HOW TO KNOW THE CLOUDS 



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which can just be seen in the distance 

 above their tops. 



At other times clouds attain such tre- 

 mendous heights that it seems almost 

 impossible to form an estimate of their 

 elevation. The upper clouds are a most 

 important factor in the science of weather 

 forecasting, and their use for this pur- 

 pose has been noticed from very early 

 times, and handed down to us in one of 

 many of the old sayings, " the higher the 

 clouds, the finer the weather." 



By means of careful measurements 

 with appropriate instruments, the heights 

 of various types of clouds have been 

 accurately determined. On the basis of 

 these data three layers, so to speak, 

 have been adopted— namely, the upper, 

 the intermediate, and the lower layer. 

 The average height of the upper layer is 

 taken as about 9,000 metres, or about 

 30,000 feet. The intermediate extends 

 from about 3,000 to 7,000 metres, or it 

 ma}^ be said to have a mean height of 

 5,000 metres, or about 16,000 feet. The 

 lower layer has a height of 2,000 metres, 

 or 6,500 feet. 



In cloud classification, the nomencla- 

 ture is so adopted that in addition to the 

 form of the cloud the height is always 

 included. This procedure is found most 

 useful, because the same form of cloud 

 frequently occurs at different levels. 



Broadly speaking, it may be said that 

 clouds in the upper layer may be iden- 

 tified by the prefix cirro to their type, 

 and those in the intermediate layer by 

 the prefix alto. In the lower layer no 

 prefix is necessary, but the clouds assume 

 their single names — i.e., cumulus and 

 stratus. The above, however, is not 

 strictly true, but may be taken as a 

 second step towards mastering the 

 adopted international scheme, which is 

 as follows : — 



A. — Upper Clouds (average height 

 9,000 metres). 

 a I. Cirrus. 

 h 2. Cirro-stratus. 



B. — Intermediate Clouds (between 

 3,000 and 7,000 metres), 

 rt f 3. Cirro-cumulus. 

 I 4. Alto-cumulus. 

 b 5. Alto-stratus. 



C. — Lower Clouds (2,000 metres). 

 a 6. Strato-cumulus. 

 b 7. Nimbus. 



D. — Clouds of Diurnal Ascending 

 Currents. 



8. Cumulus. 



9. Cumulo-nimbus. 



E. — High Fogs. 



10. Stratus. 



If this classification be carefully exam- 

 ined it will be noticed that the words 

 cumulus and stratus are used to define 

 the form that clouds assume at even 

 high elevations. Thus, for instance, if 

 one observes cirrus cloud — that is, cloud 

 say five miles high — and it appears in 

 the form of sheets, it is termed cirro- 

 stratus ; while if it takes the form of 

 small, lumpy masses, as in the case of 

 mackerel sky, it is called cirro-cumulus. 

 Cirro-cumulus does not, however, occur 

 at quite so great an elevation as the cirro- 

 stratus, and so is classed at the head of 

 the intermediate layer, as will be seen 

 from the foregoing classification. 



Two other points in the classification 

 require, perhaps, further explanation. The 

 first is the term nimbus. This name 

 simply implies a cumulus cloud from which 

 rain or snow is falling, and is generally a 

 very thick, dark cloud. The second 

 point refers to the small letters a and b 

 before the cloud names. The former 

 means that the clouds are in separate 

 or globular masses, which is generally 

 the case in dry weather ; while h indicates 

 that the forms are widely extended or 

 completely cover the sky, as in wet 

 weather. 



Let us now take each of the types of 

 clouds in turn and, where available, 

 illustrate them by photographs, so that 

 a typical jiicture may be retained for 

 purposes of reference. It must, however, 

 be borne in mind that the varieties of 

 cloud forms are really inmmaerable, and 

 that no two pictures of the same kind 

 are alike, for, as already pointed out, 

 there are all stages of change from one 

 t\pe to the next. 



To begin with, let us start with the 

 high or ujiper clouds, which consist of 

 the cirrus and cirro-stratus tyi)es. 



