WORLD METEOROLOGICAL ORGANIZATION INTERNATIONAL CL·OUD ATLAS Volume I Revised edition 1975 MANUAL ON THE OBSERVATION OF CLOUDS AND OTHER METEORS (Partly Annex I to WMO Technical Regulations) WMO - No. 407 Secretariat of the World Meteorological Organization - Geneva - Switzerland 1975 ( ( ( © 1975, World Meteorological Organization ISBN 92-63-10407-7 NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. CONTENTS Pages Preface to the 1939 edition IX Preface to the 1956 edition xv Preface to the present edition XIX Introductory note ..... XXIII 1. PARTl DEFINITION OF A METEOR AND GENERAL CLASSIFICATION OF METEORS Ll Definition of a meteor .... 3 1.2 General classification of meteors 3 1.2.1 Hydrometeors 3 1.2.2 Lithometeors 5 1.2.3 Photometeors 5 1.2.4 Electrometeors 5 II.PARTII CLOUDS ILl INTRODUCTION 11.1.1 Definition of a cloud 9 II.1.2 Appearance of clouds 9 II.1.2.1 Luminance .... 9 II.1.2.2 Colour . 10 11.1.3 Principles of cloud classification 11 II.1.3.1 Genera. 11 II.1.3.2 Species .......... 11 II.1.3.3 Varieties ......... 11 II.1.3.4 Supplementary features and accessory clouds 12 II.1.3.5 Mother-clouds ....... 12 lLl.4 Table of classification of clouds 13 II.1.5 Table of abbreviations and symbols of clouds 14 11.2 DEFINITION OF CLOUDS II.2.1 Some useful concepts 15 11.2.1.1 Height, altitude, vertical extent 15 ( IV CONTENTS Pages 11.2.1.2 Etages . 15 II.2.2 Observational conditions to which definitions of clouds apply . 16 II.2.3 Definitions of clouds 16 II.2.3.1 Genera . 16 II.2.3.2 Species . 17 II.2.3.3 Varieties . 20 II.2.3.4 Supplementary features and accessory clouds 22 11.3 DESCRIPTIONS OF CLOUDS II.3.1 Cirrus (Ci) .... 25 II.3.2 Cirrocumulus (Cc) 27 II.3.3 Cirrostratus (Cs) 29 n.3.4 Altocumulus (Ac) 31 II.3.5 Altostratus (As). 35 II.3.6 Nimbostratus (Ns) 37 II.3.7 Stratocumulus (Sc) 39 II.3.8 Stratus (St). ... 43" II.3.9 Cumulus (Cu). .. 45 II.3.10 Cumulonimbus (Cb) . 48 II.4 OROGRAPHIC INFLUENCES n.4.1 Occurrence, structure and shapes <?f orographic clouds. ........ 51 Il.4.2 Changes in the shape and structure of clouds due to orographic influences 52 n.5 CLOUDS AS SEEN FROM AIRCRAFT II.5.1 Special problems involved ..... 53 n.5.1.1 Differences between the observation of clouds from aircraft and from the Earth's surface ... : 53 n.5.1.2 Field of vision ... 53 II.5.1.3 Appearance of clouds 53 ( II.5.1.4 Icing . 54 n.5.1.5 Turbulence in clouds and in their vicinity. 54 n.5.1.6 Visibility in clouds .......... 55 n.5.!.7 Photometeors associated with clouds. .. 55 IJ .5.2 Descriptions of clouds as observed from aircraft 55 n.5.2.1 Cirrus .... 55 II.5.2.2 Cirrocumulus . 55 II.5.2.3 Cirrostratus 56 II.5.2.4 Altocumulus . 56 n.5.2.5 Altostratus. 58 n.5.2.6 Nimbostratus . 59 II. 5.2.7 Stratocumulus 59 II.5.2.8 Stratus .. 60 n.5.2.9 Cumulus ... 61 ( CONTENTS v Pages n.5.2.10 Cumulonimbus 0 0 • • • 0 • • • 62 n.5.3 Fog and haze as seen from aircraft 62 n.5.3.1 Fog . 62 n.5.3.2 Haze aloft . 63 n.6 SPECIAL CLOUDS n.6.1 Nacreous clouds .. 65 n.6.2 Noctilucent clouds . 66 n.6.3 Condensation trails (contrails) 66 n.6.4 Clouds from waterfalls. ... 67 n.6.5 Clouds from fires 0 0 • 0 • 0 67 n.6.6 Clouds from volcanic eruptions . 67 n.6.7 Clouds resulting from industry . 68 n.6.8 Clouds resulting from explosions 68 n.7 OBSERVATION OF CLOUDS FROM THE EARTH'S SURFACE n. 7.1 Introduction 0 • • • • 69 11.7.2 Identification of clouds 69 11.7.2.1 Identifying the genus . 70 H.7.2.2 Identifying the species . 71 H.7.2.3 Identifying the varieties 72 H. 7.2.4 Identifying·the supplementary features and the accessory clouds . 72 n.7.2.5 Determining the mother-cloud ....... 72 H.7.2.6 Identifying meteors associated with the clouds 72 n.7.3 Total cloud cover and cloud amount. 72 n.7.4 Height and altitude ....... 73 H. 7.5 Direction and speed of movement . 73 n.7.6 Optical thickness ....... 0 73 n. 7.7 Observations of clouds made from mountain stations 74 n.7.8 Observation of special clouds. 74 n.7.8.1 Nacreous and noctilucent clouds .... 0 • 0 • • 74 n.7.8.2 Other special clouds. .......... 0 • • • 74 n.8 THE CODING OF CLOUDS IN THE CODES CL, CM AND ~ AND CORRESPONDING SYMBOLS no 8. I Introduction to the coding of clouds. ... 0 • • • O' • 0 0 • • 0 • • • 75 n.8.2 Code specifications and coding procedures ............... 0 75 n.8.2.1 CL-clouds of the genera Stratocumulus, Stratus, Cumulus and Cumulonimbus 75 n.8.2.2 CM-clouds of the genera Altocumulus, Altostratus and Nimbostratus . 84 II.8.2.3 CH-clouds of the genera Cirrus, Cirrocumulus and Cirrostratus ... 91 n.8.3 Pictorial guides for the coding of clouds in the codes CL, CM and CH' 97 n.8.3.1 Description and procedure ... 0•••••••••• 97 II.8.3.2 Pictorial guide for the coding of clouds in the code CL 99 II.8.3.3 Pictorial guide for the coding of clouds in the code CM 100 n.8.3.4 Pictorial guide for the coding of clouds in the code CH 101 n.8.4 Symbols for clouds corresponding to the figures of the CL, CM and CH codes 102 ( VI CONTENTS Pages III. PART III METEORS OTHER THAN CLOUDS Ill. 1 CLASSIFICATION AND SYMBOLS OF METEORS OTHER THAN CLOUDS Ill. 1. 1 Classification of meteors other than clouds 105 IlL 1.2 Symbols for meteors other than clouds. 106 lll.2 DEFINITIONS AND DESCRIPTION OF METEORS OTHER THAN CLOUDS Ill.2.1 Hydrometeors other than clouds 109 Ill. 2.1.1 Hydrometeors consisting of a suspension of particles in the atmosphere 109 (1) Fog ("fog" and "mist") . 109 (2) Ice fog 109 ( IIl.2.1.2 Hydrometeors consisting of a fall of an ensemble of particles (precipitation) 110 (1) Rain . 110 (2) Supercooled rain . 110 (3) Drizzle . 110 (4) Supercooled drizzle. 111 (5) Snow. 111 (6) Snow grains . 111 (7) Snow pellets . 112 (8) Diamond dust . 112 (9) Hail 112 (10) Small hail . 113 (11) Ice pellets. 113 Ill.2. 1.3 Hydrometeors consisting of ensembles of particles raised by the wind 114 (1) Drifting snow and blowing snow. 114 (2) Spray. 114 Ill. 2. 1.4 Hydrometeors consisting of a deposit of particles 115 (1) Deposit of fog droplets 115 (2) Dew 115 ( (3) White dew 116 (4) Hoar frost 116 (5) Rime. 117 (6) Glaze. 118 lll.2.1. 5 Spout 118 Ill.2.2 Lithometeors . 119 Ill.2.2.1 Lithometeors consisting of a suspension of particles in the atmosphere . 119 (1) Haze. 119 (2) Dust haze . 119 (3) Smoke 119 III.2.2.2 Lithometeors consisting of ensembles of particles raised by the wind . 120 (1) Drifting and blowing dust or sand 120 (2) Dust storm or sandstorm 120 (3) Dust whirl or sand whirl (dust devil) 121 CONTENTS VII Pages IIl.2.3 Photometeors. .... 121 (1) Halo phenomena. 121 (2) Corona. 122 (4) Irisation 122 (4) Glory .. 123 (5) Rainbow 123 (6) Bishop's ring 124 (7) Mirage .. 124 (8) Shimmer . 124 (9) Scintillation 124 (10) Green flash i25 (11) Twilight colours 125 IIl.2.4 Electrometeors ... 126 (1) Thunderstorm. 126 (2) Saint Elmo's fire . 127 (3) Polar aurora. .. 127 IIl.3 OBSERVATION OF METEORS OTHER THAN CLOUDS FROM THE EARTH'S SURFACE Ill. 3.1 Introduction ................ 129 Ill.3.2 Observation of hydrometeors other than clouds 129 IIl.3.3 Observation of lithometeors 129 IlL3.4 Observation of photometeors 129 Ill. 3. 5 Observation of electrometeors . 130 ApPENDICES Appendix I- Etymology of latin names of clouds .... 133 Appendix Il - Historical bibliography of cloud classification 135 Appendix In - Bibliography of cloud nomenclature 139 ALPHABETICAL INDEX OF WORDS AND EXPRESSIONS . 147 ( ( ( ( IX PREFACE TO THE 1939 EDITIONl The first published 2 classification of clouds only dates back to the beginning of the 19th cen­ tury and was the work of Lamarck (1802). This celebrated naturalist did not set out to classify all possible clouds; he confined himself to distinguishing certain forms which seemed to him to be the manifestation of general causes which it would be useful to recognize. But this work, in spite of its real value, did not make any impression even in France and his nomenclature does not seem to have been used by anyone. Perhaps this was due to his choice of somewhat peculiar French names which would not readily be adopted in other countries or perhaps the paper was discredited through appearing in the same publication (Annuaire Meteorologique), as forecasts based upon astrological data. One year later Luke Howard published in England a cloud classification which, in striking contrast, achieved very great success and which is the basis of the existing classification. Whereas Lamarck contented himself with defining and naming a certain number ofinteresting forms, Howard set out to establish a complete classification covering all possible cases.