INTRODUCTION 



At Copenhagen, Denmark, in 1929, the International Meteorologi- 

 cal Organization adopted code specifications for clouds including 

 lower, middle, upper, and predominating clouds. The third edition 

 of the International Atlas of Clouds and of the States of the Sky which 

 was published in 1932 and the abridged edition which was published 

 in 1930, contain detailed instructions for observing and coding cloud 

 forms in accordance with the specifications adopted at Copenhagen. 

 The cloud codes form a part of the International figure code for syn- 

 optic messages. 



Definitions, descriptions, and codes in the following pages are 

 based on the third edition of the International Atlas and are intended 

 for the use of observers who report clouds in synoptic messages in 

 the International Code. This publication is not designed for public 

 distribution. The cloud codes are of little interest to the general 

 public. The Weather Bureau has prepared a separate publication 

 entitled "Cloud Forms," which is especially designed for general 

 use in the identification and study of the forms of clouds. 



Observers who have access to the International Atlas should also 

 study the illustrations, diagrams, and comments contained therein. 



The observer will note the following details in which International 

 practices differ from those heretofore in effect in the Weather Bureau: 



1. The definition of cirrocumulus clouds has been restricted more 

 than in previous editions of the International Atlas. Real cirrocu- 

 mulus is now considered to be uncommon; it must have an evident 

 connection with cirrus or cirrostratus ; it must result from a change 

 in cirrus or cirrostratus; and it must show some characteristics of 

 ice crystal clouds. 



2. The distinction between cumulus and cumulonimbus has been 

 modified. By the new definitions, masses of cumulus clouds, however 

 heavy they may be, and however great their vertical development, are 

 not classed as cumulonimbus unless the whole or a part of their tops is 

 transformed or is in process of transformation into a cirrus mass. 

 In the species cumulonimbus calvus, however, no cirriform parts can 

 be detected at first but nevertheless the freezing of the upper parts 

 has already begun and the tops are beginning to lose their rounded 

 outlines and clear-cut contours; the hard and "cauliflower" swellings 

 soon become confused and melt away so that nothing can be seen 

 in the white mass but more or less vertical fibers. Hence, the cri- 

 terion for distinction in cases difficult to identify is the fibrous summit 

 that must be present if the cloud is to be called cumulonimbus with- 

 out having the clearly defined cirriform top. Precipitation may faU 

 from cumulus clouds, though only in relatively small quantities. 



3. Altocumulus clouds are now distinguished from stratocumulus 

 by the specific criterion that the well-defined, regularly arranged 

 elements which are observed in the layer are not altocumulus if they 

 are greater than 10 solar diameters in their smallest diameters, i. e., 

 the apparent width of three fingers when the arm is held extended. 



(Ill) 



