atlas are well known under this title, the 618

same title has been retained for the present edition. But for reasons of consistency with other 619 WMO publications constituting annexes to the Technical Regulations and accordingly described 620 as "Manuals", this publication also bears the subtitle "Manual on the observation of clouds and 621 other meteors". 622

623 Moreover, those parts of the book having the legal status of Technical Regulations are 624

distinguished from the rest by a different type of print. Similarly, a system of paragraph numbering 625 similar to that used in the Technical Regulations has been employed. 626

627 As a consequence of the adoption of the new definition of "cloud", which is now regarded as 628

a hydrometeor, and having regard to the corresponding change in the definition of "meteor", it was 629 thought necessary to re-arrange the layout of the Atlas completely, by changing the order of the 630 parts and chapters so as to give the definitions of "meteor" and "hydrometeor" before dealing in 631 detail with clouds and other meteors. 632

633 The present edition of Volume I thus consists of three parts. The first, in addition to the 634

new definition of "meteor", contains a general classification of meteors into hydrometeors 635 (including clouds), lithometeors, photometeors and electrometeors, as well as definitions of 636 each of these four groups of phenomena. These various texts have been taken from Chapter I of 637 Part II of the 1956 edition of Volume I, taking into account, in their wording, the new concepts set 638 forth in Recommendation 41 (CSM-V) approved by Resolution 14 (EC-XXII). 639

640 Part II deals exclusively with clouds. It recapitulates, with a few essentially drafting changes, 641

the various chapters dealing with clouds in the 1956 edition of Volume I: definition of a cloud and 642 classification of cloud; definitions of the genera, species and varieties, etc., of clouds; descriptions 643 of clouds; orographic influences; clouds as seen from aircraft; special clouds; observation of 644 clouds from the Earth's surface; the coding of clouds in the codes CL, CM and CH and the 645 corresponding symbols. 646

647 Part III deals with meteors other than clouds. It consists of three chapters: classification of, 648

and symbols for, meteors other than clouds; definitions and description of meteors other than 649 clouds; and observation of these meteors from the Earth's surface. The texts relating to hydro 650 meteors have been introduced in the new version given in Recommendation 41 (CSM-V), with 651 certain drafting amendments. 652

653

17

654 The three appendices to the 1956 edition of Volume I, namely: Appendix I - 655

Etymology of Latin names of clouds, Appendix II - Historical bibliography of cloud 656 classification, and Appendix III - Bibliography of cloud nomenclature, have been 657 maintained unchanged in the present edition of the Atlas. The alphabetical index of 658 words and expressions has also been maintained with appropriate updating. 659

660 On behalf of the World Meteorological Organization, I should like here to express 661

my gratitude to Mr. L. Dufour and to the members of his working group, as well as 662 to Mr. A. Durget, for their valuable contributions to the preparation of this edition of 663 the volume. 664

665

666

D. A. DAVIES 667

Secretary-General 668

18

18 (temporary page number to facilitate discussions)

669 FOREWORD TO THE 1987 EDITION OF VOLUME II 670

671 672 673 674

With this new, thoroughly revised edition of Volume II of the International 675 Cloud Atlas a key publication is once again made available for professional 676 meteorologists as well as for a wide circle of interested amateurs. For meteorologists 677 this is a fundamental handbook, for others a source of acquaintance with the 678 spectacular world of clouds. 679

680 The present internationally adopted system of cloud classification is the result of 681

work which started in the nineteenth century. The first studies on the topic were 682 published by J. B. Lamarck (1802) and L. Howard (1803). The first attempt to use 683 photography for cloud classification was made by H. Hildebrandsson ( 1879), in 684 Uppsala, who prepared a cloud atlas of 16 photographs. The further development of 685 this work, following the recommendation of an International Meteorological Conference 686 which took place in Munich in 1891, resulted in the publication in 1896 of the first 687 International Atlas, containing 28 coloured plates accompanied by definitions and 688 descriptions of clouds and instructions on cloud observations in three languages 689 (French, German, English). The first International Atlas, which was then adopted in 690 almost all countries, was a great step forward in making internationally comparable 691 cloud observations. This Atlas was reprinted in 1910, without substantial 692 amendments. The subject of further refinement of cloud classification still remained to 693 the fore, however, during the following decades. As a result the International Atlas of 694 Clouds and Study of the Sky, Volume I, General Atlas was published in 1932 by the 695 International Commission for the Study of Clouds. A modified edition of the same 696 work appeared in 1939, under the title lnternational Atlas of Clouds and of Types 697 of Skies, Volume I, General Atlas. The latter contained 174 plates: 101 cloud 698 photographs taken from the ground and 22 from aeroplanes, and 51 photographs of 699 types of sky. From those photographs. 31 were printed in two colours (grey and blue) 700 to distinguish between the blue of the sky and the shadows of' the clouds. Each plate 701 was accompanied by explanatory notes and a schematic drawing on the same scale as 702 the photograph, showing the essential characteristics of the type of cloud. 703

704 When the World Meteorological Organization (WMO) came into being in 1951 in 705

place of the non-governmental International Meteorological Organization, the First 706 Meteorological Congress noted the need for a new International Cloud Atlas and 707 referred the task to the Commission for Synoptic Meteorology. Within a relatively short 708 time very substantial work was accomplished and the new Atlas was published in 709 1956 in two volumes: Volume I contained a descriptive and explanatory text on the 710 whole range of hydrometeors (including clouds), lithometeors, photometeors and 711 electrometeors; Volume II contained a collection of 224 plates (123 in black and white 712 and 101 in colour) of photographs of clouds and of certain meteors. Each photograph 713 in Volume II was accompanied by an explanatory text, to enable the pictures in 714 Volume II to be understood without the detailed technical definitions and 715 descriptions contained in Volume I. 716

The 1956 edition of Volume II has not been reprinted or revised until the 717 preparation of the present edition. A revised version of Volume l, however, was 718 published in 1975 under the title Manual on the Observation of Clouds and Other 719 Meteors. ln the meantime there have been substantial advances in techniques of 720 cloud photography and a growing requirement for more photographs taken at locations 721 outside Europe. 722

ln 1981 a WMO Informal Planning Meeting on Volume II of the International Cloud 723 Atlas drew up a plan for the preparation of a new edition. It recommended the deletion 724 of 26 black-and-white plates and eight in colour, and their replacement by 41 new 725 colour plates selected from a large number of photographs received from various 726 countries. The section containing illustrations of certain meteors was also expanded by 727 the addition of nine more plates. The legends for the new plates selected by the 728 Informal Planning Meeting were edited by the chairman of the meeting, Mr. R. L. Holle, 729

19


of the U.S. National Oceanic and Atmospheric Administration, and those for the new 730 plates in the section on meteors by Mr. C. S. Broomfield, of the U.K. 731 Meteorological Office. 732

Later it became apparent that many of the original photographs of the 1956 733 edition had deteriorated with time to an extent excluding the possibility of their inclusion 734 in the new edition. Moreover, it was felt that the geographical distribution of the 735 photographs was still somewhat restricted and that the balance between the various 736 sections could be improved. With the approval of the president of the Commission for 737 Basic Systems, it was therefore decided to revise the Atlas extensively, bearing in 738 mind the urgent requirement for the new edition, and Mr. Holle kindly agreed to 739 undertake this complex task, including the soliciting at short notice of new photographs 740 from specialists. The final editorial work was carried out by the WMO Secretariat. The 741 result of the work, the present Volume II of the International Cloud Atlas, contains 196 742 pages of photographs, 161 in colour and 35 in black and white. Each illustration is 743 accompanied by an explanatory text. 744

The excellent work of the consultants and the authorization willingly given by all 745 contributors for publication of photographs in both the original volume and this new 746 edition are gratefully acknowledged. Particular thanks are due to the printer, whose 747 painstaking work permitted much of the original material to be conserved and blended 748 harmoniously with the new contributions. 749

It is felt that this new edition of the Atlas, besides being a most valuable reference 750 work for meteorologists and those working in aviation, in agriculture and at sea, will 751 also be a fascinating addition to the amateur's bookshelf. 752

753

754

755

(G.O.P. 756

Obasi) 757

Secretary-General 758

759

760

761

20


PREFACE TO THE PRESENT EDITION 762

763

764

765

[To be provided prior to publication] 766

767

768

769

770

771

772

773

21


774

INTRODUCTORY NOTE 775

776

Certain parts of this publication constitute Annex I to the Technical 777 Regulations and have the legal status of standard practices and procedures. 778

779

The sections, paragraphs and sub-paragraphs having the status of an Annex to 780 the Technical Regulations, except for the footnotes, are numbered in bold type. In 781 Chapter III. 2 only the definitions printed in italics have this status. 782

783

784

22


PART I –METEOROLOGICAL METEORS: DEFINITION AND GENERAL CLASSIFICATION 785

786

23


I.1 - METEOROLOGICAL DEFINITION OF A METEOR 787

In meteorology, a phenomenon observed in the atmosphere or on the surface of the Earth is 788

known as a meteor. It may be a form of precipitation, a suspension, or a deposit of liquid or solid 789

particles. It is generally visible to human observers, but in the case of thunder it is audible. 790

Sometimes names of meteors are also applied to related concepts. For instance, the word "snow" 791

may refer to a type of hydrometeor (ensemble of falling particles), to snow cover (ensemble of 792

particles lying on the ground), or to the substance snow (as in "snow blown from mountains", 793

snowball). The constituent particles of snow in these three cases are either snow crystals or 794

snowflakes. 795

Also, certain meteors are named based on their constituent particles; for example, the 796

hydrometeor “snow grains" is an ensemble of falling snow grains. 797

I.2 - GENERAL CLASSIFICATION OF METEORS 798

The nature of meteorological meteors is quite varied. However, by considering their constituent 799

particles or the physical processes surrounding their occurrence, meteors are classified into four 800

groups, namely hydrometeors, lithometeors, photometeors and electrometeors. 801

I.2.1 802

Hydrometeors 803

A hydrometeor consists of liquid or solid water particles. Hydrometeors may fall through the 804

atmosphere or be suspended in it, may be blown by the wind from the Earth's surface, or may be 805

deposited on other objects. Snow or water on the ground is, by convention, not considered a 806

hydrometeor. 807

We describe the following five types of hydrometeors: 808

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 non-showery 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,

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 Phenomena

Suspended particles 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

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)

Spouts funnel cloud: o water spout over water o tornado over land

813

814

815

25


816

Cloud forms (genera) associated with each hydrometeor type. 817

Falling hydrometeor

Cloud Genera associated with Falling hydrometers (precipitation)

Alt

ost

ratu

s

Nim

bo

stra

tus

Stra

tocu

mu

lus

Stra

tus

Cu

mu

lus

Cu

mu

lon

imb

us

No

clo

ud

Rain Possible Usual Possible Possible showers

Usual showers

Drizzle Possible

Snow Possible Possible Possible Possible Possible showers

Possible showers

Snow grains Possible

Snow pellets Possible Possible showers

Possible showers

Diamond dust Possible

Hail (large and small)

Possible showers

Ice pellets Possible Possible

818

I.2.2 Lithometeors 819

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

I.2.3. 821

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

26


824

I.2.4 Electrometeors 825

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

828

27


829 830 831 832 833 834

835

PART II- CLOUDS 836

837

838

839

28

II.1 – INTRODUCTION AND PRINCIPLES OF CLOUD CLASSIFICATION 840 841 II.1.1 842 843 Definition of a cloud 844 845 A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both, 846 suspended in the atmosphere and usually not touching the ground. It may also include larger 847 particles of liquid water or ice as well as non-aqueous liquid or solid particles such as those 848 present in fumes, smoke or dust. 849 850 II.1.2 851 852 Appearance of clouds 853 854 Appearance is best described by dimensions, shape, structure, texture, luminance and colour of 855 the cloud. These factors are considered for each of the characteristic cloud forms. 856 857 II.1.2.1 858 859

LUMINANCE 860

Light reflected, scattered and transmitted by its constituent particles determines the luminance of 861 a cloud. This light comes, for the most part, direct from the luminary (sun or moon or stars) or 862 from the sky; it may also come from the surface of the earth, being particularly strong when ice 863 fields or snowfields reflect sunlight or moonlight. 864 865 The luminance of a cloud may be modified by intervening haze. When haze is present between the 866 observer and the cloud, it may, depending on its thickness and the direction of the incident light, 867 either diminish or increase the luminance of the cloud. Haze also diminishes the contrast that 868 reveals the shape, structure and texture of the cloud. Luminance may also be modified by optical 869 phenomena such as haloes, rainbows, coronae, glories, etc. ("Photometeors" in Part III of this 870 Atlas). 871 872 During daytime, the luminance of the clouds is sufficiently high to make them easily observable. 873 On a moonlit night, clouds are visible when the moon is more than a quarter full. In its darker 874 phases, the moon is not bright enough to reveal clouds far from it, especially when the clouds are 875 thin. On a moonless night, clouds are generally invisible; sometimes, however, their presence may 876 be deduced from the obscuration of stars (noting that stars near the horizon can be obscured due 877 to haze), of polar aurora, of zodiacal light etc. 878 879 Clouds are visible at night in areas with sufficiently strong artificial lighting. Over large cities, 880 clouds may be revealed by direct illumination from below. An artificially illuminated layer of clouds 881 may provide a bright background against which lower cloud fragments stand out. 882 883

1 When a slightly opaque cloud is illuminated from behind, its luminance is at a maximum in the 884 direction of the luminary. It decreases away from the luminary; the thinner the cloud, the more 885

rapid the decrease. Clouds of greater optical thickness (measure of the degree to which the cloud 886 prevents light from passing through) show only a slight decrease in luminance with distance from 887 the luminary. Still greater thickness and opacity make it impossible even to determine the position 888 of the luminary. When the sun or moon is behind a dense isolated cloud, the latter shows a 889 brilliantly illuminated border, and luminous streaks alternating with shadowed bands may be seen 890 around it. 891 892

29

As the optical thickness of a cloud layer often varies from one portion of the layer to another; the 893 luminary may therefore be perceptible through one part of the cloud but not through another. 894 The varying optical thickness and the luminance of the cloud layer, especially at short angular 895 distances from sun or moon, may change considerably with time owing to the movement of the 896 cloud. 897 898 In the case of a uniformly and sufficiently opaque cloud layer, the luminary may be perceptible 899 when it is not too far from the zenith, but may be completely masked when close to the horizon. 900 Sufficiently opaque cloud layers sometimes show a maximum luminance at the zenith when the 901 sun or moon is at low elevation. 902 903 The light reflected from a cloud to the observer is at a maximum when the cloud is opposite to the 904 luminary. The luminance is stronger the greater the denseness of the cloud and its thickness in the 905 line of sight. When sufficiently dense and deep, the cloud reveals shades of grey showing a more 906 or less clear relief; the more tangential the direction of illumination, the more extensive the range 907 of shades. 908 909 Finally, appreciable differences in luminance exist between clouds composed of water droplets 910 and clouds composed of ice crystals. Ice crystal clouds are usually more transparent than water 911 droplet clouds owing to their thinness and to the sparseness of the ice particles. Certain ice crystal 912 clouds, however, occur in thick patches and, can have a high concentration of ice particles. When 913 these clouds are illuminated from behind, they show marked shading. They are, however, 914 brilliantly white in reflected light. 915 916

II.1.2.2 917 918 COLOUR 919 920

Since light of all wavelengths is almost equally strongly diffused by clouds, the colour of 921 the clouds depends primarily on that of the incident light. Haze between observer and cloud may 922 modify cloud colours; e.g., it tends to make distant clouds look yellow, orange or red. Cloud 923 colours are also influenced by special luminous phenomena ("Photometeors" in Part III of this 924 Atlas). 925 926 When the sun is sufficiently high above the horizon, clouds or portions of clouds that chiefly 927 diffuse light from the sun are white or grey. Parts that receive light mainly from the blue-sky are 928 bluish grey. When the illumination by the sun and the sky is extremely weak, the clouds tend to 929 take the colour of the surface below them. 930 931

When the sun approaches the horizon, its colour may change from yellow through orange 932 to red; the sky in the vicinity of the sun and the clouds show a corresponding coloration. The blue 933 of the sky and the colour of the underlying surface may still influence the colours of the clouds. 934 Cloud colours also vary with the height of the cloud and its relative position with regard to 935 observer and sun. 936 937 When the sun is close to the horizon, high clouds may still look almost white whilst low clouds 938 show a strong orange or red coloration. These differences in colour make it possible to obtain an 939 idea of the relative altitudes of the clouds; noting that clouds at the same level appear less red 940 when seen towards the sun than when viewed away from it. 941 942 When the sun is just above or on the horizon, it may redden the under surface of a cloud; when 943 this surface is corrugated, its coloration is distributed in bands alternately lighter (yellowish or 944 reddish tint) and darker (other tints), which make the relief more apparent. 945 946

30

When the sun is just below the horizon, the lowest clouds, in the shadow of the earth, 947 are grey; clouds at the middle levels are rose coloured and those very high may be whitish. 948 949

At night, the luminance of clouds is usually too weak for colour vision; all perceptible 950 clouds appear black to grey, except those illuminated by the moon, which present a whitish 951 appearance. Special illumination (fires, lights of large cities, polar aurora, etc.) may sometimes give 952 a more or less marked colouring to certain clouds. 953 954 II.1.3 955 956 Principles of cloud classification 957 958 Clouds continuously evolve and appear in an infinite variety of forms. However, there is a limited 959 number of characteristic forms frequently observed all over the world, into which clouds are 960 broadly grouped into a classification scheme. The scheme uses genera, species and varieties. This 961 is similar to the systems used in classification of plants or animals, and similarly uses Latin names. 962 963 There are some intermediate or transitional forms of clouds, although observed fairly frequently, 964 that are not described in the classification scheme. The transitional forms are of little interest; 965 they are less stable and in appearance are not very different from the definitions of the 966 characteristic forms. 967 968 There are also clouds, rarely or occasionally observed and usually only in certain parts of the 969 world, that are not included in the present classification scheme. Special clouds are described in 970 Chapter II.6. 971

972 II.1.3.1 973 GENERA 974 975 The classification of clouds has ten main groups, called genera. Each observed cloud is a member 976 of one, and only one, genus. 977 978 II.1.3.2 979 SPECIES 980 981 Most of the genera are subdivided into species, based on the shape of the clouds or their internal 982 structure. A cloud, observed in the sky and identified as a specific genus, may bear the name of 983 only one species. 984 985 II.1.3.3 986 VARIETIES 987 988 Varieties are different arrangements of the visible elements of clouds and varying degrees of 989 transparency. 990 991 A variety may be common to several genera and a cloud may show characteristics of more than 992 one variety. When this is the case all the observed varieties are included in the name of the cloud. 993 994 II.1.3.4 995 996 SUPPLEMENTARY 'FEATURES AND ACCESSORY CLOUDS 997 998 A cloud may show supplementary features attached to it, or may be accompanied by accessory 999 clouds, sometimes partly merged with its main body. "Supplementary features" and "accessory 1000 clouds" may occur at any level of the cloud, or above or below it. 1001

31

1002 One or more supplementary features or accessory clouds may be observed simultaneously with 1003 the same cloud. 1004 1005 II.1.3.5 1006 1007 MOTHER-CLOUDS and SPECIAL CLOUDS 1008 1009 Clouds may form in clear air. They may also form or grow from other clouds, called "mother 1010 clouds”. Depending on the change, one of two suffixes may be used. 1011

1012 a) “genitus” suffix. A part of a cloud may develop and more or less pronounced extensions may 1013

form. These extensions, whether attached to the mother-cloud or not, may become clouds 1014 of a genus different from that of the mother-cloud. They are then given the name of the 1015 appropriate genus, followed by the name of the genus of the mother-cloud with the 1016 addition of the suffix "genitus" (e.g. Cirrus altocumulogenitus, Stratocumulus 1017 cumulogenitus). 1018 1019

b) “mutatus” suffix: The whole or a large part of a cloud may undergo complete internal 1020 transformation, changing from one genus into another. The new cloud is given the name 1021 of the appropriate genus, followed by the name of the genus of the mother-cloud with the 1022 addition of the suffix "mutatus" (e.g. Cirrus cirrostratomutatus, Stratus 1023 stratocumulomutatus). The internal transformation should not be confused with changes 1024 in appearance resulting from the relative movement of clouds and observer. 1025

In addition, there are special cases where clouds may form or grow as a consequence of 1026 certain, often localised, generating factors. These may be either natural, or the result of 1027 human activity. Several cases of "Special clouds" can be distinguished: 1028

a) "flammagenitus": Clouds may develop as a consequence of convection initiated by heat 1029 from forest fires, wild-fires or from volcanic eruption activity. Clouds that are clearly 1030 observed to have originated as a consequence of localised natural heat sources, such as 1031 forest fires, wild-fires or volcanic activity and which, at least in part, consist of water drops, 1032 will be given the name relevant to the genus followed, if appropriate, by the species, 1033 variety and any supplementary features, and finally by the Special Cloud name 1034 "flammagenitus", (e.g. Cumulus congestus flammagenitus, Cumulonimbus calvus 1035 flammagenitus). [Note: cumulus flammagenitus is also known by the unofficial, common 1036 name, 'pyrocumulus']. 1037 1038

b) "homogenitus": Clouds may also develop as a consequence of human activity. Examples 1039 are aircraft condensation trails (Contrails), or clouds resulting from industry, such as 1040 cumuliform clouds generated by rising thermals above power station cooling towers. 1041 Clouds that are clearly observed to have originated specifically as a consequence of human 1042 activity will be given the name of the appropriate genus, followed by the Special Cloud 1043 name "homogenitus". For example: Cumulus cloud formed above industrial plants will be 1044 known as Cumulus (and, if appropriate, the species, variety and any supplementary 1045 features) followed by the Special Cloud name homogenitus; e.g. Cumulus mediocris 1046 homogenitus. 1047 1048

c) Aircraft condensation trails (Contrails) that have persisted for at least 10 minutes will be 1049 given the name of the genus, Cirrus, followed only by the Special Cloud name 1050 "homogenitus", i.e. a Contrail will be known only as Cirrus homogenitus. Because new, or 1051 recently formed aircraft condensation trails may undergo a fairly rapid state of change and 1052 may display a variety of transient shapes, no species, varieties or supplementary features 1053 will be applied to the name. 1054

32

1055 d) "homomutatus": Persistent contrails (Cirrus homogenitus) may be observed, over a period 1056

of time and under the influence of strong upper winds, to grow and spread out over a 1057 larger portion of sky, and undergo internal transformation such that the cloud eventually 1058 takes on the appearance of more natural cirri-form clouds. In this instance, the resulting 1059 clouds will be given the name of the appropriate genus (e.g. cirrus, cirrocumulus, or 1060 cirrostratus) followed by any appropriate species, variety and supplementary features, 1061 followed by the Special Cloud name "homomutatus", (e.g. cirrus floccus homomutatus, 1062 cirrus fibratus homomutatus). 1063

1064 e) "cataractagenitus": Clouds may develop locally in the vicinity of large waterfalls as a 1065

consequence of water broken up into spray by the falls. The downdraft caused by the 1066 falling water is compensated by locally ascending motion of the air. These Special Clouds 1067 will be given the name of the appropriate genus, followed by any appropriate species, 1068 variety and supplementary feature, and followed by the Special Cloud name 1069 "cataractagenitus" (e.g. Cumulus mediocris cataractagenitus, Stratus fractus 1070 cataractagenitus). 1071

1072 f) "silvagenitus": Clouds may develop locally over forests as a result of increased humidity 1073

due to evaporation and evapotranspiration from the tree canopy. These Special Clouds 1074 will be given the name of the appropriate genus, followed by any appropriate species, 1075 variety and supplementary feature, and followed by the Special Cloud name "silvagenitus" 1076 (e.g. Stratus fractus silvagenitus). 1077

1078

1079

33

II.1.4 1080 1081 Table of classification of clouds 1082 1083 [Editorial Note: New classifications in this table are denoted by yellow highlighting] 1084

GENERA SPECIES VARIETIES

Supplementary Features and

Accessory Clouds

MOTHER-CLOUDS and SPECIAL CLOUDS (most commonly occurring mother-clouds

are listed in the same order as genera)

(listed by frequency of observation) GENITUS MUTATUS

Cirrus fibratus uncinus spissatus castellanus floccus

intortus radiatus vertebratus duplicatus

mamma fluctus

Cirrocumulus Altocumulus Cumulonimbus Homo

Cirrostratus Homo

Cirrocumulus stratiformis lenticularis castellanus floccus

undulatus lacunosus

virga mamma cavum

_ Cirrus Cirrostratus Altocumulus Homo

Cirrostratus fibratus nebulosus

duplicatus undulatus

- Cirrocumulus Cumulonimbus

Cirrus Cirrocumulus Altostratus Homo Altocumulus stratiformis

lenticularis castella nus floccus volutus

translucidus perlucidus opacus duplicatus undulatus radiatus lacunosus

virga mamma cavum fluctus asperitas

Cumulus Cumulonimbus

Cirrocumulus Altostratus Nimbostratus Stratocumulus

Altostratus - translucidus opacus duplicatus undulatus radiatus

virga praecipitatio pannus mamma

Altocumulus Cumulonimbus

Cirrostratus Nimbostratus

Nimbostratus - - praecipitatio virga pannus

Cumulus Cumulonimbus

Altocumulus Altostratus Stratocumulus

Stratocumulus stratiformis lenticularis castellanus floccus volutus

translucidus perlucidus opacus duplicatus undulatus radiatus lacunosus

virga mamma praecipitatio fluctus asperitas cavum

Altostratus Nimbostratus Cumulus Cumulonimbus

Altocumulus Nimbostratus Stratus

Stratus nebulosus fractus

opacus translucidus undulatus

praecipitatio fluctus

Nimbostratus Cumulus Cumulonimbus Homo Silva Cataracta

Stratocumulus

Cumulus humilis mediocris congestus fractus

radiatus virga praecipitatio pileus velum arcus pannus fluctus tuba

Altocumulus Stratocumulus Flamma Homo Cataracta

Stratocumulus Stratus

34

Cumulonimbus calvus capillatus

- praecipitatio virga pannus incus mamma pileus velum arcus murus cauda flumen tuba

Altocumulus Altostratus Nimbostratus Stratocumulus Cumulus Flamma Homo

Cumulus

1085

1086

35

II.1.5 1087

1088

Table of Abbreviations and Symbols of Clouds 1089

1090

GENERA SPECIES

Designations Abbreviations Symbols Designations Abbreviations

Cirrus

Cirrocumulus

Cirrostratus

Altocumulus

Altostratus

Nimbostratus

Stratocumulus

Stratus

Cumulus

Cumulonimbus

Ci

Cc

Cs

Ac

As

Ns

Sc

St

Cu

Cb

fibratus

uncinus

spissatus

castellanus

floccus

stratiformis

nebulosus

lenticularis

fractus

humills

mediocris

congestus

volutus

calvus

capillatus

fib

unc

spi

cas

flo

str

neb

len

fra

hum

med

con

vol

cal

cap

VARIETIES

SUPPLEMENTARY FEATURES and ACCESSORY

CLOUDS

Designations

Abbreviations Abbreviations Designations Abbreviations

intortus

vertebratus

undulatus

radiatus

lacunosus

duplicatus

translucidus

perlucidus

opacus

in

ve

un

ra

Ia

du

tr

pe

op

incus

mamma

virga

cavum

fluctus

asperitas

praecipitatio

arcus

murus

tuba

pileus

velum

pannus

cauda

flumen

inc

mam

vir

cav

flu

asp

pra

arc

mur

tub

pil

vel

pan

cau

flm

MOTHER-CLOUDS and SPECIAL CLOUDS

GENITUS MUTATUS Designations Abbreviations Designations Abbreviations

36

cirrocumulogenitus

altocumulogenitus

altostratogenitus

nimbostratogenitus

stratocumulogenitus

cumulogenitus

cumulonimbogenitus

flammagenitus

homogenitus

silvagenitus

cataractagenitus

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

1091

37

II.2- DEFINITIONS OF CLOUDS 1092

II.2.1 1093

Some useful concepts 1094

II.2.1.1 1095

HEIGHT, ALTITUDE, VERTICAL EXTENT 1096

HEIGHT: Vertical distance from the point of observation (which may be on a hill or a mountain) to the point being measured.

HEIGHT OF CLOUD BASE: Surface observations - height above ground level of the cloud base; aircraft observers – height above sea level of the 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

II.2.1.2 1098

LEVELS 1099

Clouds 1 are generally encountered over a range of altitudes varying from sea level to the top of 1100 the troposphere (the tropopause). The troposphere has been vertically divided into three levels: 1101 high , middle and low. Each level is defined by the range of levels at which clouds of certain 1102 genera occur most frequently.The levels overlap and their limits vary with latitude. 1103

1104

1105

Table #: Approximate heights, and genera in each, of the levels 1106

1107

Most clouds are confined within their level with a few notable exceptions: 1108

(a) Altostratus is usually found in the middle level, but it often extends higher; 1109

(b) Nimbostratus is almost invariably found in the middle level, but it usually extends into 1110

the other two levels; 1111

(c) Cumulus and Cumulonimbus usually have their bases in the low level, but their vertical 1112

extent is often so great that their tops may reach into the middle and high levels. 1113

Levels Genera Polar Regions Temperate Regions Tropical Regions

High Cirrus

Cirrocumulus

Cirrostratus

3 – 8 km

(10 000 – 25 000 ft)

5 – 13 km

(16 500 – 45 000 ft)

6 –18 km

(20 000 – 60 000 ft)

Middle Altocumulus

Altostratus

Nimbostratus

2 – 4km

(6 500 – 13 000 ft)

2 – 7 km

(6 500 – 23 000 ft)

2 – 8 km

(6 500 – 25 000 ft)

Low Stratus

Stratocumulus

Cumulus

Cumulonimbus

From the Earth's

surface to 2 km

(0 – 6 500ft)

From the Earth's surface

to 2 km

(0 – 6 500ft)

From the Earth's

surface to 2 km

(0 – 6 500ft)

38

1114

When the height of a particular cloud is known, the concept of levels may be of some help to the 1115

observer in identifying this cloud. The genus can be determined by making a choice from among 1116

the genera normally encountered in the level corresponding to its height. 1117

1118

II.2.2 1119

Definitions of clouds 1120

[Editorial Comment: the definitions of this section from the previous edition of the ICA have been 1121

moved to the tables below with very little change. New definitions are highlighted in yellow in 1122

their respective table] 1123

11.2.2.1 1124

GENERA 1125

Consideration of the most typical forms of clouds leads to the recognition of ten genera. The 1126

definitions of the genera given below do not cover all possible aspects, but are limited to a 1127

description of the main types and of the essential characteristics necessary to distinguish a given 1128

genus from genera having a somewhat similar appearance. 1129

1130

Cirrus

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.

Cirrocumulus

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.

Cirrostratus

Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance,

totally or partly covering the sky, and generally producing halo phenomena.

Altocumulus 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.

Altostratus

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.

Nimbostratus

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

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.

Stratus

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

II.2.2.2 1132

SPECIES 1133

Most cloud genera are subdivided into species due to peculiarities in the shape of clouds and 1134

differences in their internal structure. 1135

A cloud belonging to a certain genus, may bear the name of one species only; this means that the 1136

species are mutually exclusive, On the other hand, certain species may be common to several 1137

genera. When none of the definitions of the species are relevant to a genus, no species is 1138

indicated. 1139

1140

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

TABLE OF SPECIES AND THE GENERA WITH WHICH THEY MOST FREQUENTLY 1143

OCCUR 1144

[Editorial Note: The following table from the 1975 edition did not map across well 1145

from the original pdf file to the word equivalent. The version of the table proposed 1146

for the new edition contains the same information, but simply has a different 1147

appearance] 1148

Genera Ci Cc Cs Ac As Ns Sc St Cu Cb

Species

fibratus (fib) ● ●

uncinus (unc) ●

spissatus (spi) ●

castellanus (cas) ● ● ● ●

floccus (flo) ● ● ● ●

stratiformis (str) ● ● ●

nebulosus (neb) ● ●

lenticularus (len) ● ● ●

volutus (vol) ● ●

fractus (fra) ● ●

humilis (hum) ●

mediocris (med) ●

congestus (con) ●

calvus (cal) ●

capillatus (cap) ●

1149

1150

42

11.2.2.3 1151

VARIETIES 1152

Varieties are the arrangements of the macroscopic elements and the degree of transparency of the 1153

genera. 1154

A given cloud may bear the names of different varieties, which means that varieties are not 1155

mutually exclusive. 1156

The exceptions to this are the varieties translucidus and opacus both of which are mutually 1157

exclusive. 1158

On the other hand, certain varieties may be present in several genera. 1159

The fact that a number of varieties has been established does not imply that a specific cloud 1160

must necessarily receive the name of one or more of those varieties 1161

1162

Arr

an

ge

me

nt

of

ma

cro

sco

pic

ele

me

nts

Intortus

Cirrus, the filaments of which are very irregularly curved and often

seemingly entangled in an erratic and unpredictable (known as

capricious) manner.

Vertebratus

Clouds, the elements of which are arranged in a manner suggestive of

vertebrae, ribs, or a fish skeleton.

This term applies mainly to Cirrus.

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. D

eg

ree

of

tra

nsp

are

ncy

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

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.

Opacus

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

VARIETIES AND THE GENERA WITH WHICH THEY MOST FREQUENTLY OCCUR 1166

1167

[Editorial Note: The following table from the 1975 edition did not map across well from the 1168

original pdf file to the word equivalent. The version of the table proposed for the new 1169

edition contains the same information, but simply has a different appearance] 1170

1171


Varieties

intortus (in) ●

vertebratus (ve) ●

Undulatus (un) ● ● ● ● ● ●

radiatus (ra) ● ● ● ● ●

lacunosus (la) ● ● ●

dublicatus (du) ● ● ● ● ●

translucidus (tr) ● ● ● ●

perlucidus (pe) ● ●

opacus (op) ● ● ● ●

1172

II.2.2.4 1173

SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS 1174

44

Clouds sometimes have supplementary features attached to them or may be accompanied by 1175

other usually smaller clouds, known as accessory clouds which are separate from their main body 1176

or partly merged with it. 1177

A given cloud may present simultaneously one or more supplementary features or accessory 1178

clouds, which means that supplementary features and accessory clouds are not mutually exclusive. 1179

1180

Su

pp

lem

en

tary

fe

atu

res

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

Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.

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'.

Acc

esso

ry c

lou

ds

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.

1181

1182

1183

46

SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS AND THE GENERA WITH 1184

WHICH THEY MOST FREQUENTLY OCCUR 1185

[Editorial Note: The following table from the 1975 edition did not map across well from 1186

the original pdf file to the word equivalent. The version of the table proposed for the new 1187

edition contains the same information (except for the inclusion of the new classifications) , 1188

but simply has a different appearance] 1189


Supplementary

Features and

Accessory Clouds

incus (inc) ●

mamma (mam) ● ● ● ● ● ●

virga (vir) ● ● ● ● ● ● ●

cavum (cav) ● ● ●

fluctus (flu) ● ● ● ●

asperitas (asp) ● ● ●

praecipitatio (pra) ● ● ● ● ● ●

arcus (arc) ● ●

murus (mur) ●

tuba (tub) ● ●

pileus (pil) ● ●

velum (vel) ● ●

pannus (pan) ● ● ● ●

cauda (cau) ●

flumen (flm) ●

1190

1191

47

II.3 - DESCRIPTIONS OF CLOUDS 1192

II.3.1 1193

Cirrus (Ci) 1194

(Howard 1803) 1195

1196

II.3.1.1 1197

DEFINITION 1198

1199

Detached clouds in the form of white, delicate filaments or white or mostly white patches or 1200 narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both. 1201

1202

1203

II.3.1.2 1204

SPECIES 1205

1206

Cirrus fibratus (Ci fib) - BESSON 1921, CCH 1953 1207

1208

Nearly straight or more or less irregularly curved white filaments, which are always fine 1209

and do not terminate in hooks or tufts. The filaments are, for the most part, distinct from 1210

one another. 1211

1212

Cirrus uncinus (Ci unc) - MAZE 1889 1213

1214

Cirrus without grey parts, often shaped like a comma, terminating at the top in a hook, or in a 1215

tuft the upper part of which is not in the form of a protuberance. 1216

1217

Cirrus spissatus (Ci spi) - CCH 1953 1218

1219

Cirrus in patches, sufficiently dense to appear greyish when viewed towards the sun; it may 1220 also veil the sun, obscure its outline or even hide it. Cirrus spissatus often originates from the 1221 upper part of a Cumulonimbus. 1222

1223

Cirrus castellanus (Ci cas) - CCH 1953 1224

1225

Fairly dense Cirrus in the form of small rounded and fibrous turrets or masses rising from a 1226

common base, and sometimes having a crenelated (castle battlement) appearance. The 1227

apparent width of the turret like protuberances may be smaller or greater than 1° when 1228

observed at an angle of more than 30° above the horizon; distinct to Cirrocumulus castellanus 1229

48

where the width is less than 1°. A width of 1° is approximately the width of the little finger at 1230

arm’s length. 1231

1232

Cirrus floccus (Ci flo) - VINCENT 1903, CEN 1930 1233

1234

Cirrus in the form of more or less isolated, small, rounded tufts, often with trails. The 1235

apparent width of the tufts may be smaller or greater than one degree when observed at 1236

an angle of more than 30 degrees above the horizon; distinct to Cirrocumulus castellanus 1237

where the width is less than 1°. A width of 1° is approximately the width of the little finger at 1238

arm’s length. 1239

1240

1241

II.3.1.3 1242

1243

VARIETIES 1244

1245

Cirrus intortus (Ci in) - CCH 1953 1246

Cirrus, the filaments of which are very irregularly curved and often seemingly 1247 entangled in an irregular manner. 1248

49

1249

Cirrus radiatus (Ci ra) - CEN 1926 1250

Cirrus arranged in parallel bands which, owing to the effect of perspective; appear to 1251 converge towards one point or towards two opposite points of the horizon. These 1252 bands are often partly composed of Cirrocumulus or Cirrostratus. 1253 1254

Cirrus vertebratus (Ci ve) - MAZE 1889, OSTHOFF 1255

1905 1256

Cirrus, the elements of which are arranged in a manner suggestive of vertebrae, ribs, or a fish skeleton. 1257 1258

Cirrus duplicatus (Ci du) - MAZE 1259

1889 1260

Cirrus arranged in superposed layers at slightly different levels, sometimes merged in 1261 places. Most Cirrus fibratus and Cirrus uncinus belong to this variety. 1262 1263

II.3.1.4 1264

1265

SUPPLEMENTARY FEATURES AND ACCESSORY 1266

CLOUDS 1267

1268

Cirrus, usually of the species spissatus, sometimes shows mamma. 1269 1270 II.3.1.5 1271 1272

CLOUDS FROM WHICH CIRRUS MAY FORM 1273

1274

Cirrus often evolves from: 1275

virga of Cirrocumulus (Ci cirrocumulogenitus); or 1276

virga of Altocumulus (Ci altocumulogenitus), or 1277

the upper part of a Cumulonimbus (Ci cumulonimbogenitus) 1278 1279 1280

Cirrus clouds may also form as a result of the transformation of non-uniform 1281 Cirrostratus by evaporation of its thinner parts (Ci cirrostratomutatus). 1282 1283

II.3.1.6 1284

1285

MAIN DIFFERENCES BETWEEN CIRRUS AND SIMILAR CLOUDS OF OTHER GENERA 1286

1287 Cirrus is distinguished from Cirrocumulus by: 1288

its mainly fibrous or silky appearance; 1289

the absence of small cloud elements in the form of grains, ripples, etc; 1290

the absence of a corona or irisation; 1291

possible shading in thick Cirrus and no shading in Cirrocumulus; 1292

50

the absence of virga 1293 1294 1295 Cirrus is distinguished from Cirrostratus by: 1296

its discontinuous structure or, if they are in patches or bands, by their small horizontal extent 1297 or the narrowness of their continuous parts; 1298

sometimes being thick enough to mask the sun or moon, Cirrostratus is always thin enough to 1299 disk of the sun or moon; 1300

the absence of undulations; 1301

possible mamma, usually in the species spissatus 1302

the absence of a corona; 1303

only having, at best, a partial circular halo; 1304

1305 Cirrus near the horizon may be difficult to distinguish from Cirrostratus, owing to the 1306 effect of perspective. 1307 1308 Cirrus is distinguished from Altocumulus by: 1309

its mainly fibrous or silky appearance; 1310

the absence of cloud elements in the form of laminae, rolls, etc; 1311

the usual absence of any shading and only partial shading in the species spissatus; 1312

the absence of undulations; 1313

the absence of virga; 1314

the absence of a corona 1315 1316 Thick Cirrus is distinguished from Altostratus patches by: 1317

smaller horizontal extent; 1318

mostly white appearance; 1319

the absence of precipitation or virga; 1320

the absence of undulations 1321 1322 II.3.1. 7 1323 1324

PHYSICAL CONSTITUTION 1325

Cirrus is composed almost exclusively of ice crystals. These crystals are in general very small; 1326 together with their sparseness this accounts for the transparency of most Cirrus clouds. 1327

1328

Dense Cirrus patches or Cirrus in tufts may contain ice crystals large enough to acquire an appreciable 1329 terminal velocity. Trails of considerable vertical extent fall from the base of Cirrus. 1330

Infrequently, the ice crystals in the trails melt into small water droplets; the trails are then greyish, in 1331 contrast with their usual white appearance. A rainbow may form. 1332

1333

The trails curve irregularly or slant as a result of wind shear and of the variation in size of the constituent 1334 particles; consequently, Cirrus filaments near the horizon do not appear parallel to it. 1335

1336

Halo phenomena may occur; circular halos almost never show a complete ring, due to the narrowness 1337 of the Cirrus clouds. 1338

1339

51

1340

II.3.1.8 1341 EXPLANATORY REMARKS 1342

Cirrus tufts with rounded tops often form in clear air. Fibrous trails may appear under the tufts; 1343 the tops then gradually lose their roundness. Subsequently, the tufts may disappear completely; 1344 the clouds are then in the form of filaments (species fibratus or uncinis). 1345

1346

Cirrus in the form of filaments may develop also from dense Cirrus patches, from Altocumulus 1347 castellanus and floccus and, occasionally at very low temperatures, from Cumulus congestus. 1348

1349 Cirrus may also form from aircraft condensation trails (contrails) that have persisted for at least 10 1350 minutes (Ci homogenitus). No species, varieties or supplementary features are identified with Cirrus 1351 homogenitus as new or recently formed aircraft condensation trails usually undergo a fairly rapid 1352 state of change and may display a variety of transient shapes. 1353

1354

Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong 1355 upper winds, take on the form of species and/or varieties of Cirrus. When this occurs, the Cirrus is 1356 identified by relevant species and varieties followed by homomutatus. 1357

1358

Colour: 1359

At all times of day, Cirrus not too close to the horizon is white, whiter than any other cloud in the same 1360 part of the sky [Fig 26]. 1361

With the sun on the horizon it is whitish, while lower clouds may be tinted yellow or orange [Fig 27]. 1362

When the sun sinks below the horizon, Cirrus high in the sky is yellow, then pink, red and finally grey. 1363 The colour sequence is reversed at dawn. 1364

Cirrus near the horizon often takes a yellowish or orange tint [Fig 28] owing to the great thickness of 1365 air traversed by the light in passing from the cloud to the observer. These tints are less conspicuous in 1366 clouds in the low and middle levels. 1367 1368

II.3.2 1369

Cirrocumulus (Cc) (HOWARD 1803; RENOU 1855) 1370

II.3.2.1 1371

DEFINITION 1372

Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the 1373 form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the 1374 elements have an apparent width of less than one degree. 1375 1376

1377

II.3.2.2 1378

SPECIES 1379

Cirrocumulus stratiformis (Cc str) - CCH 1953 1380

Cirrocumulus in the form of a relatively extensive sheet or layer, sometimes showing breaks. 1381 1382

52

Cirrocumulus lenticularis (Cc len) - LEY 1894, CEN 1930 1383

Lens or almond shaped patches of Cirrocumulus; often very elongated and usually with well-defined 1384 outlines. The patches are more or less isolated, mostly smooth and are very white throughout. 1385 Irisation is sometimes observed in these clouds. 1386 1387

Cirrocumulus castellanus (Cc cas) - CCH 1953 1388

Cirrocumulus where some elements are vertically developed in the form of small turrets, rising from a 1389 common horizontal base. The apparent width of the turrets is always less than one degree, when 1390 observed at an angle of more than 30 degrees above the horizon. Castellanus develops due to 1391 instability at that level. 1392 1393

Cirrocumulus floccus (Cc flo) - VINCENT 1903, CCH 1953 1394

Very small cumuliform tufts, the lower parts of which are more or less ragged. The apparent width of 1395 each tuft is always less than one degree, when observed at an angle of more than 30 degrees above the 1396 horizon. Floccus develops due to instability at that level. Cirrocumulus fioccus sometimes evolves form 1397 Cirrocumulus castellanus the base of the latter having dissipated. 1398 1399

II.3.2.3 1400

VARIETIES 1401

1402

Cirrocumulus undulatus (Cc un) - CLAYTON 1896, CCH 1953 1403

Cirrocumulus showing one or two systems of undulations. 1404

1405

Cirrocumulus lacunosus (Cc Ia) - CCH 1953 1406

Cirrocumulus in a patch, sheet or layer, showing small more or less regularly distributed round holes, 1407 many of them with fringed edges. Cloud elements and clear spaces are often arranged like a net or a 1408 honeycomb. 1409 1410

II.3.2.4 1411


1413

Small virga may be present, particularly falling from Cirrocumulus castellanus and floccus. 1414

Cirrocumulus occasionally shows mamma. 1415

Cavum is rarely observed and only when the Cirrocumulus consists of strongly supercooled water 1416 droplets. 1417 1418

II.3.2.5 1419

53

CLOUDS FROM WHICH CIRROCUMULUS MAY FORM 1420

1421

Cirrocumulus often forms from the: 1422

transformation of Cirrus (Cc cirromutatus); or 1423

transformation of Cirrostratus (Cc cirrostratomutatus); or 1424

decrease in size of the elements of a patch, sheet or layer of Altocumulus (Cc 1425 altocumulomutatus); 1426

Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong 1427 upper winds, transform into Cirrocumulus (Cc homomutatus). 1428 1429

II.3.2.6 1430

MAIN DIFFERENCES BETWEEN CIRROCUMULUS AND SIMILAR CLOUDS OF OTHER GENERA 1431

1432

Cirrocumulus is distinguished from Cirrus by: 1433

having ripples or grains; any fibrous, silky or smooth portions do not collectively constitute its 1434 greater part; 1435

possible virga; 1436

no shading; 1437

no partial circular halo or any other halo phenomena 1438 1439

Cirrocumulus is distinguished from Cirrostratus by: 1440

having ripples or grains; any fibrous, silky or smooth portions do not collectively constitute its 1441 greater part; 1442

usually being thin enough to show the disk of the sun or moon but sometimes thicker so that 1443 only the position of the sun or moon is revealed; 1444

possible irisation; 1445

no halo phenomena; 1446

possible virga; 1447

possible mamma 1448 1449 Cirrocumulus differs from Altocumulus by: 1450

most of its elements being very small (an apparent width of less than one° when observed at 1451 an angle of more than 30° above the horizon); 1452

not having any shading; 1453

no halo (uncommon in Altocumulus and usually only a partial halo when of circular nature) 1454 1455

II.3.2. 7 1456


Cirrocumulus is composed almost exclusively of ice crystals; strongly supercooled water droplets may 1458 occur but are usually rapidly replaced by ice crystals. 1459

A corona or irisation may sometimes be observed. 1460 1461

54


Lens or almonds shaped Cirrocumulus elements may be produced by local orographic lifting of a layer 1464 of moist air. 1465

In middle and high latitudes, Cirrocumulus is usually observed with Cirrus and/or Cirrostratus. 1466

In low latitudes, Cirrocumulus is less often accompanied by Cirrus or Cirrostratus. 1467

A cloud is not identified as Cirrocumulus if it consists of a patch of incompletely developed small 1468 elements, such as often observed on the edges of a patch or sheet of Altocumulus or those sometimes 1469 present in separate patches at the same level as Altocumulus. 1470

In case of doubt, a cloud should be identified as Cirrocumulus only when it has evolved from, or is 1471 obviously connected with Cirrus or Cirrostratus. 1472 1473

II.3.3 1474 1475 Cirrostratus (Cs) (HOWARD 1803; RENOU 1855) 1476

II.3.3.1 1477

DEFINITION 1478 1479 Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly 1480 covering the sky, often producing halo phenomena. 1481 1482 II.3.3.2 1483 SPECIES 1484 1485 Cirrostratus fibratus

(Cs fib) - BESSON 1921, CCH 1953 1486

A fibrous veil of Cirrostratus in which thin striations can be observed. Cirrostratus fibratus may develop 1487 from Cirrus fibratus or less likely, Cirrus spissatus. 1488

1489

Cirrostratus nebulosus (Cs neb) - CLAYDEN 1905 1490

A nebulous veil of Cirrostratus with no distinct detail. Sometimes the veil is so light that it is barely visible; 1491 it may also be relatively dense and easily visible. 1492 1493

II.3.3.3 1494 VARIETIES 1495

1496 Cirrostratus duplicatus (Cs du) - MAZE 1889, DE QUERVAIN 1908, CCH 1953 1497

Cirrostratus arranged in superposed sheets or layers, at slightly different levels, sometimes partly merged. 1498

1499 Cirrostratus undulatus (Cs un) - CCH 1953 1500

Cirrostratus showing undulations. 1501

1502 II.3.3.4 1503 SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS 1504

1505 None 1506 1507 II.3.3.5 1508 CLOUDS FROM WHICH CIRROSTRATUS MAY FORM 1509 1510

55

Cirrostratus may form by: 1511

● merging of elements of Cirrus (Cs cirromutatus); 1512

● merging of elements of Cirrocumulus (Cs cirrocumulomutatus); 1513

● ice crystals falling from Cirrocumulus (Cs cirrocumulogenitus); 1514

● the thinning of Altostratus (Cs altostratomutatus); 1515

● the spreading out of the anvil of a Cumulonimbus (Cs cumulonimbogenitus) 1516 1517

Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong upper 1518 winds, transform into Cirrostratus (Cs homomutatus). Occasionally, multiple persistent contrails (Ci 1519 homogenitus) may spread and give almost complete sky coverage of Cirrostratus. 1520 1521 MAIN DIFFERENCES BETWEEN CIRROSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA 1522

1523

Cirrostratus is distinguished from Cirrus by: 1524 ● being in the form of a veil, usually of great horizontal extent; when seen in patches it is usually 1525

forming or dissipating; 1526 ● having no distinct details such as filaments and undulations when of the species nebulosus; 1527 ● always being thin enough to show the disk of the sun or moon; 1528 ● displaying numerous halo phenomena and when of the circular form, the halos are usually 1529

complete circles; 1530 ● possibile corona; 1531 ● no mamma. 1532

1533

Cirrostratus is distinguished from Cirrocumulus by: 1534

● the absence of very fine elements in the form of grains; 1535

● possibility of being partly shaded; 1536

● no irisation; 1537

● halo phenomena; 1538

● no mamma 1539

● no virga; 1540 1541

Cirrostratus is distinguished from Altocumulus by: 1542

● the absence of rounded masses or turrets; 1543

● never being completely shaded; 1544

● no irisation; 1545

● no mamma; 1546

● no virga; 1547

1548

Cirrostratus is distinguished from Altostratus by: 1549 ● being thin enough to show the disk of the sun or moon, except when the sun or moon is low on 1550

the horizon; Altostratus translucidus will reveal the position of the sun or moon; Altostratus 1551 opacus will obscure the sun or moon; 1552

● being partly shaded or having no shading; Altostratus translucidus is partly shaded; Altostratus 1553 opacus is shaded throughout; 1554

● usual presence of halo phenomena; 1555 ● no mamma, 1556 ● no precipitation; 1557 ● no virga 1558

1559

56

Cirrostratus can be confused with very thin Stratus that, at angular distances of less than 45° from the 1560 sun, may appear very white. Cirrostratus: 1561

● is whitish throughout; 1562 ● may have a fibrous appearance; 1563 ● often displays halo phenomena; Stratus only displays halo phenomena on rare occasions when 1564

it consist of small ice particles 1565 1566

Cirrostratus can be confused for a veil of haze and is difficult to discern through haze. Haze is opalescent or 1567 has a dirty yellowish to brownish colour. Cirrostratus is greyish in areas if there is partial shading; else it is 1568 white throughout. 1569 1570 Il.3 .3.7 1571

1572 PHYSICAL CONSTITUTION 1573

1574

Cirrostratus is a cloud of moderate vertical extent and is composed of a relatively sparse number of mainly 1575 small ice crystals. These factors combine to account for the transparency of this cloud through which the 1576 disk (outline) of the sun is visible, at least when the sun is not too close to the horizon. 1577

Sometimes Cirrostratus ice crystals are large enough to acquire an appreciable terminal velocity and trailing 1578 filaments form. This gives these Cirrostratus clouds a fibrous appearance. 1579

Halo phenomena are often observed in thin Cirrostratus; sometimes the veil of Cirrostratus is so thin that a 1580 halo provides the only indication of its presence. 1581 1582 Il.3.3.8 1583 1584 EXPLANATORY REMARKS 1585

1586 Cirrostratus, not completely covering the sky usually has an irregular border fringed with Cirrus; or may 1587 infrequently be straight edged and clear cut 1588 1589

Cirrostratus is never thick enough to prevent objects on the ground from casting shadows, at least when 1590 the sun is high above the horizon. When the sun is low (less than about 30°) the relatively longer light 1591 path through a Cirrostratus veil may reduce the light intensity so much that shadows do not exist. 1592

The remarks about the colours of Cirrus are, to a great extent, also valid for Cirrostratus. 1593 1594 II.3.4 1595

1596

Altocumulus (Ac) (RENOU 1870) 1597

1598

II.3.4.1 1599

DEFINITION 1600

1601

White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed 1602 of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or 1603 may not be merged; most of the regularly arranged small elements usually have an apparent width 1604 between one and five degrees. 1605 1606

1607

57

II.3.4.2 1608

SPECIES 1609

1610

Altocumulus stratiformis (Ac str) - CCH 1953 1611

An extensive sheet or layer of separate or merged elements. This is by far the most frequent species. 1612 1613

Altocumulus lenticularis (Ac len) - LEY 1894, CEN 1930 1614

A patch in the shape of a lens or almond, often very elongated and usually with well defined outlines. 1615 The patch can be small elements, closely grouped together; or one generally smooth unit with 1616 pronounced shadings. 1617

Irisation is occasionally visible. 1618 1619

Altocumulus castellanus (Ac cas) - CCH 1953 1620

Cumuliform turrets rising vertically from cloud elements connected by a common horizontal base. The 1621 turrets: 1622

seem to be arranged in lines; 1623

give the cloud a crenelated (castle battlement) appearance; 1624

are sometimes taller than they are wide; 1625

are especially evident when the cloud is seen from the side. 1626

Altocumulus castellanus is an indicator of instability at that level; when it acquires considerable 1627 vertical extent, it is classified as: 1628

Cumulus congestus altocumulogenitus if it is strongly sprouting or is of great vertical extent; 1629

Cumulonimbus altocumulogenitus if part of its upper portion is smooth, fibrous or striated, or 1630 if the cloud produces lightning, thunder or showers of hail. 1631

1632

Altocumulus floccus (Ac flo) - VINCENT 1903 1633

Small tufts of cumuliform appearance; the lower parts of the tufts are generally ragged and often 1634 accompanied by fibrous trails (ice crystal virga). Altocumulus floccus is an indication of instability at 1635 that level. Altocumulus floccus sometimes forms as a result of the dissipation of the base of 1636 Altocumulus castellanus. 1637

1638

Altocumulus volutus (Ac vol) - ___ 2016 1639

A long, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal 1640

axis. It usually occurs as a single line and seldom extends from horizon to horizon. 1641

This species of Altocumulus is rare. 1642

1643

II.3.4.3 1644 VARIETIES 1645

1646

Altocumulus translucidus (Ac tr) - CEN 1930 1647

58

A patch, sheet or layer of Altocumulus, the greater part is sufficiently translucent to reveal the 1648 position of the sun or moon. This variety often occurs in the species stratiformis and lenticularis. 1649

1650

Altocumulus perlucidus (Ac pe) - CCH 1953 1651

A patch, sheet or layer of Altocumulus where the spaces between the elements allow the sun, the 1652 moon, the blue of the sky or higher clouds to be seen. This variety often occurs in the species 1653 stratiformis. 1654 1655

Altocumulus opacus (Ac op) - CEN 1930 1656

A patch, sheet or layer of Altocumulus, the greater part is sufficiently opaque to mask completely the 1657 sun or moon. Most often, the base of this variety of Altocumulus is even and its apparent subdivision 1658 into merged elements results from the irregularity of its upper surface. The base is sometimes 1659 uneven and the elements then stand out in true relief, particularly with a low sun. The variety opacus 1660 occurs fairly often in the species stratiformis. 1661 1662

Altocumulus duplicatus (Ac du) - MAZE 1889, DE QUERVAIN 1908, CEN 1926 1663

Altocumulus of two or more superposed patches, sheets or layers, close together and sometimes 1664 partly merged. This variety occurs in the species stratiformis and lenticularis. 1665

1666

Altocumulus undulatus (Ac un) - CLAYTON 1896, CEN 1930 1667

Altocumulus composed of separate or merged elements, either elongated and broadly parallel, or 1668 arranged in ranks and files having the appearance of two distinct systems of undulations. 1669

1670

Altocumulus radiatus (Ac ra) - CEN 1926 1671

Altocumulus with approximately straight parallel bands. The bands appear to converge towards one 1672 point, or two opposite points of the horizon. 1673 1674

Altocumulus lacunosus (Ac la) - CCH 1953 1675

Sheet, layer or patches of Altocumulus showing more or less regularly distributed round holes, many 1676 of them with fringed edges. The cloud elements and clear spaces are often arranged in a net or 1677 honeycomb manner. The details change rapidly. 1678 1679

II.3.4.4 1680


1682

Virga appears with most species of Altocumulus. Altocumulus floccus frequently dissipates, leaving 1683

very white trails of ice crystal virga. If the Altocumulus floccus completely dissipates, the virga is 1684

identified as Cirrus altocumulogenitus 1685

1686 Mamma, cavum, fluctus and asperitas are sometimes visible in Altocumulus. 1687 1688

59

II.3.4.5 1689 CLOUDS FROM WHICH ALTOCUMULUS MAY FORM 1690

1691

Altocumulus may form by: 1692

an increase in size or a thickening of at least some elements of a patch, sheet or layer of 1693 Cirrocumulus (Ac cirrocumulomutatus); 1694

subdivision of a layer of Stratocumulus (Ac stratocumulomutatus); 1695

transformation of Altostratus (Ac altostratomutatus); 1696

transformation of Nimbostratus (Ac nimbostratomutatus); 1697

spreading of the tops of Cumulus when they reach a middle level stable layer (Ac 1698 cumulogenitus). The vertical development of the Cumulus may: 1699

o cease at the stable layer, resulting in patches of Altocumulus spreading out from the 1700 top of the Cumulus; 1701

o temporarily stop at the stable layer and then resume their growth in places or 1702 throughout, resulting in Altocumulus on the sides of the Cumulus; 1703

1704

Altocumulus may also be observed on or near the sides of Cumulonimbus. This Altocumulus often 1705 forms while the mother-cloud is still in the Cumulus stage. Nevertheless, the Altocumulus is classified 1706 as cumulonimbogenitus, not cumulogenitus. 1707 1708

1709

II.3.4.6 1710

MAIN DIFFERENCES BETWEEN ALTOCUMULUS AND SIMILAR CLOUDS OF OTHER GENERA 1711

1712

Altocumulus is distinguished from Cirrus by: 1713

the absence of a mainly fibrous or silky appearance; 1714

the presence of cloud elements in the form of laminae, rolls, etc; 1715

possible shading throughout which never occurs with Cirrus; 1716

usually being partly shaded as against Cirrus usually being without shading; 1717

the possibility of cloud elements arranged in undulations; 1718

possibile virga; 1719

the usual presence of a corona 1720 1721

Altocumulus sometimes produces descending trails of fibrous appearance (ice 1722 crystal virga). The cloud is identified as Altocumulus and not as Cirrus, as long as the 1723 cloud has a part without a fibrous appearance or a silky sheen 1724

1725 Altocumulus is distinguished from Cirrocumulus by: 1726

the presence of any shading, even if most of the regularly arranged cloud elements have an 1727 apparent width of less than 1°; 1728

most of the regularly arranged elements having an apparent width of between 1 and 5°, even 1729 if the cloud has no shading; 1730

possibile halo phenomena 1731 1732 1733 1734 Altocumulus is distinguished from Altostratus by: 1735

the presence of thin plates, rounded masses, rolls, etc; 1736

60

possibly being thin enough to see the disk of the sun or moon; Altostratus will only reveal the 1737 position of the sun or moon or completely mask the sun or moon; 1738

the absence of the accessory cloud pannus; 1739

the absence of any precipitation; Altostratus may produce rain, snow or ice pellets; 1740

the absence of halo phenomena or irisation 1741 1742 Altocumulus is distinguished from Stratocumulus by: 1743

most of the regularly arranged elements having, when observed at an angle of more than 30° 1744 above the horizon, an apparent width between 1 and 5°; 1745

the absence of any precipitation; Stratocumulus can have weak falls of rain, snow or snow 1746 pellets 1747

1748 Altocumulus floccus is distinguished from Cumulus fractus, humilis and mediocris by: 1749

often having fibrous trails (virga) compared to the often flat base of the Cumulus; 1750

generally being smaller than the Cumulus 1751 1752

1753

II.3.4. 7 1754


1756

Altocumulus is invariably composed of water droplets. This is evident from the fairly low 1757 transparency of the elements and that the elements show sharp outlines when separate. When the 1758 temperature is very low, ice crystals may form. If the droplets then evaporate, the cloud becomes 1759 entirely an ice cloud and its elements cease to present sharp outlines. The formation of ice crystals 1760 may take place in all species of Altocumulus; it occurs most frequently in Altocumulus castellanus and 1761 floccus. 1762 1763 A corona or irisation is often observed in thin parts of Altocumulus. 1764 1765 The halo phenomena parhelia or luminous pillars are sometimes seen in Altocumulus, indicating the 1766 presence of tabular-shaped ice crystals. 1767 1768

II.3.4.8 1769

EXPLANATORY REMARKS 1770

1771

During the initial stages of its formation, Altocumulus is frequently a fairly smooth cloud of moderate 1772 horizontal extent. This cloud then subdivides into more or less regularly arranged small elements, in 1773 the form of laminae or tessellations. 1774 1775

Altocumulus in the shape of lenses or almonds often forms in clear air as a result of local orographic 1776 lifting of a layer of moist air. 1777 1778

Altocumulus frequently occurs at different levels in the same sky and is in many instances associated 1779 with Altostratus. In this case, the air is often hazy immediately below the sheets or layers of 1780 Altocumulus or between the elements constituting them. 1781

II.3.5 1782

Altostratus (As) (RENOU 1877) 1783

1784

II.3.5.1 1785

61

DEFINITION 1786

Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly 1787 covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground 1788 glass. Altostratus does not show halo phenomena. 1789

1790

II.3.5.2 1791

SPECIES 1792

Altostratus is not subdivided into species owing to the uniformity of its appearance and general 1793 structure. 1794

1795

1796

II.3.5.3 1797

VARIETIES 1798

1799

Altostratus translucidus (As tr) - CEN 1926 1800

Altostratus, the greater part of which is sufficiently translucent to reveal the position of the sun 1801 or moon. 1802

1803

Altostratus opacus (As op) - BESSON 1921 1804

Altostratus, the greater part of which is sufficiently opaque to mask completely the sun or moon. 1805

1806

Altostratus duplicatus (As du) - MAZE 1889, DE QUERVAIN 1908, CEN 1926 1807

Altostratus composed of two or more superposed layers, at slightly different levels, sometimes partly 1808 merged. This variety is rarely seen in Altostratus. 1809

1810

Altostratus undulatus (As un) - CLAYTON 1896, CEN 1930 1811

Altostratus showing undulations. 1812

1813

Altostratus radiatus (As ra) - CEN 1926, CCH 1953 1814

Altostratus showing broad parallel bands that appear to converge towards one point or 1815 towards two opposite points of the horizon. This variety is rarely seen in Altostratus. 1816

1817

II.3.5.4 1818


1820

Virga and praecipitatio may be clearly visible. Pannus clouds may be observed under Altostratus. 1821

62

1822

Altostratus may show mamma. 1823

1824

II.3.5.5 1825

CLOUDS FROM WHICH ALTOSTRATUS MAY FORM 1826

1827

Altostratus may form: 1828

by the thickening of a veil of Cirrostratus (As cirrostratomutatus); 1829

sometimes by the thinning of a layer of Nimbostratus (As nimbostratomutatus) 1830

from an Altocumulus layer when widespread ice crystal virga falls from the latter (As 1831 altocumulogenitus); 1832

sometimes, particularly in the tropics, by the spreading out of the middle or upper part of 1833 Cumulonimbus (As cumulonimbogenitus). 1834

1835

II.3.5.6 1836

MAIN DIFFERENCES BETWEEN ALTOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA 1837

1838

Altostratus breaking up into patches is distinguished from Cirrus spissatus (dense) by: 1839

having greater horizontal extent; 1840

being predominently grey; 1841

the possibility of precipitation in the form of rain, snow or ice pellets; 1842

the possibility of coronae 1843

1844

High Altostratus translucidus (thin) is distinguished from Cirrostratus by: 1845

Altostratus preventing objects on the ground from casting shadows; 1846

the possibility of the sun having a ground glass effect (blurred outline of the 1847 sun); 1848

the absence of any halo phenomena 1849

1850

Altostratus sometimes has gaps or breaks where the Altostratus could be confused for a 1851 similar layer of Altocumulus (Image 18) or Stratocumulus. Altostratus has a more uniform 1852 appearance. 1853

1854

A low layer of Altostratus opacus (thick) is distinguished from similar Nimbostratus by: 1855

the presence of thinner parts through which the sun is, or could be, vaguely revealed; 1856

being a lighter shade of grey; 1857

having a less uniform base than the Nimbostratus; 1858

possible mamma with the Altostratus 1859

1860

63

Visually distinguishing Nimbostratus from Altostratus on moonless nights is difficult, if possible at 1861 all. When rain, snow or snow pellets reach the Earth’s surface, the cloud is identified as 1862 Nimbostratus. 1863

1864

1865

1866

1867

Altostratus translucidus (thin) is distinguished from Stratus by: 1868

the sun having a ground glass effect (blurred outline of the sun); 1869

always having some shading and not being white, as thin Stratus may be when observed 1870 toward the sun 1871

1872

Altostratus opacus (thick) is distinguished from Stratus nebulosus opacus (thick) by: 1873

the layer being thin enough somewhere to reveal the position of the sun; 1874

the type and intensity of precipitation. Snow is common to both clouds but will be of a greater 1875 intensity with Altostratus. Rain and ice pellets are unique to Altostratus; weak drizzle and 1876 snow grains are unique to Stratus; 1877

possible mamma 1878

In the absence of thin parts near the sun, precipitation or mamma, it can be difficult to distinguish 1879

between these two clouds. A continuous watch will assist, as Stratus opacus usually does not 1880

occur in combination with other cloud genera. Altostratus rarely forms without the presence of 1881

high level clouds. 1882

1883

II.3.5.7 1884

1885


1887

Altostratus is generally a layer of great horizontal extent [several tens or hundreds of kilometres] and 1888

fairly considerable vertical extent [several hundreds or thousands of metres]. 1889

Alltostratus is composed of water droplets and ice crystals. In the most complete case [Figure 1], three 1890

superposed parts can be distinguished: 1891

(a) an upper part, composed wholly or mainly of ice crystals; 1892

(b) a middle part, composed of a mixture of ice crystals, snow crystals or snowflakes and supercooled 1893 water droplets; 1894

(c) a lower part, composed wholly or mainly of ordinary or supercooled water droplets or 1895 drops 1896

Sometimes the cloud may consist of only two parts, either: 1897

an upper part like (a) and a lower part like (c) [Figure 2]; or 1898 an upper part like (b) and a lower part like (c) [Figure 3] 1899

Less frequently, the entire cloud may also be like (a) [Figure 4] or like (b) [Figure 5] alone. 1900

1901

64

The constituent particles in the lower part of Altostratus are so numerous that the outline 1902

of the sun or moon is always blurred. Surface observers wi l l never see halo phenomena. In 1903

the thickest parts, the position of the sun or moon may be completely concealed. 1904

Raindrops or snowflakes are often present in Altostratus and below its base. When 1905

precipitation reaches the ground, it is generally of the "continuous" type and in the form 1906

of rain, snow or ice pellets. 1907

1908

II.3.5.8 1909


1911

The base of Altostratus occasionally shows a mamma-like or ragged appearance due to rain 1912

or snow virga. Isolated virga are clearly seen when rain, before evaporating, falls farther in 1913

some places than in others. 1914

Precipitation sometimes makes it difficult to distinguish a cloud base; particularly when uniformly 1915

falling snow completely evaporates before reaching the ground. If, however, snow melts rapidly 1916

into rain, an apparent base may be observed at the melting level, as the visibility through rain is 1917

greater than through snow. This "base" is very clearly visible when the rain layer is thin, which is 1918

the case for instance if the raindrops quickly evaporate; it may be completely obscured when the 1919

rain layer is thick. 1920

1921

Pannus clouds may be present. They; 1922

occur under the Altostratus in the lower turbulent layers when these are moistened by 1923 evaporation from precipitation; 1924

show a tendency to form near the 0° C level where the cooling of the air by melting snow 1925 increases the instability of the layer underneath; 1926

In the initial stage of formation, pannus clouds are small, sparse and well separated. They usually 1927

occur at a considerable distance below the under surface of the Altostratus; 1928

Later, with a thickening Altostratus and a lowering of its base, this distance is greatly reduced. At 1929

the same time, the pannus clouds increase in size and number and may merge into a quasi-1930

continuous layer. 1931

1932

II.3.6 1933

Nimbostratus (Ns) (CEN 1930) 1934

1935

II.3.6.1 1936

DEFINITION 1937

1938

65

Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less 1939

continuously falling rain or snow, which in most cases reaches the ground. It is thick enough 1940

throughout to blot out the sun. 1941

Low, ragged clouds frequently occur below the layer, with which they may or may not merge. 1942

1943

II.3.6.2 1944

SPECIES 1945

No species are distinguished in Nimbostratus. 1946

II.3.6.3 1947

VARIETIES 1948

Nimbostratus has no varieties. 1949

1950

II.3.6.4 1951


1953

Virga and praecipitatio in the form of rain, snow or ice pellets are the supplementary features 1954

associated with Nimbostratus. 1955

The accessory cloud pannus is frequently observed under Nimbostratus. 1956

1957

II.3.6.5 1958

CLOUDS FROM WHICH NIMBOSTRATUS MAY FORM 1959

1960

Nimbostratus develops: 1961

usually from thickening Altostratus (Ns altostratomutatus); 1962

rarely from the thickening of a layer of Stratocumulus (Ns stratocumulomutatus); 1963

rarely from the thickening of a layer of Altocumulus (Ns altocumulomutatus); 1964

sometimes by the spreading out of Cumulonimbus (Ns cumulonimbogenitus); 1965

very rarely, when these clouds produce rain, by the spreading out of Cumulus congestus (Ns 1966 cumulogenitus). 1967

1968

II.3.6.6 1969

MAIN DIFFERENCES BETWEEN NIMBOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA 1970

1971

66

Nimbostratus is distinguished from Altostratus opacus (thick) by: 1972

generally having a darker grey colour than Altostratus; 1973

being thick enough throughout to hide the sun or moon; only the thicker parts of Altostratus 1974 opacus hide the sun or moon. On moonless nights there may be doubt regarding the 1975 identification of Nimbostratus or Altostratus. The cloud is identified as Nimbostratus when 1976 precipitation (rain, snow or ice pellets) reaches the ground; 1977

When discernable, usually having a lower base than Altostratus; 1978

sometimes having the appearance of being illuminated from the inside; this feature is never 1979 found in Altostratus; 1980

1981

Nimbostratus is distinguished from Altocumulus stratiformis opacus (thick layer): 1982

by falls of rain, snow or ice pellets; no precipitation falls from Altocumulus; 1983

having no features and usually no discernible base; Altocumulus has clearly defined elements 1984 and a distinct lower surface; 1985

1986

Nimbostratus is distinguished from Stratocumulus stratiformis opacus (thick layer) by: 1987

having no features and usually no discernible base; Stratocumulus has clearly defined elements 1988 (merged or separated) and a distinct lower surface 1989

falls of rain, snow or ice pellets; Stratocumulus has infrequent weak falls of rain, snow or 1990 snow pellets 1991

1992

Nimbostratus is distinguished from Stratus nebulosus opacus (thick layer without distinct detail): 1993

falls of rain, snow or ice pellets; Stratus has weak falls of drizzle, snow or snow grains ;. 1994

thick Stratus usually develops in the absence of other low or middle level clouds; Nimbostratus 1995 usually evolves from an overcast layer of Altostratus 1996

1997

When the observer is beneath a cloud having the appearance of Nimbostratus, but accompanied by 1998

lightning, thunder or hail, the cloud is classified as Cumulonimbus. 1999

2000

II .3.6. 7 2001


2003

Nimbostratus generally covers a wide area and is of great vertical extent. It is composed of water 2004

droplets (sometimes supercooled) and raindrops, of snow crystals and snowflakes, or of a mixture of 2005

these liquid and solid particles. The high concentration of particles and the great vertical extent of the 2006

cloud prevent direct sunlight from being observed through it. The cloud produces rain, snow or ice 2007

pellets which, however, do not necessarily reach the ground. 2008

2009

67

Il.3.6.8 2010


2012

Nimbostratus usually develops from thickening Altostratus, the base of which gradually lowers. 2013 When the cloud becomes thick enough throughout to mask the sun, it is classified as Nimbostratus. 2014

Nimbostratus usually appears as if illuminated from inside. This is a result of the absence of small 2015 cloud droplets in its lower parts, whereby more light penetrates from above than in the case of non-2016 precipitating clouds of the same depth. The small cloud droplets in the lower parts of the cloud are 2017 swept out by precipitation or they evaporate owing to the presence of colder raindrops or 2018 snowflakes in the cloud. 2019

Although Nimbostratus generally has no clear under surface, an apparent base is sometimes 2020 discernible. This "base" is situated at the level where the snow melts into rain and is due to the poorer 2021 visibility in snow than in rain. The melting level can be seen only when it is sufficiently low and when 2022 the precipitation is not too heavy. 2023

The under surface of Nimbostratus is often partially or totally hidden by pannus clouds resulting 2024 from turbulence in the layers under its base, which are moistened by partial evaporation of 2025 precipitation. At first, these pannus clouds consist of separate units; they may later merge into a 2026 continuous layer extending up to the Nimbostratus. When the pannus covers a large expanse of the 2027 sky, care should be exercised in order not to confuse it with the under surface of Nimbostratus. 2028 Although pannus clouds have a tendency to dissipate, chiefly by the coalescence of their small 2029 particles with raindrops or snowflakes falling through them, they continue to reform. In heavy 2030 precipitation, however, the pannus particles are swept out faster than they can be replaced and the 2031 pannus clouds disappear. 2032

In the tropics, particularly during short lulls in the rainfall, Nimbostratus can be seen breaking up into 2033 several different cloud layers, which rapidly merge again. The clouds then often show a very 2034 characteristic livid colour with variations of luminance, probably due to internal gaps. 2035

2036

II.3.7 2037

Stratocumulus (Sc) (KAEMTZ 1841) 2038

2039

II.3.7.1 2040

DEFINITION 2041

2042

Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has 2043 dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for 2044 virga) and which may or may not be merged; most of the regularly arranged small elements have an 2045 apparent width of more than five degrees. 2046 2047

2048

II.3.7.2 2049

SPECIES 2050

2051

Stratocumulus stratiformis (Sc str) - CCH 1953 2052

68

Rolls or large rounded masses arranged in an extended sheet or layer. The elements are more or less 2053 flattened. This species is the most common. 2054 2055

2056

Stratocumulus lenticularis (Sc len) - LEY 1894, CEN 1930 2057

A patch of Stratocumulus, in the shape of a lens or almond, often very elongated and usually with well 2058 defined outlines. The patch is either composed of small elements (an apparent width greater than 5° 2059 degrees when observed at an angle of more than 30° above the horizon), closely grouped together, or 2060 consists of one more or less smooth and usually dark unit. Irisation is possible. 2061

This species of Stratocumulus is fairly rare. 2062 2063

Stratocumulus castellanus (Sc cas) - CCH 1953 2064

Cumuliform turrets rising vertically from cloud elements connected by a common horizontal base. The 2065 turrets: 2066

seem to be arranged in lines; 2067 give the cloud a crenelated (castle battlement) appearance; 2068 are sometimes taller than they are wide; 2069 are especially evident when the cloud is seen from the side. 2070

Stratocumulus castellanus can grow to a considerable size and develop into: 2071 Cumulus congestus stratocumulogenitus if it is strongly sprouting or is of great vertical extent; 2072 Cumulonimbus stratocumulogenitus if part of its upper portion is smooth, fibrous or striated, 2073

or if the cloud produces lightning, thunder or showers of hail. 2074

2075 Stratocumulus volutus (Sc vol) - ??? 2016 2076

A long, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal 2077

axis. They usually occur singularly but are occasionally observed in successive lines of clouds. 2078

This species of Stratocumulus is rare. 2079

2080

Stratocumulus floccus (Sc flo) - ??? 2016 2081

2082

Small tufts of cumuliform appearance; the lower parts of the tufts are generally ragged and, at very 2083 low temperatures, accompanied by fibrous trails (ice crystal virga). Stratocumulus floccus is an 2084 indication of instability at that level. Stratocumulus floccus often forms as a result of the dissipation of 2085 the base of Stratocumulus castellanus. 2086

2087

II.3.7.3 2088

VARIETIES 2089

2090

Stratocumulus translucidus (Sc tr) - CEN 1930 2091

A patch, sheet or layer of Stratocumulus that is nowhere very dense. The greater part of the cloud is 2092 sufficiently translucent to reveal the position of the sun or moon; the cloud may even allow the blue of 2093 the sky to be faintly distinguished where its elements join. 2094

69

2095

Stratocumulus perlucidus (Sc pe) - CCH 1953 2096

2097

A patch, sheet or layer of Stratocumulus where the spaces between the elements allow the sun, the 2098 moon, the blue of the sky or higher clouds to be seen. 2099

2100

Stratocumulus opacus (Sc op) - CEN 1930 2101

Dense Stratocumulus composed of a continuous or nearly continuous sheet or layer of large dark rolls 2102 or rounded masses, most of which are sufficiently opaque to hide the sun or moon. The base of 2103 Stratocumulus opacus is sometimes even, and its apparent subdivision into merged elements results 2104 from the irregularity of its upper surface. More often, however, the under surface is uneven, and the 2105 elements stand out in true relief. 2106 2107

Stratocumulus duplicatus (Sc du) - CCH 1953 2108

Stratocumulus comprising two or more broadly horizontal superposed patches, sheets or layers, close 2109 together, sometimes partly merged. This variety occurs in the species stratiformis and lenticularis. 2110

2111

Stratocumulus undulatus (Sc un) - CLAYTON 1896, CEN 1930 2112

A layer composed of fairly large and often grey elements, arranged in a system of nearly 2113 parallel lines. A double system of undulations is sometime visible in the form of 2114 transverse lines that cross the main system. The elements then appear to be arranged in 2115 “rank and file”. 2116

Stratocumulus undulatus occurs in the species stratiformis. 2117 2118

Stratocumulus radiatus (Sc ra) - CEN 1926 2119

Stratocumulus showing broad, nearly parallel bands that, owing to the effect of perspective, appear to 2120 converge towards a point or towards opposite points of the horizon. This variety looks similar to 2121 Cumulus arranged in files ("cloud streets"), the difference being the Cumulus is individual cells 2122 arranged in files. Stratocumulus radiatus occurs in the species stratiformis. 2123

2124

2125

Stratocumulus lacunosus (Sc la) - CCH 1953 2126

Stratocumulus, in a sheet or layer or in patches, with more or less regularly distributed round holes, 2127 many of them with fringed edges. The cloud elements and clear spaces are often arranged in a net 2128 or honeycomb manner. The details change rapidly. 2129 2130

II.3.7.4 2131


2133

Stratocumulus may show asperitas and fluctus and at very low temperatures cavum. 2134

Stratocumulus may also how mamma; its under surface then presents an accentuated relief in the 2135 form of udders or inverted mounds that even appear to be on the point of detaching themselves from 2136

70

the cloud. 2137

Stratocumulus mamma can be confused with Altostratus opacus with a mammilated (wrinkled) 2138 appearance. 2139

Virga may also occur under Stratocumulus, especially at very low surface temperatures. 2140

The feature praecipitatio rarely occurs. When it does, the precipitation (rain, snow or snow pellets) is 2141 only of weak intensity. 2142 2143

II.3.7.5 2144 CLOUDS FROM WHICH STRATOCUMULUS MAY FORM 2145

2146

Stratocumulus may form: 2147

from Altocumulus when the small elements grow to a sufficient size (Sc altocumulomutatus); 2148

sometimes near the base of Altostratus, as a result of turbulence or convection in the layers 2149 moistened by evaporating precipitation (Sc altostratogenitus) 2150

more often near the base of Nimbostratus, as a result of turbulence or convection in the 2151 layers moistened by evaporating precipitation (Sc nimbostratogenitus); 2152

from the thinning of Nimbostratus, signifying the end of a rain event (Sc 2153 nimbostratomutatus); 2154

from the of the lifting of a layer of Stratus (Sc stratomutatus); 2155

from the convective or turbulent transformation of Stratus, with or without change of height 2156 (Sc stratomutatus); 2157

by spreading of the tops of Cumulus when they reach a stable layer (Sc cumulogenitus); 2158 usually the mid to upper part of the Cumulus gradually widens in proximity to the stable 2159 layer. The vertical development of Cumulus may: 2160

o cease at the stable layer, resulting in patches of Stratocumulus spreading out from the 2161 top of the Cumulus; often the Cumulus dissipates completely from the base upwards; 2162

o temporarily stop at the stable layer and then resume their growth in places or 2163 throughout, resulting in Stratocumulus on the side/s of the Cumulus; 2164

from Cumulus (Sc cumulogenitus) when towers lean and spread or are detached and spread due 2165 to strong wind shear; 2166

from Cumulus (Sc cumulogenitus) when convection ceases in the late afternoon and evening 2167 and the domed tops of the Cumulus clouds flatten; 2168

by the spreading out of Cumulonimbus (Sc cumulonimbogenitus). Stratocumulus may be 2169 observed on or near the sides of Cumulonimbus and often forms while the Cumulonimbus is 2170 still in the Cumulus stage. Nevertheless, the Stratocumulus is classified as 2171 cumulonimbogenitus, not cumulogenitus; 2172

by the spreading out of some of the lower parts of an existing Cumulonimbus (Sc 2173 cumulonimbogenitus). 2174 2175 2176

Cumulus or Cumulonimbus towers can also pass through a preexisting layer of Stratocumulus 2177 formed independently of them. When this occurs: 2178

the Cumulus or Cumulonimbus do not widen upward towards the Stratocumulus layer; 2179

a thinned or even a cleared zone frequently appears in the Stratocumulus around the 2180 cumuliform towers. 2181

2182

I.I.3.7.6 2183

MAIN DIFFERENCES BETWEEN STRATOCUMULUS AND SIMILAR CLOUDS OF 2184

OTHER GENERA 2185

71

2186

Stratocumulus can, in extremely cold weather, produce abundant ice crystal virga from which halo 2187 phenomena may be observed. This Stratocumulus is distinguished from Cirrostratus nebulosus by: 2188

showing some evidence of elements, rounded masses, rolls, etc; 2189

greater opacity than that of Cirrostratus 2190 2191 Stratocumulus is distinguished from Altocumulus by; 2192

most of the regularly arranged elements having, when observed at an angle of more than 30° 2193 above the horizon, an apparent width greater than 5° (3 fingers at arm’s length); 2194

possible precipitation in the form of weak falls of rain, or snow; 2195

2196 An extensive layer of Stratocumulus is distinguished from Altostratus by; 2197

having a less uniform base; 2198

the presence of elements, rounded masses, rolls, etc; 2199

appearing non-fibrous, except at extremely low temperatures; Altostratus often has a fibrous 2200 appearance; 2201

very weak and infrequent precipitation in the form of rain, snow or snow pellets, Altostratus 2202 can have light, or occasionally moderate, falls of rain, snow or ice pellets; 2203

2204 Stratocumulus stratiformis opacus (thick extensive layer) is distinguished from Nimbostratus by; 2205

having a less uniform base; 2206


precipitation in the form of rain, snow or snow pellets being weak in intensity and infrequent 2208 in occurrence; Nimbostratus usually has falls of rain, possible falls of snow or ice pellets that 2209 can be up to heavy in intensity; 2210

possible asperitas or mamma; 2211

absence of pannus, a usual feature of Nimbostratus 2212 2213 Stratocumulus is distinguished from Stratus by; 2214


usual absence of a ragged structure, a feature of Stratus fractus; Stratocumulus floccus has a 2216 ragged lower part with a cumuliform tuft at the top; 2217

weak falls of rain, snow or snow pellets; Stratus has falls of drizzle, snow or snow grains; 2218

possible mamma; 2219

possible virga; the low base of Stratus ensures precipitation reaches the ground; 2220

possible corona; 2221 2222 Stratocumulus is distinguished from Cumulus by; 2223

elements usually occurring in groups or patches and generally have flat tops; 2224

occasionally having tops in the form of domes or turrets that rise from merged bases; 2225

possibily being shaded throughout, Cumulus is noted for its brilliant white sunlight parts; 2226

the absence of arcus, tuba, pileus, velum and pannus; 2227

precipitation falling uniformly in an intermittent or continuous nature; Cumulus is noted for 2228 precipitation falling in the form of showers; 2229

usual absence of any form of rainbow 2230

2231

Stratocumulus floccus may bear a strong resemblance to Cumulus fractus frayed by a fairly strong wind or 2232 Cumulus mediocris with irregular bases on days of fresh to strong winds. Stratocumulus floccus is 2233

72

distinguished from these species of Cumulus by: 2234

usually having a very ragged lower part; if there is evidence of a partial flat base it rapidly 2235 dissipates; 2236

base unusually high compared to an expected base for Cumulus; Cumulus may even be 2237 observed at a lower base 2238

If there is doubt as to the identification, the cloud is identified as the appropriate Cumulus species. 2239

2240

2241

2242

I.I.3.7.7 2243


2245

Stratocumulus is composed of water droplets, sometimes accompanied by raindrops or, more rarely, 2246 by snow pellets, snow crystals and snowflakes. Any ice crystals present are usually too sparse to give 2247 the cloud a fibrous appearance. During extremely cold weather, Stratocumulus may produce 2248 abundant ice crystal virga that may be accompanied by halo phenomena. When Stratocumulus is not 2249 very thick, a corona or irisation is sometimes observed. 2250 2251

2252


2255

The appearance of Stratocumulus is similar to that of Altocumulus. Owing to its generally lower 2256 height, the elements of Stratocumulus look larger and, at times, smoother than those of Altocumulus. 2257 2258 Stratocumulus elements are often arranged in lines or groups, showing a single or double system of 2259 undulations. Elements may be more or less separate; however the cloud layer is more often 2260 continuous, sometimes with gaps. The under surface of a continuous cloud layer is often uneven and 2261 presents a relief in the form of creases, mamma, etc. 2262

2263

II.3.8 2264

2265

Stratus (St) (HOWARD 1803; HILDEBRANDSSON AND ABERCROMBY 1887) 2266 2267

2268

II.3.8.1 2269 2270

DEFINITION 2271

2272

Generally grey cloud layer with a fairly uniform base, which may give drizzle, snow or snow grains. 2273 When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not 2274 produce halo phenomena except, possibly, at very low temperatures. 2275 Sometimes Stratus appears in the form of ragged patches. 2276 2277

73

II.3.8.2 2278

SPECIES 2279

2280

Stratus nebulosus (St neb) - CLAYDEN 1905, CCH 1953 2281

Nebulous, grey, fairly uniform layer of Stratus. This is the most common species. 2282 2283

Stratus fractus (St fra) - CEN 1930, CCH 1953 2284

Stratus occurring in the form of irregular ragged shreds the outlines of which change continuously and 2285 often rapidly. 2286 2287

II.3.8.3 2288

VARIETIES 2289

2290

Stratus opacus (St op) – BESSON 1921, CCH 1953 2291

Patch, sheet or layer of Stratus, the major part of which is so opaque that it completely masks the 2292 sun or moon. This is the most common variety. 2293 2294

Stratus translucidus (St tr) – CEN 1926, CCH 1953 2295

Patch, sheet or layer of Stratus, the major part of which is sufficiently translucent to reveal the 2296 outline of the sun or moon. 2297 2298

Stratus undulatus (Stun) – CLAYTON 1896, CCH 1953 2299

Patch, sheet or layer of Stratus showing undulations. This variety does not occur very often. 2300 2301

2302

II.3.8.4 2303


2305

The only supplementary feature of Stratus is praecipitatio (precipitation in the form of drizzle, 2306 snow and snow grains). 2307 2308

2309

II.3.8.5 2310

CLOUDS FROM WHICH STRATUS MAY FORM 2311

2312

74

Stratus may form from the transformation of Stratocumulus (St stratocumulomutatus) when it 2313

thickens and lowers forming a fairly uniform base; or the base stays at the same height but loses its 2314

relief or its subdivisions. 2315

Precipitating Stratocumulus can thicken and lower and lose its relief or subdivisions. This is 2316

not transformation to Stratus. If the precipitating Stratocumulus is devoid of relief or 2317

subdivisions for an extended period of time, consideration must be given to the cloud having 2318

transformed to Nimbostratus. 2319

2320

Stratus often forms from the slow lifting of a fog layer, due to warming of the Earth's surface or an 2321

increase in wind speed. 2322

2323

Stratus fractus of wet weather (the conditions which generally exist during precipitation and a 2324

short time before and after) is often produced by Altostratus, Nimbostratus or Cumulonimbus (St 2325

fra altostratogenitus, St fra nimbostratogenitus or St fra cumulonimbogenitus); it may also result 2326

from precipitating Cumulus (St fra cumulogenitus). 2327

2328

2329

II.3.8.6 2330

MAIN DIFFERENCES BETWEEN STRATUS AND SIMILAR CLOUDS OF OTHER GENERA 2331

2332

Stratus can occasionally resemble Cirrus when it is frayed by the wind into the form of coarse fibres 2333 (Stratus fractus). The fibres: 2334

are not as white (except when viewed towards the sun) as Cirrus fibres; 2335

are not as spread out as Cirrus fibres; 2336

change in appearance rapidly; Cirrus is noted for slow change. 2337 2338

Thin Stratus (translucidus) is distinguished from Cirrostratus by: 2339

not being so completely white except when viewed towards the sun; 2340

possible coronae 2341 2342

Stratus is distinguished from Altostratus by; 2343

the outline of the sun not being blurred; that is the disk or partial disk of the sun is 2344 discernible; 2345

the absence of shading in thin Stratus when viewed toward the sun; Altostratus always has 2346 shading; 2347

precipitation in the form of drizzle or snow grains; Altostratus has rain and ice pellets; snow 2348 can occur with both clouds 2349

2350

75

Stratus nebulosus opacus (thick) closely resembles Nimbostratus and may very easily be confused 2351 with it. It is distinguished from Nimbostratus by; 2352

having a more clearly defined and more uniform base than Nimbostratus; 2353

having a "dry" appearance, contrasting to the “wet” appearance of Nimbostratus; 2354

producing only weak falls of drizzle, snow or snow grains, whereas Nimbostratus nearly 2355 always produces rain, snow or ice pellets; 2356

not usually being preceded by other clouds of the low and middle levels; Nimbostratus nearly 2357 always succeeds other clouds, usually of the middle level, or develops from a pre-existing 2358 cloud 2359

2360

Stratus translucidus (thin) is distinguished from Nimbostratus by the disk of the sun being 2361 discernible, at least through its thinnest parts; Nimbostratus masks the sun or moon throughout. 2362

2363

Stratus is distinguished from Stratocumulus by there being no evidence of elements, merged or 2364 separate. 2365

2366

Stratus fractus is distinguished from Cumulus fractus in that it is less white and less dense. It also 2367

has less vertical development, since it owes its formation mainly to turbulence, not heating of the air 2368

near the earth’s surface. 2369

2370

II.3.8. 7 2371 PHYSICAL CONSTITUTION 2372

2373

Stratus is usually composed of small water droplets. When very thin, Stratus may produce a corona 2374 around the sun or moon. At low temperatures, Stratus may consist of small ice particles. The ice 2375 cloud is usually thin and on rare occasions, may produce halo phenomena. 2376 2377 Stratus, when dense or thick, often contains drizzle droplets and sometimes snow or snow 2378 grains. It may then have a dark or even a threatening appearance. 2379 2380 Stratus with a low optical thickness, usually of the variety translucidus, often shows a smoky greyish 2381 tint like that of fog when viewed at more than 90° from the sun, 2382 2383

2384


2387

Stratus forms under the combined effect of cooling in the lower layers of the troposphere and 2388 turbulence due to the wind 2389 2390

Over land, the cooling may be a result of nighttime cooling which is particularly marked when the 2391 sky is clear and the wind is weak, or by advection of relatively warm air over colder ground. Over 2392 sea, the cooling is mainly due to advection of relatively warm air over colder water surface. 2393 2394

Stratus is sometimes observed as more or less joined cloud fragments with varying luminance. These 2395

76

Stratus fractus clouds constitute a transitory stage during the formation or the dissipation of the 2396 more common extensive uniform Stratus layer. The transitory stage is usually very short. 2397 2398

Stratus fractus clouds may also form as accessory clouds (pannus) under Altostratus, Nimbostratus, 2399 Cumulonimbus and precipitating Cumulus. They develop as a result of turbulence in the moistened 2400 layers under these clouds. 2401 2402

Stratus may develop locally in the vicinity of large waterfalls as a consequence of water broken up 2403 into spray by the falls. The Stratus will be classified by any appropriate species, variety and 2404 supplementary feature, followed by cataractagenitus. 2405

2406

Stratus may develop locally over forests as a result of increased humidity due to evaporation and 2407 evapotranspiration from the tree canopy. The Stratus will be classified by any appropriate species, 2408 variety and supplementary feature followed by silvagenitus. 2409 2410

II.3.9 2411

2412

Cumulus (Cu) 2413

(HOWARD 1803) 2414

2415

2416

II.3.9.1 2417

DEFINITION 2418

2419

Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising 2420 mounds, domes or towers, of which the bulging upper part often resembles a cauliflower. The sunlit 2421 parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal. 2422

Sometimes Cumulus is ragged. 2423

2424

II.3.9.2 2425

SPECIES 2426

2427

Cumulus humilis (Cu hum) - VINCENT 1907 2428

Cumulus characterized by only a small vertical extent and appearing generally as if flattened. Cumulus 2429 humilis clouds never produce precipitation. 2430 2431

Cumulus mediocris (Cu med) - CCH 1953 2432

Cumulus of moderate vertical extent, with small protuberances and sproutings at their tops. Cumulus 2433 mediocris generally produce no precipitation. 2434

2435

Cumulus congestus (Cu con) - MAZE 1889 2436

77

Strongly sprouting Cumulus with generally sharp outlines and often great vertical extent. The bulging 2437 upper part of Cumulus congestus frequently resembles a cauliflower. Cumulus congestus may produce 2438 precipitation in the form of showers of rain, snow or snow pellets. In the tropics they often release 2439 abundant rain in the form of showers. 2440

Cumulus congestus sometimes resemble narrow, very high towers. The tops of these towers may 2441 detach themselves successively from the main body of the cloud. They are then carried away by the 2442 wind and usually rapidly disintegrate, occasionally producing virga. 2443

Cumulus congestus usually results from the development of Cumulus mediocris or infrequently, 2444 Altocumulus castellanus or Stratocumulus castellanus. 2445

Cumulus congestus often develops into Cumulonimbus; this transformation is: 2446

visually revealed by the smooth appearance or by the fibrous or striated texture of its upper 2447 portion; 2448

apparent when lightning, thunder or showers of hail are observed. 2449 2450

Cumulus fractus (Cu fra) – POEY, 1863, CCH 1953 2451

Small Cumulus with very ragged edges and with outlines continuously undergoing changes that are 2452 often rapid. 2453

2454

II.3.9.3 2455 VARIETIES 2456

2457

Cumulus radiatus (Cu ra) - CCH 1953 2458

Cumulus arranged in lines nearly parallel to the wind direction (cloud streets) and usually of the 2459 species mediocris. Due to perspective, these lines seem to converge towards a point or towards 2460 opposite points of the horizon. 2461 2462

II.3.9.4 2463 SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS 2464

2465

Cumulus may show: pileus, velum, virga, praecipitatio (the precipitation occurs in the form of 2466 showers), fluctus (occasionally in the species humilis), arcus (rarely), pannus (rarely) and tuba (very 2467 rarely). 2468 2469

2470

II.3.9.5 2471 CLOUDS FROM WHICH CUMULUS MAY FORM 2472

2473

Cumulus is often preceded by hazy spots in the sky out of which the clouds develop. 2474 2475 Cumulus may form: 2476

from the development of Altocumulus castellanus (Cu congestus altocumulogenitus); 2477

from the development of Stratocumulus castellanus (Cu congestus stratocumulogenitus); 2478

by the transformation of Stratocumulus (Cu stratocumulomutatus); or 2479

by the transformation of Stratus (Cu stratomutatus); this frequently occurs in the morning over 2480

78

land. 2481 2482

Cumulus fractus of wet weather forms under Altostratus, Nimbostratus, Cumulonimbus or 2483 precipitating Cumulus (Cu fra altostratogenitus, nimbostratogenitus, cumulonimbogenitus or 2484 cumulogenitus). 2485 2486

II.3.9.6 2487

MAIN DIFFERENCES BETWEEN CUMULUS AND SIMILAR CLOUDS OF OTHER GENERA 2488

2489

Cumulus is distinguished from most Altocumulus and Stratocumulus by being detached and dome-2490 shaped. When viewed from a distance, Cumulus clouds may appear merged, owing to the effect of 2491 perspective. Care should be taken not to confuse distant Cumulus with Altocumulus or Stratocumulus. 2492 2493

Cumulus tops may: 2494

spread and form Stratocumulus cumulogenitus or Altocumulus cumulogenitus; 2495

penetrate and pass through existing layers of Stratocumulus or Altocumulus; or 2496

merge with Altostratus or Nimbostratus. 2497

The Cumulus clouds that are spreading, penetrating or merging continue to be identified as Cumulus as 2498 long as they remain detached from one another or they have relatively considerable vertical extent. 2499 2500

When an extensive precipitating Cumulus is directly above, it can be confused with Altostratus or 2501 Nimbostratus or perhaps Stratocumulus stratiformis opacus (thick layer). If the precipitation is in the 2502 form of showers the cloud is Cumulus. A continuous watch will also assist as Altostratus, Nimbostratus 2503 and Stratocumulus stratiformis opacus are noted for their longevity; a feature not characteristic of 2504 Cumulus cloud. 2505

2506

Cumulus congestus of great vertical extent is distinguished from Cumulonimbus by: 2507

the edges of the bulging upper parts being sharply defined and having no fibrous or striated 2508 texture; 2509

no lightning, thunder or showers of hail. 2510 2511 Cumulus mediocris (with irregular bases and ragged in parts due to fresh to strong winds) and 2512 Cumulus fractus (frayed by strong winds) are distinguished from Stratocumulus floccus by: 2513

part of the base being flat; Stratocumulus floccus usually has a very ragged lower part; if there 2514 is evidence of a flat base it rapidly dissipates; 2515

the vertical structure of the cloud leaning strongly downwind; 2516

continuous and rapid changes in the outlines of Cumulus fractus 2517

If there is doubt as to the identification, the cloud is identified as the appropriate Cumulus species. 2518

2519

Cumulus fractus is distinguished from Stratus fractus by: 2520

generally greater vertical extent; 2521

usually whiter and less transparent appearance; 2522

possible rounded or dome shaped tops, Stratus fractus always occurs as ragged shreds. 2523 2524

II.3.9.7 2525


79

Cumulus is composed mainly of water droplets. When of great vertical extent, Cumulus may release 2527 precipitation in the form of showers of rain, snow or snow pellets. 2528

Ice crystals may form in those parts of a Cumulus where the temperature is well below 0° C. The ice 2529 crystals grow at the expense of evaporating super-cooled water droplets, transforming the cloud into 2530 Cumulonimbus. In cold weather, when the temperature in the entire cloud is well below 0° C, this 2531 process leads to the degeneration of the cloud into diffuse trails of snow. 2532 2533

II.3.9.8 2534


2536

Cumulus develops: 2537

in convection currents resulting from heating near the Earth’s surface; 2538

due to cooling or advection of cold air (instability) in higher layers; 2539

from lifting of air layers where vertical expansion results in cooling; 2540 2541

The characteristics of Cumulus clouds depend essentially on their vertical extent; that is the vertical 2542 distance between their base and the stable layer that has inhibited vertical development. 2543

The degree of stability and thickness of the stable layer determine how effective this layer is at 2544 inhibiting vertical development. When it is very stable, for example a strong inversion, the tops of 2545 Cumulus clouds will spread out forming Stratocumulus cumulogenitus or Altocumulus cumulogenitus . 2546 When it is stable but not very thick, the spreading out of the tops of Cumulus clouds may be only in 2547 parts or momentary and then some tops may penetrate it. 2548

2549

Comments on vertical extent. 2550

When Cumulus have: 2551

great vertical extent (a low base and a high stable layer) the Cumulus is of the species 2552 congestus. The vertical extent of Cumulus in tropical regions (when not under the influence of 2553 a trade wind inversion) is generally much greater than elsewhere; 2554

moderate vertical extent (base and stable layer are reasonably close together) Cumulus 2555 mediocris tops may spread out forming either Stratocumulus or Altocumulus; 2556

small vertical extent (base and stable layer are very close together) the Cumulus has a flattened 2557 appearance (Cumulus humilis). They may even spread out in their entirety into Stratocumulus 2558 or Altocumulus. 2559

Cumulus may dissipate during the day as the surface air temperature rises and the Cumulus base rises 2560 until its height exceeds, sometimes considerably, that of the stable layer. 2561

When there is no vertical extent (stable layer is lower than the level where sufficient cooling had 2562 occurred for condensation to take place) Cumulus may be present but only if there is a mechanism 2563 forcing air to rise to reach the level where condensation could have occurred. Orographic lift is an 2564 example of a forcing mechanism 2565

2566

Diurnal variation in Cumulus activity: 2567

is generally pronounced over land. On clear mornings, with the sun rapidly heating the surface 2568 of the ground, conditions are favourable for the formation of Cumulus. This formation may 2569 begin early, when the lapse rate is steep and the relative humidity is high; it begins late, if it 2570 occurs at all, when the lapse rate is small and the relative humidity is low. After having reached 2571 a maximum, usually in mid-afternoon, the Cumulus activity decreases and finally the clouds 2572 disappear in the late afternoon or early evening. 2573

Is so small over the open oceans that its existence is sometimes doubtful. When it exists, 2574

80

maximum Cumulus activity appears to occur in the late hours of the night. 2575

near coasts has Cumulus forming over the land by day in connection with the sea breeze and 2576 over the sea by night in connection with the land breeze. 2577

2578

Comments on illumination 2579

When a well-developed Cumulus is: 2580

observed opposite the sun, the diffuse reflection of the sunlight falling on the surface of the 2581 cloud reveals the relief of the protuberances by very pronounced differences in luminance; 2582

illuminated from the side, Cumulus shows strongly contrasted shading; 2583

illuminated from behind, the Cumulus appears relatively dark, with an extremely brilliant 2584 border (every cloud has a sliver lining); 2585

against a background of cirriform (ice) clouds and away from the horizon, Cumulus appears a 2586 little less white than the cirriform clouds and its margins appear grey, even when the Cumulus 2587 is directly illuminated by the sun. 2588

Whatever the illumination of the Cumulus may be, its base is generally grey. 2589

2590

Cumulus may develop in convection initiated by heat from forest fires, wild-fires or from volcanic 2591 eruption activity. The Cumulus will be classified by any appropriate species, variety and supplementary 2592 feature, followed by flammagenitus (e.g. Cumulus congestus flammagenitus). 2593

2594

Cumulus may develop as a consequence of human activity such as forming in convection initiated 2595 above power station cooling towers. When Cumulus is clearly observed to have originated as a 2596 consequence of human activity it will be classified by any appropriate species, variety and any 2597 supplementary features followed by the homogenitus (e.g. Cumulus mediocris homogenitus). 2598

2599

Cumulus may develop locally in the vicinity of large waterfalls as a consequence of water broken up 2600 into spray by the falls. The Cumulus will be classified by any appropriate species, variety and 2601 supplementary feature, followed by cataractagenitus (e.g. Cumulus mediocris cataractagenitus). 2602

2603

II.3.10 2604

Cumulonimbus (Cb) (WEILBACH 1880) 2605

2606

II.3.10.1 2607

DEFINITION 2608

2609

Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. 2610 At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; 2611 this part often spreads out in the shape of an anvil or vast plume. 2612

2613

Under the base of this cloud which is often very dark, there are frequently low ragged clouds either 2614 merged with it or not, and precipitation sometimes in the form of virga. 2615 2616

2617

81

II.3.10.2 2618

SPECIES 2619

2620

Cumulonimbus calvus (Cb cal) - CEN 1926 2621

Cumulonimbus where the sproutings of the upper parts are indistinct and flattened and have the 2622 appearance of a whitish mass without sharp outlines. No fibrous or striated parts are visible. 2623 Cumulonimbus calvus usually produces precipitation; when it reaches the ground it is in the form of 2624 showers. 2625 2626

Cumulonimbus capillatus (Cb cap) - CEN 1926 2627

Cumulonimbus where the upper portion has cirriform parts of clearly fibrous or striated structure, 2628 frequently in the shape of an anvil (Cumulonimbus capillatus incus), a plume or a vast disorderly 2629 mass of hair. In very cold air masses the fibrous structure often extends virtually throughout the 2630 cloud. 2631

Cumulonimbus capillatus is usually accompanied by a shower or by a thunderstorm, often with wind 2632 squalls and sometimes with hail; it frequently produces very distinct virga. 2633 2634

2635

II.3.10.3 2636

2637

VARIETIES 2638

2639

Cumulonimbus has no varieties. 2640 2641

2642

II.3.10.4 2643

2644


2646

Cumulonimbus may show: 2647 praecipitatio (showers of rain, snow, snow pellets, or hailstones); 2648 virga; 2649 pannus; 2650 incus; 2651 mamma (observed frequently on the base of the projecting portion of the anvil, less 2652

frequently on the base of the cloud); 2653 pileus, velum, arcus and murus; 2654 cauda, flumen and tuba are rarely observed. 2655

2656

2657

II.3.10.5 2658

82

2659

CLOUDS FROM WHICH CUMULONIMBUS MAY FORM 2660

2661

Cumulonimbus most commonly evolves from Cumulus congestus (Cb cumulogenitus or Cb 2662 cumulomutatus) that was formed in the normal manner [see paragraphs II.3.9.7 and II.3.9.8]. 2663 2664 Cumulonimbus also forms: 2665

from Altocumulus castellanus (Cb altocumulogenitus); the base of the Cumulonimbus is 2666 unusually high 2667

from Stratocumulus castellanus (Cb stratocumulogenitus); 2668 by the transformation and development of a portion of Altostratus or Nimbostratus (Cb 2669

altostratogenitus or Cb nimbostratogenitus). 2670 2671 In the majority of these cases, the transformation into Cumulonimbus passes through the Cumulus 2672 congestus stage. 2673 2674

2675

II.3.10.6 2676

2677

MAIN DIFFERENCES BETWEEN CUMULONIMBUS AND SIMILAR CLOUDS OF OTHER GENERA 2678

2679

When Cumulonimbus covers a large expanse of the sky, it can easily be confused with Nimbostratus, 2680 especially when identification is based solely on the appearance of the under surface. The character of 2681 the precipitation may help to distinguish Cumulonimbus from Nimbostratus. If the precipitation is of 2682 the showery type, or if it is accompanied by lightning, thunder or hail, the cloud is Cumulonimbus. 2683

2684

Certain Cumulonimbus clouds appear nearly identical with Cumulus congestus. The cloud is identified 2685

as Cumulonimbus as soon as at least a part of its upper portion loses the sharpness of its outlines or 2686

presents a fibrous or striated texture. If it is not possible to decide on the basis of the above criteria 2687

whether a cloud is a Cumulonimbus or Cumulus, it is identified as Cumulonimbus if it is accompanied 2688

by lightning, thunder or hail. 2689

2690

II.3.10.7 2691

2692


2694

Cumulonimbus is composed of water droplets and, especially in its upper portion, of ice crystals. It 2695

also contains large raindrops and, often, snowflakes, snow pellets, or hailstones. The water droplets 2696

and raindrops may be substantially supercooled. 2697

2698

2699

83

II.3.10.8 2700

2701


2703

The conditions under which Cumulonimbus clouds occur are similar to those ·which are favourable 2704

for the development of Cumulus congestus (see paragraph II.3.9.8). The transformation of Cumulus 2705

congestus into Cumulonimbus is due to the formation of ice particles in its upper part. The presence 2706

of ice particles is evident when some or all of the upper part loses the sharpness of its outlines or 2707

acquires a fibrous or striated texture. 2708

2709

Cumulonimbus clouds may appear either as isolated clouds or in the form of a continuous line of 2710

clouds resembling a very extensive wall. 2711

2712

In certain cases, the upper portion of Cumulonimbus clouds may be merged with Altostratus or 2713

Nimbostratus. Cumulonimbus may also develop within the general mass of Altostratus or 2714

Nimbostratus. 2715

2716

Low, ragged accessory clouds (pannus) often develop under ·Cumulonimbus; these clouds are at first 2717

separated from one another, but they may later merge so as to form a continuous layer partially or 2718

totally in contact with the Cumulonimbus base. 2719

2720

Cumulonimbus may be described as a "cloud factory"; it can produce thick patches or sheets of Cirrus 2721

spissatus, Altocumulus, Altostratus or Stratocumulus by the spreading out of its upper portions and 2722

by the dissipation of the lower parts. The spreading of the highest part usually leads to the formation 2723

of an anvil; if the wind increases strongly with altitude, the cloud top spreads only downwind, 2724

assuming the shape of a half anvil or in some cases of a vast plume. 2725

2726

Cumulonimbus is rare in polar regions and more frequent in temperate and tropical regions. 2727

2728

Cumulonimbus may develop from convection initiated by heat from forest fires, wild-fires or from 2729

volcanic eruption activity. Cumulonimbus that is clearly observed to have originated as a consequence 2730

of localised natural heat sources will be classified by any appropriate species, variety and 2731

supplementary feature, followed by flammagenitus. 2732

2733

2734

2735

2736

2737

PREFACE TO THE 1939 EDITION1 - World … · 18 One year later Luke Howard published in England a...

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Transcript of PREFACE TO THE 1939 EDITION1 - World … · 18 One year later Luke Howard published in England a...