THE STATE OF THE SKY AND THE EVOLUTION 

 OF THE CLOUDS 



The necessity of notiiuj the state oj the sky as a whole. — It is evident 

 from the specifications of the cloud code that to describe the sky at the 

 station at a particular time logically and completely it is not enough to 

 know the genera or even the species of the clouds present ; for example 

 altocumulus appears in seven specifications of the code, and cirrus in 

 nine. In reality each specification of the code, as the explanations 

 show, is not so much a dry enumeration of the genera or species of 

 clouds in the sky as a general indication of the structure, the organiza- 

 tion, and the evolution of the cloud complex which makes up the state 

 of the sky. Some specifications only refer to the general structure; 

 for example Cii/= 9 is a thundery sky ; everyone knows that in thundery 

 conditions degenerate cloud forms are met with wliich are very difficult 

 to classify, wlule the thimdery look of the sky is apparent immediately 

 and without any doubt. 



Each specification of the code corresponds to a state of _ the sky — 

 lower, middle, or high.^ The observer should have at his "fingers' 

 ends" the commentaries accompanying the definitions; he should 

 consider as a whole the lower, middle, or high clouds there described, 

 and try to make a considered judgment of the observed sky as a whole, 

 so that he can directly apply the proper numbers of the code. 



The detailed analysis of the individual clouds should follow and not 

 precede this recognition of the state of the sky as a whole. If the observer 

 becomes accustomed to this course he will find in a short time that the 

 different states of the sky — lower, middle, and high — corresponding 

 with the code, will seem just as "live" as the typical cloud forms, and it 

 will be just as easy to identify a state of the sky as the form of a cloud. 



Necessity of following the evolution of the sky. — The aspect of the sky 

 IS continually changing, and many transitional forms exist between 

 the different types of cloud. It is relatively rare for the observer to see 

 typical clouds of one genus which float past, or persist in the sky for 

 any considerable time; in most cases he will find that he has difficulties 

 at the time of observation if he has not taken the trouble to watch the 

 sky since the last observation. If, however, he has taken this precau- 

 tion he will often be able to refer a confusing state of the sky, or a 

 particular cloud, to a previous state which was typical and easy to 

 identify. Moreover, most of the specifications of the cloud code take 

 into account the evolution of the clouds. A single isolated observation 

 is insufficient. 



As regards evolution, the recognition of the state of the sky, recom- 

 mended in the previous paragraph, is better than the identification of 

 clouds considered by themselves, for, as a matter of fact, the evolution 

 of the state of the sky can be followed indefinitely at one station, wliile 

 the evolution of a cloud, if, as is usual, it is a "migrant" can only be 

 observed during the relatively short time that it takes to cross the sky. 



1 In the International Atlas, the term "state of the sky" has two distinct meanings. (1) The form and ar- 

 rangement of the clouds in the sky at any given level — lower, middle, or high— is a "state of the sky" at that 

 level; this is the meaning of "state of the sky" in the code specifications. (2) The combination of all clouds 

 in the sky is also described as a "state of the sky," for example, a "front zone sky," an "emissary sky," etc. 

 In the latter sense it is used by the forecaster as an interpretation of the combination of code figures in the 

 synoptic message. 



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